Patent Publication Number: US-6699132-B2

Title: Apparatus for damping vibrations

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a division of U.S. application Ser. No. 09/417,814, filed Oct. 14, 1999 now abandoned, which is a division of U.S. application Ser. No. 08/906,555, filed Aug. 5, 1997 now U.S. Pat. No. 5,980,387, which is a division of U.S. application Ser. No. 08/856,213, filed May 14, 1997 now U.S. Pat. No. 5,971,857, which is a division of U.S. application Ser. No. 08/320,732, filed Oct. 7, 1994 now abandoned, which is a division of U.S. application Ser. No. 08/060,490, filed May 7, 1993 now U.S. Pat. No. 5,487,704, which is a continuation of U.S. application Ser. No. 07/626,384, filed Dec. 12, 1990, now abandoned, which is a continuation of U.S. application Ser. No. 07/434,524, filed Nov. 7, 1989, now abandoned, which is a continuation of U.S. application Ser. No. 07/063,301, filed Jun. 17, 1987, now abandoned, each of these prior applications is hereby incorporated herein by reference it is entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to apparatus for damping vibrations, especially torsional vibrations between the output element (e.g., a crankshaft) of an engine and the power train in a motor vehicle. More particularly, the invention relates to improvements in apparatus of the type having at least two flywheels which are rotatable relative to each other against the opposition of damper means wherein one flywheel is the input member and the other flywheel is the output member of the damper means. The output member can be coupled to the power train by a clutch, particularly a friction clutch. 
     Heretofore known vibration damping apparatus of the above outlined type employ dampers which have energy storing elements acting in the circumferential direction of the flywheels and normally including coil springs which store elastic energy, and additional energy storing elements which act in the axial direction of the flywheels and cooperate with friction pads and/or linings to produce friction (i.e., hysteresis). The means for generating friction operate in parallel with energy storing means which act in the circumferential direction of the flywheels. 
     It has been found that certain conventional vibration damping apparatus can operate satisfactorily (i.e., they are capable of damping torsional vibrations as well as noise) but only under specific circumstances. Thus, the mode of operation of such conventional apparatus is not entirely satisfactory under many operating conditions because their design is a compromise due to an attempt to ensure satisfactory or acceptable operation under a variety of different conditions. For example, a purely mechanical solution does not suffice to cover a wide spectrum of operating conditions entailing the development of many basically different stray movements and noise levels. Moreover, purely mechanical solutions are quite expensive, especially if they are to adequately suppress stray movements and noise under a variety of different operating conditions. This is due to the fact that, if a mechanically operated vibration damping apparatus is to counteract a wide range of amplitudes of undesirable stray movements of the flywheels relative to each other, such undertaking greatly increases the cost, bulk, complexity and sensitivity of the apparatus. Moreover, even a very complex and expensive mechanical vibration damping apparatus is incapable of operating satisfactorily under any one of a wide range of different operating conditions because the individual damper stages (i.e., hystereses produced by individual energy storing elements which act in the circumferential direction of the flywheels) cannot be altered as a function of changes in operating conditions. Still further, presently known apparatus are subject to extensive wear so that their useful life is relatively short, and they are also prone to malfunction. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of the invention is to provide a vibration and noise damping apparatus whose versatility exceeds that of heretofore known apparatus and which can be used in a wide variety of systems for transmission of torque, especially between the engines and power trains of motor vehicles. 
     Another object of the invention is to provide an apparatus whose damping characteristics (i.e., the rate of energy dissipation) can conform to the vibration and/or noise generating behavior of motor vehicles under a wide variety of different operating conditions and/or other influences. 
     A further object of the invention is to provide an apparatus which can be used to connect existing engines or other prime movers with existing power trains. 
     An additional object of the invention is to provide an apparatus which operates properly at low or high rotational speeds as well as at resonance RPM and during starting or stoppage of the engine in a motor vehicle. 
     Still another object of the invention is to provide an apparatus which can properly prevent transmission of undesirable stray movements between an engine and a power train under a variety of apparently contradictory or conflicting circumstances without affecting the quality, reliability and/or reproducibility of the vibration- and/or noise-suppressing action. 
     Another object of the invention is to provide a relatively simple, compact and inexpensive apparatus which can be readily assembled or taken apart and whose useful life is eminently satisfactory for utilization between the engines and power trains of motor vehicles of all or nearly all kinds. 
     An additional object of the invention is to provide an apparatus which comprises a relatively small number of relatively simple and inexpensive parts and wherein the percentage of components which need not undergo secondary treatment in material removing tools and the like is higher than in heretofore known apparatus. 
     A further object of the invention is to provide an apparatus wherein the wear upon the parts which move relative to each other is not pronounced and whose utilization entails minimal losses in the driving system. 
     Another object of the invention is to provide novel and improved flywheels for use in the above outlined apparatus. 
     A further object of the invention is to provide a novel and improved method of broadening the range of utility of apparatus for counteracting vibrations and the transmission of noise between the engines and power trains of motor vehicles. 
     An additional object of the invention is to provide a motor vehicle which embodies the above outlined apparatus. 
     A further object of the invention is to provide the apparatus with novel and improved means for suppressing stray movements of several flywheels which are rotatable relative to each other and serve to transmit torque between a prime mover and a transmission or the like. 
     Still another object of the invention is to provide the apparatus with novel and improved means for damping stray movements of several flywheels with reference to each other. 
     A further object of the invention is to provide an apparatus which can generate a variety of damping actions, either simultaneously or during selected stages of transmission of torque between a prime mover and a power train or the like. 
     An additional object of the invention is to provide an apparatus wherein a highly satisfactory damping action can be generated by the medium which is used to prolong the useful life of moving parts. 
     A further object of the invention is to provide the apparatus with novel and improved means for transmitting torque between its components in such a way that the components can be readily separated, reassembled and inspected in a time-saving operation. 
     Another object of the invention is to provide the apparatus with novel and improved means for varying the vibration- and/or noise-damping action in automatic response to changes in operating conditions. 
     The invention is embodied in an apparatus for damping vibrations, especially between an engine and a power train. The apparatus comprises a composite flywheel including a first flywheel which is connectable with the engine (e.g., with a crankshaft which is driven by the engine) and a second flywheel which is connectable with the power train (e.g., by way of a friction clutch which is installed between the second flywheel and the input shaft of a change-speed transmission of the power train). The flywheels are rotatable relative to each other against the opposition of damper means which operates between the flywheels, the first flywheel constituting the input member and the second flywheel constituting the output member of the damper means. One of the first and second flywheels includes or carries a housing which defines at least one annular compartment having a substantially closed (e.g., substantially circular) cross-sectional outline. The damper means includes at least one damper having a plurality of deformable energy storing elements (such as coil springs) which are installed in the compartment, and the housing preferably closely conforms to the outlines of the energy storing elements (i.e., the energy storing elements of the one damper are snugly received in the compartment). The one damper further comprises means for deforming the energy storing elements in the compartment, and such deforming means includes first abutment means provided on the housing and located in the compartment and a deforming member (hereinafter called flange for short) which is rotatable with the other of the first and second flywheels and has second abutment means in the compartment. Still further, the damper means comprises a supply of viscous fluid medium (such as a paste) which at least partially fills the compartment. 
     The flange and the housing can define a narrow gap which communicates with the compartment, and the second abutment means preferably extends substantially radially of the one flywheel. The second abutment means can include radially outwardly extending arms which are integral parts of the flange and form an annulus in a plane making an angle of 90 degrees with the axes of the flywheels. At least one of the arms can include an extension which is disposed in the compartment radially outwardly of the adjacent energy storing element or elements and is preferably received in a portion of the compartment so that its inner side is adjacent the radially outermost portion or portions of the adjacent energy storing element(s). 
     The housing can include two substantially shell-shaped parts or sections and at least one of these sections can consist of a deformable (ductile) metallic sheet material which can be shaped in a press or a like machine. Each section can constitute a half shell. 
     The compartment is or can constitute a circumferentially complete annulus, and the first abutment means can constitute discrete stops in the compartment. Such stops can be riveted, welded or otherwise fixedly secured to the respective sections of the housing to alternate with the second abutment means (such as the aforementioned arms of the flange) in the neutral position of the one damper. 
     At least those portions of the abutment means which actually contact the energy storing elements can have a pronounced hardness. Such pronounced hardness can be achieved as a result of thermal treatment of the aforementioned portions of the abutment means. Alternatively or in addition to such thermal treatment, selected portions of the abutment means and/or of the energy storing elements and/or of the sections of the housing can be provided with coatings of a material which exhibits a pronounced hardness. 
     The first abutment means can be integral with the housing; for example, such integral first abutment means can include pockets which are provided on one or both sections of the housing and extend into the compartment. 
     The apparatus can comprise separately produced means for reducing frictional engagement of the housing with the energy storing elements, and such means is preferably disposed radially outwardly of the energy storing elements in the compartment and can include at least one insert in the form of a strip or band of steel or the like. The insert or inserts are received in suitable recess(es) of the housing. For example, the entire frictional engagement reducing means can include a single steel band whose end portions are anchored in the housing and whose material exhibits a pronounced hardness. 
     The band can have a concave side which faces the energy storing elements in the compartment and extends along an arc of 45-120 degrees, preferably along an arc of 60-90 degrees in the circumferential direction of the normally circular cross-sectional outline of the compartment. 
     The apparatus can further comprise retainer means interposed between at least one of the abutment means and the energy storing elements, particularly between the energy storing elements and the second abutment means. Each retainer means can have an outline which closely conforms to that of the surfaces forming part of the housing and bounding the compartment. The energy storing elements are preferably springs (such as coil springs) having hollow end portions and at least one of the retainer means has an extension in the end portion of the adjacent spring. Such extension can have a substantially conical shape to be readily receivable in the end portion of the adjacent spring. The conicity of the extension can be such that it automatically reenters the end portion of the adjacent spring upon each separation of such end portion from the extension in response to subsequent movement of the end portion of the spring toward the extension and/or vice versa. Each retainer means can act not unlike a piston for the fluid medium in the compartment, and at least one of the retainer means can define a path for the flow of fluid medium therethrough (e.g., through an opening or hole or notch or recess in the retainer means) substantially in the circumferential direction of the flywheels. 
     The compartment can have a varying cross-sectional area in the region of at least one energy storing element to influence the flow restricting action of the housing in such region and hence the damping action of the damper as a result of different resistance to the flow of fluid medium. 
     The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is an axial sectional view of a vibration damping apparatus which embodies one form of the invention and wherein the damper means comprises two dampers; 
     FIG. 2 is a fragmentary end elevational view as seen in the direction of arrow II in FIG. 1; 
     FIG. 3 is a fragmentary axial sectional view of a second apparatus wherein one section of the housing for the dampers serves to center the other section; 
     FIG. 4 is a fragmentary axial sectional view of a third apparatus wherein both sections of the housing are made of deformable metallic sheet material; 
     FIG. 5 is a fragmentary sectional view as seen in the direction of arrows from the line V—V of FIG. 4; 
     FIG. 6 is an axial sectional view of a fourth apparatus with a different sealing device between the flywheels radially inwardly of the inner damper; 
     FIG. 6 a  is an enlarged view of the detail within the phantom-line circle “X” of FIG. 6; 
     FIG. 7 is a fragmentary end elevational view of the fourth apparatus as seen in the direction of arrow VII in FIG. 6; 
     FIG. 7 a  illustrates the manner on anchoring the ends of a frictional engagement reducing band in the housing of the apparatus which is shown in FIGS. 6,  6   a  and  7 ; 
     FIG. 8 is a fragmentary axial sectional view of a fifth apparatus wherein the compartment for the outer damper is sealed from the compartment for the inner damper in a different way. 
     FIG. 9 is a fragmentary axial sectional view of a sixth apparatus wherein the sections of the housing for the damper or dampers are coupled to each other by a ring-shaped cage; 
     FIG. 10 is an axial sectional view of a seventh apparatus wherein the inner and outer dampers are connected in series; 
     FIG. 11 is a fragmentary axial sectional view of an eighth apparatus with a single damper; 
     FIG. 12 is a fragmentary sectional view as seen in the direction of arrows from the line XII—XII of FIG. 11; 
     FIG. 13 is a fragmentary sectional view as seen in the direction of arrows from the line XIII—XIII of FIG. 12; 
     FIG. 14 is a fragmentary axial sectional view of a ninth apparatus; 
     FIG. 15 is a fragmentary sectional view as seen in the direction of arrows from the line XV—XV of FIG. 14; 
     FIG. 16 is a fragmentary sectional view of a tenth apparatus with an abutment which can be used in the apparatus shown in other Figures; 
     FIG. 17 is a fragmentary axial sectional view of an eleventh apparatus wherein the radially outermost portions of the housing sections are configurated in a different way; 
     FIG. 18 is a fragmentary axial sectional view of a twelfth apparatus wherein each section of the housing for the damper means has an inner layer and an outer layer; 
     FIG. 19 is a fragmentary axial sectional view of a thirteenth apparatus wherein the damper means comprises a hydraulic damper and a dry friction generating device; 
     FIG. 20 is a fragmentary axial sectional view of a fourteenth apparatus wherein the damper means comprises a hydraulic damper and a slip clutch; 
     FIG. 21 is a fragmentary axial sectional view of a fifteenth apparatus with three concentric dampers disposed at different distances from the axes of the flywheels; and 
     FIG. 22 is a fragmentary axial sectional view of a sixteenth apparatus wherein two dampers are disposed at the same distance from the axes of the flywheels. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The apparatus  1  which is shown in FIGS. 1 and 2 is used to damp torsional vibrations between the internal combustion engine and the power train including a change-speed transmission of a motor vehicle. The apparatus  1  can be considered a unit of the power train and comprises a composite flywheel  2  having a first component  3  and a second component  4 . The components  3  and  4  (hereinafter called flywheels) are rotatable relative to each other and the flywheel  3  is non-rotatably affixed to the output element  5  (such as a crankshaft) of the engine by an annulus of bolts  6  or analogous fastener means. The flywheel  4  is connectable to the input shaft  10  of a change-speed transmission in response to engagement of a friction clutch  7 . 
     The friction clutch  7  comprises an axially movable pressure plate  8  which is disposed between the flywheel  4  and a clutch cover  11  and is non-rotatably but axially movably secured to the flywheel  4  and/or cover  11  by a set of leaf springs (not shown) in the customary way. A diaphragm spring  12  at the inner side of the cover  11  is tiltable between two annular seats and normally bears against the pressure plate  8  to urge the latter toward the friction surface  70  of the flywheel  4  whereby the friction surface  70  cooperates with the adjacent surface of the pressure plate  8  to clamp the friction linings at the periphery of a clutch plate or clutch disc  9  having a hub which is non-rotatably mounted on the input shaft  10  of the transmission. The means for disengaging the clutch  7  can comprise an antifriction bearing which is movable in the direction of arrow II in FIG. 1 in order to engage the radially inwardly extending prongs of the diaphragm spring  12  and to thereby tilt the spring so as to allow the pressure plate  8  to move axially and away from the flywheel  4 . 
     The apparatus  1  further comprises damper means including a first or outer damper  13  and a second or inner damper  14 . The dampers  13 ,  14  are disposed between the flywheels  3 ,  4  and serve to oppose but to permit angular movements of the flywheel  3  relative to the flywheel  4  and/or vice versa. The damper  14  operates in parallel with the damper  13 . 
     The means for rotatably mounting the flywheel  4  on the flywheel  3  or vice versa comprises anti-friction bearing means  15  including an antifriction ball or roller bearing  16  having a single annulus of rolling elements. The illustrated rolling elements are balls which are mounted between an outer race  17  and an inner race  19  of the bearing  16 . The outer race  17  is installed in an axial recess  18  of the flywheel  4 , and the inner race  19  surrounds an axial protuberance  20  which is an integral part of the flywheel  3 , which extends axially in a direction away from the output element  5  of the engine, and which is received in the recess  18  of the flywheel  4 . The inner race  19  is a press fit on a cylindrical peripheral surface or seat  20   a  of the protuberance  20  and is held against axial movement relative to the flywheel  3  by an external annular shoulder  21  of the protuberance  20  and a washer-like retaining ring  22  which is secured to the end face of the protuberance  20  by a set of screws or other suitable fastener means. 
     The means for holding the outer race  17  of the bearing  16  against axial movement relative to the flywheel  4  comprises two rings  23 ,  24  each of which has a substantially L-shaped cross-sectional outline and which extend into the recess  18 . A disc  27  cooperates with the rings  23 ,  24  to hold the outer race  17  against axial movement relative to the flywheel  4 . The disc  27  can be considered an integral part of the flywheel  4 ; it is permanently (or more or less permanently) secured to the flywheel  4  by rivets  26  or other suitable fastener means. The radially extending portion  23   a  of the ring  23  abuts the adjacent side of the disc  27 , and the radially extending portion  24   a  of the ring  24  abuts a shoulder  25  which is machined into or is otherwise formed in the recess  18 . Thus, the outer race  17  is confined between the rings  23 ,  24  and these rings are respectively flanked by the disc  27  and shoulder  25 . The rings  23 ,  24  together form a thermal insulator which prevents or reduces the transfer of heat between the friction surface  70  of the flywheel  4  and the bearing means  15 . Each of these rings further includes an axially extending cylindrical portion which surrounds the adjacent part of the peripheral surface of the outer race  17 . The portions  23   a ,  24   a  of the rings  23 ,  24  preferably extend radially inwardly beyond the outer race  17  so that they are adjacent the respective end faces of the inner race  19 . It is preferred to configurate and mount the radially extending portions  23   a ,  24   a  of the rings  23 ,  24  in such a way that they actually bear against (i.e., sealingly engage) the respective end faces of the inner race  19  so as to confine the lubricant (e.g., a suitable grease) for the rolling elements of the antifriction bearing  16 . The sealing action of the radially extending portions  23   a ,  24   a  can be enhanced by resilient elements  28 ,  29  (e.g., diaphragm springs) which are provided to urge the radially innermost parts of the portions  23   a ,  24   a  against the respective end faces of the inner race  19 . The resilient element  28  reacts against the disc  27  and bears against the radially innermost part of the radially extending portion  23   a , and the resilient element  29  reacts against the radially innermost portion of the flywheel  4  and bears against the radially innermost part of the radially extending portion  24   a.    
