Patent Publication Number: US-6699131-B2

Title: Torsional vibration damping apparatus

Description:
CROSS-REFERENCE TO RELATED CASES 
     This is a division, of application Ser. No. 07/963,109, filed Oct. 19, 1992, now U.S. Pat. No. 6,558,260, which is a division of application Ser. No. 07/617,918, filed Nov. 21, 1990, now U.S. Pat. No. 5,194,044 which is a continuation of application Ser. No. 07/069,708, filed Jul. 2, 1987 now abandoned. Each of these prior applications is hereby incorporated herein by reference, in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to improvements in apparatus for damping vibrations, especially between the output element of an engine (such as the internal combustion engine of a motor vehicle) and a power train (particularly the power train including the change-speed transmission in a motor vehicle). More particularly, the invention relates to improvements in torsional vibration damping apparatus of the type wherein at least two flywheels are rotatable relative to each other against the opposition of damper means, wherein one of the flywheels is connectable to the output element of the engine, and wherein another flywheel is connectable with the input element of a change-speed transmission, especially by way of a clutch (such as a friction clutch). 
     Vibration damping apparatus of the above outlined character normally employ one or more dampers which comprise elastic energy storing elements (such as circumferentially extending coil springs) installed or operating between the flywheels in such a way that they oppose rotation of the flywheels relative to each other and undergo compression and store energy when one of the flywheels is caused to change its angular position with reference to the other flywheel, and/or energy storing elements which act in the axial direction and employ or cooperate with friction pads or linings to generate friction (i.e., hysteresis). As a rule, or in many instances, the energy storing elements which act in the axial direction of the flywheels are connected in parallel with the damper or dampers acting in the circumferential direction of the flywheels. 
     It has been found that, though the just described conventional vibration damping apparatus are quite satisfactory under certain operating conditions (i.e., they can damp certain types of vibrations and they can also reduce noise which develops in response to angular movements of the flywheels relative to each other), the operation of all presently known apparatus constitutes a compromise between an optimum operation under first circumstances and a less satisfactory operation under different second circumstances. For example, purely mechanical vibration damping apparatus cannot satisfactorily oppose a full spectrum of vibrations which are likely to develop at different rotational speeds of the engine and/or under different loads and/or on different types of terrain and/or in different types of motor vehicles. The same applies for the reduction of noise under such widely different circumstances. The bulk and cost of mechanical vibration damping apparatus increase considerably if such apparatus are to be designed with a view to satisfactorily oppose vibrations and to reduce noise under two or more different circumstances which require different modes of vibration damping and/or different modes of noise reduction. Another drawback of purely mechanical vibration damping apparatus is that they cannot conform their damping characteristics to a variety of widely different operating conditions which vary within wide ranges (for example, to different operating conditions which arise as a result of acceleration of the engine-driven flywheel from a relatively low speed to a much higher speed or vice versa). One of the reasons for such lack of versatility of mechanical vibration damping apparatus is that the histeresis of their energy storing elements which act in the circumferential direction of the flywheels cannot adequately conform to changing operating conditions. Moreover, mechanical vibration damping apparatus are prone to malfunction and their parts are subject to extensive wear. 
     Another drawback of presently known vibration damping apparatus is that they do not allow for extensive angular movements of the flywheels relative to each other. In other words, the damping action of the damper or dampers must be very pronounced, at least during the major part of the extent of angular displacement of the flywheels relative to each other. This prevents the conventional apparatus from effectively damping large-amplitude vibrations. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of the invention is to provide a novel and improved vibration damping apparatus which can be used as a superior substitute for heretofore known apparatus between the engines and the power trains of motor vehicles. 
     Another object of the invention is to provide an apparatus which can effectively filter vibrations between the engine and the change-speed transmission of a motor vehicle under a wide range of operating conditions. 
     A further object of the invention is to provide an apparatus which is effective at low, medium and high rotational speeds of its flywheels as well as at resonance RPM and during starting and stoppage of the engine. 
     An additional object of the invention is to provide an apparatus whose damping characteristics (i.e., its ability to dissipate energy) can readily conform to different vibration generating and/or noise generating parameters of the vehicle. 
     Still another object of the invention is to provide a relatively simple, compact and inexpensive apparatus which can oppose vibratory movements and the generation of noise in a number of different ways (including hydraulically and mechanically) and at least as effectively as specially designed inflexible (low-versatility) conventional vibration damping apparatus. 
     A further object of the invention is to provide an apparatus whose parts can be mass-produced in available machines and with a minimum of material removing treatment. 
     Another object of the invention is to provide an apparatus whose useful life is long and wherein the parts are subject to less wear than in conventional apparatus. 
     An additional object of the invention is to provide the apparatus with novel and improved flywheels. 
     Another object of the invention is to provide a novel and improved method of assembling the above outlined apparatus and a novel and improved method of coordinating the action of two or more hydraulic and/or mechanical dampers. 
     A further object of the invention is to provide the apparatus with novel and improved damper means and to provide the apparatus with novel and improved means for confining and shielding the damper means. 
     Another object of the invention is to provide a motor vehicle which embodies the above outlined apparatus and to provide a novel and improved torque-transmitting connection between the engine and the change-speed transmission of a motor vehicle. 
     An additional object of the invention is to provide a novel and improved torque-transmitting connection between the relatively movable parts of the above outlined apparatus. 
     The invention is embodied in an apparatus which can be used to damp vibrations, particularly torsional vibrations between an engine (such as the internal combustion engine of a motor vehicle) and a power train (particularly a power train including a change-speed transmission and a friction clutch which can establish a torque-transmitting connection between the input element of the transmission and the output element of the engine), wherein a first flywheel is connectable with the engine, wherein a second flywheel is rotatable relative to the first flywheel and is connectable with the power train (particularly by way of a clutch, such as the aforementioned friction clutch), and wherein a damper means operates between and yieldably opposes rotation of the first and second flywheels relative to each other. More particularly, the invention resides in improvements in the above outlined apparatus, the improvements including at least three of the following features: 
     (a) One of the flywheels includes sections which define an annular compartment for a supply of viscous fluid medium (preferably a lubricant of pasty consistency) which at least partially fills the compartment, the compartment has an at least substantially closed cross-sectional outline and the damper means comprises at least two energy storing elements (such as springs, especially coil springs) which are disposed in the annular compartment at the same distance from the axis of the one flywheel and are directly or indirectly engageable or engaged by the sections of the one flywheel; 
     (b) a flange (e.g., a flat metallic disc) extends radially into the compartment and engages the energy storing elements and at least substantially seals the compartment, and means (such as a coupling including a portion of the flange) is provided to transmit torque between the flange and the other of the first and second flywheels; 
     (c) the flywheels are rotatable relative to each other against the opposition of the energy storing elements through angles of at least 25 degrees in clockwise and counterclockwise directions starting from a neutral position which may but need not always be the same; 
     (d) the total number of energy storing elements is less than five and such elements jointly extend along an arc which approximates between 70 and 96% of a complete circle; 
     (e) the energy storing elements jointly extend along an arc of 70-96% of the circumferential of the one flywheel; and 
     (f) the energy storing elements are prefabricated or pre-curved to exhibit a curvature with a radius of curvature which equals or approximates the radius of the annular compartment (this simplifies the assembly of the damper means with the sections of the one flywheel). 
     The flange is preferably provided with substantially radial outwardly extending arms which project into the compartment and engage the energy storing elements. The flange can be further provided with one or more ribs which extend in the circumferential direction of the one flywheel and merge into the arms. The sections preferably further define an annular passage which communicates with the annular compartment and receives the ribs of the flange. 
     The sections of the one flywheel preferably include two substantially shell-shaped sections at least one of which can consist of sheet metal. Alternatively, at least one of the sections can constitute a metallic casting. 
     The sections are provided with integral or separately produced inserts which constitute abutments for the energy storing elements and extend into the compartment. The latter preferably extends along an arc of 360°, i.e., it can constitute a circumferentially complete annular compartment. The abutments can include or constitute rivets which are secured to the sections of the one flywheel, and such abutments are or can be disposed at opposite sides of the arms of the flange in the neutral positions of the flywheels. The inserts can constitute integral pocket-like portions of the sections of the one flywheel. 
     The abutments flank the arms of the flange and at least one arm of the flange can be shorter or longer than the adjacent abutments (as seen in the circumferential direction of the one flywheel). Each energy storing element of the damper means is located between an arm of the flange and a pair of abutments, and the damper means can further comprise retainer means (e.g., cup-shaped spring retainers) between at least one of the arms and the respective energy storing element. The arrangement is preferably such that each retainer means at least substantially fills the respective portion of the annular compartment so that each such retainer means can act not unlike a piston or plunger to displace the fluid medium in the compartment in response to angular displacement of the flange and the one flywheel relative to each other. Such piston or pistons can be provided with one or more peripheral recesses or notches and/or one or more holes for the passage of fluid medium therethrough (the fluid-displacing action of such notched, recessed or hollow piston or pistons is less satisfactory than that of a piston which is devoid of holes, notches and/or recesses and has a peripheral surface in immediate or close proximity to the surfaces bounding the adjacent portion or portions of the annular compartment). 
     The compartment can have a substantially constant cross-sectional area all the way from one of its ends to the other end (if it is not a circumferentially complete compartment) or in each and every portion thereof (if it constitutes an endless annular compartment). Alternatively, the compartment can have at least one first portion with a first cross-sectional area and at least one second portion with a different second cross-sectional area. The second cross-sectional area can exceed the first cross-sectional area and can be adjacent one end portion of one of the energy storing elements (e.g., in the form of arcuate coil springs) in the neutral positions of the first and second flywheels. 
     The damper means can include a first damper which comprises the aforementioned energy storing elements in the annular compartment, and at least one second damper which is preferably disposed radially inwardly of the first damper and can include additional energy storing elements. The flange is preferably provided with recesses for the energy storing elements in the compartment and with arcuate windows for the additional energy storing elements. The aforementioned arms alternate with the recesses and the flange is further provided with substantially radially extending webs which alternate with the windows and engage the additional energy storing elements. At least one spring retainer (such as the aforementioned cupped piston-like retainers) can be provided between at least one arm and the adjacent energy storing element in the annular compartment and/or between at least one of the webs and the adjacent additional energy storing element. The retainer or retainers can be provided with sockets and the adjacent arms or webs of the flange can be provided with projections (e.g., in the form of lobes) which extend into the sockets of the adjacent retainers in the circumferential direction of the one flywheel. Such projection(s) of the arm(s) or web(s) serves or serve to maintain the respective energy storing element(s) out of contact with the sections of the one flywheel or out of contact with the flange radially outwardly of the respective energy storing element or elements. 
     The second damper can be connected in parallel with the first damper, e.g., by way of the aforementioned flange. Alternatively, the first and second dampers can operate in series. 
     The energy storing elements of the damper means can form several groups (the energy storing elements of the first damper can form at least one group and the energy storing elements of the second damper can form one or more groups), and the flange and the sections of the one flywheel include means (such as the aforementioned abutments of or on the sections and the aforementioned arms and webs of the flange) for engaging at least two groups of energy storing elements during different stages of angular movement of the first and second flywheels relative to each other. The arrangement may be such that one group of energy storing elements of the second damper begins to store energy in immediate response to angular displacement of at least one flywheel from the neutral position, that another group of energy storing elements of the second damper begins to store energy after at least one of the flywheels completes a certain angular movement from the neutral position, and that the energy storing elements of the first damper begin to store energy simultaneously with the one or the other group or in response to a different angular displacement of at least one of the first and second flywheels from neutral position. 
     If the damper means includes two dampers, the windows for the energy storing elements of the second damper can be distributed in the flange in such a way that each window is located radially inwardly of a different recess for an energy storing element of the first damper. The length of each window (as seen in the circumferential direction of the one flywheel) can equal or approximate the length of a recess. The number of energy storing elements in the first and/or second damper need not exceed four. 
     Those sections of the one flywheel which define the annular compartment preferably include portions which are disposed radially inwardly of the compartment and define a preferably circumferentially complete annular passage which communicates with the compartment and is at least substantially filled by the rib or ribs of the flange. The flange can fill the passage to such an extent that it establishes with the one or the other section an annular gap having a width of 0.1-2 mm in the axial direction of the one flywheel. The gap can be a single gap or a composite gap having a first portion at one side and a second portion at the other side of the flange. 
     The energy storing elements of the second damper are preferably confined in arcuate grooves which are defined by the sections forming the annular compartment, and such grooves together form a second compartment for the respective energy storing elements. The aforementioned passage is disposed between the annular compartment and the grooves and communicates with the compartment as well as with the grooves. Those surfaces of the sections which bound the grooves can closely conform to the outlines of energy storing elements of the second damper. The energy storing elements of the second damper can also constitute coil springs which are prefabricated or pre-curved so as to have a curvature (prior to installation in the one flywheel) which equals or approximates the curvature of the arcuate grooves. The additional energy storing elements (of the second damper) can abut the rib or ribs of the flange under the action of centrifugal force when the one flywheel is set in rotary motion at an RPM which suffices to subject the additional energy storing elements to the action of a centrifugal force strong enough to tend to propel the additional elements radially outwardly and against the rib or ribs of the flange. Alternatively, or in addition to abutting the rib or ribs of the flange, the additional energy storing elements can abut the surfaces which bound the grooves, at least while the flywheels rotate and the additional elements are acted upon by centrifugal force. Each groove can constitute a circumferentially complete groove and can contain abutments for the additional energy storing elements; such abutments are provided on the sections of the one flywheel and cooperate with the webs of the flange to cause the additional elements to store energy in response to angular displacement of the abutments relative to the webs and/or vice versa. Each abutment can include one or more rivets which connect it to the one flywheel. Each abutment can be provided with a substantially flat surface which is in relatively large-area contact with the end portion of the adjacent additional energy storing element. 