     The flywheel  3  constitutes or forms part of a housing defining an annular chamber  30  for the dampers  13  and  14 . This flywheel comprises two substantially shell-shaped parts or sections  31 ,  32  having radially outermost portions which are secured to each other by threaded fasteners  33  in the form of screws or the like. These fasteners ensure that the inner side or surface  34  of the part  31  abuts the adjacent side or surface  35  of the part  32 . The sides  34 ,  35  of the parts  31 ,  32  of the flywheel  3  are located radially outwardly of the chamber  30  and of the dampers  13 ,  14  therein. The means for sealing the chamber  30  in the region of the abutting sides  34 ,  35  of the parts  31 ,  32  comprises at least one sealing ring  36  which is recessed into the side  34  and/or  35  and is deformed in response to the application of fasteners  33 . Such fasteners are disposed radially outwardly of the sealing ring  36 . In the embodiment of FIGS. 1 and 2, the sealing ring  36  is recessed into a groove  37  in the side  34  of the part  31 . In order to ensure accurate positioning of the parts  31 ,  32  relative to each other during assembly of the apparatus  1 , these parts are provided with registering axially parallel bores or holes which are disposed radially outwardly of the sealing ring  36  and receive centering pins  38 . 
     The radially outermost portion of the part  31  of the flywheel  3  is formed with a circumferentially extending cylindrical surface  39  which is surrounded by a ring-shaped starter gear  40 . The parts  31 ,  32  of the flywheel  3  can be made of cast iron. However, if it is desirable to reduce the inertia of the flywheel  3 , at least one of the parts  31 ,  32  (particularly the part  31 ) can be made of a light metal alloy, particularly a casting of aluminum alloy. An advantage of such cast lightweight parts is that they can be mass-produced in accordance with a compression, molding, stamping or like technique and require a minimum of secondary treatment. 
     The axial position of the gear  40  can be selected by causing such gear to abut the tips of the fasteners  33 , i.e., the fasteners can serve as a means for locating the gear  40  in a predetermined axial position with reference to the flywheel  3 . 
     The dampers  13 ,  14  comprise a common output member in the form of a radial flange  41  which is disposed axially between the parts  31 ,  32  of the flywheel  3 . As shown in FIG. 2, the radially innermost portion of the flange  41  is non-rotatably but axially movably connected to the disc  27  by a torque transmitting connection  42 . The disc  27  is secured to the flywheel  4 , and more specifically to the end face of the axially extending projection  43  of the flywheel  4  by means of the aforementioned rivets  26 . The projection  43  extends toward the output element  5  of the engine. In order to facilitate and ensure accurate centering of the disc  27  on the projection  43  during assembly of the apparatus  1 , the projection  43  is or can be provided with a centering seat  43   a  for the disc  27 . 
     The flange  41  comprises radially outwardly extending abutments or arms  44  which alternate with energy storing elements  45  of the outer damper  13 . Each such energy storing element  45  constitutes an arcuate coil spring. The arms  44  alternate with recesses  46  for the respective coil springs  45 , and each such recess is disposed radially outwardly of one of three arcuate windows  47  for energy storing elements  48  (preferably coil springs) of the inner damper  14 . The flange  41  further comprises arcuate webs or ribs  49  which extend in the circumferential direction of the flywheel  3  between the recesses  46  and the windows  47 . The ribs  49  connect the neighboring arms  44  to each other and which also connect to each other radially extending partitions or webs  50  which are provided between neighboring windows  47  of the flange  41 . The coil springs  45  of the outer damper  13  can bear against the arms  44 , and the coil springs  48  of the inner damper  14  can bear against the radially extending webs  50 . The ribs  49  and webs  50  together form a ring between the dampers  13  and  14 . 
     The radially outermost portion of the chamber  30  has a substantially circular cross-sectional outline and forms a compartment  51  for the arms  44  of the flange  41  as well as for the coil springs  45  of the outer damper  13 . The compartment  51  is formed primarily by arcuate grooves  52 ,  53  which are mirror symmetrical to each other and are respectively provided in the sides or surfaces  34 ,  35  of the parts  31 ,  32 . The grooves  52 ,  53  flank the radially outermost portion (including the arms  44 ) of the flange  41  and each thereof receives a little less than one-half of each coil spring  45 . The radially innermost portion of the compartment  51  of the chamber  30  is substantially sealed by the ribs  49  of the flange  41  save for a relatively narrow radially extending clearance or gap  54  at one side of the flange  41 . The compartment has a circular cross-sectional outline which is complete (closed) save at the location of entry of the flange  41 . 
     As shown in FIG. 1, the configuration of the grooves  52 ,  53  in the parts  31 ,  32  of the flywheel  3  is selected in such a way that the corresponding portions of the coil springs  45  are received therein with a minimum of play. The surfaces bounding the radially outermost portions of the grooves  52  and  53  can serve to guide and confine the adjacent radially outermost portions of the convolutions of coil springs  45  in the compartment  51  of the chamber  30 . The arrangement is such that the radially outermost portions of the convolutions of coil springs  45  abut or can abut the surfaces bounding the adjacent radially outermost portions of the grooves  52  and  53 , at least when the composite flywheel  2  rotates and the coil springs  45  are acted upon by centrifugal force. A reasonably large surface-to-surface contact between the convolutions of the coil springs  45  and the parts  31 ,  32  of the flywheel  3  is often desirable in order to achieve a substantial reduction of wear and, more specifically, to distribute the wear over larger portions of the convolutions and surfaces bounding the grooves  52 ,  53  in the radially outermost portion of the compartment  51 . 
     The end convolutions of the coil springs  45  bear against abutments or stops  55 ,  55   a  which are provided in the compartment  51  so that they extend into the grooves  52 ,  53  and flank the adjacent radially outwardly extending arms  44  of the flange  41 . As shown in FIG. 2, the abutments  55  and  55   a  can be oriented in the same way as the adjacent arms  44 , i.e., substantially radially of the flywheel  3 . The abutments  55 ,  55   a  have substantially mirror symmetrical parts  56 ,  57  which are respectively received in the grooves  52 ,  53  and are affixed to the corresponding parts  31 ,  32  of the flywheel  3  by rivets  58  or in any other suitable way. Portions of the abutments  55 ,  55   a  are preferably flattened to ensure a more satisfactory engagement with the respective coil springs  45 . Each arm  44  is located between two abutments  55 ,  55   a  as seen in the axial direction of the apparatus  1 . 
     Cup-shaped spring retainers  59  are provided at the circumferential ends of the arms  44  (see FIG.  2 ). The cross-sectional area of each retainer  59  matches or approximates the cross-sectional area of the compartment  51 . Each retainer  59  is located between an arm  44  and one end of the respective coil spring  45 . 
     The sides or surfaces  34 ,  35  of the parts  31 ,  32  include circumferentially complete ring-shaped portions  60 ,  61  which are disposed radially inwardly of the compartment  51  and flank a ring-shaped passage  62  for the corresponding portion of the flange  41 . The width of the passage  62  (as measured in the axial direction of the flywheel  3 ) slightly exceeds the thickness of the corresponding portion of the flange  41  (namely the thickness of the portion between the portions  60 ,  61  of the sides or surfaces  34 ,  35 ) so that the flange  41  and at least one of the parts  31 ,  32  define the afore-mentioned gap  54 . The width of the gap  54  can be 0.1-2 mm. 
     The surface portions  60 ,  61  which flank the ring-shaped passage  62  are disposed radially outwardly of a second compartment which includes arcuate grooves  63 ,  64  machined into or otherwise formed in the parts  31 ,  32  and serving to receive portions of the coil springs  48  which form part of the inner damper  14 . The diameters of convolutions of the coil springs  48  are such that each such convolution extends axially beyond both sides of the adjacent portion of the flange  41 . 
     FIG. 1 shows that at least the radially outermost portions of the grooves  63 ,  64  are bounded by surfaces which are complementary to the surfaces of adjacent coil springs  48  so that the coil springs  48  are at least partially guided and confined by such surfaces, at least in the axial direction of the apparatus  1 . Each of the grooves  63 ,  64  preferably extends through an angle of 360 degrees, the same as the grooves  52 ,  53  which form part of the compartment  51  for the coil springs  45 . This is of advantage because the grooves  52 ,  53 ,  63  and  64  can be formed during casting of the parts  31 ,  32  and can be finished by turning or in accordance with another suitable material removing technique. The means for stressing the coil springs  48  (i.e., for stressing the energy storing elements of the inner damper  14 ) comprises abutments or stops  65 ,  66  which are installed in the grooves  63 ,  64  and are preferably provided with flattened portions in contact with the hollow ends of the coil springs  48 . Thus, the configuration of each of the abutments  65 ,  66  preferably conforms to that of the adjacent portion of the surface bounding the respective groove  63 ,  64 , and these abutments are affixed to the respective parts  31 ,  32  of the flywheel  3  by rivets  67 . FIG. 2 shows that the abutments  65 ,  66  (which are disposed at opposite sides of the radially extending webs  50  of the flange  41 , as seen in the axial direction of the apparatus  1 ) are shorter than the corresponding webs  50  (as seen in the circumferential direction of the flywheel  3 ). 
     The dimensions of the ribs  49  of the flange  41  are selected (with reference to the grooves  63 ,  64 ) in such a way that the coil springs  48  abut the ribs  49 , at least when the flywheel  3  rotates and the coil springs  48  are acted upon by centrifugal force. This does not entail an excessive amount of wear, especially if the flange  41  is made of steel which is hardened at least along its surface to thus reduce wear upon those portions which are disposed radially and are engaged by the coil springs  48 . Another advantage of the feature that the ends of the coil springs  48  bear radially against the webs  49  is that the coil springs  48  can be twisted or turned with the flange  41  before they engage the abutments  65 ,  66  and this does not result in the development of excessive friction with the parts  31 ,  32  of the flywheel  3  under the action of centrifugal force. Such friction is often undesirable because it can distort the characteristics of the outer damper  13 . 
     FIG. 2 shows that the apparatus  1  comprises three coil springs  45  and three coil springs  48 . Each outer coil spring  45  extends along an arc of close to or exactly 110 degrees, and the arcs formed by the inner coil springs  48  preferably equal or approximate the arcs covered by the outer coil springs  45 . In the embodiment of FIGS. 1 and 2, each inner coil spring  48  extends along an arc of approximately or exactly 100 degrees. Thus, the three outer coil springs  45  jointly extend along an arc which approximates 91 percent of a complete circle, and the three inner coil springs  48  jointly extend along an arc which approximates 83 percent of a complete circle. 
     The coil springs  45  and/or  48  are or can be straight prior to introduction into the respective compartments of the flywheel  3 . If the coil springs are originally straight, they must be deformed during insertion into the chamber  30 . However, it is equally within the purview of the invention to make the coil springs  45  and/or  48  in such a way that their curvature matches or approximates that of the respective compartments in the flywheel  3  before the springs are installed in the chamber  30 . The utilization of “prefabricated” coil springs whose curvature matches or approximates that of the grooves  52 ,  53  and  63 ,  64  is preferred for convenience of assembly as well as to reduce internal stresses which develop during compression. Moreover, the prefabricated coil springs (i.e., springs whose curvature matches or approximates the curvature of the compartment  51  and grooves  63 ,  64  prior to installation of the springs in the housing of the flywheel  3 ) are devoid of bending moments in neutral positions of the flywheels. 
     The chamber  30  contains a supply of a viscous fluid medium which is a lubricant. For example, the chamber  30  can be partially filled with silicon oil or grease. The quantity of viscous fluid medium is preferably selected in such a way that, when the apparatus  1  is idle (i.e., when the flywheels  3  and  4  do not rotate), the upper level of the supply of fluid medium extends at least to the level of the axis of the lowermost outer coil spring  45  so as to ensure that such spring will dip into the supply of lubricant. It is presently preferred to select the upper level of the fluid medium in such a way that the lowermost inner coil spring  48  dips into the fluid medium, at least at the lowermost point of its axis and at the very least to such an extent that at least a portion of one or more lowermost convolutions of the lowermost coil spring  48  dips into the supply of lubricant. This ensures the development of an adequate film of lubricant between the lowermost convolutions of the lowermost spring  48  and the adjacent surfaces, particularly the surfaces of the respective web or webs  49 , with attendant reduction of wear upon such surfaces. It is assumed that the axis of the apparatus  1  which is shown in FIGS. 1 and 2 is horizontal and that the supply of fluid medium in the chamber  30  extends to the level of the lowermost portion of the axis of the lowermost coil spring  48 . 
     An advantage of the feature that the chamber  30  is provided in the flywheel  3 , which is connected with the output element  5  of the engine, is that the chamber  30  is as remote from the flywheel  4  and from the friction clutch  7  as possible, i.e., that the supply of fluid medium in the chamber  30  is less likely to be affected by heat which is generated at the friction surface  70  of the flywheel  4  when the clutch  7  is being engaged or disengaged. 
     The apparatus  1  further comprises means for ventilating the region of the chamber  30  in the flywheel  3 . Such ventilating means includes a radially extending annular channel  68  which is provided between the part  32  and the flywheel  3  and the radially innermost portion of which communicates with passages  69  provided in the flywheel  4  radially inwardly of the friction surface  70 . The radially outermost portion of the channel  68  is open to the surrounding atmosphere. 
     As can be seen in FIG. 2, the flange  41  has a centrally located opening  71  bounded by a surface which is provided with radially inwardly extending tooth-like projections  72  mating with complementary tooth-like projections  73  at the periphery of the disc  27 . The projections  72  and  73  together constitute the aforementioned connection  42  which ensures that the flange  41  shares all angular movements of the disc  27  but that the flange  41  can find an optimum axial position between the parts  31 ,  32  of the flywheel  3 . This renders it possible to ensure that the width of the aforementioned clearance  54  between the part  32  and the flange  41  in the passage  62  can be very narrow or extremely narrow. Moreover, the connection  42  including the projections  72  and  73  renders it possible to readily compensate for eventual manufacturing tolerances of the parts whose surfaces are adjacent the flange  41 . 
     The means for sealing the chamber  30  further comprises a sealing device  74  which is installed between the disc  27  and the adjacent radially innermost portion of the part  32 . The sealing device  74  comprises a substantially washer-like sealing member  75  having an inner portion which engages a shoulder  76  on the aforementioned projection  43  of the flywheel  4 . A radially outer or outermost portion of the sealing member  75  bears against the adjacent ring-shaped surface  77  of the radially innermost portion of the part  32 . The sealing member  75  can undergo axial deformation not unlike a diaphragm spring. A diaphragm spring  78  is provided to bias the sealing member  75  axially against the shoulder  76  as well as against the surface  77 . The spring  78  is installed in prestressed condition between the sealing member  75  and the flange  41 . This spring further serves to bias the flange  41  axially toward the surface portion  60  so that the gap  54  develops only between the flange  41  and the surface portion  61 . This gap communicates with the compartment  51  and with those portions of the chamber  30  which are located radially inwardly of the compartment  51 . FIG. 1 shows that the device  74  seals the annular chamber  30  from the aforementioned channel  68  of the ventilating means for the housing (flywheel  3 ) of the chamber  30 . The inner diameter of the sealing member  75  exceeds the outer diameter of the annulus of tooth-shaped projections  73  on the disc  27  i.e., of the connection  42 . 
     The provision of the torque transmitting connection  42  and sealing device  74  renders it possible to simplify the assembly of the apparatus  1 . Thus, the flywheels  3  and  4  are assembled with certain parts in a first step, and the thus assembled flywheels are thereupon fixed in optimum axial positions relative to each other by fastening the retaining ring  22  to the protuberance  20  of the flywheel  3 . The sealing device  74  is mounted on the flywheel  3  prior to assembly of the flywheels with each other and the bearing means  15  is or can be form-lockingly mounted in the recess  18  of the flywheel  4  before the recess  18  receives the protuberance  20  of the flywheel  3 . When the flywheel  4  is being assembled with the flywheel  3 , the inner race  19  of the antifriction bearing  16  is slipped onto the protuberance  20  so that the internal surface of the race  19  surrounds the cylindrical external surface  20   a  of the protuberance  20 . The projections  73  are brought into mesh with the complementary projections  72  and, as the flywheels  3  and  4  are being assembled, the radially innermost portion of the sealing member  75  forming part of the sealing device  74  comes into abutment with the shoulder  76  so that the sealing member  75  is deformed (distorted) by the diaphragm spring  78  and bears against the shoulder  76 . The final axial fixing of the flywheels  3  and  4  relative to each other involves attachment of the retaining ring  22  to the protuberance  20  of the flywheel  3 . 
     The utilization of torque transmitting connection  42  between the flywheels  3  and  4  is particularly desirable and advantageous when the housing  31 + 32  forms an integral or detachable part of the flywheel  3 , i.e., of that flywheel which is connectable to the output element  5  of the engine, and when the sealing device  74  is constructed and assembled in the aforedescribed manner so that the sealing member  75  is axially stressed directly between the flywheels  3 ,  4  or between elements which are integral with or attached to the two flywheels. The connection  42  renders it possible to preassemble the apparatus  1  into two units or subassemblies each of which includes one of the flywheels  3 ,  4  and to thereupon connect the subassemblies to each other by slipping the inner race  19  of the bearing  16  onto the seat  20   a  of the protuberance  20  of the flywheel  3  prior to attachment of the retaining ring  22  to the end face of the protuberance  20 . As shown in FIG. 1, the bolts  6  which secure the flywheel  3  to the output element  5  of the engine can also serve as a means for attaching the retaining ring  22  to the protuberance  20 . 
     The aforediscussed mounting of the flange  41  between the parts or sections  31 ,  32  of the housing for the chamber  30  in such a way that the flange has a certain freedom of axial movement against the opposition of the diaphragm spring  78  in the sealing device  74  is desirable and advantageous on the additional ground that the flange  41  can find for itself an optimum orientation between the parts  31 ,  32  without unduly stressing the adjoining elements of the apparatus. Moreover, such mounting of the flange  41  ensures that the apparatus cannot generate a pronounced frictional hysteresis in response to small angular displacements of the flywheels  3 ,  4  relative to each other when the engine of the motor vehicle is idling and/or under certain other circumstances when the development of pronounced hysteresis is not desirable. 
     In order to reduce wear upon the convolutions of the coil springs  45 ,  48  and upon the adjacent portions of surfaces bounding the respective grooves  52 ,  53  and  63 ,  64 , it is often desirable and advantageous to harden the corresponding portions of the parts  31  and  32 . Such hardening can involve any suitable surface hardening procedure. Certain presently preferred treatments include induction hardening, case hardening, hardening with laser beams or flame hardening. If the wear upon the surfaces which are adjacent the coil springs  45  and  48  is expected to be or is indeed very pronounced, it is advisable to coat the corresponding portions of the parts  31 ,  32  with layers of highly wear-resistant material. Such layers can be applied to the surfaces bounding the entire grooves  52 ,  53  and  63 ,  64  or to selected portions of such surfaces, e.g., in regions where the wear as a result of rubbing contact with the coil springs  45 ,  48  is expected to be very pronounced. For example, the surfaces bounding the grooves  52 ,  53  and/or  63 ,  64  can be chemically coated with layers of chromium, nickel, monybdenum or a plastic or ceramic substance. The layers preferably consist of or contain a material which reduces the coefficient of friction between the convolutions of the coil springs and the housing  31 + 32  to a minimum. The applied layers of such material can undergo a secondary treatment, particularly a polishing or other smoothing treatment, in order to enhance the quality of the surfaces in the regions of rubbing contact with the coil springs. The secondary treatment can be carried out in a grinding, a milling or a like machine. 
     The recesses  46  in the flange  41  preferably extend along circular arcs of at least 45 degrees, more preferably 65-115 degrees and most preferably 80-100 degrees. When the flywheels  3  and  4  assume their idle or neutral positions, the coil springs  45  jointly extend along a circular arc which is between 70 and 96 percent of a complete circle. As mentioned above, the coil springs  45  are or can be prefabricated in such a way that their curvature matches or at least approximates the curvature of the recesses  46  and ribs  49 . 
     Angular displacement of the flywheels  3  relative to the flywheel  4  and/or vice versa through a relatively large angle is desirable and advantageous in most instances because the resistance to such angular displacement increases rather gradually, at least during the initial stage or stages of angular displacement, which is desirable for the damping of large torsional vibrations and/or abrupt changes in angular velocity of one of the flywheels relative to the other flywheel. Furthermore, this enables the viscous fluid medium in the chamber  30  to dissipate large quantities of energy, i.e., to produce a pronounced hysteresis. 
     However, the fluid medium is equally capable of damping angular displacements of small amplitude which require small hystereses and develop during operation under load. This is believed to be attributable to the fact that the pressure which develops in the fluid medium depends on momentary speed at which a certain volume of the fluid medium is being displaced. Thus, the damping capacity of the fluid medium (which fills at least the compartment  51  of the channel  30 ) depends on the nature (extent and velocity) of angular displacement of the flywheels relative to each other. This renders it possible to achieve a substantially automatic regulation of the damping action. 
     Each of the sections  31 ,  32  of the housing for the chamber  30  can include an inner layer which defines the compartment  51  and the compartment including the grooves  63 ,  64 , and an outer layer which surrounds the inner layer. A somewhat similar apparatus ( 1101 ) is shown in FIG.  18 . It is also possible to provide inner and outer layers in the region of the compartment  51  alone, or only in the region of the compartment which includes the grooves  63 ,  64  for the energy storing elements  48  of the inner damper  48 . The inner layers can be made of a highly wear-resistant material which contains or consists of nickel, chromium, molybdenum of a suitable synthetic plastic substance. 
     The number of recesses for the coil springs of the outer damper and the number of windows for the coil springs of the inner damper need not exceed four. This ensures that each coil spring is relatively long and allows for a large annular displacement of the flywheels  3  and  4  relative to each other. 
     The apparatus  1  of FIGS. 1 and 2 operates as follows: 
     When the flywheel  4  is caused to turn relative to the flywheel  3  from the illustrated idle position, the flange  41  is compelled to rotate with the disc  27  by way of the connection  42  whereby the outer coil springs  45  are compressed between the abutments  55 ,  55   a  and the arms  44  and store energy. After the flywheel  4  has covered the angle  79  (see FIG.  2 ), in one direction or the angle  80  in the opposite direction, the abutments  65 ,  66  reach the adjacent coil springs  48  so that, if the flywheel  4  continues to turn relative to the flywheel  3 , the coil springs  48  also begin to store energy. It goes without saying that the situation is analogous if the flywheel  3  turns relative to the flywheel  4  or if both flywheels turn but in opposite directions. The coil springs  45  and  48  continue to store energy at the same time until the compression of the inner coil springs  48  is completed, i.e., when each of the springs  48  resembles, or acts not unlike, a solid block which cannot undergo additional shortening in the circumferential direction of the flywheels. This terminates the angular movement of the flywheel  4  relative to the flywheel  3 . In the embodiment of FIGS. 1 and 2 (and starting from the idle position of FIG.  2 ), the flywheel  4  can turn with reference to the flywheel  3  through an angle of 47 degrees in either direction. The coil springs  45  rub against the surfaces surrounding the grooves  52 ,  53  of the housing (flywheel  3 ) for the chamber  30  when the flywheel  4  turns relative to the flywheel  3  and/or vice versa so that the springs  45  cooperate with the parts  31 ,  32  to produce a frictional damping action. Additional friction is generated as a result of rubbing contact between the flange  41  and the portion  60  of the surface  34  on the part  31  under the bias of the diaphragm spring  78 . Still further, frictional damping action develops as a result of sliding contact between the inner coil springs  48  and the adjacent surfaces bounding the grooves  63  and  64  of the parts  31 ,  32 , respectively. 
     The frictionally induced damping action between the coil springs  45 ,  48  on the one hand and the surfaces bounding the grooves  52 ,  53  and  63 ,  64  on the other hand varies as a function of changes of rotational speed. Thus, the frictionally induced damping action increases in response to increasing RPM of the flywheels  3 ,  4  because the coil springs  45 ,  48  are acted upon by centrifugal force which urges their convolutions against the outer portions of surfaces bounding the respective grooves  52 ,  53  and  63 ,  64 . 
     Additional damping action is generated as a result of turbulence in and displacement of viscous fluid medium in the chamber  30 . The body of viscous fluid medium in the compartment  51  produces a more pronounced (hydraulic or viscous) damping action because the compartment  51  is practically sealed from the remainder of the chamber  30  and the cup-shaped spring retainers  59  act not unlike pistons which slide in the arcuate cylinder-like compartment  51  of the chamber  30 . When the outer coil springs  45  undergo compression, the cup-shaped retainers  59  share the movements of the respective arms  44  toward the corresponding abutments  55 ,  55   a  (such abutments also carry, or they can also carry, cup-shaped retainers) so that the viscous fluid medium which is confined in the compartment  51  can escape (in response an abrupt change of the angular positions of flywheels  3 ,  4  relative to each other) only by way of the very narrow clearance or gap  54  which connects the compartment  51  with other portions of the chamber  30  radially inwardly of the springs  45 . The retainers  59  expel primarily viscous fluid which has filled the coil springs  45  prior to compression of such springs as a result of angular displacement of the flywheel  4  relative to the flywheel  3  and/or vice versa. The surfaces bounding the gap  54  act not unlike a flow restrictor. Some fluid medium is also forced to pass or leak between the cup-shaped retainers  59  and the surfaces surrounding the compartment  51 . The fluid medium which has been expelled from the compartment  51  radially inwardly is redistributed uniformly in the radially outermost portion of the chamber  30  under the action of centrifugal force. 
     When the springs  45  are allowed to dissipate energy, some viscous fluid medium in the compartment  51  is again caused to leak between the cup-shaped retainers  59  and the adjacent surfaces bounding the grooves  52 ,  53  and flows through the gap  54  prior to returning into the radially outermost portion of the chamber  30  under the action of centrifugal force to fill the compartment  51  so that the springs  45  are fully embedded in the fluid medium. The damping action of the fluid medium is a function of the centrifugal force, i.e., such damping action becomes more pronounced when the RPM of the flywheels  3  and  4  increases. 
     The inner coil springs  48  dip into the supply of viscous fluid medium in the chamber  30 , at least in part, to thereby generate turbulence which, in turn, produces a hydraulic or viscous damping action. 
     The damping action of viscous fluid medium can be altered within a wide range by the expedient of providing one or more cup-shaped retainers  59  with axially extending channels, recesses, grooves or holes and/or by the expedient of altering the width of the gap  54 . This renders it possible to conform the damping action to requirements in a particular power train. Additional regulation of the damping action which is furnished by the viscous fluid medium can be achieved by removing one or more retainers  59 , i.e., by providing cup-shaped retainers only for selected coil springs. It is further possible to provide cup-shaped retainers for one or both ends of one or more inner coil springs  48  and one or more webs  50  of the flange  41 . This renders it possible to carry out additional adjustments of the damping action which is furnished by the fluid medium in the chamber  30 . 
     The abutments  55 ,  55   a ,  65  and  66  (and/or the cup-shaped retainers  59 ) can be used as a means for determining and regulating the rate of fluid flow in the respective compartment(s) during certain stages of angular movement of the flywheels  3  and  4  relative to each other to thus ensure the establishment of a predetermined characteristic progress of damping action in dependency on certain operating parameters. Additional regulation can be achieved by appropriate selection of constrictions and/or enlargements in the housing including the parts or sections  31  and  32 , i.e., such housing can be configurated in such a way that the compartment  51  and/or the compartment  63 + 64  includes portions of constant cross-sectional area and portions of varying cross-sectional area. This will be described in greater detail with reference to FIG.  7 . 
     An advantage of the feature that the coil springs  45  and  48  practically fill the respective compartments  51  and  63 + 64  is that the surfaces bounding these compartments provide a highly satisfactory guidance for the respective coil springs so that each of the dampers  13 ,  14  can employ very long coil springs. Relatively long coil springs  45  and  48  allow for larger angular displacements of the flywheels  3  and  4  relative to each other. Furthermore, relatively long coil springs which undergo extensive compression and thereupon expand extensively back to their original length agitate and generate pronounced turbulence in the supply of viscous fluid medium. The turbulence is also generated by the arms  44  and webs  50  of the flange  41  and by the abutments  55 ,  56  and  65 ,  66  on the housing parts or sections  31 ,  32 . The hydraulic or viscous damping action which is generated in the just outlined manner varies as a function of the amplitude and frequency of angular displacement of the flywheel  4  relative to the flywheel  3  and/or vice versa and also as a function of the abruptness of such relative movements i.e., as a function of the angular velocity and acceleration. The damping action which is caused by the viscous fluid medium in the chamber  30  is also a function of the RPM of the engine, i.e., this damping action can be varied in dependency on a number of parameters including the angular velocity of movement of the flywheels relative to each other, the acceleration of the flywheels relative to each other and the angular velocity of the composite flywheel  2 ; each of these parameters can alter the damping characteristics and hysteresis of the apparatus  1 . 
     An advantage of the ribs  49  is that they guide the radially innermost portions of the coil springs  45  in the compartment  45  and the radially outermost portions of coil springs  48  in the compartment including the grooves  63  and  64  of the housing  31 + 32 . The radially outermost portions of the coil springs  48  bear against and are guided by the ribs  49  while they undergo compression or expansion and they merely bear against the ribs  49  when they are not in the process of storing or dissipating energy. The ribs  49  serve a useful purpose also during the intervals when the coil springs  48  do not undergo compression, i.e., while the webs  50  of the flange  41  move relative to the coil spring  48  and/or vice versa. Furthermore, all of the coil springs  48  need not be expanded or compressed at the same time (this will be explained with reference to FIGS.  6  and  7 ), i.e., such coil springs can be grouped to operate during different stages of angular movement of the flywheels  3  and  4  relative to each other. There is no relative sliding movement between the convolutions of the coil springs  48  and the ribs  49  while the coil springs  48  merely rotate with the flange  41 . Thus, no frictional damping action is generated by the flange  41  and coil springs  48  during the just discussed stage or stages of operation of the apparatus  1 . 
     As shown in FIG. 1, the two sections or parts  31 ,  32  of the housing for the chamber  30  constitute the entire flywheel  3 . However, and as shown for example in FIG. 4, the flywheel  3  can include the sections or parts of the housing plus one or more additional parts. 
     The abutments  55 ,  55   a ,  65  and  66  can constitute plates, blocks or heads of rivets whose shanks are anchored in the respective parts  31 ,  32  of the housing for the chamber  30 . Furthermore, the abutments  55 ,  55   a ,  65  and/or  66  can be welded to the respective parts of the housing. As mentioned above, wear upon the abutments and on the arms  44 , ribs  49  and webs  50  of the flange  41  can be reduced considerably if at least the most affected portions of the surfaces of such elements are surface hardened or coated with layers of hard wear-resistant material. Chromium, molybdenum, nickel and certain plastic substances are presently preferred coating materials. Moreover, it is possible to use certain ceramic materials which can be treated to a high degree of finish and can stand long periods of use without extensive wear. 
     As a rule, or at least in many instances, the frictional and/or hydraulic damping action of the inner damper  14  is much less pronounced than that of the outer damper  13  which is in parallel with the damper  14 . This can be achieved by providing cup-shaped and/or otherwise configurated spring retainers only in the compartment  51  and/or by designing the retainers for the coil springs  45  in such a way that they fit more snugly in the compartment  51  than the retainers which are received in the compartment including the grooves  63 ,  64  for the coil springs  48 . In other words, the displacement of fluid medium in the outer compartment should be more pronounced than the displacement of fluid medium in the inner compartment and the flow restrictor means for the fluid in the outer compartment should produce a throttling action which is more pronounced than the throttling action of flow restrictor means in the inner compartment. If the coil springs of the inner damper  14  are assembled into several groups, only one of these groups can be provided with retainers so that the throttling action varies in response to progressing angular displacement of the flywheels relative to each other. Moreover, the coil springs  48  or at least some of the coil springs  48  can be received in the housing  31 + 32  with a play which exceeds the play between the coil springs  45  and the parts or sections  31 ,  32 . As mentioned above, the damping action can also be influenced by appropriate selection of the quantity of fluid medium in the chamber  30 , e.g., in such a way that the compartment  51  is invariably filled when the flywheels  3 ,  4  rotate but the compartment for the coil springs  48  is filled only in part. This ensures that the damping action of the damper  13  is very pronounced in immediate response to start of angular displacement of at least one flywheel with reference to the other flywheel. The damping action of coil springs  48  (which are normally only partly immersed in the fluid medium) is less pronounced (in fact, it can be much less pronounced than that of the coil springs  45 ). 
     The apparatus of FIGS. 1 and 2 can be modified in the following way: The surfaces  60 ,  61  bounding the passage  62  and/or the adjacent surfaces of the flange  41  (including the surfaces of the ribs  49 ) can include or constitute ramps which extend in the circumferential direction of the flywheel  3  and are designed to alter the effective area of the gap  54  in response to angular displacement of the flywheel  3  and/or  4  from its neutral position with reference to the other flywheel, preferably in such a way that the effective area of the gap  54  decreases with increasing angular displacement from the neutral position. In other words, the flow restrictor including the flange  41  and the adjacent parts  31 ,  32  of the housing for the chamber  30  become more effective with increasing angular displacement of one of the flywheels with reference to the other flywheel. The aforementioned ramp or ramps can be provided at one side, at the other side or at both sides of the flange  41  and the height of such ramp or ramps varies in the axial direction of the flywheels  3  and  4 . 
     FIG. 3 shows a portion of a second apparatus  101  wherein the part  132  of the flywheel  3  is made of a deformable metallic sheet material and includes a cylindrical portion  132   a  which surrounds the part  131 . The part  132  is adjacent but spaced apart from the flywheel  4 . The internal surface  135  of the portion  132   a  is adjacent the peripheral surface  134  of the part  131 ; the surface  134  serves as a means for centering the part  132  with reference to the part  131  and flywheel  4 . A sealing ring  136  (e.g., an O-ring) is recessed into a groove  137  in the surface  134  of the part  131  to seal the radially outermost portion of the chamber  130  from the atmosphere. A radially extending shoulder  135   a  of the part  132  is located radially outwardly of the compartment for the coil springs  45  and abuts a portion of the radially extending side or surface  134   a  of the part  131 . The shoulder  135   a  is closely or immediately adjacent the internal surface  135 . 
     The means for holding the parts  131 ,  132  of the flywheel  3  against axial movement away from each other comprises radially extending centering members or pins  138  in holes which extend transversely of the surfaces  134 ,  135 . The pins  138  are preferably so-called heavy type dowel pins and their outer end portions are surrounded by the starter gear  140 . Each of these pins extends radially across the portion  132   a  of the part  132  and into the radially outermost portion of the part  131 . It will be noted that the sealing ring  136  is disposed between the annulus of pins  138  (only one pin  138  is actually shown in FIG. 3) and the radially outermost portion of the chamber  130 . The starter gear  140  surrounds a cylindrical seat  139  forming part of the peripheral surface of the portion  132   a . The gear  140  serves its primary purpose and also as a means for holding the properly inserted pins  138  against movement radially and away from the axis of the flywheel  3 . 
     It is clear that the connection between the parts  131 ,  132  of the flywheel  3  which is shown in FIG. 3 can be used with equal or similar advantage between the parts  31 ,  32  of the flywheel  3  which is shown in FIG. 1 as well as between analogous parts of flywheels in other embodiments of the improved apparatus. In other words, the connection of FIG. 3 can be used between parts which are made by casting or in sheet deforming or like machines. 
     An advantage of housing parts which are made of deformable metallic sheet material is that they can be produced at a fraction of the cost of making such parts in a casting machine or in a material removing machine. Moreover, the making of such parts of deformable metallic sheet material renders it possible to impart thereto practically any desired shape. The shaping operation can be carried out in a stamping, embossing, drawing, coining or other suitable machine. The making of one or both parts of the housing for the chamber  130  from such materials is especially desirable and advantageous when the compartment for the coil springs of the outer damper and/or the compartment for the coil springs of the inner damper is not a complete annulus, e.g., if the parts or sections of the housing must be provided with constrictions of the type shown, for example, in FIG.  5 . Still further, such mode of making the parts of the housing renders it possible to produce the aforediscussed abutments or stops (corresponding to the abutments  55 ,  55   a  and  65 ,  66  shown in FIG. 1) of the flywheel  3  as integral constituents of the respective sections or parts. This obviates the need for the utilization of rivets  58 ,  67  and analogous fasteners and contributes significantly to lower initial and assembly cost of the entire apparatus. 