     The additional energy storing elements can be located in the windows of two substantially disc-shaped members which flank the flange and are connected to the other flywheel. The flange then comprises means for connecting the first damper in series with the second damper. 
     The apparatus further comprises a coupling or connection which includes a first half on the first flywheel and a second half on the other flywheel. The coupling serves to transmit torque between the two halves which are in torque transmitting engagement with each other in predetermined axial positions of the first and second flywheels relative to each other. One half of the coupling can be provided on the flange and the other half of the coupling can be provided on the other flywheel (e.g., on a disc-shaped member which is bolted, riveted or otherwise secured to the other flywheel). The annular compartment can constitute a portion of an annular chamber which is defined by the one flywheel and which further includes the aforementioned passage for the flange and the aforementioned grooves for the additional energy storing elements. Such apparatus can further comprise means for sealing the chamber from the atmosphere, and the sealing means can include a sealing member on one of the flywheels and a sealing surface provided on the other flywheel and being engaged by the sealing member when the two halves of the coupling are assembled and can transmit torque. The halves of the coupling can comprise mating tooth-like projections which are separable from each other in response to axial shifting of at least one flywheel relative to the other flywheel from a predetermined axial position in which the projections of one half mate with the projections of the other half. 
     The compartment or compartments are preferably provided in the first flywheel, i.e., in that flywheel which can be driven by the output element of the engine if the apparatus is installed in a motor vehicle. 
     The fluid medium in the aforementioned chamber of the one flywheel preferably fills the annular compartment and the passage as well as a portion at least of the second compartment (including the aforementioned annular grooves) so that the additional energy storing elements are contacted by the fluid medium. 
     The apparatus can further comprise at least one friction generating device which operates between the two flywheels to oppose angular movements of such flywheels relative to each other, either during each stage or during selected stages of such angular movements. In other words, the friction generating device or devices can include means for opposing one or more predetermined portions of angular movement of the first and second flywheels relative to each other. The friction generating device or devices can be installed in or externally of the aforementioned chamber. Each friction generating device can operate in series with the first and/or second damper of the damper means. For example, at least one first friction generating device can operate in parallel with the first damper so as to oppose rotation of the flywheels relative to each other with a first force, and one or more additional friction generating devices can operate in parallel with the second damper to oppose rotation of the flywheels relative to each other with a different second force, preferably a lesser force. 
     The mutual spacing of abutments in the annular compartment and/or in the second compartment of the aforementioned chamber can exceed the length of at least one energy storing element in the respective compartment (as considered in the circumferential direction of the one flywheel). 
     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 torsional vibration damping apparatus which embodies one form of the invention; 
     FIG. 2 is a fragmentary end elevational view as seen in the direction of arrow II of FIG. 1, with certain parts broken away; 
     FIG. 3 is an axial sectional view of a second torsional vibration damping apparatus; 
     FIG. 3 a  is an enlarged view of a detail within the phantom-line circle “X” of FIG. 3; 
     FIG. 4 is a fragmentary end elevational view as seen in the direction of arrow IV in FIG. 3, with certain parts broken away; 
     FIG. 5 is a fragmentary axial sectional view of a third torsional vibration damping apparatus; 
     FIG. 6 is a fragmentary sectional view as seen in the direction of arrows from the line VI—VI of FIG. 5; 
     FIG. 7 is a fragmentary sectional view, substantially as seen in the direction of arrows from the line VII—VII of FIG. 6, but showing a slightly modified housing for the fluid-containing chamber; 
     FIG. 8 is a fragmentary sectional view of a further torsional vibration damping apparatus; 
     FIG. 9 is a fragmentary axial sectional view of an additional torsional vibration damping apparatus; and 
     FIG. 10 is a similar fragmentary axial sectional view of still another torsional vibration damping apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show a torsional vibration damping apparatus  1  which comprises a composite flywheel  2  including a first component or flywheel  3  which is connectable to the output element  5  (such as a crankshaft) of an internal combustion engine by a set of bolts  6  or analogous fasteners, and a second component or flywheel  4  which is connectable to the input element  10  of a change-speed transmission by a friction clutch  7 . 
     The friction clutch  7  comprises a pressure plate  8  which is axially movably coupled to the flywheel  4  by a clutch cover  11 . The means for coupling the pressure plate  8  to the cover  11  includes a set of leaf springs two of which are shown in the upper right-hand portion of FIG.  1 . The means for biasing the pressure plate  8  toward the friction surface  70  of the flywheel  4  comprises a diaphragm spring  12  which is tiltable between two ring-shaped seats at the inner side of the clutch cover  11 . The friction clutch  7  further comprises a clutch plate or clutch disc  9  having a hub which is axially movably but non-rotatably mounted on the input element  10  of the transmission and has two sets of friction linings engageable with the surface  70  of the flywheel  4  and with the adjacent surface of the pressure plate  8 , respectively. The means (not shown) for disengaging the clutch  7  can comprise an antifriction bearing which can be moved axially in the direction of arrow II to engage the tips of radially inwardly extending prongs forming integral parts of the diaphragm spring  12  and serving to change the conicity of the diaphragm spring so as to allow the pressure plate  8  to move axially and away from the friction surface  70  of the flywheel  4  and to thus enable the flywheel  4  to rotate relative to the clutch plate  9  and input element  10  of the transmission. The means for biasing the pressure plate  8  axially and away from the flywheel  4  includes the aforementioned leaf springs. 
     The flywheels  3  and  4  are rotatable relative to each other against the opposition of two dampers including an outer damper  13  and an inner damper  14 . The two dampers are connected in parallel and include a common flange-like output member  41  (hereinafter called flange for short). 
     The apparatus  1  further comprises a bearing unit  15  including an antifriction bearing  16  with a single row of antifriction rolling elements in the form of spheres. The bearing  16  comprises an outer race  17  which is mounted in a centrally located recess  18  of the flywheel  4 , and an inner race  19  which is mounted on a cylindrical peripheral surface or seat  20   a  of an axial protuberance  20  of the flywheel  3 . The protuberance  20  extends into the recess  18  which is defined by an axial projection or extension  43  of the flywheel  4 . The means for connecting the flywheels  3  and  4  to each other in predetermined axial positions comprises a retaining ring  22  which is affixed to the end face of the protuberance  20  so that it overlies the adjacent end face of the inner race  19  of the bearing  16 . The inner race  19  is held in a predetermined axial position between a circumferential shoulder  21  of the protuberance  20  and the radially outermost portion of the retaining ring  22 . This ring is secured to the protuberance  20  by the aforementioned bolts  6  which further serve to secure the flywheel  3  to the output element  5  of the internal combustion engine. The inner race  19  is preferably a press fit on the cylindrical seat  20   a  of the protuberance  20 . 
     The means for locating the outer race  17  of the bearing  16  in a predetermined axial position with reference to the extension or projection  43  of the flywheel  4  includes a thermal barrier  25  comprising two rings  23 ,  24  which surround the periphery and the end faces of the race  17  and are recessed into the extension or projection  43 . The ring  23  has a radially inwardly extending portion  23   a  which constitutes a means for sealing the space between the races  17 ,  19  from an annular internal chamber  30  of the flywheel  3 . To this end, the ring portion  23   a  extends radially inwardly beyond the respective end face of the outer race  17 , across the space between the races  17 ,  19  and along a portion at least of the respective end face of the inner race  19 . The ring  23  can be installed in prestressed condition so that its portion  23   a  invariably bears against the respective end face of the inner race  19  and thus prevents leakage of lubricant (such as a grease) from the space between the races  17 ,  19  and into the radially innermost portion of the annular chamber  30  or vice versa. The ring  24  also comprises a radially inwardly extending portion  24   a  which extends along the respective end face of the outer race  17 , across the space between the races  17 ,  19  and along the respective end face of the inner race  19 . The portion  24   a  can bear against the inner race  19  due to innate elasticity of the ring  24  and due to mounting of such ring in prestressed condition. The outer race  17  is held against axial movement relative to the flywheel  4  by an internal shoulder of the extension  43  and by a disc-shaped member  27  (hereinafter called disc for short) which is rigidly secured to the extension  43  by a set of rivets  26  and engages a centering seat  43   a  of the extension  43 . 
     In order to further enhance the sealing action of the radially inwardly extending portion  23   a  of the ring  23 , the apparatus  1  further comprises a resilient element  28  in the form of a diaphragm spring which reacts against an internal shoulder of the disc  27  and bears against the radially innermost part of the ring portion  23   a  so that the latter is held in adequate sealing engagement with the inner race  19 . Analogously, the radially innermost part of the ring portion  24   a  is biased against the respective end face of the inner race  19  by a resilient element in the form of a diaphragm spring  29  which reacts against an internal shoulder of the extension  43  and applies pressure to the radially innermost part of the ring portion  24   a.    
     The chamber  30  contains a supply of preferably highly viscous fluid medium and is sealed from the surrounding atmosphere. In order to prevent leakage of lubricant from the space between the races  17 ,  19  of the antifriction bearing  16  into the innermost portion of the chamber  30  and/or to prevent escape of fluid medium from the chamber  30  into the space between the races  17  and  19 , the ring portion  23   a  preferably establishes a reliable seal between the respective end faces of the two races. This is particularly important if the fluid medium in the chamber  30  is not compatible with the lubricant for the rolling elements of the bearing  16 . The provision of a reliable seal before the right-hand side of the space between the races  17  and  19  is also important in order to prevent flow of lubricant from the bearing  16  into the adjacent region around the hub of the clutch plate  9 . 
     The thermal barrier  25  reduces the likelihood of overheating of lubricant in the bearing  16  and/or overheating of fluid medium in the chamber  30  as a result of repeated engagement or disengagement of the friction clutch  7 . In other words, this thermal barrier prevents the transfer of excessive quantities of heat from the friction surface  70  of the flywheel  4  toward an annular inner section  32  which forms part of the flywheel  3  and constitutes the right-hand sidewall for the chamber  30 . The left-hand sidewall or section is shown at  31 . In addition, the thermal barrier  25  shields the bearing  16  from overheating as a result of repeated engagement and disengagement of the clutch  7 . 
     The dampers  13 ,  14  which oppose rotation of the flywheels  3  and  4  relative to each other are installed in the chamber  30  of the flywheel  3 . The radially outermost portions of the sections  31 ,  32  of the housing which defines the chamber  30  and forms part of the flywheel  3  are secured to each other by a set of screws  33  or analogous fasteners. This establishes a reliable seal between the radially extending abutting surfaces  34 ,  35  of the sections  31 ,  32 , respectively. The escape of fluid medium from the radially outermost portion of the chamber  30  is further prevented by a sealing element  36  (for example, an O-ring) which is installed in a circumferentially complete groove  37  in the surface  34  of the section  31 . The ring  36  is located radially outwardly of an annular compartment  51  which constitutes the radially outermost portion of the chamber  30  and receives the component parts of the outer damper  13 . The screws  33  connect the sections  31 ,  32  to each other radially outwardly of the sealing element  36 . 
     The apparatus  1  further comprises a set of axially parallel centering pins  38  which are received in registering holes or bores provided therefor in the radially extending surfaces  34 ,  35  of the sections  31  and  32 . The centering pins  38  facilitate the assembly of parts which form the apparatus  1 , and more specifically the assembly of parts which form the flywheel  3 . 
     The section  31  is nearer to the engine than the section  32  and has a cylindrical peripheral surface  39  which is surrounded by a ring-shaped starter gear  40 . The gear  40  constitutes a stop against excessive leftward axial movement of centering pins  38  with reference to the section  31 . This gear can be a press fit on the surface  39  or it can be welded or otherwise reliably secured to the section  31 . 
     The sections  31 ,  32  can constitute castings. If it is desired to utilize a low-inertia flywheel  3 , the section  31  and/or  32  of the flywheel  3  can be made of a light metal, such as an aluminum alloy. An advantage of such sections is that they can be shaped in response to the application of pressure in a press, in a stamping machine or in a like machine with a minimum of secondary treatment. 
     The output member or flange  41  of the dampers  13 ,  14  is disposed axially between the sections  31 ,  32  of the flywheel  3 . As can be seen in FIG. 2, the radially innermost portion of the flange  41  has a central opening  71  which is surrounded by an annulus of tooth-like projections  72  constituting one portion or half of a coupling or connection  42  which can transmit torque between the flange  41  and the aforementioned disc  27 . The second half or portion of the coupling  42  includes a set of radially outwardly extending tooth-like projections  73  at the periphery of the disc  27 . The aforementioned cylindrical seat  43   a  of the extension  43  facilitates centering of the disc  27  in the radial direction of the flywheel  4 . 
     The flange  41  has radially outwardly extending projections in the form of arms  44  which alternate with energy storing resilient elements  45  of the outer damper  13  in the annular compartment  51 . The energy storing elements  45  are coil springs which are received in arcuate recesses  46  provided in the periphery of the flange  41  and alternating with the arms  44 . The recesses  46  are located radially outwardly of arcuate windows  47  which are machined into or are otherwise formed in the flange  41  and receive energy storing elements in the form of coil springs  48  constituting component parts of the inner damper  14 . The recesses  46  and windows  47  of the flange  41  are separated from each other by arcuate webs or ribs  49  which alternate with radially extending webs  50 . The webs  50  further alternate with the windows  47  and perform the same function as the arms  44  of the flange  41  except that they abut the adjacent end convolutions of the coil springs  48 . 
     The annular compartment  51  of the chamber  30  is defined primarily by two circumferentially complete annular grooves  52 ,  53  which are respectively machined into or are otherwise formed in the surfaces  34 ,  35  of the sections  31 ,  32  radially inwardly of the sealing element  36  and radially outwardly of similar grooves  63 ,  64  for the coil springs  48  of the inner damper  14 . The grooves  52 ,  53  receive those portions of the coil springs  45  which extend axially beyond the respective sides of the flange  41 . Analogously, the grooves  63 ,  64  receive those portions of the coil springs  48  which extend beyond the respective sides of the flange  41  radially inwardly of the ribs  49 . The compartment  51  for the outer damper  13  can communicate with the compartment for the coil springs  48  of the inner damper  14  by way of a relatively narrow annular clearance or gap  54  which constitutes a small portion of a ring-shaped passage  62  between the circumferentially complete portions  60 ,  61  of surfaces  34 ,  35  of the sections  31  and  32 . The gap  54  can be provided at the one or at the other side of the flange  41 , or it can comprise two portions, one between the surface portion  60  and the flange  41  and the other between the surface portion  61  and the flange  41 . 
     FIG. 1 shows that the grooves  52 ,  53  of the sections  31 ,  32  are bounded by arcuate surfaces which conform rather closely to the adjacent surfaces of the coil springs  45  in the annular compartment  51 . This enables the sections  31  and  32  to act as a means for guiding the convolutions of the coil springs  45  when these springs expand or contract in response to angular displacement of the flange  41  and sections  31 ,  32  relative to each other. The convolutions of the coil springs  45  will tend to abut or will actually abut the surfaces bounding the grooves  52 ,  53  of the sections  31 ,  32  at least when the flywheel  3  rotates, i.e., when the coil springs  45  are acted upon by centrifugal force. It has been found that excessive localized wear upon the convolutions of the coil springs  45  can be reduced considerably if such coil springs are properly guided in the annular compartment  51 . This is due to the fact that the area of contact between the sections  31 ,  32  and the coil springs  45  is increased considerably if the surfaces bounding the grooves  52 ,  53  can contact a substantial number of convolutions of each coil spring  45 . 
     The coil springs  45  are acted upon by the arms  44  of the flange  41  and by pairs of inserts in the form of abutments or stops  55 ,  55   a  which are respectively provided in the grooves  52 ,  53  and are secured to the respective sections  31 ,  32  by rivets  58 . The length of the illustrated abutments  55 ,  55   a  (as seen in the circumferential direction of the flywheel  3 ) equals or closely approximates the length or width of the arms  44  on the flange  41 . These abutments respectively comprise separately produced parts  56 ,  57  which are riveted (at  58 ) to the respective sections  31 ,  32  of the flywheel  3 . It is preferred to flatten those surfaces of the abutments  55 ,  55   a  which come into actual contact with the adjacent end convolutions of the respective coil springs  45 . 
     FIG. 2 shows that the apparatus  1  can further comprise cup-shaped spring retainers  59  which are interposed between the arms  44  of the flange  41  and the adjacent end portions of the coil springs  45 . The peripheral surfaces of the spring retainers  59  preferably conform to the outlines of the adjacent portions of surfaces bounding the grooves  52  and  53  of the sections  31 ,  32 . 
     FIG. 1 further shows that the surfaces bounding the grooves  63 ,  64  of the sections  31 ,  32  closely conform to the outlines of coil springs  48  in the respective compartment of the chamber  30 . This enables the convolutions of the coil springs  48  to abut and to be guided by the surfaces of the sections  31 ,  32 , at least when the flywheel  3  rotates and the coil springs  48  are acted upon by centrifugal force. It is further desirable to properly guide the coil springs  45  and  48  against stray movements in the axial direction of the flywheel  3 . Such stray movements could cause undesirable buckling of the coil springs. 
     It is preferred to provide the sections  31  and  32  with arcuate grooves  52 ,  63  and  53 ,  64 , respectively, which are circumferentially complete recesses. This is advantageous if the surfaces of castings (sections  31  and  32 ) must be treated upon completion of the casting operation and prior to insertion of coil springs  45 ,  48  into the respective compartments of the chamber  30 . Surfaces bounding circumferentially complete grooves can be more readily treated in available machine tools. 
     The grooves  63 ,  64  of the inner annular compartment of the chamber  30  respectively contain inserts in the form of abutments or stops  65 ,  66  which cooperate with the webs  50  of the flange  41  to deform the coil springs  48  of the inner damper  14 . The abutments  65 ,  66  are preferably inserted in such a way that they fill the respective portions of the grooves  63  and  64 , and they are secured to the sections  31 ,  32  of the flywheel  3  by rivets  67 . As can be seen in FIG. 2, the length of abutments  65 ,  66  in the circumferential direction of the flywheels is less than the length or width of the webs  50  which form part of the flange  41 . Each coil spring  48  is confined in its window  47  between a web  50  and a pair of abutments  65 ,  66 . The coil springs  48  preferably abut the internal surfaces of the respective ribs  49  of the flange  41 , at least when the flywheel  3  rotates so that the coil springs  48  are subjected to the action of centrifugal force. 
     It is preferred to make the flange  41  of steel or of a similar strongly wear resistant material. Furthermore at least a portion of the surface of the flange  41  (such as the internal surfaces of the ribs  49 ) is preferably hardened so as to further reduce the likelihood of pronounced wear upon the flange  41  when the apparatus is in use. The ribs  49  are preferably positioned in such a way that they reduce the area of contact between the coil springs  48  and the surfaces bounding the recesses  63 ,  64  in order to ensure that the sections  31 ,  32  of the flywheel  3  are not subjected to extensive wear. Another advantage of the feature that the convolutions of the coil springs  48  abut the internal surfaces of the respective ribs  49  is that the coil springs can share the angular movements of the flange  41  relative to the corresponding abutments  65 ,  66  without sliding along the ribs  49 . Unnecessary slippage of coil springs  48  relative to the ribs  49  is undesirable because it can distort the characteristics of the outer damper  13 . 
     FIG. 2 shows that the dampers  13  and  14  respectively comprise three coil springs  45  and  48 . When the flywheels  3  and  4  assume the neutral positions of FIG. 2, each of the coil springs  45  extends along an arc of approximately 110° and each of the coil springs extends along a similar arc, preferably not less than 100°. The three coil springs  45  together form approximately 91% of a complete circule, and the three coil springs  48  together form approximately 83% of a complete circle. 
     The coil springs  45  and  48  can be furnished while straight and are then bent during insertion into the respective grooves  52 ,  53  and  64 ,  65 . This can result in the development of certain internal stresses which can be avoided if the coil springs  45  and  48  are shaped (prefabricated) so as to assume an arcuate shape even before they are inserted into the chamber  30 . The curvature of pre-curved or prefabricated coil springs  45  and/or  48  can but need not exactly match the curvature of the respective annular compartments of the chamber  30 . The utilization of pre-curved or prefabricated coil springs is desirable on the additional ground that it is much simpler to install them in the respective grooves of the sections  31  and  32 . 
     The viscous fluid medium in the chamber  30  can constitute a lubricant, such as silicon oil or grease. The quantity of the fluid medium in the chamber  30  can be selected in such a way that, when the apparatus rotates, the supply of fluid medium fills the outer compartment  51  at least to the level of the axes of the coil springs  45 . It is normally preferred to introduce a larger quantity of fluid medium so that the fluid medium preferably also fills the gap or clearance  54  between the dampers  13  and  14 . In accordance with a presently preferred embodiment, the supply of fluid medium is selected in such a way that the medium fills the entire compartment  51 , the entire gap  54  and the compartment for the coil springs  48  of the inner damper  14  to the level of the axes of coil springs  48 . This ensures adequate lubrication of the coil springs  48 , of the webs  50  and of the internal surfaces of ribs  49  which are normally engaged by the convolutions of the adjacent coil springs  48 . It is often sufficient to select the quantity of the fluid medium in the chamber  30  in such a way that it fills the compartment  51  and the gap  54  and contacts at least the outermost portions of convolutions of the coil springs  48 . 
     As mentioned before, the provision of the chamber  30  in that flywheel ( 3 ) which is more remote from the friction clutch  7  is advantageous and desirable on the ground that heat which is generated along the friction surface  70  of the flywheel  4  is less likely to adversely influence the characteristics (such as the viscosity) of the fluid medium in the chamber  30 . Additional heat can be withdrawn and thus prevented from reaching the flywheel  3  due to the provision of an annular ventilating channel  68  which is disposed between the section  32  of the flywheel  3  and the flywheel  4  and is open along its radially outermost portion. The radially innermost portion of the channel  68  communicates with passages  69  which are provided in the flywheel  4  radially inwardly of the friction surface  70 . As shown in FIG. 2, the passages  69  can be elongated in the circumferential direction of the flywheel  3 . FIG. 1 shows that the passages  69  can comprise portions which are elongated in the radial direction of the flywheel  3 . 
     An advantage of the aforementioned connection or coupling  42  is that the flange  41  can be properly positioned between the sections  31  and  32  of the flywheel  3  and also that the width of the gap  54  between the compartment  51  and the inner compartment of the chamber  30  can be reduced to a minimum. This enables the parts which define the gap  54  to constitute a highly effective flow restrictor which opposes the flow of viscous fluid medium between the dampers  13  and  14 . An additional advantage of the coupling  42  is that it allows for the machining of certain parts, including the flange  41  and the adjoining parts, with larger tolerances which contributes to lower cost of the entire apparatus. 
     The means for preventing communication between the chamber  30  and the annular ventilating channel  68  comprises a sealing device  74  which operates between the radially innermost portion of the section  32  and the axial extension  43  of the flywheel  4 . The sealing device  74  comprises a washer-like sealing member  75  having an inner marginal portion abutting a circumferentially complete surface  77  of the section  32 . The radially innermost portion of the sealing member  75  surrounds a centering shoulder  76  of the extension  43 . The sealing member  75  is biased axially against the surface  77  by a diaphragm spring  78  which reacts against the flange  41  and urges the sealing member  75  in a direction to the right, as seen in FIG.  1 . The diaphragm spring  78  also biases the flange  41 , namely against the portion  60  of the surface  34  of the section  31  so that the gap  54  normally develops only at one side of the flange  41 , i.e., between this flange and the portion  61  of the surface  35  on the section  32 . 
     The inner diameter of the disc-shaped member  75  which seals the chamber  30  from the annular ventilating channel  68  is greater than the outer diameter of the annulus of projections  73  on the disc  27  of the connection or coupling  42 . The coupling  42  and the sealing device  74  allow for an extremely simple assembly of the apparatus  1 . Thus, it is necessary to first assemble the parts of the apparatus  1  into two subassemblies one of which includes the flywheel  3  and the other of which includes the flywheel  4 . The two subassemblies are then connected to each other by inserting the protuberance  20  into the inner race  19  of the bearing  16  and by attaching the retaining ring  22  to the protuberance  20  so that the outer marginal portion of the ring  22  overlies the inner race  19 . The sealing member  75  is mounted on the flywheel  3  prior to attachment of the flywheels  3  and  4  to each other, and the bearing  16  is mounted in the flywheel  4  prior to such attachment. During assembly, the inner race  19  is slipped onto the seat  20   a  of the axial protuberance  20  of the flywheel  3 , and more particularly of the section  31  of the flywheel  3 , whereby the projections  73  of the disc  27  move into mesh with the projections  72  of the flange  41  so that the coupling or connection  42  is ready to transmit torque between the disc  27  and the flange  41 . At the same time, the sealing member  75  comes into sealing engagement with the shoulder  76  and surface  77  of the projection  43  of the flywheel  4  so that the sealing member  75  is tilted relative to the diaphragm spring  78  and bears against the surface  77  with a force which is required to establish a satisfactory sealing action. As mentioned above, the final axial positioning of the flywheels  3 ,  4  relative to each other is effected by the retaining ring  22  which must be affixed to the protuberance  20  of the flywheel  3 . In the embodiment which is shown in FIGS. 1 and 2, the bolts  6  are used to affix the ring  22  to the protuberance  20 ; however, it is equally possible to employ a set of separate bolts, screws, or rivets, not shown. 
     In order to reduce wear upon the surfaces bounding the grooves  52 ,  53  and  63 ,  64  of the sections  31 ,  32  as a result of repeated frictional engagement with the convolutions of the coil springs  45  and  48 , it is advisable to harden the respective portions of the sections  31  and  32 . This is possible by treating the corresponding portions of sections  31  and  32  in an induction hardening, insert hardening, laser beam hardening or flame hardening apparatus. The exact nature of the hardening treatment forms no part of the invention. All that counts is to ensure that the wear upon the sections  31 ,  32  is reduced and that the apparatus  1  can stand long periods of use. It is also possible to avoid actual hardening of selected portions of sections  31  and  32  if such selected portions are provided with coats or layers of wear-resistant material. The coating can be effected by providing selected portions of the sections  31 ,  32  with layers of chemically applied nickel, with layers of chromium, with layers of molybdenum or with layers of a synthetic plastic material. The thus applied coats or layers can be subjected to a suitable smoothing treatment in order to enhance the surface quality of the sections  31 ,  32  in the regions of the grooves  52 ,  53  and  63 ,  64 . For example, the surface finish can be improved by treating the material around the grooves  52 ,  53  and  64 ,  65  in a suitable grinding or milling machine. 