     Proper angular positioning of the parts or sections  131 ,  132  relative to each other can be ensured by providing the cylindrical surfaces  134 ,  135  with non-uniformly distributed holes or bores for the centering pins  138 , i.e., by selecting the distribution of the holes in such a way that each hole in the surface  134  registers with a hole in the surface  135  only in a single angular position of the part  131  with reference to the part  132 . 
     The sealing device  174  is located radially inwardly of the inner damper between the sections or parts  131 ,  132 . Thus, the coil springs of the inner damper are or can be contacted by the viscous fluid medium. However, it is also possible to install one or two sealing devices between the compartment for the coil springs  145  and the compartment for the coil springs  148 , i.e., the inner damper can remain dry. 
     FIGS. 4 and 5 show a portion of a third apparatus  201  wherein the housing for the chamber  230  includes two parts  231 ,  232  each of which is made of a deformable metallic sheet material. The parts  231 ,  232  are portions of the flywheel  3 . The radially outermost portions of the parts  231 ,  232  are permanently connected to each other at  238 , e.g., by welding. This even more reliably ensures that the surfaces  234 ,  235  of the parts  231 ,  232  remain in permanent sealing engagement with each other and this also obviates the need for a sealing ring (such as  36  or  136 ). The permanent connection at  238  can be established in an electron beam welding, resistance butt welding, pressure welding or other suitable welding machine. The radially innermost portion of the part  231  is connected with an axial extension  220  of the flywheel  3  which performs the function of the protuberance  20  shown in the apparatus of FIGS. 1-2 and is surrounded by the antifriction bearing means  16 . The extension or protuberance  220  has a centering seat  220   b  for the part  231  of the flywheel  3 , and the part  231  abuts a radially extending shoulder  220   c  of the extension  220 . Rivets  200  are provided to ensure that the part  231  remains in contact with the shoulder  220   c . These rivets can be replaced by welds analogous to the connections  238  between the surfaces  234 ,  235  of the parts  231  and  232 . Alternatively, a portion of the extension  220  can be upset to the left of the radially innermost portion of the part  231  to thus establish a permanent and wobble-free connection between  220  and  231 . 
     If the parts which form the housing for the chamber  30 ,  130  or  230  are made of deformable metallic sheet material, the aforediscussed abutments or stops for the coil springs of the inner and/or outer dampers in the respective chambers can constitute integral parts of the housing. This is shown in FIG. 5 wherein the abutments  255 ,  255   a  are integral parts of  231 ,  232 , respectively. These abutments resemble pockets which are formed as a result of suitable deformation of the corresponding parts  231  and  232 . This simplifies the making and assembly of the apparatus, i.e., the number of parts which must be separately produced and assembled is reduced considerably. 
     For the purpose of satisfactory welding, the material (such as steel) of the parts or sections  231 ,  232  should have a relatively low carbon content. It suffices if the carbon content is low in those regions of the surfaces  234 ,  235  which are actually welded to each other (at  238 ). 
     The apparatus  301  which is shown in FIGS. 6,  6   a ,  7  and  7   a  again comprises a composite flywheel having two discrete components or flywheels  3 ,  4  which are rotatable relative to each other with the respective races of an antifriction ball bearing  16 . The means for holding the flywheels  3  and  4  against axial movement away from each other comprises a ring-shaped retainer  322  which is affixed to the end face of the axial protuberance  320  of the flywheel  3  by a set of rivets  322   a  or the like. The manner in which the flywheels  3 ,  4  are assembled with each other is or can be the same as described in connection with FIGS. 1 and 2. Thus, the antifriction bearing  16  can be mounted in the flywheel  4  and is thereupon pushed onto the protuberance  320  of the flywheel  3  so that its inner race surrounds the cylindrical surface  320   a  at the periphery of the protuberance  320 . A sealing device  374  is mounted on the flywheel  3  (i.e., on the flywheel which is nearer to the output element of the engine) before the protuberance  320  is inserted into the axial recess of the flywheel  4 . The connection  342  between the radially innermost portion of the flange  341  and the radially outermost portion of the disc  327  facilitates assembly of the apparatus  321 . The flange  341  again constitutes the output element of the outer damper  13  as well as of the output element of the inner damper  14 . The disc  327  is secured to the flywheel  4  by rivets  326 . 
     The parts  331 ,  332  which constitute the housing for the chamber  330  are castings. The radially outermost portion  332   a  of the part  332  is a cylinder having a cylindrical internal surface  335  which is adjacent a sealing ring  336  and surrounds the complementary cylindrical external surface  334  of the part  331  so that the latter centers the part  332  and its cylindrical portion  332   a . Radially extending pins  338  are provided to hold the parts  331 ,  332  against axial movement away from each other; such pins are received in registering bores or holes provided therefor in the cylindrical surfaces  334  and  335 . The starter gear  340  surrounds the cylindrical portion  332   a  and prevents expulsion of the pins  338  from their respective bores. 
     The torque transmitting connection  342  again comprises tooth-like projections  372  which extend radially inwardly from the internal surface of the flange  341  and complementary tooth-like projections  373  which are provided on the disc  327  and mate with the projections  372 . 
     FIG. 6A shows the details of the sealing device  374  which is installed between the radially innermost portion of the part  332  on the one hand and the axial protuberance or projection  343  of the flywheel  4  and disc  327  on the other hand. The device  374  comprises washer-like sealing member  375  which is elastically deformable in the axial direction and has an inner portion engaging a ring-shaped insert  376  on the projection  343 . The outer portion of the sealing member  375  is coupled to the radially innermost portion of the part  332  so that it is held against movement in the axial direction of the apparatus  301 . The sealing member  315  is formable not unlike a diaphragm spring and its radially outermost and innermost portions are coated with layers  375   a ,  375   b  which can consist of or contain synthetic plastic material and can be applied by spraying. The material of the layers  375   a ,  375   b  should have a low coefficient of sliding friction and should exhibit a certain amount of elastic or plastic deformability. A ring-shaped coupling and centering member or carrier  380  is provided on the part  332  and is configurated to form an annular groove or socket receiving the radially outermost portion (including the layer  375   a ) of the sealing member  375 . The confinement of the radially outermost portion of the sealing member  315  can change its conicity. That portion ( 380   b ) of the carrier  380  which defines the aforementioned groove or socket is received in a ring-shaped centering notch  377  provided therefor in the radially innermost portion of the part  332 . The carrier  380  comprises two radially outwardly extending collars  380   a  which flank the annular innermost portion  332   b  of the part  332  so as to securely locate the carrier  380  in a desired axial position. The carrier  380  can be said to constitute a swivel bearing for the sealing member  375  of the sealing device  374 . 
     The ring-shaped insert  376  has a surface which is adjacent a surface of the sealing member  375  to form therewith a seal against penetration of foreign matter into the radially innermost portion of the chamber  330  as well as against escape of viscous fluid medium from the chamber. A disc-shaped radially innermost portion  376   a  of the insert  376  is clamped between the projection  343  of the flywheel  4  and the disc  327 , and a dished radially outermost portion  376   b  of the insert  376  engages the radially innermost portion of the sealing member  375  so that the latter is held in axially stressed position and the insert and sealing member define the aforementioned seal at the radially innermost locus of the chamber  330 . 
     The portions  376   a  and  376   b  of the insert  376  are offset with reference to each other in the axial direction of the apparatus  301  in such a way that the portion  376   a  is immediately adjacent the tooth-like projections  373  of the disc  327  but the portion  376   b  is axially offset in a direction away from the disc  327  and toward the flywheel  4 . The insert  376  cooperates with the sealing member  375  not only to seal the radially innermost portion of the chamber  330  from the atmosphere but also to seal such radially innermost portion of the chamber  330  from the radially extending ventilating channel  368  between the parts  331 ,  332  on the one hand and the flywheel  4  on the other hand. 
     In order to facilitate assembly of the flywheels  3  and  4  into the apparatus  301  which is shown in FIGS. 6 and 7, the inner diameter of the sealing member  375  exceeds the outer diameter of the annulus including the radially outwardly extending tooth-like projections  373  which are provided on the disc  327  and form part of the connection  342 . The portion  376   b  of the insert  376 , which is in engagement with and stresses the sealing member  375  in the axial direction of the apparatus  301 , extends radially outwardly beyond the tooth-like projections  373  of the disc  327 . 
     When the antifriction bearing  16  is slipped onto the cylindrical portion  320   a  of the peripheral surface of the protuberance  320  on the flywheel  3 , the projections  373  move into mesh with the projections  372  of the flange  341  to thereby establish the connection  342 . At the same time, the portion  376   b  of the insert  376  engages and stresses the sealing member  375  to ensure the establishment of a seal between  376   b  and  375   b.    
     In order to prevent or reduce wear upon the surfaces which bound the annular compartment  351  (including the grooves  352 ,  353  in the parts  331 ,  332 ) of the chamber  330  and are contacted by the convolutions of coil springs  345  forming part of the outer damper  13  in the chamber  330 , there is provided a strip- or band-shaped frictional engagement reducing member or insert  381  of hardened metallic material (such as steel). The member  381  surrounds the radially outermost portion of the compartment  351  and is adjacent the radially outermost portions of the coil springs  345 . In accordance with a presently preferred embodiment, the member  381  constitutes a short cylinder which is received in a shallow recess  382  provided therefor in the part  331  of the flywheel  3 . The recess  382  can be formed during casting of the part  331  or is machined into the part  331  thereafter. When the apparatus  301  rotates, the convolutions of the coil springs  345  tend to move radially outwardly under the action of centrifugal force and thus bear against the internal surface of the member  381 . The manner in which the member  381  is secured against slippage with reference to the flywheel  3  is shown in FIG. 7 a . Thus, the member  381  is a split ring with end portions  381   a  bent radially outwardly into a notch  383  provided in the part  331  of the flywheel  3 . 
     The circumferential abutments or stops  355 ,  355   a  for the coil springs  345  of the outer damper  13  and the circumferential abutments or stops  365 ,  366  for the coil springs  348  of the inner damper  14  in the chamber  330  are separately produced forgings, stampings or like elements which are respectively provided with one-piece rivets  358 ,  367  for attachment to the respective parts  331 ,  332  of the flywheel  3 . 
     FIG. 7 shows that the abutments  355 ,  355   a  which flank the arms  344  of the flange  341  extend beyond the respective arms  344  in the circumferential direction of the flywheel  3 . In the idle or neutral position which is shown in FIG. 7, the arms  344  are disposed centrally of the respective abutments  355 ,  355   a , i.e., such abutments extend circumferentially beyond both ends of the respective arms  344  through identical distances. 
     The abutments  365 ,  366  which flank the radially extending webs  350  of the flange  341  also extend circumferentially beyond the respective webs  350 . The webs  350  alternate with the coil springs  348  of the inner damper  14  in the chamber  330 . In contrast to the positions of abutments  355 ,  355   a  with reference to the respective arms  344  in the idle position of the flange  341 , the abutments  365 ,  366  then extend beyond one end only of the respective webs  350  (as seen in the circumferential direction of the flywheel  3 ). The other end of each web  350  can be flush with the respective ends of the associated abutments  365 ,  366 . The selection of the circumferential offset of the abutments  365 ,  366  and the respective webs  350  relative to each other is such that the neighboring abutments  365  as well as the neighboring abutments  366  are offset in opposite directions. This ensures that the inner coil springs  348  (there are four coil springs  348 ) form two groups  348   a ,  348   b  which become effective during different stages of angular displacement of the flywheels  3 ,  4  relative to each other. 
     The chamber  330  again contains a supply of viscous fluid medium (such as silicon oil or grease) which should at least fill the compartment  351  when the apparatus  301  is rotated. It is presently preferred to select the quantity of fluid medium in such a way that it not only fills the compartment  351  but also contacts at least the radially outermost portions of the coil springs  348  when the flywheels  3  and  4  are driven. A filling to the level such that the supply of fluid medium extends radially inwardly to the axes of the coil springs  348  has been found to be quite satisfactory. 
     Cup-shaped spring retainers  359  are installed in the compartment  351  between the arms  344  and abutments  355 ,  355   a  on the one hand and the respective end portions of the coil springs  345  on the other hand. The dimensions of the retainers  359  are or can be selected in such a way that they at least substantially fill the respective portions of the compartment  351 , i.e., the retainers  359  can act not unlike pistons when they are caused to move along the surfaces bounding the compartment  351  or vice versa. This ensures that the retainers  359  can produce a desirable damping action by throttling the flow of viscous fluid medium between their peripheries and the surfaces bounding the respective portions of the compartment  351 . The damping action is or can be the same as described in connection with FIGS. 1 and 2. 
     Each cup-shaped retainer  359  is provided with a slightly conical extension or stub  359   a  which normally extends into the adjacent end convolutions of the respective coil spring  345 . This can be seen in the top portion of FIG.  7 . Each stub  359   a  has a tip  359   b  which is conical but can also be roof-shaped. The just described design of the retainers  359  ensures that the stubs  359   a  automatically find their way back into the end convolutions of the adjacent coil springs  345  even if the stubs  359   a  are completely separated from the neighboring springs  345  during certain stages of angular movement of the flywheels  3  and  4  relative to each other. Thus, when a spring  345  is free to expand, its end convolutions automatically receive the stubs  359   a  of the adjacent retainers  359 , and the same holds true when the retainers  359  move toward the adjacent end portions of the respective coil springs  345 . This not only guarantees a more reliable operation but also reduces the likelihood of damage to the springs  345  and/or to the retainers  359 . The stubs  359   a  of the retainers  359  are likely to leave the adjacent end portions of the respective coil springs  345  when the springs  345  are compressed and the apparatus  301  is driven at a relatively high RPM. Under such operating conditions, friction between the convolutions of the springs  345  and the adjacent surfaces bounding the compartment  351  of the chamber  330  is or can be so pronounced that the springs  345  cannot expand or cannot fully expand in immediate response to an abrupt change of load. During such abrupt change of load, the arms  344  of the flange  341  displace the viscous fluid medium in the compartment  351  and the fluid medium thereupon flows back radially outwardly under the action of centrifugal force. Such flow of fluid medium during an abrupt change of load and the resulting angular displacement of the flywheels  3 ,  4  relative to each other can result in expulsion of stubs  359   a  of the retainer  359  from the adjacent end portions of the respective coil springs  345  because the springs  345  are slow to expand for the aforediscussed reasons. 
     It will be noted that the layer or coating  375   a  on the radially outermost portion of the sealing member  375  is not in the path of flow of viscous fluid medium radially inwardly in response to an abrupt angular displacement of the flywheel  3  relative to the flywheel  4  and/or vice versa. This ensures that the fluid medium (e.g., grease) which flows radially inwardly along the right-hand side of the flange  341  (as seen in FIG. 6 or  6   a ) cannot penetrate into the notch  377  of the part  332  to escape into the channel  368  between the part  332  and the flywheel  4 . The notch  377  is sufficiently deep (as seen in the axial direction of the flywheels) to ensure that the major part at least of the layer  375   a  can be received therein to thus maintain the sealing member  375  away from the path of the fluid medium when the latter flows radially inwardly along the sealing device  374 . An additional advantage of the construction which is shown in FIG. 6 a  is that, when the fluid medium in the chamber  330  is pressurized in response to abrupt angular displacement of the flywheels relative to each other, the fluid medium acts upon the entire left-hand side of the sealing member  375  so as to urge the latter into a more pronounced sealing engagement with the part  332  (at  375   a ) as well as against the flywheel  4  (i.e., against the insert  376  which can be said to constitute a portion of the flywheel  4 ). Thus, the sealing action of the member  375  is enhanced in automatic response to pressurization of fluid medium in the chamber  330 . 
     The apparatus  301  operates as follows: 
     When the flywheel  4  is caused to turn with reference to the flywheel  3  so that it leaves the idle position of FIG. 7, the flange  341  is compelled to rotate through the medium of the connection  342  (i.e., the projections  373  of the disc  27  on the flywheel  4  transmit torque to the projections  372  of the flange  341 ). This results in compression of coil springs  348  which form the group  348   b  because the corresponding webs  350  of the flange  341  move with reference to the abutments  365 ,  366  on the parts  331 ,  332  of the flywheel  3 . When the flywheel  4  completes an angular displacement through the angle  379  in one direction or through the angle  390  in the opposite direction, the webs  350  of the flange  341  (which turns with the flywheel  4 ) engage the adjacent ends of the coil springs  348  which form the group  348   a  so that, if the flywheel  4  continues to turn with reference to the flywheel  3 , the coil springs  348  of the group  348   a  are compressed and store energy jointly with the coil springs  348  of the group  348   b . When the flywheel  4  thereupon completes an additional angular displacement through the angle  379   a  in one direction or through the angle  390   a  in the opposite direction, the arms  344  of the flange  341  engage and begin to compress the respective coil springs  345  so that the springs  345  begin to store energy (or to store additional energy, depending upon their initial condition) because they are acted upon by the arms  345  in conjunction with the respective abutments  355 ,  355   a  on the parts  331 ,  332  of the flywheel  3 . In the embodiment which is shown in FIGS. 6 to  7   a , the angle  379  equals or closely approximates the angle  379   a , and the angle  390  equals or closely approximates the angle  390   a . Thus, the coil springs  345  store energy simultaneously with the coil springs  348  of the group  348   a . Therefore, the characteristic curve of these springs is a two-stage curve. 
     It is equally within the purview of the invention to design the apparatus  301  in such a way that the angles  379 ,  390  merely approximate the respective angles  379   a ,  390   a  or that the angles  379 ,  390  are entirely different from the angles  379   a ,  390   a , respectively, i.e., the characteristic curve can have three or more stages or steps in one direction of rotation and two stages or steps in the opposite direction or the characteristic curve can have more than two stages or steps in one direction and three or more stages or steps in the opposite direction. 