     The mode of operation of the apparatus  1  of FIGS. 1 and 2 is as follows: 
     When the flywheel  4  is caused to leave the neutral angular position of FIG. 2 with reference to the flywheel  3 , the coupling  42  transmits torque to the flange  41  so that the coil springs  45  of the outer damper  13  begin to store energy because they are compressed between the arms  44  of the flange  41  and the abutments  55 ,  55   a  in the grooves  52 ,  53  of the sections  31 ,  32 . When the flywheel  4  completes an angle  79  in one direction or an angle  80  in the other direction, the abutments  65 ,  66  in the grooves  63 ,  64  engage the respective end portions of coil springs  48  of the inner damper  14  so that, if the flywheel  4  continues to turn relative to the flywheel  3 , the coil springs  48  are compressed by the webs  50  and abutments  65 ,  66  simultaneously with further compression of coil springs  45  by the arms  44  and abutments  55 ,  55   a . Such angular displacement of the flywheel  4  relative to the flywheel can continue until the coil springs  45  reach the stage of maximum compression so that each of these coil springs constitutes or acts not unlike a solid block which cannot undergo any additional compression in the circumferential direction of the flywheels  3  and  4 . 
     In the embodiment of FIGS. 1 and 2, the maximum angle through which the flywheel  4  can turn relative to the flywheel  3  and/or vice versa (starting from the neutral positions of the flywheels  3  and  4  shown in FIG. 2) is approximately or exactly 47°. 
     The apparatus  1  produces a frictional damping action as a result of angular displacement of the flywheel  3  relative to the flywheel  4  and/or vice versa because the coil springs  45  and/or damper  13  rub against the surfaces bounding the grooves  52 ,  53  of the section  31  and  32 . At the same time, the flange  41  rubs axially against the portion  60  of the internal surface  34  of the section  31  under the action of the diaphragm spring  78  for the sealing member  75 . Additional frictional damping action takes place as a result of rubbing contact between the coil springs  48  of the inner damper  14  and the surfaces which are adjacent thereto. The frictional damping action between the coil springs  45 ,  48  on the one hand and the adjacent surfaces on the other hand is a function of the rotational speed of the apparatus  1 . Thus, as the RPM of the apparatus  1  increases, the frictional camping action also increases because the magnitude of the centrifugal force acting upon the coil springs  45 ,  48  increases and these springs are biased against the adjacent surfaces with a progressively increasing force. 
     Additional damping action is generated as a result of turbulence in and displacement of the viscous (normally pasty) fluid medium in the annular chamber  30 . The fluid medium which is confined in the nearly completely sealed annular compartment  51  for the coil springs  45  of the outer damper  13  produces a highly pronounced viscous or hydraulic damping action because the cup-shaped spring retainers  59  act not unlike pistons or plungers and displace the fluid medium in the circumferential direction of the flywheel  3 . When the coil springs  45  in the compartment  51  are caused to store energy, the spring retainers  59  which are shifted by the arms  44  of the flange  41  are moved in a direction toward the spring retainers  59  (if any) which are in abutment with the corresponding stops  55 ,  55   a  so that the viscous fluid medium which is expelled from the interior of coil springs  45  is forced to flow into the gap  54  wherein the flow of fluid medium is restricted due to the narrowness of the gap. In other words, the parts which define the gap  54  act not unlike a flow restrictor. A certain amount of fluid medium is also caused to flow around the peripheral surfaces of the spring retainers  59  and this also produces a desirable hydraulic or viscous damping action. 
     The fluid medium which has flown radially inwardly is compelled to flow back into the compartment  51  as soon as possible because it is being acted upon by centrifugal force whereby the parts which define the gap  54  again perform a damping action. Additional damping action is generated during expansion of coil springs  45  as a result of the flow of fluid medium around the spring retainers  59 . The spaces within the convolutions of the coil springs  45  are again filled with fluid medium, partly as a result of the flow of fluid medium around the spring retainers  59  and mainly as a result of the flow of fluid medium through the gap  54  and into the annular compartment  51 . The damping action which is generated by the viscous fluid medium is a function of the rotational speed of the apparatus  1 , namely of the centrifugal force which increases with increasing rotational speed of the flywheels  3  and  4 . 
     Additional viscous or hydraulic damping action is generated by the coil springs  48  of the inner damper  14 , partly as a result of agitation of the fluid medium in the compartment including grooves  63 ,  64  and in part as a result of expulsion of such fluid medium from or as a result of return flow of fluid medium into the convolutions of the coil springs  48 . 
     The damping action can be regulated and varied in a number of ways. For example some or all of the spring retainers  59  can be provided with notches, grooves or similar recesses in their peripheral surfaces so as to facilitate the flow of fluid medium around such spring retainers. Additional reduction of resistance of the spring retainers  59  to the flow of fluid medium can be achieved by providing such spring retainers with through holes or bores which may but need not be identical in all or some of the spring retainers. Still further, the damping action can be regulated by a proper selection of the total area of the gap  54  between the flange  41  and the section  32  of the flywheel  3 . Moreover, the damping action can be regulated by removing one or more spring retainers  59  and/or by adding spring retainers for those coil springs  45  and/or  48  which are not provided with such spring retainers from the start. Spring retainers can be provided on some or all of the coil springs  48  and/or on all or some of the coil springs  45 . 
     An important advantage of the improved apparatus is that the coil springs  45  and  48  of the dampers  13  and  14  are properly guided in their respective compartments  51  and  63 ,  64  even if they are relatively long or very long. This, in turn, renders it possible to allow for large angular displacements of the flywheels  3  and  4  relative to each other, namely through angles of at least 25 degrees and, if necessary, well in excess of 25 degrees. The ability of the flywheels  3  and  4  to perform large angular movements relative to each other enhances the ability of the apparatus to damp vibrations because the damping angle per increment can be relatively small but the overall damping action (through an angle of 25 or more degrees) is still highly satisfactory and, in fact, much more satisfactory than that which can be obtained with conventional vibration damping apparatus. The arrangement is such that the damping rate is small over a relatively large portion of the angle, or even over the entire angle, which can be covered by the flywheel  3  and/or  4  relative to the other flywheel. The damping rate can be lowered proportionally with an increase of the extent of angular movability of the flywheels relative to each other. 
     The damper  13  and/or  14  can employ relatively long coil springs or other suitable energy storing elements each of which is a one-piece body and each of which is or can be relatively soft and can undergo a pronounced expansion or contraction. This renders it possible to achieve the aforementioned desirable low damping rate. The ability of the flywheels  3  and  4  to perform large angular movements relative to each other and the possibility of using long coil springs having a low damping rate render it possible to damp large-amplitude vibrations, i.e., to compensate for peaks of torque acting in a clockwise or in a counterclockwise direction, as well as to damp small-amplitude vibrations, i.e., relatively small fluctuations of torque which is transmitted by the output element of the engine to the power train including the change-speed transmission or vice versa. The just outlined features of the apparatus render it possible to effectively damp practically all types of vibrations which are likely to develop between the engine and the power train of a motor vehicle. 
     It has been found that the operation of the improved apparatus is particularly satisfactory if the stiffness of the coil springs is between 2-20 Nm/°. It is further advantageous if such spring rate or stiffness is effective through an angle of at least 15 degrees in both directions, i.e., when the engine drives the input element of the transmission as well as when the vehicle is coasting. 
     Another important advantage of relatively long energy storing elements and of the feature that the arms  44  and webs  50  of the flange  41  can cover long distances with reference to the sections  31 ,  32  of the flywheel  3  and/or vice versa is that the viscous fluid medium in the chamber  30  is subjected to a pronounced agitating action and that large quantities of fluid medium can be expelled from and caused to flow back into the respective compartments of the chamber to be thereby subjected to a pronounced throttling action during flow through the gap  54 . This results in the generation of a pronounced hydraulic or viscous damping action whose intensity fluctuates as a function of changes of rotational speed of the composite flywheel and resulting changes of the magnitude of centrifugal force. The hydraulic or viscous damping action also varies as a function of the extent and abruptness of fluctuations of torque which is being transmitted between the flywheels  3  and  4 , i.e., as a function of the extent of angular movement and the extent of acceleration or deceleration of the flywheels relative to each other. 
     As mentioned above, the intensity of the hydraulic or viscous damping action is dependent on the RPM of the flywheels  3  and  4 , i.e., not the RPM of the output element of the engine. Thus, the damping ratio or hysteresis and hence the overall damping characteristics of the apparatus can be varied in dependency on the angular velocity of the flywheels relative to each other and also as a function of changes of the RPM of the engine. It has been found that the apparatus can effectively damp large-amplitude vibrations as well as vibrations of small amplitude, i.e., vibrations which are caused by pronounced surges of torque as well as vibrations which must be counteracted with a relatively small hysteresis. Small-amplitude vibrations are likely to develop when the engine is operated under load. One of the reasons for the above outlined advantages of the improved apparatus is believed to be that the pressure which develops in the fluid medium depends upon the velocity with which a certain volume of the fluid medium is being displaced. In other words, the ability of the fluid medium in the chamber  30 , and particularly in the compartment  51 , depends upon the nature and magnitude of variations of transmitted torque. This enables the apparatus to automatically conform the damping action to the prevailing requirements. In other words, the damping action is regulated automatically as a function of changes in the magnitude and/or nature of deviations of transmitted torque from that which does not involve any angular displacement of the flywheels  3  and  4  relative to each other. 
     The length of the coil springs  48 ,  45  and of the windows  47  and recesses  46  is preferably selected in such a way that each coil spring is normally in contact with the adjacent webs  50  and arms  44 , at least when the flywheels  3  and  4  rotate. This is particularly desirable and advantageous if the coil springs are arranged to undergo compression and to store energy during different stages of angular movement of the flywheels  3  and  4  relative to each other, i.e., if the damper means including the dampers  13 ,  14  is to produce a multi-stage damping action. Consequently, those springs which do not store energy in immediate response to angular movements of the flywheels relative to each other can share the angular movements of the flange to avoid the development of undesirable frictional damping action between such springs and the flange. 
     The abutments  55 ,  55   a  and  65 ,  66  can constitute mass-produced plate-like, rivet-shaped or analogous parts which are riveted, welded, screwed or otherwise reliably affixed to the respective sections  31 ,  32  of the flywheel  3 . The making of separate abutments renders it possible to form the sections with circumferentially complete annular grooves  52 ,  53  and  63 ,  64  which simplifies the making of the sections, especially in a casting machine. 
     The number of stages of operation of the damper means can be varied practically at will, e.g., by appropriate distribution of the abutments in the grooves  52 ,  53  and  63 ,  64  with reference to the arms  44  and webs  50 . As mentioned above, the abutments can be flush with the arms at one side but project beyond or are recessed with reference to the arms and webs at the other side, or they can be recessed or can project beyond both sides of the respective arms and webs. As also mentioned above, the arrangement can be such that the coil springs  45  and/or  48  are not compressed at all during the initial stage of angular movement of at least one of the flywheels  3 ,  4  relative to the other flywheel from the neutral position of FIG.  2 . At such time, the apparatus  1  merely produces a hydraulic or viscous damping action or a frictional damping action, such as due to rubbing contact between the sealing member  75  and the surface  76  of the flywheel  4 . This hydraulic or viscous and/or frictional damping action can be small or very small. 
     It is preferred to use relatively narrow arms  44  and/or webs  50  (in comparison with the abutments in the respective compartments of the chamber  30 ) in apparatus wherein the coil springs are installed in unstressed condition (in the neutral positions of the flywheels relative to each other) and are engaged and held in selected positions (circumferentially of the flywheel  3 ) by the abutments in the respective compartments. 
     If the apparatus is designed to have one or more coil springs turn with the flange during the initial stage of angular displacement of one of the flywheels with reference to the other flywheel before these coil springs begin to store energy, it is desirable that at least one arm  44  or web  50  of the flange  41  be dimensioned to have a width (in the circumferential direction of the flywheel  3 ) which is greater than that of the adjacent abutments in the compartment  51  or in the compartment including the grooves  63 ,  64 . The arrangement can be such that the one arm or web is flush with the abutment at one of its sides but out of register with the abutment at the other side. 
     The dampers  13  and  14  can be connected in parallel or in series. The coil springs of each of these dampers are connected in parallel, and such coil springs can form two or more groups which are caused to store energy during different stages of angular displacement of the flywheel  3  relative to the flywheel  4  and/or vice versa. For example, each of the coil springs  45  can be constructed and mounted to proceed to store energy in response to a different angular displacement of the one or the other flywheel from its neutral position. 
     It is presently preferred to employ a relatively small number of coil springs in each of the dampers  13  and  14 . This brings about the aforediscussed advantage that the dampers can employ relatively long coil springs which can undergo extensive compression and thus enable the flywheels  3  and  4  to turn angles with reference to each other. The number of coil springs  45  or  48  need not exceed four. 
     It is further possible to design the flange  41  and/or the sections  31 ,  32  of the flywheel  3  in such a way that the width of the gap  54  (or the cross-sectional area of the entire gap) changes in response to angular displacement of the flywheels  3  and  4  relative to each other. For example, the cross-sectional area of the gap  54  can decrease if the flywheel  3  and/or  4  is caused to leave the neutral position of FIG.  2 . In other words, the damping action of the flow restrictor means including the parts which define the gap  54  increases with increasing angular displacement of the flywheels with reference to each other. For example, at least one side of the flange  41  can be provided with circumferential extending and axially sloping ramps which cooperate with complementary ramps on the surface portion  60  or  61  in such a way that the area which is available for the flow of viscous fluid medium to or from the compartment  51  is reduced as the flywheel  3  and/or  4  continues to move away from its neutral position. 