     The arrangement may be such that each coil spring  345  begins to store energy during a different stage of angular displacement of the flywheels relative to each other. The same applies for the coil springs  348  of the inner damper. This applies regardless of whether the dampers are connected in series or in parallel. 
     It is further possible to shift the abutments  365 ,  366  with reference to the coil springs  348  of the group  348   b  to positions corresponding to that which is shown in FIG. 7 by phantom lines, as at  365   a , to thus ensure that the bias of the springs  348  in the group  348   b  does not change in immediate response to angular displacements of the flywheel  3  and/or  4  from the idle position of FIG. 7 in either direction. At such time, the apparatus  301  merely produces a hydraulic or viscous damping action and/or a frictional damping action. 
     The magnitude or characteristics of the hydraulic or viscous damping action can be varied in a number of ways. For example, the number of cup-shaped retainers  359  can be reduced (i.e., only certain coil springs  345  can be provided with such retainers or only one end of each coil spring can be provided with a retainer). Furthermore at least one coil spring  348  in the group  348   a  and/or  348   b  can be provided with one or two cup-shaped retainers corresponding to the retainers  359  or analogous retainers. Other factors which influence the hydraulic or viscous damping action include the selected quantity of fluid medium in the chamber  330  and/or the width of the clearance or gap between the flange  341  and the portion  360  or  361  of the surface on the part  331  or  332  of the flywheel  3 . Additional damping action is produced as a result of turbulence of the viscous fluid medium in the chamber  330 . The exact manner in which such damping action is produced is the same as described in connection with FIGS. 1 and 2. 
     FIG. 7 shows that the outer damper  13  of the apparatus  301  comprises four equidistant coil springs  345  and the inner damper  14  comprises four equidistant coil springs  348 . Each of the coil springs  345  extends along an arc of or close to 78 degrees, each coil spring  348  in the group  348   b  extends along an arc of or close to 74 degrees, and each coil spring  348  of the group  348   a  extends along an arc of or close to 68 degrees. Thus, the four coil springs  345  jointly extend along approximately 86 percent of a complete circle, and the four coil springs  348  jointly extend along approximately 79 percent of a complete circle. 
     The flywheel  4  includes a portion  4   b  which has radially outwardly extending projections or lugs  386  (FIG. 7) each of which has a tapped axially parallel bore  387  to facilitate the attachment of a friction clutch. One or more lugs  386  are further provided with bores or holes  388  which are parallel to the respective tapped bores  387  and serve for reception of pins (not shown) which facilitate centering of the clutch cover on the flywheel  4  during assembly of the friction clutch with the flywheel  4 . 
     The lugs  386  contribute to a reduction of the weight of the flywheel  4  as a result of removal or absence of material in recesses or tooth spaces  386   a  which alternate with the lugs  386 , as seen in the circumferential direction of the flywheel  4 . Moreover, the recesses  386   a  provide paths for the flow of air which cools the flywheel  4 , the supply of fluid medium in the chamber  330  and the clutch which is affixed to the flywheel  4 . Atmospheric air which flows through the recesses  386   a  contacts the flywheel  4  and the aforementioned cover of the clutch which is attached to the flywheel  4  by threaded fasteners extending into the tapped bores  387  of the lugs  386 . The flywheel  4  is further formed with air-conveying passages  369  which communicate with the radially innermost portion of the channel  368  between the flywheels  3  and  4 . 
     The thickness of the lugs  386  on the portion  4   b  of the flywheel  4  can exceed the thickness of the remaining portion of the flywheel  4 . Such design of the flywheel  4  can be resorted to in order to ensure that the mass or weight of the flywheel  4  will equal or approximate a preselected value and/or to prevent overheating of the portion  4   b  (which is outwardly adjacent the friction surface  4   a ). 
     The damping action which is provided by the viscous fluid medium can be further varied by forming the parts  331 ,  332  of the flywheel  3  with grooves  352 ,  353  which have portions of different cross-sectional areas. Thus, the compartment  351  can include at least one portion of larger cross-sectional area and at least one portion of smaller cross-sectional area. The damping action in the portion or portions of larger cross-sectional area is less pronounced. The compartment  351  can be configurated in the just described manner in the region of one, two, three or all four coil springs  345 . The portions of larger cross-sectional area can be provided anywhere along the length of one or more coil springs  345  but are preferably provided in regions receiving the end portions of the coil springs  345  when such springs-are not compressed or store a minimum of energy. The transitions from the portions of smaller cross-sectional area to the portions of larger cross-sectional area or vice versa can be gradual or abrupt. It is presently preferred to provide the enlarged portions of the compartment  351  in the region or regions of the smaller-diameter portion of such compartment. This can be seen in FIG. 7, as at  389 , where the enlarged portion of the cross-sectional area of the compartment  351  is close to the axis of the composite flywheel  3 + 4  and includes a first part with abrupt transition from the larger cross-sectional area to the smaller cross-sectional area as well as a portion with a gradual transition. The enlarged portion of the compartment  351  can be formed by removing material from the part  331  or  332  of the flywheel  3  and/or from the flange  341  (this is actually shown in FIG.  7 ). The damping action of viscous fluid medium can be varied within a rather wide range by appropriate selection of the length and/or cross-sectional area of the enlarged portion or portions of the compartment  351 , i.e., by enlarging one or more portions of the groove  351  and/or  352  and/or by removing more or less material from the flange  341  in the region of the outer damper  13 . 
     FIG. 8 shows a portion of an apparatus  401  wherein the part  432  of the housing for the chamber which confines the inner and outer dampers has a circumferentially complete groove  460  for a sealing ring  460   a . The sealing ring  460   a  is elastic in the radial direction and can constitute an open wire ring or it can be made of a synthetic plastic material. The cross-sectional configuration of the groove  360  is oval or otherwise elongated and this groove extends at an angle outwardly from the locus (adjacent the flange  441 ) where it ends in the left-hand side or surface of the part  432 . The sealing ring  460   a  tends to contract to normally engage the adjacent side or surface of the flange  441 , but the ring  460   a  expands under the action of centrifugal force when the RPM of the apparatus  401  is increased so that it ceases to establish a seal between the part  432  and the flange  441 . This entails an increase in the effective cross-sectional area of the clearance or gap  454  between the flange  441  and the part  432 , i.e., the sealing action of the ring  460   a  is reduced or terminated and the gap  454  offers a lesser resistance to the flow of viscous fluid medium into and from the radially outermost portion (compartment  451 ) of the chamber which is defined by the parts  431  and  432 . The diameter of the sealing ring  460   a  is reduced automatically when the RPM of the apparatus  401  is reduced so that the viscous fluid medium urges the sealing ring into more pronounced sealing engagement with the flange  441  in response to increasing angular displacement of the one flywheel relative to the other and/or vice versa. 
     The part  531  of that flywheel which is affixed to the output element of the engine has an axial extension  431   a  in the form of a short cylinder which surrounds the radially outermost portion of the part  432 . The extension  431   a  serves as a means for centering the part  432  and it also cooperates with the sealing ring  436  to prevent escape of fluid medium from the compartment  351  radially outwardly between the abutting sides or surfaces of the parts  431  and  432 . 
     The apparatus  501  of FIG. 9 comprises a strip- or band-shaped frictional engagement reducing member or insert  581  which corresponds to the member  381  of the apparatus  301  and has an arcuate cross-sectional outline with a concave side facing the coil springs  545 . This member is made of a hard or hardened material which can resist extensive wear as a result of repeated frictional engagement with the convolutions of the coil springs  545  in the compartment  551 . The member  581  can be made of steel and can be hardened in any suitable way. The curvature of the concave side of the member  51  preferably equals or approximates the curvature of convolutions of the adjacent springs  545 . As shown in FIG. 9, the member  581  can surround the radially outermost portion of the compartment  551  along an arc of approximately 90 degrees; this member is received in shallow recesses  531   a ,  532   a  which are provided therefor in the respective parts  531 ,  532  of the housing which forms part of the left-hand flywheel and defines the chamber for the inner and outer dampers. The aforementioned are can be in the range of 45-120 degrees, preferably 60-90 degrees. 
     Instead of using a member  581  which is hardened, either entirely or along its surface, one can employ a member which has a relatively soft core and a coating of highly wear-resistant material such as hard or solid nickel or chromium. It is also possible to make the member  581  of a highly wear-resistant plastic material. 
     The apparatus  501  of FIG. 9 exhibits the advantage that the useful life of the flywheel which is connected with the output element of the engine or of the entire apparatus can be prolonged by the simple expedient of replacing a worn or damaged member  581  with a fresh member. In other words, the parts  531 ,  532  (which can constitute solid castings) are not subject to any wear, or to extensive wear, because they are not in intensive friction engagement with the coil springs  545 . 
     The means for coupling the parts  531  and  532  to each other comprises a ring-shaped member or cage  533  which is or can be made of suitable metallic sheet material and surrounds the radially outermost portion  532   b  of the part  532  as well as a portion  531   a  of the part  531  adjacent the starter gear  540 . The radially inwardly extending portions  533   a  and  533   b  are integral with a cylindrical web of the cage  533  and flank the portions  531   b ,  532   b  to thus prevent the parts  531 ,  532  from moving axially and away from each other. 
     The means for preventing angular displacements of the parts  531 ,  532  relative to each other comprises axially parallel centering members or pins  538  each of which can constitute a so-called heavy type dowel pin and which are received in registering axially parallel bores or holes of the portions  531   b ,  532   b . The radially inwardly extending portions  533   a  and  533   b  of the cage  533  hold the pins  538  against axial movement. The left-hand radially extending portion  533   a  of the cage  533  is adjacent the starter gear  540  which is connected to and surrounds the part  531 . 
     The utilization of a frictional engagement reducing member in the form of a strip or band having a concave side or surface which is adjacent the convolutions of the coil springs  545  is desirable and advantageous because this greatly enlarges the area of contact between the member  581  and the coil springs with attendant reduction of pressure per unit area of the abutting surfaces and a considerable reduction of wear. It normally suffices if the member  581  extends around the coil springs  545  along an arc of between 45 and 120 degrees, normally between 60 and 90 degrees. 
     The one-piece member  581  (or the one-piece member  381  of FIG. 6) can be replaced with a composite member including a plurality of arcuate portions each of which is or can be separately embedded in the housing for the damper or dampers. The length of each arcuate portion can equal or approximate the maximum length of a coil spring in the respective compartment (as measured in the circumferential direction of the flywheels). 
     The flywheel  3  of the apparatus  601  which is shown in FIG. 10 comprises a housing including the parts  631  and  632  which define an annular chamber  630  for the inner damper  14  and the outer damper  13 . The coil springs  645  (only one shown) of the outer damper  13  are installed in the compartment  651  of the chamber  630 . The dampers  13 ,  14  are connected in series. One coil spring of the inner damper  14  is shown at  648 . The parts  631 ,  632  constitute the input element of the outer damper  13  and are provided with abutments or stops  655 ,  655   a  for the end portions of the coil springs  645 . As shown, the abutments  655  and  655   a  are respectively riveted to the parts  631  and  632 . 
     A flange  641  constitutes the output element of the outer damper  13  and the input element of the inner damper  14 . 
     The apparatus  601  further comprises disc-shaped members in the form of washers  665 ,  666  which flank the flange  641  radially inwardly of the compartment  651  and are rigidly connected to each other by distancing elements in the form of rivets  667  which are anchored in the flywheel  4 . The washers  665 ,  666  are provided with windows  665   a ,  666   a  which register with windows  641   a  in the flange  641  and receive the coil springs (energy storing elements)  648  of the inner damper  14 . The coil springs  648  serve to yieldably oppose angular movements of the flange  641  and washers  665 ,  666  relative to each other. The flange  641  is further provided with radially outwardly extending arms  644  which alternate with the coil springs  645  of the outer damper  13 , i.e., the arms  644  extend into the compartment  651  of the chamber  630 . 
     The apparatus  601  also comprises an antifriction ball bearing  16  which is installed between the flywheels  3  and  4  in the same way as described in connection with FIGS. 1 and 2. A sealing device  674  operates between the radially innermost portion of the part  632  and the adjacent portion of the washer  666 . The parts  631  and  632  are formed with grooves which jointly define the compartment  651  as well as a second compartment for portions of the coil springs  648 . 
     A friction generating device  690  is provided between the flywheels  3  and  4  adjacent the antifriction bearing  16 . This device is also confined in the chamber  630  and surrounds the protuberance  620  of the flywheel  3  between the bearing  16  and washer  665  on the one hand and the radially extending portion  961  of the section or part  631  on the other hand. The friction generating device  690  comprises a prestressed energy storing element  692  which is composed of two neighboring diaphragm springs and operates between the inner race of the bearing  16  and a pressure-applying ring  693 . A friction pad  694  in the form of a washer is disposed between the ring  693  and the radially innermost portion  691  of the part  631 . The pad  694  can be made of synthetic plastic material and has radially outwardly extending projections or prongs  694   a  which alternate with spaces for the heads  667   a  of the aforementioned distancing elements or rivets  667 . The spaces between the prongs  694   a  provide room for some angular movement of the pad  694  relative to the flywheel  4  and vice versa. Thus, the flywheel  4  can turn the pad  694  with reference to the adjacent flywheel  3  when the heads  667   a  of rivets  667  come into abutment with the prongs  694   a  at the one or the other end of the respective spaces between prongs  694   a . It will be noted that, when the direction of angular movement of the flywheel  4  relative to the flywheel  3  or vice versa is reversed, the friction generating device  690  is ineffective during the initial stage of angular movement of the flywheel  3  or  4  in the opposite direction. The extent of that angular displacement of the flywheel  3  or  4  during which the friction generating device  690  is ineffective is determined by the diameters of the heads  667   a  of the rivets  667  and by the length of the spaces between neighboring prongs  694   a  (as seen in the circumferential direction of the flywheels  3  and  4 ). The play between the heads  667   a  of the rivets  667  and the prongs  694   a  of the friction pad  694  renders it possible to shift that portion of the total angular displacement of the flywheel  3  relative to the flywheel  4  and/or vice versa during which the friction generating device  690  is effective with reference to the angular positions in which the energy storing coil springs  645 ,  648  begin to store energy. 
     Confinement of the friction generating device  690  in the chamber  630  is desirable and advantageous because this ensures that the moment of friction which is generated by the device  690  is constant or practically constant during the entire useful life of the apparatus  601 . 
     It is possible to dimension the prongs  694   a  of the friction pad  694  and/or the heads  667   a  of the rivets  667  in such a way that the pad  694  is compelled to share all angular movements of the flywheel  4 , i.e., that the friction generating device  690  is effective during each and every stage of angular movement of the flywheel  3  relative to the flywheel  4  and/or vice versa. Alternatively, the friction pad  694  can be extended radially outwardly into the region of the coil springs  648  and can have one or more windows for a corresponding number of coil springs  648 . This enables the coil spring or springs  648  in such window or windows to restore the angular position of the friction pad  694 , either entirely or in part. 
     The radially outermost portions  631   b ,  632   b  of the parts  631 ,  632  are coupled to each other by a ring-shaped cage  633  which is or can be made of a metallic sheet material. The radially inwardly extending portions of the cage  633  flank the portions  631   b ,  632   b  of the parts  631  and  632  to thereby hold such parts against axial movement away from each other. The means for holding the parts  631 ,  632  against rotation relative to each other comprises axially parallel pins  638 , such as heavy type dowel pins, which are received in registering bores or holes of the portions  631   b ,  632   b . Each pin  638  further extends into a registering bore or hole in the right-hand radially extending portion  633   b  of the cage  633 . Thus, the pins  638  also serve to hold the cage  633  against angular movement relative to the parts  631 ,  632  of the flywheel  3 ; this is desirable because the cage  633  is provided with means (to be described hereinafter) which limits the extent of angular movability of the flywheels  3  and  4  relative to each other. 
     The radially inwardly extending portion  633   b  of the cage  633  is disposed between the flywheel  4  and the part  632  and its radially innermost part has profiled portions in the form of teeth  633   c  which cooperate with pin- or stud-shaped projections  658  on the flywheel  4  to determine the extent of angular movability of the flywheels  3  and  4  relative to each other. The projections  658  cannot move with reference to the flywheel  4  and the cage  633  and its teeth  633   c  cannot move relative to the flywheel  3  (because the pins  638  are anchored in the parts  631 ,  632  as well as in the portion  633   b  of the cage  633 ). The projections  658  (each of which can constitute a dowel pin) cooperate with the teeth  633   c  to determine the extent of angular movability of the flywheels  3 ,  4  relative to each other and they also serve as a means for centering the cover (not shown) of a friction clutch which can be mounted on the flange  4   a  of the flywheel  4  in the same way as described in connection with FIGS. 1 and 2. The left-hand end portions of the projections  658  extend into the radially outermost portion of the radially extending ventilating channel  668  between the flywheel  4  and the part  632 . The channel  668  communicates with passages  669  which are provided in the flywheel  4  radially inwardly of the inner damper  14 . 
     The friction generating device  690  (or an analogous friction generating device) and/or the cage  633  (or an analogous cage) with means ( 633   c ) for limiting the extent of angular movability of the flywheels  3  and  4  relative to each other can be employed with equal or similar advantage in apparatus wherein the dampers  13 ,  14  are connected in parallel rather than in series. For example, the friction generating device  690  and the cage  633  with its teeth  633   c  can be used in the apparatus  1  of FIGS. 1 and 2. 
     Referring to FIGS. 11 and 12, there is shown an apparatus  701  having a composite flywheel  702  with two components or flywheels  703  and  704 . Antifriction bearing means  15  is interposed between the flywheels  703 ,  704  which can rotate relative to each other. The flywheel  703  includes a housing which defines an annular chamber  730  for a damper  713 . The housing of the flywheel  703  includes two sections or parts  731 ,  732  having radially outermost portions which are outwardly adjacent the chamber  730  and are connected to each other. Each of the parts  731 ,  732  can be made of deformable metallic sheet material and their radially outermost portions can be bonded (e.g., welded) to each other, as at  738 . The weld  738  simultaneously serves as a means for sealing the radially outermost portion of the chamber  730  from the surrounding atmosphere. The welding operation can be carried out in a resistance butt welding machine or in a so-called stored-energy high-rate discharge welding machine. In each of these machines, the welding operation is carried out by placing the parts to be welded against each other and by applying to them low-voltage high-amperage alternating current to heat the parts to welding temperature and to unite such parts in response to the application of pressure. In order to allow for the carrying out of such welding operation, the parts  731 ,  732  of the flywheel  703  are provided with surfaces  734 ,  735  which can be placed into abutment with each other and each of which has a predetermined area for the required current strength. The surfaces  734 ,  735  are welded to and abut each other in a plane which extends at right angles to the axis of the composite flywheel  702 . 
     In order to properly center the part  732  relative to the part  731  in the course of the welding operation, the part  731  comprises a cylindrical portion  731   a  which surrounds the cylindrical peripheral surface  735   a  of the part  732 . Accurate angular positioning of the parts  731 ,  732  relative to each other during welding of the surfaces  734 ,  735  to each other is ensured by welding pins (not shown) which project into axially parallel recesses or sockets  765 ,  766  of the parts  731 ,  732  in the course of the welding operation. This ensures that the parts  731 ,  732  are bonded to each other in optimum angular and radial positions. 
     The making of the weld  738  between the surfaces  734 ,  735  of parts  731 ,  732  involves a certain axial displacement of these parts relative to each other. Therefore it may be desirable or advantageous to provide the part  731  and/or  732  with one or more axially extending stops which become effective only in the course of the welding operation. FIG. 11 shows, by broken lines, a stop  767  which is provided on the part  732  of the flywheel  703 . The provision of stops  767  renders it possible to make the quality of the welding operation less dependent upon the exact current strength, i.e., it is possible to operate with greater current strengths because the axial positions of parts  731 ,  732  with reference to each other are determined by the stops  767  rather than by the selected current strength and the axial pressure which is applied to the parts  731 ,  732  in the course of the welding operation. 
     The output element of the damper  713  is a radially disposed flange  741  which is installed between the parts  731 ,  732  of the flywheel  703 . The connection  742  between the radially innermost portion of the flange  741  and a disc  727  at the end face of the axial protuberance or projection  743  of the flywheel  704  is of such nature that the flange  741  can move axially with reference to the disc  727  and vice versa. The means for securing the disc  727  to the projection  743  (which extends toward the part  731  of the flywheel  703 ) includes rivets  726  or analogous fastener means. 
     The flange  741  is formed with radially outwardly extending arms  744  which alternate with the energy storing coil springs  745  of the damper  713  and extend into the annular compartment  751  of the chamber  730 . The compartment  751  constitutes the radially outermost portion of the chamber  730  and receives the coil springs  745 . This compartment includes two annular grooves  752 ,  753  which are respectively provided in the radially extending surfaces of the parts or sections  731 ,  732  at the level of the damper  713 . The making of grooves  752 ,  753  presents no problems since the parts  731 ,  732  consist of a deformable metallic sheet material. The central portions of the coil springs  745  (as seen in the axial direction of the apparatus  701 ) are located in the plane of the flange  741 , and the outer portions of such springs extend into the respective grooves  752 ,  753 . The flange  741  comprises an arcuate portion in the form of a rib  749  which is disposed radially inwardly of the compartment  751  and seals this compartment from the remainder of the chamber  730  save for the provision of a relatively narrow clearance or gap  754 . 
     The configuration of surfaces bounding the grooves  752 ,  753  in the parts  731 ,  732  of the flywheel  703  is preferably such that their curvature conforms to that of the adjacent portions of coil springs  745  in the compartment  751 . Thus, the radially outermost portions of the coil springs  745  could come into actual contact with the adjacent portions of the surfaces bounding the grooves  752 ,  753 , at least when the apparatus  701  rotates so that the coil springs  745  are acted upon by centrifugal force. 
     In order to reduce or avoid wear upon the just discussed surfaces of the parts  731  and  732 , the apparatus  701  can comprise a band- or strip-shaped member  781  which is recessed into the part  731  radially outwardly of the compartment  751  and is thus adjacent those portions of convolutions of the coil springs  745  which are most likely to rub against the flywheel  703  under the action of centrifugal force. The hardness of the material of the band-like member  781  can greatly exceed the hardness of the parts  731 ,  732 . The illustrated member  781  is a short cylinder and is received in a recess  782  of the part  731 . The recess  782  can be formed during casting of the part  731  or it can be machined into the part  731  in a grinding, milling or other suitable machine which employs material removing tools. The axial length of the member  781  suffices to ensure that it is contacted by the radially outermost portions of convolutions of the coil springs  745  when the apparatus  701  is in use, i.e., at least while the flywheels  703  and  704  rotate with and/or relative to each other. 
     The parts  731 ,  732  respectively carry abutments or stops  755 ,  755   a  which extend into the respective grooves  752 ,  753  and cannot rotate relative to the flywheel  703 . These abutments can be engaged by the end convolutions of the coil springs  745  in the compartment  751 . Each arm  744  of the flange  741  is flanked by two abutments  755 ,  755   a . In the embodiment which is shown in FIGS. 11 to  13 , the length of the abutments  755 ,  755   a  in the circumferential direction of the compartment  751  equals or closely approximates the length of the respective arms  744 . FIG. 12 shows cup-shaped retainers  759  which are disposed between the arms  744  and the adjacent end portions of the respective coil springs  745 . The configuration of the retainers  759  is preferably such that they at least substantially fill the corresponding portions of the compartment  751  in order to offer a substantial resistance to the flow of a viscous fluid medium along their peripheral surfaces, i.e., between the retainers and the adjacent portions of surfaces bounding the grooves  752  and  753 . 
     The compartment  751  is disposed radially outwardly of two annular portions  760 ,  761  of the radially extending surfaces of the parts  731 ,  732 , and the portions  760 ,  761  define a ring-shaped passage  762  which is largely filled by the corresponding portion of the flange  741  so that the flange and the part  731  define the aforementioned narrow clearance or gap  754  providing a path for the flow of fluid medium between the compartment  751  and the radially inner portions of the chamber  730 . The distance between the annular portions  760 ,  761  of radially extending surfaces of the parts  731 ,  732  need not appreciably exceed the thickness of the corresponding portion of the flange  741 , i.e., the flange  741  and the part  731  can constitute a highly effective flow restrictor which offers a pronounced resistance to the flow of fluid medium into and from the compartment  751  of the chamber  730 . 
     The compartment  751  accommodates four equidistant coil springs  745  each of which extends along an arc of or close to 82 degrees. Thus, the combined length of the four coil springs  745  in the circumferential direction approximates 90 percent of a complete circle. As already explained in connection with the coil springs  45  in the apparatus  1  of FIGS. 1 and 2, the coil springs  745  are preferably curved prior to insertion into the compartment  751  because this reduces the internal stresses which develop when the springs  745  are acted upon by the abutments  755 ,  755   a  and by the arms  744  in order to store energy. Furthermore, such shaping of the coil springs  745  facilitates their installation in the compartment  751 . The initial curvature of the coil springs  745  can match or merely approximate the curvature of the compartment  751 . 
     The supply of viscous fluid medium in the chamber  730  is preferably a lubricant, and its quantity is preferably selected in such a way that it fills at least the compartment  751  when the apparatus  701  is caused to rotate. 
     As can be seen in FIG. 12, the flange  741  has a central opening  771  bounded by a set of radially inwardly extending tooth-like projections  772  in mesh with complementary tooth-like projections  773  of the disc  727 . The projections  772  and  773  together form the aforementioned connection  742 . This connection ensures that the disc  727  and flange  741  can be readily separated from each other by moving the flange and/or the disc axially of the apparatus  701  as well as that, when the apparatus  701  is assembled, the flange  741  is compelled to share all angular movements of the flywheel  704  which carries the disc  727 . The projections  773  fit into recesses or tooth spaces  772   a  which alternate with the projections  772  in the circumferential direction of the surface bounding the opening  771 . The rivets  726  extend through the projections  773  of the disc  726  and are anchored in the flywheel  704 . As described in connection with FIGS. 1 and 2, the projections  772 ,  773  render it possible to locate the flange  741  in an optimum position between the parts  731 ,  732  of the housing which defines the chamber  730 ; this, in turn, renders it possible to keep the width of the clearance or gap  754  between the flange  741  and the part  731  to a minimum. The connection  742  serves the additional useful purpose of allowing to compensate for machining tolerances of elements which include the flange  741  and the elements adjacent thereto. 
     The apparatus  701  further comprises a sealing device  774  which prevents or reduces to a minimum the escape of viscous fluid medium radially inwardly beyond the innermost portion of the chamber  730 . This sealing device is installed between the radially innermost portion of the part  732  and the flywheel  704 . The main difference between the sealing device  774  and the sealing device  374  of FIG. 6 is that the entire sealing member  775  is coated with highly wear-resistant material. The radially outermost portion of the sealing member  775  is held against axial movement by the portion  732   a  of the part  732  and by a ring-shaped carrier  780  which latter is affixed to portion  732   a  of the part  732  by rivets  732   b  or similar fasteners. 
     The portion  732   a  of the part  732  extends radially inwardly beyond the radially outermost part of the axially resilient sealing member  775  so that the portion  732   a  and the member  775  define an annular space  732   c  having a wedge-like cross-sectional outline and an open innermost portion of maximum width which is located directly radially outwardly of the location of sealing engagement between the radially innermost portion of the member  775  and an insert  776 . Such mounting of the sealing member  775  and such configuration of the part  732  ensure that minute quantities of viscous fluid medium which happen to escape from the chamber  730  in the region  776   b  between the sealing member  775  and insert gather in the space  732   c  and are forced back into the chamber  730  under the action of centrifugal force when the apparatus  701  is set in rotary motion. As mentioned above, the region  776   b  of sealing engagement between the insert  776  and the sealing member  775  is located radially inwardly of the space  732   c  and at the same distance from the flange  741 , as seen in the axial direction of the flywheels  703  and  704 . This ensure that any fluid medium which has managed to pass between the insert  776  and the sealing member  775  invariably enters the space  732   c  under the action of centrifugal force when the flywheels  703 ,  704  are caused to rotate at an elevated speed. 
     The reference character  791  denotes in FIG. 11 a notch which is formed in the part  732  of the housing for the chamber  730  and serves to receive the radially outermost portion of the sealing member  775  and the carrier  780 . The notch  791  extends in the axial direction of the flywheel  704  from the left-hand side of the part  732  and away from the flange  741 . 
     The radially innermost portion of the part  731  is secured to an extension  720  which is a functional equivalent of the protuberance  20  on the flywheel  3  of FIG.  1  and is surrounded by the anti-friction bearing  16  of the bearing means  15 . The protuberance  720  has a cylindrical surface  720   b  which serves to center the part  731 , and a circumferentially extending shoulder  720   c  which determines the axial position of the part  731 . The manner in which the bearing  16  is installed between the flywheels  703  and  704  is or can be the same as described in connection with FIG. 1 or  6 . The means for permanently or separably connecting the part  731  with the protuberance  720  can include a set of screws or rivets, a weld or an upset portion of the protuberance  720  at the left-hand side of the part  731  (as seen in FIG.  11 ). 
     The manner of assembling the flywheels  703 ,  704  of the apparatus  701  is analogous to the afore-described manner of assembling the flywheels  3  and  4  of FIGS. 1 and 2. Thus, the antifriction bearing  16  is first mounted in the flywheel  704  and the sealing device  774  is mounted on the flywheel  703 . The bearing  16  is thereupon slipped onto the protuberance  720  of the flywheel  703  so that the inner race of the bearing surrounds the cylindrical peripheral surface  720   a  of the protuberance  720  whereby the projections  773  of the disc  727  on the flywheel  704  move into mesh with the projections  772  of the flange  741  to establish the connection  742 . Moreover, the sealing member  775  of the device  774  is stressed in the axial direction as a result of engagement with the insert  776 . The retaining ring  722  is then affixed to the end face of the protuberance  720  to locate the inner race of the bearing  16  in a desired axial position. This also ensures that the flywheels  703  and  704  are maintained in predetermined axial positions with reference to each other. The retaining ring  722  can be riveted, bolted, screwed or otherwise securely affixed to the protuberance  720  of the flywheel  703 . 
     The manner in which the viscous fluid medium which partially fills the chamber  730  performs a desirable hydraulic or viscous damping action is the same as or similar to that described in connection with FIGS. 1 and 2. The damping action is attributable to the establishment of several flow restrictors as well as to turbulence in the fluid medium. 
     It is desirable to provide layers of electric insulating material between the parts  731 ,  732  on the one hand and the adjacent movable or other elements or components of the apparatus  701  on the other hand, at least for the duration of the welding operation to attach the radially outermost portions of the parts  731 ,  732  to each other. The absence of electric insulating layers could result in partial bonding of movable elements or components of the apparatus  701  (especially of the damper means) to the part  731  and/or  732  as well as in an undesirable change of texture of elements which are sufficiently close to the part  731  and/or  732  to be likely to be overheated during making of the welded connection  738 . The elements which are most likely to be affected by overheating are the coil springs  745  in the compartment  751  of the chamber  730 , the cup-shaped retainers  759  in the compartment  751  and the flange  741 . 
     The layers or coats of insulating material can be provided on the part  731 , on the part  732  and on the elements or components (such as  741 ,  745 ,  755 ,  755   a  and  759 ) which are adjacent the parts  731 ,  732 . It is not always necessary to completely coat the part  731  and/or  732  and/or the element  741 ,  745 ,  755 ,  755   a  and/or  759  with electrically insulating material, i.e., it often suffices to coat the parts  731 ,  732  only in regions where they contact the aforeenumerated elements and/or to coat the elements only in regions where they are nearest to the part  731  and/or  732 . The insulating operation can involve phosphatizing of metallic parts. An additional solution is to make certain parts (such as the cup-shaped retainers  759  and/or the abutments  755 ,  755   a ) from a non-conductive material. The springs  745  can be provided with coats of lacquer but the majority of elements which are likely to be affected by overheating or which are likely to overheat the neighboring elements are preferably phosphatized. The elements to be phosphatized preferably include the parts  731 ,  732  (which are made of metallic sheet material) and the flange  741 . It is also possible to provide the parts  731 ,  732  and/or the elements or components which are in contact therewith with coats of suitable ceramic material, plastic material and/or grease. Such coats will normally be applied to the parts  731  and  732 . It often suffices to phosphatize the parts  731 ,  732  and to merely apply layers of lacquer to the coil springs  745 . 
     In order to simplify the phosphatizing or coating with a layer of ceramic or like material, the corresponding elements or components (such as the parts  731 ,  732 ) are preferably coated in their entirety and the thus applied coats are thereupon removed in the course of a secondary treatment in order to expose those portions which are to be welded to each other as well as those portions which are to be connected with the source of electrical energy. The secondary treatment can involve mechanical removal of ceramic material, phosphate or the like in a machine tool. This ensures that the parts  731 ,  732  are electrically conductive at  738  and at the location or locations of connection to the energy source. The selection of insulating material should be made with a view to ensure that the selected material is compatible with viscous fluid medium which is thereupon admitted into the chamber  730  to fill at least the compartment  751 . 
     It is further preferred to select the insulating layer or layers (especially phosphatized coats) in such a way that they exhibit highly satisfactory wear resistant and self-lubricating properties. 
     The periphery of the part  731  defines a cylindrical seat  739  for the starter gear  740 . The latter is preferably welded, as at  740   a , to the part  731 . The welded connection can be established all the way around the part  731  or it can consist of several spot welds. A connection which involves welding is preferred at this time in view of the limited thickness of the metallic sheet material which is used for the making of the part  731 . As can be seen in FIG. 11, the axial length (thickness) of the gear  740  is greater than the thickness of the material of the part  731 . FIG. 11 further shows that the thickness of the part  731  exceeds the thickness of the part  732 . 
     Referring to FIG. 13, the abutments or stops  755 ,  755   a  of the apparatus of FIGS. 11 and 12 can be replaced with pocket-like abutments  755   c ,  755   d  which are integral portions of the respective parts  731  and  732 . This simplifies the making and assembly of the respective apparatus because the number of separately produced parts is reduced and the abutments are invariably located in optimum positions for engagement with the end portions of the adjacent coil springs. The recesses which are formed in the outer sides of the parts  731 ,  732  shown in FIG. 13 as a result of the making of pockets  755   c ,  755   d  can be used for reception of centering devices (not shown) which ensure that the parts  731 ,  732  are properly centered and otherwise positioned relative to each other in the course of the welding operation. The centering devices are normally provided in or on the welding apparatus which is used to connect the parts  731 ,  732  to each other, i.e., to form the welded connection  738  shown in FIG.  11 . The dimensions of the centering devices are preferably selected with a view to ensure that such devices fill or practically fill the recesses at the outer sides of the pockets which constitute the abutments  755   c  and  755   d  of FIG.  13 . Such centering devices can constitute electrodes which supply the welding current and/or the means for pressing the parts  731 ,  732  against each other in the course of the welding operation. It is particularly advantageous if the centering devices are constructed, configurated and mounted in the welding apparatus in such a way that they are invariably located at a predetermined distance from each other (i.e., that the centering devices which enter the recesses outside of the pockets  755   c  and  755   d  are located at a preselected axial distance from one another); this ensures that the parts  731 ,  732  are located at an optimum axial distance from one another when the welding operation is completed. This is important in view of the aforediscussed need for proper dimensioning of the compartment  751  (to avoid excessive or insufficient rubbing contact between the surfaces surrounding the grooves  752 ,  753  on the one hand and the surfaces of convolutions of the coil springs  745  on the other hand). Moreover, proper positioning of the parts  731 ,  732  is important in view of the aforediscussed need to ensure that the elements defining the gap  754  will constitute effective flow restrictors by offering an optimum resistance to the flow of viscous fluid medium into and from the compartment  751 . 