     Pre-bending or pre-curving of coil springs  45  and/or  48  is desirable and advantageous for the aforediscussed reasons as well as because these springs are preferably long. Moreover, such pre-bending ensures that the installed coil springs  45  and/or  48  are not subjected to any or to any appreciable bending stresses. 
     The coil springs  45  and  48  are preferably mounted in such a way that they are guided primarily by the surfaces bounding the radially outermost portion of the compartment  51  and by the internal surfaces of the ribs  49 . This reduces the wear upon the surfaces surrounding the grooves  52 ,  53  and  63 ,  64 . In other words, the coil springs  48  are guided by the sections  31 ,  32  solely against deflection in the axial direction of the flywheel  3 , and the coil springs  45  are guided by the sections  31 ,  32  solely or primarily against movement radially outwardly (under the action of centrifugal force) so that the wear upon the major portions of surfaces bounding the grooves  52 ,  53  is not extensive. The end portions of the recesses  46  and/or windows  47  can be configurated in such a way that the end portions of the respective coil springs  45 ,  48  are pulled radially inwardly and are out of frictional contact with the radially outermost portions of surfaces bounding the compartment  51  and/or with the end portions of ribs  49  adjacent the webs  50 . To this end, the end portions of the recesses  46  and/or the end portions of the windows  47  can be bent inwardly toward the axis of the flywheel  3 . 
     However, and especially if the coil springs  48  are to become effective only after the flywheel  3  and/or  4  already completes a certain angular displacement from its neutral position, the mounting of such coil springs is or can be such that they bear against the adjacent ribs  49  with a force which increases with increasing rotational speed of the flywheel  3 . This ensures that the coil springs  48  will not slide relative to the adjacent ribs  49  except when necessary in order to enable or cause them to store energy. 
     The manner in which the parts of the apparatus  1  can be connected with each other to form two subassemblies which are ready to be connected to each other by causing the projections  72  of the coupling  42  to engage the projections  73  is disclosed in the commonly owned copending patent application Ser. No. 069,525, filed Jul. 2, 1987 now abandoned. 
     The feature that the two halves of the coupling  42  are not fixedly secured to each other in the axial direction of the flywheel  3  is desirable and advantageous because the flange  41  is free to find for itself an optimum position between the sections  31 ,  32  of the flywheel  3  and because it is not necessary to machine the flange and/or the parts which are adjacent thereto with a very high degree of precision. Moreover, such construction of the coupling  42  renders it possible to compensate for certain machining tolerances. Still further, such design of the coupling  42  ensures that the apparatus  1  does not develop a pronounced frictional hysteresis in response to small angular displacements of the flywheel  3  relative to the flywheel  4  and/or vice versa while the engine is idling. Highly satisfactory results are obtained if the flywheel  41  is mounted in such a way that it actually floats between the sections  31  and  32  of the flywheel  3 . 
     Referring to FIGS. 3,  3   a  and  4 , there is shown a second torsional vibration damping apparatus  101  wherein nearly all such parts which are identical with or clearly analogous to the corresponding parts of the apparatus  1  are denoted by similar reference characters plus  100 . The antifriction bearing  16  is interposed between the flywheels  3  and  4  substantially in the same way as described in connection with FIG. 1. A ring-shaped retainer  122  is used to engage the inner race of the bearing  16  when the apparatus  101  is fully assembled, and such retainer is secured to the axial protuberance  120  of the flywheel  3  by a set of rivets  122   a  or analogous fasteners. These fasteners ensure that the retainer  122  abuts the end face of the protuberance  120 . 
     The flywheels  3  and  4  are assembled into the apparatus  101  in a manner which is similar to that described above in connection with FIGS. 1 and 2. In other words, the bearing  16  is installed first in the flywheel  4 , and the inner race of the thus installed bearing  16  is thereupon slipped onto the cylindrical seat  120   a  of the axial protuberance  120  of the flywheel  3 . A sealing device  174  is installed on the flywheel  3  before the two flywheels are connected to each other, and such connection involves the activation or complete assembly of the coupling or connection  142  which can transmit torque between the disc  127  on the flywheel  4  and the flange  141  which constitutes the output member of the dampers  13  and  14 . 
     The sections  131  and  132  of the flywheel  3  define the annular chamber  130  and each of these sections is a casting. The periphery of the section  132  is provided with an axially extending cylindrical portion  132   a  having a cylindrical internal surface  135  which is centered by the cylindrical peripheral surface  134  of the section  131 . The sections  131 ,  132  are held against axial movement relative to each other by radially extending centering members or pins  138  received in radially extending recesses or holes provided therefor in the surfaces  134  and  135 . The section  132  carries a ring-shaped starter gear  140  which partially overlies the radially outermost portions of the centering pins  138  so that such pins are held against expulsion radially outwardly under the action of centrifugal force when the apparatus  101  is driven by the engine or by the change-speed transmission. 
     The coupling  142  includes an annulus of tooth-like projections  173  at the periphery of the disc  127 , and a set of complementary tooth-like projections  172  surrounding the central opening of the flange  141 . 
     As shown in FIG. 3 a , the sealing device  174  for the radially innermost portion of the chamber  130  comprises a washer-like sealing member  175  which operates between the radially innermost portion  132   b  of the section  132  and the dished outer marginal portion  176   b  of a sealing member or insert  176 . The sealing member  176  has an inner marginal portion  176   a  which is clamped between the projection or extension  143  of the flywheel  4  and the disc  127 . The sealing member  175  is deformable and is elastic in the axial direction of the flywheels  3 ,  4  and has an inner marginal portion  175   b  which bears against the outer marginal portion  176   b  of the sealing member  176 . The outer marginal portion  175   a  of the sealing member  175  is engaged and held against axial movement by a ring-shaped carrier  180  which is mounted on the radially innermost portion  132   b  of the section  132 . When deformed, the sealing member  175  resembles the frustum of a hollow cone and acts not unlike a diaphragm spring. The marginal portions  175   a  and  175   b  of the sealing member  175  are provided with layers or coats of a plastic material which has a low coefficient of friction and exhibits at least some elastic or plastic deformability. The layers of such plastic material can be sprayed onto the respective marginal portions of the sealing member  175 . The carrier  180  has a collar  180   a  which overlies the right-hand side of the section  132 , and a socket  180   b  which is received in a complementary ring-shaped notch  177  in the radially innermost portion  132   b  of the section  132  and confines the outer marginal portion  175   a  of the sealing member  175  to swiveling movements relative to the section  132 . Such swiveling enables the sealing member  175  to change its conicity during final stages of assembly of the flywheels  3  and  4  with each other, namely when the inner marginal portion  175   b  of the sealing member  175  begins to bear against the dished outer marginal portion  176   b  of the sealing member or insert  176 . The carrier  180  can be said to constitute a bearing which enables the outer marginal portion  175   a  of the sealing member  175  to perform the aforementioned tilting or swiveling movements relative to the section  132  and sealing member  176 . The dished outer marginal portion  176   b  of the sealing member  176  is offset with reference to the inner marginal portion  176   a  in a direction away from the disc  127  and flange  141 . The sealing device  174  of FIG. 3 a  establishes an airtight seal between the radially innermost portion of the annular chamber  130  and the adjacent portion of the annular ventilating channel  168  between the section  132  of the flywheel  3  and the flywheel  4 . 
     In order to allow for convenient assembly of the flywheels  3  and  4 , the inner diameter of the sealing member  175  exceeds the outer diameter of the annulus of projections  173  on the disc  127 . Furthermore, the diameter of the outer marginal portion  176   b  of the sealing member  176  exceeds the diameter of the inner marginal portion  175   b  of the sealing member  175 . This ensures that the inner marginal portion  175   b  automatically engages and is displaced by the outer marginal portion  176   b  when the projections  173  of the coupling  142  are brought into mesh with the projections  172  of the flange  141 . As mentioned above, the coupling  142  becomes operative to transmit torque between the disc  127  and the flange  141  in automatic response to slipping of the inner race of the bearing  16  onto the seat  120   a  on the protuberance  120  of the flywheel  3  so that the ring-shaped retainer  122  can be attached by the rivets  122   a  and abuts the end face of the protuberance  120 . 
     In order to reduce wear between the convolutions of the coil springs  145  (forming part of the outer damper  13 ) and the adjacent surfaces, the flywheel  3  is preferably provided with a short cylindrical member  181  of highly wear-resistant material which is installed in a shallow recess  182  forming the outermost portion of the annular compartment  151  for the coil springs  145 . When the apparatus  101  is driven, the coil springs  145  are acted upon by centrifugal force and their convolutions bear against the cylindrical internal surface of the member  181 . If desired, the member  181  can be provided with a concave internal surface so as to further increase the area of contact between the member  181  and the coil springs  145 . 
     The abutments  155 ,  155   a  for the coil springs  145  of the outer damper  13  and the abutments  165 ,  166  for the coil springs  148  of the inner damper  14  are preferably prefabricated parts which can be made by casting, forging, pressing or the like and which can be provided with integral rivets  158 ,  167 , respectively, for attachment to the respective sections  131 ,  132  of the housing defining the annular chamber  130  and forming part of the flywheel  103 . 
     FIG. 4 shows that the abutments  155 ,  155   a  at opposite sides of the arms  144  of the flange  141  are longer than the respective arms  144 . The arms  144  are disposed midway between the respective abutments  155 ,  155   a ; in other words, the abutments  155 ,  155   a  extend through identical distances beyond the respective arms  144  in the circumferential direction of the flywheels  3  and  4  when the flywheels assume the neutral positions of FIG.  4 . 
     The abutments  165 ,  166  cooperate with the webs  150  of the flange  141  to compress the coil springs  148  in response to angular displacement of the flywheel  3  relative to the flywheel  4  and/or vice versa. As can be seen in FIG. 4, the abutments  165 ,  166  are longer than the respective web  150  (as seen in the circumferential direction of the flywheels). However, the positioning of abutments  165 ,  166  with reference to the radially extending webs  150  is such that the abutments  165 ,  166  which are associated with one of the webs  150  project to one side of such web whereas the abutments  155 ,  166  which are associated with the neighboring web  150  extend beyond the other side of the associated web or are flush with the respective web. In addition, the offset of abutments  165 ,  166  relative to the radially extending webs  150  is such that neighboring webs  150  and the associated abutments  165 ,  166  are offset relative to each other in opposite directions. Consequently, the coil springs  148  of the inner damper  14  constitute two groups  148   a  and  148   b  which become active during different stages of angular displacement of the flywheel  3  relative to the flywheel  4  and/or vice versa. In other words, the coil springs of the group  148   b  begin to store energy in response to angular displacement of the flywheel  3  or  4  relative to the other flywheel through a first angular distance whereas the coil springs  148  of the group  148   a  begin to store energy when the flywheels turn through a different angle relative to each other. 
     The annular chamber  130  between the sections  131 ,  132  of the flywheel  3  contains a supply of a viscous fluid medium which is preferably a lubricant, for example, silicon oil or grease. The fluid medium fills at least the annular compartment  151  of the chamber  130 . It is presently preferred to select the quantity of fluid medium in the chamber  130  in such a way that the fluid medium is in continuous contact at least with the radially outermost portions of convolutions of coil springs  148  forming part of the inner damper  14 . In accordance to a presently preferred embodiment of the invention, the supply of viscous fluid medium fills the compartment  151 , the gap  154  and that portion of the annular compartment for the coil springs  148  which is located radially outwardly of the axes of these coil springs. 
     The apparatus  101  also comprises cupped spring retainers  159  which are interposed between the arms  144  of the flange  141  and the adjacent end portions of the coil springs  145  in the recesses  146  and/or between the abutments  155 ,  155   a  and the respective end portions of the coil springs  145 . The peripheral surfaces of the retainers  159  are or can be closely adjacent the surfaces bounding the compartment  151  so as to ensure that the retainers  159  act not unlike plungers or pistons when they are caused to move relative to sections  131 ,  132  or with such sections in response to angular displacement of the flywheel  3  and/or  4  relative to the other flywheel. As described in connection with FIG. 1, this enhances the damping action of the fluid medium in the chamber  130 . 
     As shown in FIG. 4, the retainers  159  have slightly conical extensions  159   a  which fit into the adjacent end portions of the respective coil springs  145 . Each extension  159  has a substantially conical or roof-shaped tip  159   b . Such configuration of the extensions  159   a  and their tips  159   b  facilitates penetration of the retainers  159  into the adjacent end portions of the respective coil springs  145 . It is also possible to replace the slightly conical extensions  159   a  with substantially spherical extensions. All that counts is to select the configuration of the extensions  159   a  in such a way that each extension can readily find their way into the adjacent end portion of the respective coil spring  145 . The reasons for separation of end portions of coil springs  145  from the respective retainers  159  are numerous. For example, when the apparatus  101  is driven at a very high speed, the coil springs  145  are urged against the member  181  with a pronounced force so that friction between the member  181  and the adjacent convolutions of the coil springs  145  suffices to prevent immediate expansion of these coil springs even if such expansion is permitted by the arms  145  and abutments  155 ,  155   a . At any rate, the expansion of coil springs  145  is not immediate so that the retainers  159  can become completely separated therefrom. An additional reason for potential separation of retainers  159  from the adjacent coil springs  145  is that the viscous fluid medium in the compartment  151  can oppose complete expansion of coil springs  145  or a rapid movement of retainers  159  in order to remain in engagement with the adjacent coil springs. 