     The apparatus  801  of FIGS. 14 and 15 comprises a composite flywheel  802  which includes a first component or flywheel  803  secured to the output element  805  (e.g., a crankshaft) of the internal combustion engine by a set of bolts  806  or analogous fasteners, and a second component or flywheel  804  which can be connected with the input element  810  of a change-speed transmission in response to engagement of a friction clutch  807 . The clutch  807  includes a cover  811  which is affixed to the flywheel  804 , an axially movable pressure plate  808  between the cover  811  and the flywheel  804 , a clutch plate or clutch disc  809  having a hub which is non-rotatably mounted on the input element  810  of the transmission and a set of friction linings which are disposed between the pressure plate  808  and the friction surface  804   a  of the flywheel  804 , and a diaphragm spring  812  which is tiltable between two seats at the inner side of the cover  811  and biases the pressure plate  808  against the adjacent friction lining of the clutch plate  809  when the clutch  807  is engaged. The pressure plate  808  is axially movably but non-rotatably coupled to the cover  811  and/or to the flywheel  804 . The means for engaging or dis-engaging the clutch  807  is of conventional design and is not shown in the drawing. 
     The damper means between the flywheels  803  and  804  includes a first or outer damper  813  and a second or inner damper  814 . The dampers  813 ,  814  are connected in parallel and each thereof is designed to yieldably oppose rotation of the flywheels  803 ,  804  relative to each other. 
     The bearing means  815  between the flywheels  803 ,  804  comprises an antifriction ball bearing  816  with a single annulus of rolling elements between an inner race  819  and an outer race  817 . The outer race  817  is installed in an axial recess  818  of the flywheel  804 , and the inner race  819  surrounds a cylindrical portion of the peripheral surface of an axial protuberance  820  forming part of the flywheel  803 . The protuberance  820  extends axially in a direction away from the output element  805  of the engine and is received in the recess  818  of the flywheel  804 ; this protuberance is integral with a radially outwardly extending flange  803   a  of the flywheel  803 . 
     The inner race  819  is preferably a press fit on the protuberance  820  and abuts a circumferential shoulder  821  of the protuberance under the action of a washer-like retaining ring  822  which is secured to the protuberance by the heads of the aforementioned bolts  806 . The bearing  816  is held against axial movement with reference to the flywheel  804  by being fixed between a disc  827  which is secured to the flywheel  804  by a set of distancing elements in the form of rivets  826  and by abutting an internal shoulder  825  in the recess  818 . The outer race  817  of the bearing  816  is flanked by two rings  823 ,  824  each of which has an L-shaped cross-sectional outline and which constitute a thermal barrier between the friction clutch  807  and the bearing  816 , and more particularly between the friction surface  804   a  and clutch plate  809  on the one hand and the races  817 ,  819  and rolling elements of the bearing  816  on the other hand. 
     The radially extending flange  803   a  of the flywheel  803  is integral with a cylindrical collar  828  which surrounds the radially outermost portion of an annular chamber  829 . The collar  828  extends in the axial direction of the apparatus  801  and forms part of the housing for the chamber  829 ; such housing further includes two radially extending sections or parts  831 ,  832  which flank the dampers  813  and  814 . The part  831  is an integral portion ( 833 ) of the radially extending flange  803   a  of the flywheel  803 , i.e., of the element which is integral with and extends radially outwardly from the protuberance  820 . The part  832  is a substantially or completely nonelastic rigid disc-shaped member which is disposed between the part  831  and the flywheel  804  and can be said to constitute a radially extending cover of the housing for the chamber  829 . The radially outermost portion of the part  832  abuts the end face of the collar  828  and is secured to the latter by a set of rivets  834  or analogous fastener means. 
     The dampers  813 ,  814  comprise a common output element  835  which is non-rotatably connected to the flywheel  804 . The output element  835  includes the aforementioned disc  827  which is affixed to the end face of a protuberance or projection  836  which surrounds the recess  818  and forms an integral part of the flywheel  804 . The projection  836  extends axially toward the output element  805  of the engine. The output element  835  further includes a second disc  837  which is secured to the disc  827 . FIG. 14 shows that the radially outermost portion of the disc  827  is dished or cupped in a direction toward the flange  803   a  of the flywheel  803  and that the disc  837  is affixed to the radially outermost (cupped or dished) portion  837   a  of the disc  827  by a set of rivets  838 . 
     The dished or cupped configuration of the radially outermost portion of the disc  827  results in the formation of a recess or space  839  which is disposed between the discs  827 ,  837  and receives a disc-shaped member or flange  840  constituting the input element of the inner damper  814 . The discs  827 ,  837  have registering openings or windows  841 ,  842  which register with windows  843  in the flange  840  and serve to receive energy storing elements in the form of coil springs  844  forming part of the inner damper  814 . The length of the windows  841 ,  842  (as seen in the circumferential direction of the flywheels  803  and  804 ) equals or closely approximates the length of the windows  843 , and each coil spring  844  is installed in the respective set of windows  841 - 843  in prestressed condition. This ensures that a certain moment can be transmitted between the input element or flange  840  of the inner damper  814  and the output element  835  of the outer damper  813  before the coil springs  844  undergo further compression. 
     The disc  837  (forming part of the output element  835 ), which is nearer to portion  833  of the flywheel  803  than the other disc  827 , has an inner diameter (at  845 ) which exceeds the inner diameter of the disc  827 . The latter cooperates with the shoulder  821  of the protuberance  820  to fix the bearing  816  in an optimum axial position. FIG. 15 shows that the radially innermost portion of the input element  840  of the damper  814  is provided with radially inwardly extending tooth-like projections  846  which mate with projections  847  on the portion  833  of flywheel  803 . The projections  847  are rivets which are secured to the portion  833  of the flywheel  803  and each of which includes a portion (head)  847   a  extending axially beyond the general plane of the portion  833  and toward the flywheel  804  (see FIG.  14 ). The projections  846  cooperate with the projections  847  to limit the extent of angular movability of the input element  840  of the damper  814  with reference to the flywheel  803 . 
     FIG. 15 further shows that the input element  840  of the inner damper  814  can turn relative to the flywheel  803  in the driving direction  848  (when the engine drives the input element  810  of the change-speed transmission) through a first angle  849  (before the heads  847   a  of the rivets  847  are engaged by the adjacent projections  846  of the input element  840 ) and that the input element  840  can turn with reference to the flywheel  803  through a second angle  851  in the opposite direction  850  (when the vehicle embodying the power train which employs the apparatus  801  is coasting). In the embodiment of FIGS. 14 and 15, the angle  849  equals or closely approximates the angle  851 . However, it is also possible to select a non-symmetrical positioning of the rivets  847  with reference to the projections  846  of the input element  840  so that the angle  849  deviates from the angle  851 . For example, the angle  849  could exceed the angle  851 . 
     The cupped or dished radially outermost portion  837   a  of the disc  827  need not be a circumferentially complete member; as shown in FIG. 15, the portion  837   a  consists of several radially outwardly extending arms each of which has a bend at its radially innermost end. The disc  837  comprises radially outwardly extending prongs or lugs  852  each of which is adjacent an arm ( 837   a ) of the disc  827  and is secured thereto by a rivet  838 . The length of the lugs  852  in the circumferential direction of the discs  827 ,  837  is the same as that of the arms  837   a , and each arm  837   a  is in register with a lug  852 . This renders it possible to use the coplanar edge faces of the lugs  852  and arms  837   a  as abutments or stops  853 ,  854  for the coil springs  855  of the outer damper  813 . 
     The discs  827 ,  837  are further connected to each other by distancing elements in the form of rivets  856  which are disposed at the level of coil springs  844  in the inner damper  814 , i.e., the annulus of rivets  856  has the same radius as the annulus of coil springs  844 . Portions of the rivets  856  extend with play (as seen in the circumferential direction of the flywheels  803  and  804 ) through apertures or slots  857  of the disc-shaped input element  840 . The dimensions of the slots  857  are selected in such a way that the coil springs  844  of the inner damper  814  are fully compressed (i.e., that each spring  844  acts not unlike a rigid block because its convolutions lie flush against each other) before the rivets  856  come into engagement with surfaces at the ends of the respective slots  857 . Such dimensioning of the slots  857  is preferred at this time because it ensures that abrupt shocks which develop during transmission of torque between the flywheels  803  and  804  do not entail the development of pronounced impacts. This is due to the fact that, prior to undergoing total compression (to act not unlike solid blocks), the coil springs  844  exhibit a very pronounced progressivity of their characteristics. This takes place while the springs  844  are still capable of performing a certain axial movement as a result of radial shifting. 
     FIG. 15 shows that the rivets  856  are located radially inwardly of the arms  837   a  and lugs  852 . The disc-shaped input element  840  has cutouts  858  which are located radially inwardly of the lugs  852 , arms  837   a  and slots  857 , and the cutouts  858  are flanked by the arms  846 . The cutouts  858  are provided to facilitate deformation of the rivets  826  which are disposed at the same distance from the axis of the composite flywheel  802  and whose heads are in register with the cutouts  858 . 
     The input element  840  of the inner damper  814  is clamped axially between the discs  827 ,  837  which constitute the output element  835  of the outer damper  813 . To this end, the input element  840  constitutes a diaphragm spring which is resilient in the axial direction and exhibits a certain amount of conicity prior to mounting between the discs  827 ,  837 . When properly installed, the input element  840  is stressed axially so that a friction lining or pad  859  between the radially outermost portion of the element  840  and the disc  837  is compressed as well as that a friction lining or pad  860  between the radially innermost portion of the element  840  and the disc  827  is also kept in compressed condition. In order to facilitate assembly of the apparatus  801 , the friction pads  859  and  860  are preferably bonded to the respective sides of the input element  840 . The friction pad  859  is disposed radially outwardly and the friction pad  860  is located radially inwardly of coil springs  844  in the inner damper  814 . When the input element  840  of the inner damper  814  turns relative to the output element  835  of the outer damper  813 , the friction pads  859 ,  860  produce a frictional damping action which is effective in parallel to the bias of the coil springs  844 . 
     The parts  831  and  832  of the housing for the chamber  829  are provided with arcuate grooves  861 ,  862  which together form the major part of a compartment for the coil springs  855  of the outer damper  813 . These grooves receive (either entirely or in part) those portions of the coil springs  855  which extend beyond the respective sides of the output element  835 . FIG. 14 shows that the curvature of the surfaces bounding the grooves  861 ,  862  equals or approximates the curvature of the coil springs  855 , at least in those regions which receive the radially outermost portions of convolutions of the springs  855 . This enables the convolutions of the coil springs  855  to actually contact and be guided by the adjacent portions of surfaces bounding the grooves  861 ,  862 , at least when the apparatus  801  is rotated and the coil springs  855  are acted upon by centrifugal force. It has been found that such configuration of the surfaces bounding the grooves  861 ,  862  entails a pronounced reduction of wear because wear between the convolutions of the coil springs  855  and the parts  831 ,  832  is not localized but takes place between relatively large portions of abutting surfaces on  831 ,  832  on the one hand and  855  on the other hand. Each of the grooves  861 ,  862  is a circumferentially complete recess in the respective part  831 ,  832 . This is desirable and advantageous because the grooves  861 ,  862  can be formed during casting of the respective parts  831 ,  832  and the surfaces bounding such grooves can be thereupon treated to a desired degree of finish in a suitable machine using grinding, milling or other material removing tools. 
     The grooves  861 ,  862  respectively contain abutments or stops  863 ,  864  which engage the end convolutions of the adjacent coil springs  855 . The length of the abutments  863 ,  864  in the circumferential direction of the flywheel  803  is the same as that of the arms  837   a  on the disc  827  and of the lugs  852  on the disc  837 . Each of the abutments  863 ,  864  can constitute a separately produced element which fits rather snugly into the corresponding portion of the respective groove  861 ,  862  and is riveted or otherwise reliably secured to the respective part  831 ,  832 . The end portions of the abutments  863  and  864  are preferably flattened to ensure the establishment of large-area contact with the end convolutions of the adjacent coil springs  855 . 
     FIG. 15 shows that the outer damper  813  comprises three coil springs  855  each of which extends along an arc of approximately 110 degrees. 
     The relationship between the parameters of coil springs  844  and  855  is selected in such a way that, when the angular displacement of the flywheels  803 ,  804  reaches a maximum value, the final or maximum moment which is furnished by the outer coil springs  855  is less than the corresponding moment which is furnished by the inner coil springs  844 . Furthermore, the spring rate of the inner springs  844  is greater than that of the outer springs  855 . 
     The means for sealing the radially outermost portion of the chamber  829  for the dampers  813 ,  814  from the surrounding atmosphere comprises a sealing ring  865  which is mounted between the cylindrical collar  828  of the flywheel  803  and the part  832  radially inwardly of the rivets  834 . The illustrated sealing ring  834  can constitute a simple O-ring. 
     A sealing device  866  is installed between the radially innermost portion of the part  832  and the axial projection  836  of the flywheel  804  to seal the radially innermost portion of the chamber  829  from the atmosphere. The sealing device  866  includes a ring-shaped member which has a disc-shaped inner portion clamped between the projection  836  of the flywheel  804  and the disc  827 , and a frustoconical outer portion which acts not unlike a diaphragm spring and engages, in axially stressed condition, the adjacent portion of the part  832 . Such frustoconical outer portion of the sealing device  866  is received in a radially outwardly extending ring-shaped notch  867  of the part  832  at that side of this part which faces the disc  827 . The surface bounding the notch  867  surrounds the sealing device  866  and extends along one side of the frustoconical outer portion of  866 . 
     The chamber  829  contains a supply of viscous fluid medium which is preferably a lubricant. When the apparatus  801  is rotated, the fluid medium fills the outermost portion of the chamber  829  and preferably extends radially inwardly at least to the level of the axes of coil springs  855  forming part of the outer damper  813 . 
     A radially extending ventilating or aerating channel  868  is provided between the part  832  and the flywheel  804  to ensure adequate cooling of the fluid medium in the chamber  829 . The radially outermost portion of the channel  868  is open and the radially innermost portion of this channel communicates with substantially axially extending passages  869  which are provided in the flywheel  804  radially inwardly of the friction surface  804   a.    
     The rivets  834  can be used as a means for locating the starter gear  840  in a predetermined axial position with reference to the flywheel  803 . 
     The apparatus  801  operates as follows: 
     When one of the flywheels  803 ,  804  leaves the idle position of FIG. 15, e.g., in the coasting direction  850 , the coil springs  855  of the outer damper  813  are caused to store energy. When the one flywheel (e.g., the flywheel  804 ) completes the angle  851 , the radially inwardly extending projections  846  of the input element  840  of the inner damper  814  engage the respective abutments  847  of the flywheel  803  so that any further angular displacement of the flywheel  804  in the direction  850  then entails joint compression of coil springs  855  and  844 . Such joint compression of coil springs  844  and  855  continues until the coil springs  844  begin to act not unlike solid blocks, i.e., when the convolutions of the coil springs  844  are immediately adjacent to and abut each other. This terminates the angular displacement of the flywheel  804  relative to the flywheel  803 . In the embodiment of FIGS. 14 and 15, the angle  851  equals or approximates 32 degrees and the so-called blocking angle of the coil springs  844  is approximately 4 degrees so that the total angular displacement of the flywheel  804  relative to the flywheel  803  (and/or vice versa) can be in the range of 36 degrees. A frictional damping action takes place while the coil springs  844  store energy because the friction pads  859 ,  860  on the input element  840  respectively rub along the discs  827 ,  837 . Additional frictional damping action is generated as a result of sliding contact between the convolutions of the coil springs  855  and the adjacent portions of surfaces bounding the grooves  861 ,  862  in the parts  831 ,  832 . Moreover, the radially outermost portion of the sealing device  866  slides along the part  832  to generate additional frictional damping action. The fluid medium in the chamber  829  is agitated as well as displaced from the compartment for the coil springs  844 , and this results in the development of hydraulic or viscous damping action in a manner and for reasons as explained above in connection with the apparatus  1  of FIGS. 1 and 2. 
     FIG. 16 shows a portion of an apparatus  901  having a flange  941  which has radially outwardly extending abutments or arms  944  (one shown). These arms serve to compress energy storing elements in the form of coil springs  945 ,  945   a  of a damper  913  in a manner as described in connection with the apparatus shown in FIGS. 1 through 15. The coil springs  945 ,  945   a  are confined in an annular compartment  951  forming part of a chamber between the parts of the flywheel  903 . The coil spring  945   a  is biased directly by the adjacent abutment or arm  944  and the coil spring  945   a  is acted upon by a cup-shaped retainer  959 . Each arm  944  has two projections in the form of stubs or noses  944   a ,  944   b  which extend in opposite directions (i.e., away from each other) in the circumferential direction of the flywheel  903 . The illustrated cup-shaped retainer  959  has a conical or spherical socket  959   a  in the form of a blind bore for the stub  944   a  of the arm  944 . The configuration of the stub  944   a  is such that it can hold the retainer  959  and hence the adjacent hollow end portion of the coil spring  945  in a position such that the end portion of the coil spring is out of contact with the adjacent radially innermost or outermost portion of the compartment  951  at least when the spring  945  is caused to store energy. To this end, the stub  944   a  has a sloping ramp-like cam face  944   c  along which the adjacent portion  959   b  of the internal surface of the retainer  959  slides when the retainer approaches the main portion of the arm  944  whereby the inner part of the respective end portion of the coil spring  945  is lifted off or urged toward the adjacent surface bounding the compartment  951 , i.e., such end portion of the coil spring  945  is moved radially outwardly or inwardly. The retainer  959  and its socket  959   a  have a circular cross-sectional outline. 
     The other stub  944   b  of the arm  944  which is shown in FIG. 16 has a cam face or ramp  944   d  which is adjacent the innermost portion of the compartment  951  and cooperates with the end convolution of the coil spring  945   a  to urge such end convolution radially inwardly, i.e., toward the adjacent portion of the surface bounding the compartment  951 . 
     If the apparatus  901  employs cup-shaped retainers  959 , it is advisable to ensure that the outline of the stub  944   a  (or at least the outline of the ramp  944   c ) conforms to the outline of the adjacent portion  959   b  of the internal surface of the retainer  959 ; this ensures that the adjacent end convolutions of the coil spring  945  are pulled radially inwardly even if the angular position of the retainer  959  with reference to the arm  944  and its stub changes. 
     Projections corresponding to the stubs  944   a ,  944   b  shown in FIG. 16 can be used with equal or similar advantage in apparatus which are shown in FIGS. 1 to  15 . Moreover, such projections or stubs can be provided on the aforediscussed abutments or stops in the compartments of chambers  30  . . .  829  of the previously described apparatus. 