     As shown in FIG. 4 by broken lines at  165   a , the abutments  165 ,  166  can be offset with reference to the webs  150  of the flange  141  in such a way that no compression of coil springs  148  takes place in immediate response to angular displacement of the flywheel  3  and/or  4  from the starting or neutral angular position of FIG.  4 . At such time, the apparatus  101  merely produces a hydraulic or viscous damping action and/or a frictional damping action but without any damping as a result of compression of the coil springs. 
     The extent or magnitude or the characteristic of the hydraulic or viscous damping action can be varied in a number of ways, for example by altering the total number of spring retainers  159  and/or by altering the number of spring retainers in the compartment  151  or in the compartment for the coil springs  148 . For example, at least some of the illustrated spring retainers  159  can be omitted to thus weaken the hydraulic damping action. Additional variations of the hydraulic or viscous damping action can be achieved by increasing or reducing the quantity of viscous fluid medium in the chamber  130 . Still further, the damping action can be regulated by changing the width of the gap  154  between the flange  141  and surface portion  160  and/or  161 . 
     The damping action which is attributable to agitation of viscous fluid medium in the chamber  130  is brought about in the same way as described above in connection with FIGS. 1 and 2. 
     FIG. 4 shows that the dampers  13  and  14  respectively comprise four coil springs  145  and  148 . Each coil spring  145  extends along an arc of approximately 78° (when the flywheels  3  and  4  assume the neutral positions of FIG.  4 ). Each of the coil springs  148  which form the group  148   b  extends along an arc of approximately 74° and each coil spring  148  in the group  148   a  extends along an arc of at least 68°. In other words, the four outer coil springs  145  jointly extend along an arc of approximately 86% of a complete circle, and the four inner coil springs  148  together extend along an arc which approximates 79% of a complete circle. 
     FIG. 4 shows that the portion  4   b  of the flywheel  4  has radially outwardly extending projections or lugs  186  which alternate with recesses or tooth spaces  186   a . At least some of the projections  186  have tapped bores or holes  187  for screws or other fasteners which are used to secure the clutch cover  11  (not shown in FIGS. 3 and 4) to the flywheel  4 . In addition, at least some of the projections  186  have bores or holes  188  for centering pins which facilitate assembly of the flywheel  4  with the clutch cover corresponding to the cover  11  of FIG.  1 . The projections  186  facilitate mounting of the clutch on the flywheel  4 . The recesses  186   a  serve as passages for the circulation of atmospheric air which cools the flywheel  4  at  70  and the adjacent parts of the friction clutch. Such air can flow toward and through the passages  169  which are provided in the flywheel  4  radially inwardly of the friction surface  4   a  and establish communication between the radially innermost portion of the annular ventilating channel  168  and the surrounding atmosphere. FIG. 4 shows that the passages  169  extend in the circumferential direction of the flywheel  4 . FIG. 3 shows that portions of such passages can also extend in the radial direction of the flywheel  4 . 
     The provision of recesses  186   a  renders it possible to reduce the overall mass of the flywheel  4  if such reduction of the mass and inertia is desired or necessary. In addition, the projections  186  render it possible to increase the mass of the flywheel  4  in the region of the friction surface  70 ; this reduces the likelihood of overheating of such portion of the flywheel  4 . 
     The apparatus  101  of FIGS. 3,  3   a  and  4  is operated as follows: 
     When the flywheel  4  is caused to turn relative to the flywheel  3  so that it leaves the neutral position of FIG. 4, the coupling  142  turns the flange  141  relative to the flywheel  3  whereby the coil springs  148  of the group  148   b  undergo compression and store energy as a result of engagement with the respective webs  150  of the flange  141  and the corresponding abutments  165 ,  166  in the grooves  152 ,  153  of the sections  131  and  132 . When the flywheel  4  completes an angle  179  in one direction or an angle  190  in the opposite direction, the webs  150  of the flange  141  begin to compress the coil springs  148  in the windows  147  for the group  148   a  so that, if the flywheel  4  continues to turn relative to the flywheel  3  (and/or vice versa) the coil springs  148  of the group  148   b  continue to store energy and the coil springs  148  of the group  148   a  begin to store energy. When the flywheel  4  completes the angle  179   a  in one direction or the angle  190   a  in the opposite direction, the arms  144  of the flange  141  begin to compress the coil springs  145  of the outer damper  13 . In other words, if the angular displacement of the flywheel  4  relative to the flywheel  3  continues beyond the angle  179   a  or  190   a , the flange  141  cooperates with the abutments  155 ,  155   a  and  165 ,  166  to simultaneously compress the coil springs  148  of the groups  148   a ,  148   b  as well as the coil springs  145 . It will be seen that the damper means including the dampers  13 ,  14  of FIGS. 3 and 4 has a two stage characteristic curve. The angles  179 ,  190 ,  179   a ,  190   a  can be identical or dissimilar. This enables the designer to provide a composite damper  13 ,  14  which has a three-stage or a higher-stage characteristic curve in one or both directions. For example, the arrangement can be such that the composite damper will have an at least two-stage characteristic curve in one direction and an at least three-stage characteristic curve in the opposite direction. 
     The viscous or hydraulic damping action of the fluid medium in the chamber  130  can be altered still further by utilizing sections  131 ,  132  and/or a flange  141  defining inner and/or outer annular compartments having a non-uniform cross-sectional outline. Thus, the resistance which is offered to the flow of viscous fluid medium in the inner and/or outer compartment can be reduced by increasing the cross-sectional area of one or more portions of the respective compartment. For example and as shown in the left-hand portion of FIG. 4, the compartment  151  can have at least one enlarged portion  189  which is attributable to suitable configuration of the respective rib  149  of the flange  141 . Such enlarged portion allows for ready flow of viscous fluid medium along the respective cup-shaped spring retainer  159 . The transition from the enlarged portion or portions  189  into the other portion or portions of the compartment  151  can be abrupt or gradual. The enlarged portion or portions  189  can be provided at any selected location as seen in the circumferential direction of the apparatus  1 . It is presently preferred to place such enlarged portions  189  adjacent the end convolutions of the coil springs  145 . More specifically, the enlarged portion or portions  189  will be adjacent the end portion or portions of one or more coil springs  145  in the undeformed or in the least deformed condition of such coil springs, namely when the flywheels  3  and  4  assume the neutral positions of FIG.  4 . 
     It is further preferred to place the enlarged portion or portions  189  adjacent the innermost portion or portions of the respective coil spring or coil springs  145 , i.e., at locations which are remote from the cylindrical member  181 . It is not necessary to provide each enlarged portion  189  in the web or webs  149  of the flange  141 . For example such enlarged portion or portions can be provided in the section  131 , in the section  132  or in each of these sections. 
     The spring retainers  159  render it possible to regulate the hydraulic or viscous damping action with a high degree of accuracy and in an extremely simple manner. Thus, the retainers  159  can displace predetermined quantities of viscous fluid medium in response to predetermined angular displacements of the flywheels  3  and  4  relative to each other, i.e., the retainers can regulate the damping characteristics in dependency on certain operating parameters (including the extent of angular displacement of the flywheels relative to each other). The provision of the enlarged portion or portions  189  of the compartment  151  and/or of one or more enlarged portions in the compartment for the coil springs  148  of the inner damper also contributes to a regulation of the damping action in dependency on changes in certain parameters. 
     FIGS. 5 and 6 illustrate a third torsional vibration damping apparatus  201 . Nearly all such parts of this apparatus which are identical with or clearly analogous to corresponding parts of the apparatus  1  or  101  are denoted by similar reference characters plus  200  or  100 . The apparatus  201  also comprises a composite flywheel  202  having at least two discrete components or flywheels including a first flywheel  203  which can receive torque from the output element of the engine and a flywheel  204  which can transmit torque to the input element of the change-speed transmission of the power train. The bearing unit  15  between the flywheels  203  and  204  comprises an antifriction bearing  16  with a single row of spherical antifriction rolling elements. However, it is also possible to employ two antifriction bearings or an antifriction bearing with two or more rows of spherical, roller-shaped, needle-shaped or otherwise configurated rolling elements. The flywheel  203  includes two annular sections  231 ,  232  which define an annular chamber  230  for a single damper  213 . The sections  231 ,  232  are connected to each other radially outwardly of the chamber  230 , and each such section can constitute a suitably deformed blank of sheet metal. The seal  238  between the sections  231 ,  232  is a circumferentially complete welded seam which is provided between two radially extending surfaces  234 ,  235  of the sections  231 ,  232  and replaces an O-ring or any other sealing element which is normally used in the absence of a welded connection between the sections. The welding operation is preferably carried out in a resistance butt welding machine or in a capacitor discharge welding machine, namely a machine wherein the sections  131 ,  132  are welded to each other (at  138 ) in response to the application of a high-amperage low-voltage alternating current. Such application of electrical energy entails a heating of sections  131 ,  132  along their surface portions  134 ,  135 , and the final welding step is carried out in response to the application of axial pressure. The area of surface portions  234 ,  235  is related to the strength of the applied current. 
     In order to properly position the sections  231 ,  232  relative to each other in the radial direction of the flywheel  203  preparatory to and during welding at  238 , the section  231  is provided with a ring-shaped axial extension or portion  231   a  which surrounds and centers the cylindrical peripheral surface  235   a  of the section  232 . In order to ensure accurate angular positioning of the sections  231 ,  232  relative to each other during welding, the outer surfaces of the sections  231 ,  232  are respectively provided with recesses or sockets  265 ,  266  for the tips of prongs of welding equipment which is used to bond the sections  131 ,  132  to each other at  238 . 
     As mentioned above, welding of the sections  231 ,  232  to each other at  238  involves the application of pressure in the axial direction of such sections. Since the material of the sections is soft at  234 ,  235  as a result of the application of electrical energy, the sections  231 ,  232  would be likely to move axially beyond the optimum positions relative to each other, i.e., the width of the chamber  230  (as seen in the axial direction of the flywheel  203 ) could be reduced beyond the optimum value so that the coil springs  245  of the damper  213  would be likely to jam. Therefore, the section  232  is preferably provided with one or more axial stops  267  (one shown by phantom lines in the upper portion of FIG. 5) which come into abutment with the adjacent end face of the cylindrical portion  231   a  of the section  231  when the welding operation takes place and the portions of sections  231 ,  232  are soft in the regions of the surface portions  234 ,  235 . The stops  267  ensure that the dimensions of the compartment  251  constituting the outermost portion of the chamber  230  are such that the coil springs  245  are received therein with minimal clearance which is desirable to avoid buckling of the coil springs  245  in the axial direction of the flywheel  2  or  3  and/or to ensure proper guidance of the coil springs in the circumferential direction of the chamber  230 . 
     Another advantage of the stop or stops  267  is that the strength of the current which is applied to the sections  231 ,  232  need not be regulated with a high degree of accuracy because the application of relatively strong current and the resulting softening of sections  231 ,  232  at  238  will not result in excessive narrowing of the chamber  230 . This renders it possible to avoid an extremely accurate conformance of the areas of surface portions  234 ,  235  to the selected strength of the applied current. 
     The output member of the damper  213  is radially extending flange  241  which is disposed between the sections  231 ,  232  of the housing for the chamber  230 . The radially innermost portion of the flange  241  has a central opening  271  surrounded by an annulus of tooth-like projections constituting one-half of a torque-transmitting connection or coupling  242 . The other half of such coupling is defined by tooth-like projections  273  at the periphery of the disc  227  which is secured to the end face of the extension  243  of the flywheel  204  by rivets  226 . The radially outermost portion of the flange  241  is constituted by the arms  244  which alternate with the coil springs  245  of the damper  213  in the compartment  251  of the chamber  230 . 
     The compartment  251  is defined in part by circumferentially extending arcuate grooves  252 ,  253  in the internal surfaces of the sections  231 ,  232 . The grooves  252 ,  253  can be formed during conversion of sheet metal blanks into the respective sections  231  and  232 . These grooves respectively receive those portions of the coil springs  245  which extend axially of the apparatus  201  beyond the respective sides of the flange  241 . The flange  241  comprises a ring-shaped portion  249  corresponding to the ribs  49  of the apparatus  1  and defining with the sections  231 ,  232  a relatively narrow clearance or gap  254  connecting the compartment  251  with the radially innermost portion of the chamber  230 . 
     The configuration of surfaces bounding the grooves  252 ,  253  of the sections  231 ,  232  is preferably such that these surfaces closely conform to the outlines of the adjacent coil springs  245 . In other words, the convolutions of the coil springs  245  can slide along and can be guided by the surfaces which bound the grooves  252  and  253 . Such guidance is desirable and normally takes place at least while the apparatus  201  rotates, namely when the coil springs  245  are acted upon by centrifugal force. 
     The grooves  252 ,  253  respectively contain abutments or stops  255 ,  255   a  for the adjacent end portions of coil springs  245  in the compartment  251 . FIG. 6 shows that the length of the abutments  255 ,  255   a  in the circumferential direction of the flywheel  203  equals the length or width of the arms  244  on the flange  241 . FIG. 6 further shows that the apparatus  201  comprises cup-shaped spring retainers  259  which are interposed between the arms  244  of the flange  241  and the adjacent end portions of the coil springs  245 . The configuration of the retainers  259  is preferably selected in such a way that their peripheral surfaces are immediately or closely adjacent the surfaces bounding the grooves  252 ,  253 . This enables the retainers  259  to act not unlike plungers or pistons for the supply of fluid medium in the compartment  251 . 
     The aforementioned clearance or gap  254  is defined in part by the ring-shaped portion  249  of the flange  241  and in part by the section  231  and/or  232  of the flywheel  203 . The internal surfaces of the sections  231 ,  232  respectively comprise circumferentially complete portions  260 ,  261  which together define a ring-shaped passage or channel  262  for reception of the portion  249  of the flange  241 . The gap  254  constitutes that portion of the passage or channel  262  which is not occupied by the portion  249  of the flange  241 . Such gap can be provided only between the portion  249  and the surface portion  260 , only between the portion  249  and the surface portion  261  or in part between the portion  249  and surface portion  260  and in part between the portion  249  and the surface portion  261 . The width of the passage or channel  262  exceeds only slightly the thickness of the flange  241  so that the gap  254  is relatively narrow. 
     The damper  213  comprises four coil springs  245  each of which extends along an arc of approximately 82° when the flywheels  203  and  204  assume the neutral positions of FIG.  6 . In other words, the combined length of the four coil springs  245  equals or approximates 90% of a complete circle. 
     In order to reduce the likelihood of, or to prevent, the development of internal stresses in the coil springs  245 , these coil springs can be pre-curved or prefabricated prior to their introduction into the compartment  251 . The curvature of coil springs  245  prior to insertion into the compartment  251  can equal or can merely approximate the curvature of the grooves  252  and  253 . Such pre-curving or prefabrication of the coil springs  245  facilitates and simplifies the assembly of the damper  213  with the sections  231 ,  232  of the flywheel  203 . 
     When the apparatus  201  is driven, the supply of viscous fluid medium in the chamber  230  fills at least the annular compartment  251 , i.e., the radially outermost portion of the chamber  230 . 
     FIG. 6 shows the central opening  271  of the flange  241  and the annulus of tooth-like projections  272  which surround the opening  271  and constitutes one-half of the coupling or connection  242 . The recesses  272   a  between the projections  272  of the flange  241  constitute tooth spaces for the complementary projections or teeth  273  at the periphery of the disc  227 . As mentioned above, the projections  273  constitute the other half of the connection or coupling  242 . The shanks of the rivets  226  which connect the disc  227  to the extension  243  of the flywheel  204  extend through the projections  273  of the disc  227 . 
     The coupling  242  allows for such installation of the flange  241  between the sections  231 ,  232  that the width of the gap  254  is sufficiently small to ensure that the parts which define this gap constitute an effective flow restrictor for the viscous fluid medium which tends to flow from or back into the compartment  251  of the chamber  230 . Another advantage of the coupling  242  is that it allows for the making of parts around the gap  254  with relatively large tolerances. Such parts include the disc  227 , the flange  241  and the sections  231 ,  232  of the flywheel  203 . 
     The radially innermost portion of the chamber  230  is sealed from the surrounding atmosphere (and more particularly from the ventilating channel between the section  232  and the flywheel  204 ) by a sealing device  274  which operates between the radially innermost portion of the section  232  and the flywheel  204 . The sealing device  274  distinguishes from the sealing device  174  of FIG. 3 a  in that the entire sealing member  275  is coated with a layer or film of elastically or plastically deformable material having a low coefficient of friction. Such material can be a plastic substance which is sprayed onto the sealing member  275 . Alternatively, the member  275  can be dipped into a body of liquid plastic material which hardens on the sealing member  275  to form an elastic coat. The sealing member  275  is elastically deformable in the axial direction and its inner marginal portion bears against the outer marginal portion  276   b  of a second sealing member or insert  276  the inner marginal portion of which is clamped between the disc  227  and the extension  243  of the flywheel  204 . The outer marginal portion of the sealing member  275  is tiltably held between the radially innermost portion  232   a  of the section  232  and a ring-shaped carrier  280  which is secured to the inner side of the section  232  by rivets  232   b  or other suitable fasteners. 
     The radially innermost portion  232   a  of the section  232  extends radially inwardly beyond the outer marginal portion of the sealing member  275  and defines with the latter an annular space  232   c  which is disposed radially outwardly of the locus of abutment of the inner marginal portion of the sealing member  275  with the outer marginal portion  276   b  of the sealing member  276 . This ensures that any viscous fluid medium which happens to leak between the sealing members  275 ,  276  in the region of the outer marginal portion  276   b  enters the space  232   c  under the action of centrifugal force and can be forced back into the chamber  230 , again under the action of centrifugal force, when the flywheel  203  is driven at a high speed so that the fluid medium which accumulates in the space  232   c  is compelled to creep around the outer marginal portion of the sealing member  275  and back into the chamber  230 . The outer marginal portion of the sealing member  275  is received in a circumferentially complete notch  291  in the radially innermost portion  232   a  of the section  232 . Such notch is bounded at the left-hand side (as seen in FIG. 5) by the radially innermost portion of the carrier  280  which is preferably elastic and bears against the outer marginal portion of the sealing member  275  so that the latter is held in a predetermined axial position relative to the flywheel  203  but can be tilted in its socket so as to assume the shape of a conical frustum as a result of engagement with the outer marginal portion  276   b  of the sealing member  276 . The outer marginal portion  276   b  is dished to increase its strength and hence its ability to withstand deforming forces when it is engaged by the stressed sealing member  275 . 
     The section  231  of the flywheel  203  is nearer to the engine than the section  232  and is connected with an axial protuberance  220  which constitutes a third component part of the flywheel  203 . The protuberance  220  is surrounded by the antifriction bearing  16  of the bearing unit  15  which operates between the flywheels  203  and  204 . The manner of mounting the bearing  16  between the projection  243  and the protuberance  220  is the same as or similar to that described in connection with FIG.  1 . The section  231  has a cylindrical centering surface  220   b  for the complementary cylindrical internal surface of the section  231 , and the protuberance  220  is further provided with a shoulder  220   c  serving as an abutment and axial stop for the section  231  with reference to the section or protuberance  220 . The bolts (not specifically shown) which secure the section  231  to the protuberance  220  can also serve as a means for securing the ring-shaped retainer  222  which abuts the end face of the protuberance  220  and overlies the radially innermost portion of the inner race of the bearing  16  in order to fix the bearing  16  in a predetermined axial position with reference to the flywheel  203 . The bearing  16  is held in a predetermined axial position with reference to the flywheel  204  by a thermal barrier corresponding to the thermal barrier  25  of the apparatus  1  in cooperation with an internal shoulder of the projection  243  and the radially innermost portion of the disc  227 . The aforementioned bolts which are used to connect the section  231  to the protuberance  220  can be replaced by rivets, screws or other suitable fasteners. It is also possible to weld the protuberance  220  to the section  231  or to upset the leftmost portion of the protuberance  220  (as seen in FIG. 5) around the radially innermost portion of the section  231 . 
     The apparatus of FIGS. 5 and 6 is assembled in a manner which is similar to or identical with the manner of assembling the apparatus  1  of FIGS. 1 and 2. In other words, the antifriction bearing  16  is first installed in the flywheel  204  and the sealing member  275  is first installed in the flywheel  203 . When the inner race of the bearing  16  is slipped onto the cylindrical seat  220   a  of the protuberance  20 , the coupling  242  becomes operative because the projections  273  at the periphery of the disc  227  enter the tooth spaces  272   a  between the projections  272  surrounding the central opening  271  of the flange  241 . The sealing member  275  is deformed to assume a frustoconical shape and stores energy in automatic response to shifting of the flywheel  204  to the axial position of FIG. 5 because the outer marginal portion  276   b  of the sealing member  276  is then engaged by the inner marginal portion of the sealing member  275 . The assembly of the flywheels  203  and  204  is completed when the ring-shaped retainer  222  is properly affixed to the end face of the protuberance  220 . If desired, the retainer  222  can be affixed to the protuberance  220  by a set of rivets, screws or other suitable fasteners, i.e., not necessarily those fasteners which are used to connect the section  231  to the protuberance  220 . 
     The hydraulic or viscous damping action is brought about as a result of turbulence in and displacement of fluid medium in the annular compartment  251  of the chamber  230 . The fluid medium produces the damping action in the same way as described above in connection with the apparatus  1  and  101 . 
     In order to prevent overheating of parts which must move relative to the sections  231 ,  232 , the welding of sections  231 ,  232  to each other at  238  is preferably preceded by the application of coats of electrically insulating material to certain parts of the sections  231 ,  232  and/or to other parts which are adjacent thereto and must be confined in the chamber  230  prior to start of the actual welding operation. Such parts include the flange  241  and the spring retainers  259 . The provision of coats of insulating material is desirable on the additional ground that excessive heating of parts which are adjacent the sections  231 ,  232  in the course of the welding operation could result in an undesirable change of the characteristics of the material of such parts. The coil springs  245  also include those parts which are likely to be affected by excessive heat during welding of the sections  231 ,  232  to each other. The parts  231 ,  232 ,  245 ,  259 ,  241 ,  255 ,  255   a  can be coated entirely or in part. Phosphating constitutes one of the presently preferred modes of providing selected parts with coats of electrically insulating material. Another possibility is to make certain parts, such as the spring retainers  259  and the abutments  250 ,  255   a , of a non-conductive material. In accordance with a presently preferred embodiment, the sheet-metal sections  231 ,  232  are phosphated, the same as the flange  241 . On the other hand, the coil springs  245  are preferably coated with a lacquer. However it is also possible to phosphatize the springs  245 . Another mode of providing selected portions of certain parts of the flywheel  203  with electrically insulating layers is to apply to such parts coats of a ceramic or synthetic plastic material or with layers of grease. Ceramic and/or plastic coats or coats of grease can be applied particularly to the sections  231 ,  232 . The sections  231 ,  232  can be coated except in the regions (surfaces  234 ,  235 ) where they are to be welded to each other as well as in the regions where they are to be temporarily connected to the source of electrical energy. Such regions can include the surfaces bounding the recesses or sockets  265  and  266 . Alternatively, it is possible to coat the entire section  231  and/or  232  and to thereupon remove the applied coat of electrically insulating material in the region where the section is to be bonded to the other section and in the region where the section is to be connected to the source of electrical energy. Such removal of electrically insulating material can involve a treatment in a grinding machine or another machine tool. The insulating material must be selected in such a way that it is compatible with the viscous fluid medium in the chamber  230 . 
     The making of phosphate layers is one of the presently preferred modes of applying insulating coats to sections  231 ,  232  and/or to other parts because a phosphate layer exhibits highly desirable wear-resistant and self-lubricating properties. 
     The periphery of the section  231  is provided with a cylindrical seat  239  for a ring-shaped starter gear  240  which abuts a peripheral shoulder of the section  231  and is preferably welded (at  240   a ) to the section  231 . The connection at  240   a  can constitute a series of spot welds, a plurality of arcuate welded seams or a continuous circumferentially complete welded seam. The application of spot welded or other seams is desirable because the thickness of the section  231  is normally less than the thickness of the starter gear  240  so that an annular clearance is provided along the internal surface of the gear  240  and such clearance can receive the connection  240   a.    
     The thickness of one of the sections  231 ,  232  can exceed the thickness of the other section. As shown in FIG. 5, the thickness of the section  231  exceeds the thickness of the section  232 . 
     An advantage of the flywheel  203  is that its sections  231 ,  232  can be produced at a low cost. This is due to the fact that the grooves  252 ,  253 , the recesses  265 ,  266 , the recess  282 , the centering portion  231   a , the stop or stops  267 , the notch  291  and certain other parts of these sections are or can be formed during conversion of the respective metallic blanks in a stamping, forging or like machine. This holds true even if the grooves  252 ,  253  are not circumferentially complete depressions in the sections  231 ,  232 . 
     FIG. 7 shows that the sections  231 ,  232  can be provided with integral pocket-like abutments or stops  255   c ,  255   d  which replace separately produced abutments and constitute stops for the adjacent cup-shaped spring retainers  259 . Such pocket-like abutments can be readily formed on sections which are made of sheet metal. 
     An additional advantage of integral pocket-like abutments is that their making necessarily results in the making of sockets or recesses  255   c ′,  255   d ′ and such recesses can replace the recesses  265 ,  266  of FIG.  5 . In other words, these recesses can serve to receive the tips of prongs forming part of the welding equipment which is used to bond the sections  231 ,  232  of FIG. 7 to each other. The abutments  255   c ,  255   d  thus constitute electrodes by means of which electrical energy is applied to the sections  231 ,  232  for bonding them to each other. In addition, the prongs which enter the recesses  255   c ′,  255   d ′ serve as means for applying the required axial pressure during bonding. The mutual spacing of prongs which enter the recesses  255   c ′,  255   d ′ is selected in such a way that the welded-together sections  231 ,  232  are maintained at an optimum axial distance from each other. This obviates the need for the axial stop or stops  267  of FIG.  5 . Proper axial spacing of the sections  231 ,  232  is desirable and advantageous in order to ensure that the coil springs  245  in the compartment  251  of the chamber between the sections  231 ,  232  are not held against movement in the circumferential direction of the flywheel. Additionally, proper axial positioning of the sections  231 ,  232  ensures the establishment of a relatively narrow gap (see the gap  254  in FIG. 5) so as to ensure that the viscous fluid medium in the chamber including the compartment  251  will encounter requisite resistance to the flow through the gap. 
     FIG. 8 shows a portion of a further apparatus  301  which has a flange  341  with radially outwardly extending projections or arms  344  between the neighboring coil springs  345  and  345   a . The coil springs  345  and  345   a  are installed in a circumferentially complete annular compartment  351  forming part of a chamber for a supply of viscous fluid medium. The chamber is defined by two sections of a flywheel  303 . The coil spring  345   a  is acted upon directly by the adjacent portion of the arm  344 . On the other hand, the coil spring  345  is acted upon by a cup-shaped spring retainer  359  which is slipped onto a projection or lobe  344   a  of the arm  344 . Another projection or lobe  344   b  of the illustrated arm  344  extends into the adjacent end portion of the coil spring  345   a . The coil springs  345 ,  345   a  and the arms  344  of the flange  341  together constitute a damper  313  which is installed in the compartment  351 . 