     An advantage of the stubs  944   a ,  944   b  is that they can maintain the adjacent convolutions of the coil springs  945 ,  945   a  out of contact with the radially outermost portions of surfaces bounding the compartment  951  even if the flywheel  903  is rotated at a very high speed. Consequently, the axial length of the coil springs  945 ,  945   a  can be readily changed because they cause a minimum of frictional damping. An additional advantage of the stubs  944   a ,  944   b  is that they enable the adjacent end convolutions of the coil springs  945 ,  945   a  to move in the compartment  951  (i.e., to move toward or away from the arm  944 ) even if the frictional engagement between the median convolutions of such coil springs and the surfaces bounding the compartment  951  is very high, i.e., even if such median convolutions are prevented from sliding in the circumferential direction of the flywheel  903 . This can take place when the rotational speed of the flywheel  903  is very high so that the median convolutions of the springs  945 ,  945   a  are acted upon by a very large centrifugal force. The freely slidable end convolutions of the coil springs  945 ,  945   a  are then still capable of damping high-frequency low-amplitude oscillations and similar stray movements of the flywheels. 
     The flange  941  is normally a flat stamping. The projections  944   a ,  944   b  of its arms  944  can receive a cylindrical, frustoconical or partly cylindrical or partly frustoconical shape as a result of secondary treatment in a suitable deforming machine. This enlarges the area of contact between the projections  944   a  and the internal surfaces of the respective cup-shaped retainers  959  on the one hand, and between the projections  944   b  and the hollow end portions of the respective coil springs  945   a  on the other hand. As mentioned above, the sockets  959   a  of the retainers  959  can have a conical, frustoconical or spherical outline. 
     The provision of projections  944   a  and/or  944   b  (i.e., of means for keeping at least the end portions of the coil springs  945  and/or  945   a  out of contact with the surfaces bounding the radially outermost portion of the compartment  951 ) is desirable and advantageous in many or most instances. However, certain apparatus are preferably designed in such a way that the movements of convolutions of the coil springs into frictional engagement with the adjacent outermost portions of surfaces bounding the compartment  951  (and/or the compartment for the coil springs of the inner damper) is promoted, at least when the flywheels rotate and the coil springs are acted upon by centrifugal force. This is desirable in apparatus wherein the coil springs begin to store energy only after a certain initial angular displacement of the flywheels relative to each other. 
     Referring to FIG. 17, there is shown a portion of an apparatus  1001  which has a composite flywheel including a first flywheel  1003  and a second flywheel  1004 . The flywheel  1003  is connected to the output element of the engine (not shown), and the flywheel  1004  can be connected to a change-speed transmission by way of a friction clutch, not shown, in the same way as described in connection with FIGS. 1 and 2. The flywheel  1003  comprises two sections or parts  1031 ,  1032  which are made of deformable metallic sheet material and define an annular chamber  1030  for two series-connected dampers  1013 ,  1014 . The coil springs of the dampers  1013 ,  1014  are coupled to each other by a flange  1041 . The latter is flanked by two discs  1065 ,  1066  in a manner substantially as described in connection with FIG.  10 . 
     The difference between the flange  1041  of FIG.  17  and the previously described similarly referenced flanges is that the radially outwardly located arms  1044  which extend into the compartment  1051  of the chamber  1030  have extensions  1044   a  which are disposed radially outwardly of the respective coil springs  1045  of the damper  1013 . Thus, the radially outermost portions of convolutions of the coil springs  1045  can abut the inner sides of the adjacent extensions  1044   a , at least when the flywheels  1003 ,  1004  rotate and the convolutions of the coil springs  1045  are acted upon by centrifugal force. It is rather simple to harden selected portions of or the entire flange  1041  so that it can stand extensive wear in spite of repeated and extensive frictional engagement with the coil springs  1045  of the damper  1013 . For example, the extensions  1044   a  (and, if necessary, certain other portions) of the flange  1041  can be treated by induction hardening. Moreover, and if it is more convenient or less expensive, at least the extensions  1044   a  of the flange  1041  can be coated with layers of highly wear resistant material such as solid or hard nickel or the like. 
     The extensions  1044   a  of arms  1044  forming part of the flange  1041  are received in the radially outermost portion  1051   a  of the compartment  1051 . The portion  1051   a  is also defined by the parts  1031 ,  1032  of the flywheel  1003 ; these parts include frictional engagement reducing inserts or portions  1031   a ,  1032   a  which extend radially outwardly beyond the outermost portion  1051   a  of the compartment  1051 . The portions  1031   a ,  1032   a  also extend in the axial direction of the apparatus  1001  so that they form a sleeve or shell around the adjacent part of the radially outermost portion of the flywheel  1004 . The portions  1031   a ,  1032   a  are welded to each other, as at  1038 , preferably in the region of their rightmost ends as seen in FIG.  17 . Such operation can be carried out in an electron beam welding machine. An advantage of the portions  1031   a ,  1032   a  is that they increase the moment of inertia of the flywheel  1003  without it being necessary to unduly enlarge the apparatus  1001  in the radial direction. 
     That side of the part  1031  of the flywheel  1003  which faces toward the engine is adjacent a disc  1090  which can be said to constitute a scale with graduations or other forms of indicia  1091  (e.g., projections, notches or the like) which are indicative of different parameters of the engine, for example, the timing of ignition and/or others. Reference may be had to commonly owned U.S. Pat. No. 4,493,409. 
     The flywheel  1003  further includes a centrally located axial protuberance  1020  which extends in a direction away from the output element of the engine and is secured to the scale  1090  and part  1031  by bolts, screws or other suitable fasteners  1092 . The protuberance  1020  is surrounded by an antifriction ball bearing  1016  on which the flywheel  1004  can rotate relative to the flywheel  1003  and/or vice versa. 
     The utilization of aforediscussed radially outermost portions  1031   a ,  1032   a  is not limited to those apparatus wherein the parts of the housing for the annular chamber which confines the dampers are made of deformable metallic sheet material. It is also possible to rely on such mode of shaping the radially outermost portion of the flywheel which is attached to the output element of the engine in apparatus wherein the parts of the housing are castings. 
     The apparatus  1101  of FIG. 18 comprises a housing which defines an annular chamber (including an annular compartment  1151 ) and includes two parts or sections  1131   a ,  1132   a  which are made of deformable metallic sheet material. The compartment  1151  serves to receive energy storing coil springs  1145  forming part of a damper in the chamber. The parts  1131   a ,  1132   a  further define a radially extending ring-shaped passage  1162  which is located radially inwardly of and communicates with the compartment  1151  and is substantially filled by the respective portion of the flange  1141 . Those portions ( 1165 ,  1166 ) of the parts  1131   a ,  1132   a  which extend radially inwardly beyond the passage  1162  are connected with thicker parts  1131 ,  1132  by means of rivets  1155 ,  1155   a  or the like. It can be said that each section of the housing for the chamber which includes the compartment  1151  includes two layers or strata including an outer layer  113 ,  1132  and an inner layer  1131   a ,  1132   a.    
     The portions  1165 ,  1166  of the parts  1131   a ,  1132   a  can extend radially inwardly beyond the respective rivets  1155 ,  1155   a  to define a second annular compartment (not shown) for the coil springs of a second or inner damper corresponding to the damper  14  of the apparatus  1  shown in FIGS. 1 and 2. Alternatively, the portions  1165 ,  1166  need not extend radially inwardly well beyond the rivets  1155 ,  1155   a ; instead, such portions can define arcuate compartments for the coil springs of a second or inner damper between neighboring pairs of rivets  1155 ,  1155   a  as seen in the circumferential direction of the flywheel including the parts  1131 ,  1132 ,  1131   a ,  1132   a.    
     The radially outermost portions of the parts  1131 ,  1132  are connected to each other radially outwardly of the compartment  1151  and of the parts  1131   a ,  1132   a . The connection includes suitably bent prongs  1133  which constitute relatively thin extensions of the part  1132  and overlie radially outwardly extending lugs  1134  of the part  1131 . The lugs  1134  can form a circumferentially complete rib or bead around the remaining portion of the part  1131 . Pins  1138  are used to couple the extensions  1133  to the lugs  1134  so as to hold the parts  1131 ,  1132  against angular movement relative to each other. 
     The utilization of housings with inner and outer sections or parts corresponding to the parts  1131 ,  1131   a  and  1132 ,  1132   a  of FIG. 18 is not limited to the apparatus  1101  (wherein each of the parts  113 ,  1131   a ,  1132 ,  1132   a  is made of deformable metallic sheet material) but can be used with equal or similar advantage in apparatus wherein each part or section of the housing includes a casting. If one part is a casting, it is provided with a suitable recess which accommodates the inner part (corresponding to the part  1131   a  or  1132   a ). The arrangement may be such that the recesses of the castings accommodate at least those portions of the inner parts  1131   a ,  1132   a  which surround the energy storing elements of the respective damper or dampers. 
     The apparatus  1201  of FIG. 19 has two flywheels one of which comprises a housing for an annular chamber  1230 . The housing includes two parts  1231 ,  1232  which flank a flange  1241  extending in part into an annular compartment which forms the radially outermost portion of the chamber  1230  and receives the coil springs of a damper  1213 . The chamber  1230  is at least partially filled with a viscous fluid medium, preferably a lubricant. The flange  1241  is fixedly secured to an axial protuberance or projection  1243  of the flywheel  1204  by a set of distancing elements in the form of rivets (only one shown). A sealing device  1274  is provided between the flange  1241  and the part  1232  of the housing for the chamber  1230 . 
     The apparatus  1201  further comprises a dry friction generating device  1290  which is located radially inwardly of the part  1232  (i.e., outside of the chamber  1230 ) and is installed between the flange  1241  and the radially extending flange-like portion  1204   a  of the flywheel  1204 . The friction generating device  1290  comprises a friction disc  1294  which is flanked by friction pads  1294   a ,  1294   b . The pad  1294   a  is mounted between the friction disc  1294  and the flange  1241  and the pad  1249   b  is biased by a biasing device  1293  in the form of a washer which is acted upon by a diaphragm spring  1292 . The spring  1292  is installed in prestressed condition between the radial portion  1204   a  of the flywheel  1204  and the biasing device  1293 . 
     The friction disc  1294  is provided with radially outwardly extending arms  1295  which mate with radially inwardly extending projections or prongs  1295   a  of the part  1232 . The arrangement is or can be such that the arms  1295  and the prongs  1295   a  mate without any play (as seen in the circumferential direction of the flywheels) or with a selected play, i.e., the friction disc  1294  and the part  1232  can have a certain freedom of angular movement relative to each other. Thus, the friction generating device  1290  can become effective only after at least one coil spring of the damper  1213  begins to store energy as a result of angular displacement of at least one flywheel relative to the other flywheel. 
     The apparatus  1301  of FIG. 20 comprises a damper  1313  in a compartment  1351  which is outwardly adjacent two sealing devices  1374 ,  1374   a  cooperating with the adjacent portion of the flange  1341 . The sealing device  1374  acts between the flange  1341  and the part  1332  of the housing for the annular chamber which includes the compartment  1351 , and the sealing device  1374   a  acts between the flange  1341  and the part  1331 . 
     That portion of the flange  1341  which is disposed radially inwardly of the sealing devices  1374 ,  1374   a  is flanked by and maintained in contact with two friction pads  1394   a ,  1394   b  which, in turn, are flanked by discs  1393 ,  1394 . The disc  1394  is fixedly secured to the flywheel  1304  by distancing elements in the form of rivets  1367 . The other disc  1393  is movable axially of the apparatus  1301  and is biased axially toward the friction pad  1394   b  by a diaphragm spring  1392  which reacts against the radially extending portion or flange  1304   a  of the flywheel  1304 . The diaphragm spring  1392  and the disc  1393  have cutouts in the form of openings, slots or windows for the respective portions of the distancing elements  1367  so as to ensure that the spring  1392  and the disc  1393  are compelled to share the angular movements of the flywheel  1304 . 
     The bias of the prestressed diaphragm spring  1392  determines the moment which is required to turn the flange  1341  relative to the flywheel  1304 , i.e., the spring  1392  determines that force which is required to cause the flange  1341  to slip with reference to the flywheel  1304 . It can be said that the radially innermost portions of the flange  1341  and the elements  1392  to  1394   b  jointly form a force-locking clutch or slip clutch  1390  in series with the damper  1313 . The damping action of the clutch  1390  increases with increasing angular displacement of the flywheels relative to each other. 
     In order to limit the extent of angular movability of the flange  1341  relative to the flywheel  1304 , the radially innermost portion of the flange  1341  can be provided with projections which alternate with the distancing elements  1367  (as seen in the circumferential direction of the flywheel  1304 ). Such projections then cooperate with the shanks of the distancing elements  1367  to determine the extent of angular movability of the flywheel  1304  and the flange  1341  relative to each other. The just discussed projections of the flange  1341  are optional, i.e., it is possible to mount the flange  1341  in such a way that, when a certain force is exerted, the flange has unlimited freedom of angular movement relative to the flywheel  1304 . In such apparatus, the slip clutch  1390  is designed in such a way that the moment which can be transmitted thereby exceeds the nominal torque of the engine which drives the flywheel including the parts  1331 ,  1332 . 
     In accordance with a modification which is not specifically shown in the drawing, the apparatus  1301  of FIG. 20 can be constructed in such a way that the flange  1341  is mounted with limited freedom of angular movement relative to the flywheel  1304  and the apparatus comprises a second damper having energy storing elements in the form of coil springs which are installed in windows provided therefor in the discs  1393 ,  1394  and flange  1341 . The windows for such additional coil springs are provided in the discs  1393 ,  1394  and flange  1341  between neighboring distancing elements  1367  as seen in the circumferential direction of the flywheel  1304 . It is then advisable to ensure that the spring rate of additional coil springs (which are installed in the region of the slip clutch  1390 ) be much higher than that of coil springs forming part of the damper  1313 . Moreover, the frictional damping action which is generated by the slip clutch  1390  should be much more pronounced than the frictional damping action of the damper  1313  (while this damper is active) and which is produced, among others, by the sealing devices  1374 ,  1374   a  in cooperation with the flange  1341 . 
     The apparatus  1401  of FIG. 21 comprises three dampers  1413 ,  1413   a ,  1414  which operate in parallel. The sections or parts  1431 ,  1432  of the housing for the annular chamber  1430  which accommodates the dampers define two annular compartments  1451 ,  1451   a  which respectively receive the coil springs of the dampers  1413  and  1413   a . The coil springs of the dampers can be deformed by the prongs or arms of a flange  1441  which is installed between the parts  1431 ,  143   a.    
     The parts  1431 ,  1432  define a third annular compartment or space  1452  which receives the coil springs of the innermost damper  1414 . To this end, the parts  1431 ,  1432  have arcuate grooves at the respective sides of the flange  1441 . The radially innermost portion of the compartment or space  1452  is substantially open. The quantity of viscous fluid medium in the chamber  1430  is selected in such a way that the fluid medium fills at least the outermost compartment  1451  but preferably the two outermost compartments  1451 ,  1451   a.    
     The hydraulic or viscous damping action of the damper  1413  can deviate from the damping action of the damper  1413   a . The damping action of each of the dampers  1413 ,  1413   a  can be varied in a number of ways, particularly by appropriate selection of the clearance or gap between the flange portion  1441   a  and the adjacent portions of the parts  1431 ,  1432  intermediate the compartments  1451 ,  1451   a  and/or of the clearance or gap between the flange portion  1441   b  and the parts  1431 ,  1432  radially inwardly of the compartment  1451   a . Such regulation of the hydraulic damping action of the damper  1413  and/or  1413   a  can be relied upon in order to conform the apparatus  1401  for use in a particular power train. Furthermore, the hydraulic damping action can be varied in the previously described manner by appropriate selection of flow restrictors including cup-shaped retainers for coil springs in the compartment  1451  and/or  1451   a . One or two cup-shaped retainers can be provided for one, two or more coil springs in the compartment  1451  and/or  1451   a . The same applies for regulation of the damping action of the innermost damper  1414 . 
     The apparatus  1401  can be modified by increasing the number of concentric dampers to four or even more. Furthermore, the dampers  1413 ,  1413   a  and  1414  can be connected in series rather than in parallel. It is also possible to provide a connection in parallel between two or these dampers and a series connection between another pair of these dampers. 
     FIG. 21 shows a portion of an apparatus  1501  wherein two dampers  1513 ,  1513   a  are disposed side by side, i.e., at the same or at nearly the same radial distance from the axes of the flywheels. The output elements of the dampers  1513 ,  1513   a  include two flanges  1541 ,  1541   a  having dished or cupped radially outermost portions in the respective compartments  1551 ,  1551   a . The inner portions of the flanges  1441 ,  1441   a  are adjacent one another and are attached to the section or part  1532  of the housing for the chamber which includes the compartments  1551 ,  1551   a  by distancing elements in the form of rivets  1565 . The sections or parts  1531 ,  1532  are elements of the flywheel  1503  which is affixed to the output element (not shown) of the internal combustion engine. 
     The radially innermost portions of the compartments  1551 ,  1551   a  are open; these compartments respectively receive the coil springs of the dampers  1513 ,  1513   a . At least the major portions of surfaces bounding the compartments  1551 ,  1551   a  and provided on the parts  1531 ,  1532  of the housing for the annular chamber which includes these compartments are configurated in such a way that their curvature conforms to that of coil springs forming part of the respective dampers  1513 ,  1513   a.    
     The coil springs of the dampers  1513 ,  1513   a  can be designed and mounted in such a way that they undergo compression one after the other, either individually or in groups of two or more. This renders it possible to impart to the damper means including the dampers  1513 ,  1513   a  a multistage characteristic curve. Furthermore, the arrangement may be such that the coil springs in one of the dampers (e.g., the coil springs of the damper  1513 ) become effective after the coil springs of the other damper have already undergone at least some compression, i.e., that one of the dampers is activated with a preselected delay following activation of the other damper in response to angular displacement of one flywheel relative to the other flywheel and/or vice versa. 
     The dampers  1513 ,  1513   a  can be connected in series and such dampers can be used in conjunction with one or more inner dampers (not shown). Still further, the apparatus  1501  can comprise three or more dampers at or close to the same distance from the axes of the flywheels. 
     Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic and specific aspects of our contribution to the are and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the appended claims.