     The cup-shaped spring retainer  359  has a socket  359   a  which receives the lobe  344   a . The configuration of the lobe  344   a  and of the cup-shaped retainer  359  are preferably such that the coil spring  345  is held out of contact with the surface bounding the radially outermost portion of the annular compartment  351 . To this end, the lobe  344   a  has a sloping ramp-like cam face  344   c  which engages the adjacent portion of the internal surface of the retainer  359  in such a way that the radially outermost portions of adjacent convolutions of the coil spring  345  are held out of contact with the surface bounding the radially outermost portion of the compartment  351 . The cam face  344   c  abuts a complementary portion  359   b  of the internal surface of the retainer  359 . When the lobe  344   a  is properly received in the socket  359   a  of the retainer  359  so that it bears against the portion  359   b  of the internal surface of the retainer, at least one or more end convolutions of the spring  345  are out of contact with the surface surrounding the radially outermost portion of the compartment  351 . 
     The projection or lobe  344   b  of the arm  344  which is shown in FIG. 8 has a sloping ramp-like cam face  344   d  serving to act upon the adjacent convolution or convolutions of the coil springs  345   a  in order to pull such convolution or convolutions radially inwardly and away from contact with the surface bounding the radially outermost portion of the annular compartment  351 . 
     The configurations of the lobe  344   a  and of the internal surface of the retainer  359  (namely of the surface which bounds the socket  359   a ) are preferably such that the lobe  344   a  can properly pull the adjacent end portion of the coil spring  345  radially inwardly and away from the surface bounding the outermost portion of the compartment  351 , regardless of the angular position of the retainer  359  relative to the arm  344 . Such change in the angular position of the retainer  359  can take place while the damper  313  is in actual use. 
     It goes without saying that projections or lobes corresponding to the lobes  344   a  and  344   b  can also be provided on the arms  44 ,  144  or  244  of flanges in the previously described torsional vibration damping apparatus. Such lobes ensure that the coil springs  345  and  345   a  can move relative to the sections of the flywheel  303  even if the apparatus  301  is driven at a high speed so that the coil springs  345  and  345   a  are subjected to the action of substantial centrifugal forces which tend to maintain their convolutions in strong frictional engagement with the surfaces bounding the compartment  351 . In other words, at least one or more end convolutions of each of the coil springs  345 ,  345   a  are held out of contact, or out of pronounced contact, with the adjacent portions of the surfaces bounding the compartment  351 . This allows for a much more predictable operation of the damper  313 . 
     The lobes  344   a ,  344   b  further ensure that at least the end portions of the coil springs  345 ,  345   a  retain some elasticity, even if the apparatus  301  is driven at a very high speed at which the centrifugal force acting upon the coil springs  345 ,  345   a  suffices to maintain the majority of convolutions of these springs in strong frictional engagement with the adjacent portions of the surfaces bounding the compartment  351  so that such convolutions cannot move in the compartment  351  in the circumferential direction of the flywheel  303 . The freedom of movement of at least some convolutions of the coil springs  345 ,  345   a  relative to the sections of the flywheel  303  is desirable and necessary because the coil springs can damp small-amplitude vibrations which develop as a result of minute angular displacement of the flywheels relative to each other at elevated RPM of the flywheel  3 . Such small-amplitude vibrations normally take place at a high frequency. 
     The blind bore or hole which constitutes the socket  359   a  of the spring retainer  359  is preferably configurated to have a circular cross-sectional outline. This can be achieved by deforming a metallic or plastic blank in a stamping or like machine and/or by subjecting the thus obtained blank to one or more secondary treatments, such as embossing, die stamping or the like. 
     Though FIG. 8 merely shows two coil springs  345 ,  345   a  of an outer damper, the webs (note the webs  50  in FIG. 2) can also comprise projections or lobes (corresponding to the lobes  344   a ,  344   b ) if the apparatus  301  includes a second damper radially inwardly of the damper  313 . This ensures that the end portions of coil springs forming part of the inner damper are held out of contact with the adjacent rib or ribs of the flange  341 . The coil springs of the inner damper can bear directly against the webs of the flange  341  or against retainers corresponding to the retainer  359  of FIG.  8 . 
     Referring to FIG. 9, there is shown a portion of a further apparatus  401  having a flywheel  403  and a flywheel  404 . The flywheel  403  has two sections  431 ,  432  which define an annular chamber  430  for a damper  413 . The chamber  430  is at least partially filled with a supply of fluid medium which is preferably a highly viscous substance and can fill the chamber  430  entirely or in part. The damper  413  comprises an output member in the form of a flange  441  which is mounted directly on the axial extension or projection  443  of the flywheel  404 . The arrangement is such that the rivets  426  establish a fluidtight seal between the innermost portion of the flange  441  and the end face of the projection  443 . 
     A sealing device  474  is installed between the flange  441  and the flywheel  404  radially outwardly of the projection  443 . The apparatus  401  further comprises a friction generating device  490  in the form of a slip clutch which is disposed between the flange  441  and the flywheel  404  in the region  404   a  radially outwardly of the projection  443 . The slip clutch  490  is a dry clutch and, therefore, it is out of contact with the fluid medium which is confined in the chamber  430 . In the embodiment of FIG. 9, the slip clutch  490  comprises a friction disc  494  and friction rings or pads  494   a ,  494   b  which flank the disc  494 . The friction pad  494   a  is installed axially between the disc  494  and the flange  441 . A biasing device  493  in the form of a washer is disposed at that side of the friction pad  494   b  which faces away from the disc  494 , and the biasing device  493  is acted upon by a diaphragm spring  492  which reacts against the flywheel  404  in the region  404   a . The diaphragm spring  492  ensures that the friction pad  494   a  is compressed between the flange  441  and the disc  494  as well as that the friction pad  494   b  is compressed between the disc  494  and the biasing device  493 . The friction disc  494  has radially outwardly extending tooth-like projections in mesh with complementary inwardly extending projections or prongs  495   a  of the section  432  of the flywheel  403 . The projections of the disc  494  can mate with the projections  495   a  with some play or without any play, depending upon whether it is desired that the slip clutch  490  be effective immediately or only after a certain angular displacement of the flywheels  403  and  404  relative to each other. 
     The improved apparatus can be provided with one or more friction generating devices which are effective during each and every stage of angular movement of the flywheels  403  and  404  relative to each other or only during certain stages of such angular movement. The disc-shaped members  493 ,  494  can cooperate with the diaphragm spring  492  as well as with one or more springs which operate in the circumferential direction of the flywheels  403  and  404  in such a way that torque which is applied to the disc or discs  493 ,  494  suffices, at least during certain stages of compression of such circumferentially acting spring or springs, to overcome the moment of friction of the disc or discs and to thus reset the disc or discs to its or their normal or neutral position. 
     It is often desirable to install the friction generating device in such a way that it exhibits a certain amount of play in the circumferential direction. In other words, there is a certain amount of play between one or more abutments on the friction disc or discs and the cooperating complementary abutments. This ensures that the friction generating device becomes effective with a selected delay following the start of compression of coil springs which form part of the damper or dampers. 
     The useful life of the friction generating device or devices can be prolonged and the operation of such device or devices remains unchanged if the device or devices are mounted in the fluid-containing chamber of the flywheel  403 . Of course, if the friction generating device or devices are of the type wherein the surfaces between the cooperating components must remain dry, these friction generating devices must be installed outside of the chamber, i.e., in such a way that they cannot be contacted by the viscous fluid medium. 
     A friction generating device can be connected in parallel with the damper or dampers. However, certain applications of the improved apparatus may render it necessary or desirable to employ one or more friction generating devices which are designed to operate in series with the coil springs of the damper or dampers. The arrangement may be such that the damping action of the friction generating device or devices varies in response to angular displacement of the one and/or the other flywheel from its neutral position, preferably in such a way that the damping action increases as the flywheel  403  and/or  404  continues to turn further away from its neutral position. 
     The operation can be improved and the construction of the apparatus can be simplified if the friction generating device or devices are designed in such a way that a friction generating device which cooperates with the outer damper produces a damping action which is much more pronounced than the damping action of a friction generating device which cooperates with the inner damper. This also holds true, at least in many instances, for the hydraulic or viscous damping action of the outer and inner dampers. For example, the end portions of coil springs ( 445 ) forming part of the outer damper can be engaged by spring retainers corresponding to the retainers  159  or  359  but no such retainers will be provided for the end portions of some or all coil springs which form part of the inner damper. This enables the outer damper to produce a more satisfactory hydraulic or viscous damping action. Alternatively, and if the coil springs of the inner damper form two or more groups, only the coil springs of one of these groups are provided with spring retainers. The coil springs of the other group or groups are not engaged by retainers so that their compression does not entail the displacement of large quantities of fluid medium and the corresponding stage of operation of the inner damper produces a less pronounced hydraulic or viscous damping action. The damping action of the inner and outer dampers is further regulatable by appropriate selection of the quantity of viscous fluid medium in the chamber of the respective flywheel. The compartment for the outer damper is preferably filled with fluid medium so that the viscous damping action begins immediately as soon as the one and/or the other flywheel leaves its neutral position. The viscous damping action of the fluid medium in the compartment for the coil springs of the inner damper is or can be much less pronounced if the compartment for such springs is not entirely filled with viscous fluid medium. 
     Referring to FIG. 10, there is shown a further apparatus  501  which comprises a damper  513  in the annular compartment  551  of an annular chamber between the sections  531 ,  532  of a first flywheel  503  cooperating with a second flywheel  504 . A first sealing device  574  is interposed between the radially innermost portion of the section  532  and the adjacent portion of a flange  541  which constitutes the output member of the damper  513 . A second sealing device  574   a  is installed between the section  531  and the respective side of the flange  541 . The sealing devices  574 ,  574   a  cooperate with the flange  541  and with the sections  531 ,  532  to seal the radially innermost portion of the compartment  551  (i.e., of the annular chamber between the sections  531 ,  532 ) from a force-locking or slip clutch  590  which is disposed radially inwardly of the section  532 . The inner portion of the flange  541  is flanked by two friction pads  594   a  and  594   b  which are flanked by discs  594  and  593 . The disc  594  is secured to the flywheel  504  by distancing elements  567  in the form of rivets. The disc  593  is acted upon by a diaphragm spring  592  which reacts against the portion  504   a  of the flywheel  504 . The inner marginal portions of diaphragm spring  592  and disc  593  are provided with cutouts for the shanks of distancing elements  567  so that the distancing elements  567  hold the diaphragm spring  592  and the disc  593  against angular movement relative to the flywheel  504 . 
     The diaphragm spring  592  is installed in prestressed condition, and the magnitude of such initial stress determines the torque at which the flange  541  can turn relative to the flywheel  504 . As mentioned before, the component parts  592 - 594   b  cooperate with the radially innermost portion of the flange  541  to establish a force-locking or slip clutch  590 . 
     In order to limit the extent of angular movability of the flange  541  relative to the flywheel  504 , this flange can be provided with radially inwardly extending projections which alternate with the distancing elements  567  in the circumferential direction of the flywheels  503  and  504 . When the inwardly extending projections of the flange  541  engage the distancing elements  567 , the flange is arrested in one or the other end position relative to the flywheel  504 . However, it is equally possible to omit such radially inwardly extending projections of the flange  541  in order to avoid the provision of any means which would limit angular movements of the flange  541  relative to the flywheel  504 . The slip clutch  590  is then designed in such a way that the torque which can be transmitted thereby exceeds the nominal torque which can be transmitted by the engine driving the flywheel  503 . 
     In accordance with a non-illustrated modification of the apparatus  501  of FIG. 10, the flange  541  is mounted for limited angular movement relative to the flywheel  504 , and additional energy storing elements in the form of coil springs are installed between the discs  593 ,  594  on the one hand and the flange  541  on the other hand. Such coil springs are received in suitable windows of the discs  593 ,  594  and flange  541 . The windows can be disposed between neighboring distancing elements  567 , as seen in the circumferential direction of the flywheels  503  and  504 . It is then desirable to employ additional coil springs having a spring characteristic which is much higher than that of the coil springs  545  forming part of the damper  513 . The frictional damping action of the slip clutch  519  should substantially exceed frictional damping action which develops in the region of the damper  513 . This damping action is generated primarily by the sealing devices  574  and  574   a  which rub against the flange  541  when the flange performs an angular movement relative to the sections  531 ,  532  and/or vice versa. 
     In each of the illustrated embodiments, one can achieve a multi-stage spring characteristic between the corresponding components of the flywheel  3 ,  103 ,  203 ,  303 ,  403  or  503  in that at least some coil springs of one group of springs or one damper are shorter than the angular spacing between the parts which cooperate with the coil springs to cause the springs to store energy. Moreover, the utilization of such coil springs which are shorter than the recesses or windows for their reception renders it possible to provide a certain range of angular movements of the flywheels relative to each other which does not entail a resetting or restoring of coil springs to their initial positions. For example, and referring to FIGS. 5 and 6, this can be achieved in that the length of the coil springs  245  in the circumferential direction of the flywheels  203  and  204  is less than the distance between the arms  244  and the respective abutments  255 ,  255   a.    
     Additional embodiments of the improved apparatus can include further combinations of certain parts of the illustrated apparatus. Still further, it is possible to select the materials for various component parts of the apparatus from a wide variety of substances, depending on the intended use, size and/or other characteristics of the torsional vibration damping apparatus. 
     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 art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the appended claims.