Abstract:
A dual clutch for coupling an engine drive shaft with a transmission shaft and/or an auxiliary power take-off. A first friction clutch presses a first clutch plate that is couplable with the transmission shaft between a first contact plate, displaceable in an axial direction to engage the first friction clutch and a first counter plate. A second friction clutch presses a second clutch plate that is couplable with the auxiliary power take-off between a second contact plate, displaceable in the axial direction to engage the second friction clutch and a second counter plate. First and second actuating elements move the first and second contact plates; respectively. The second contact plate is connected to a thrust ring connected to the second actuating element to transfer displacing movement. The second actuating element has a spring element, situated between the second actuating element and the thrust ring, for pre-stressing so that a reduction of contact force is avoided in case of wear and tolerance dependency of the contact force and/or actuating force is decreased.

Description:
The present application is a 371 of International application PCT/DE2013/200089, filed Aug. 1, 2013, which claims priority of DE 10 2012 213 801.3, filed Aug. 3, 2012, the priority of these applications is hereby claimed and these applications are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a dual clutch for coupling a drive shaft of a motor vehicle engine with a transmission shaft of a motor vehicle transmission and/or an auxiliary drive of the motor vehicle. Clutch units of this sort are used, for example, in tractors or other utility machines for agriculture. In such cases, a first friction clutch is used for driving operation, and a second friction clutch for independent engagement and disengagement of an additional auxiliary power take-off by means of which its additional units are drivable. The second friction clutch is usually open in a non-engaged state, for example when starting the motor vehicle engine, and in an engaged state can draw the required power directly from the drive shaft. Because of the independent actuation of the friction clutch, the auxiliary drive may be employed while standing still or also when the motor vehicle is moving. Using the second friction clutch, for example mowing machines and other agricultural attachments, such as mechanical or hydraulic winches, hydraulic pumps or compressors may be driven, which in contrast to electrical equipment offer significantly more power and do not burden an on-board electrical system of the motor vehicle. However, these dual clutches usually have a great tolerance dependency of the contact force and the engaging forces, as well as tension-critical loading of the actuating system, in particular of the second friction clutch when being engaged. 
     SUMMARY OF THE INVENTION 
     The object of the invention is therefore to prepare a dual clutch that enables the tolerance dependency of contact force and engaging force to be lowered, and enables a reduction of the tensions in the actuating system. 
     According to the invention, a dual clutch for coupling a drive shaft of a motor vehicle engine with a transmission shaft of a motor vehicle transmission and/or an auxiliary power take-off of the motor vehicle, in particular a power take-off shaft, is provided, having a first friction clutch for frictionally pressing a first clutch plate that is couplable with the transmission shaft between a first contact plate and a first counter plate, where the first contact plate is displaceable in an axial direction to engage the first friction clutch, a second friction clutch for frictionally pressing a second clutch plate that is couplable with the auxiliary power take-off between a second contact plate and a second counter plate, where the second contact plate is displaceable in the axial direction to engage the second friction clutch, a first actuating element to move the first contact plate, a second actuating element to move the second contact plate, the second contact plate being connected to a thrust ring which is operationally connected to the second actuating element to transmit the displacing movement. According to the invention, the second actuating element has at least one spring element for pre-stressing; the at least one spring element being situated between the second actuating element and the thrust ring. 
     The first clutch plate may be connected to the transmission shaft non-rotatingly but axially movably through toothing; the second clutch plate may be connected to the auxiliary drive non-rotatingly but axially movably through toothing. The particular clutch plate of the associated friction clutch may have a friction lining, in particular on each of the axial faces which face away from each other, which may come into frictionally engaged contact with a possibly provided friction lining of the associated counter plate and/or contact plate in order to engage the particular friction clutch. The dual clutch, in particular the first friction clutch of the dual clutch, may have a vibration damper. A dual-mass flywheel and/or a centrifugal force pendulum and/or a mass pendulum, for example, may be used as the vibration damper, which may be situated, for example, on the first clutch plate. The dual clutch may have an independently operable first friction clutch in the form of a sheet metal cover single clutch, in order to produce a frictional lock between the motor vehicle, engine, in particular the drive shaft of the motor vehicle engine, and the transmission shaft, to propel the motor vehicle. The second friction clutch may be situated in the form of an auxiliary power take-off clutch as an understructure on the first friction clutch, for example between the first friction clutch and a flywheel of the motor vehicle engine. The auxiliary power take-off clutch, for example a power take-off shaft of a tractor, may be designed as a safety power take-off shaft clutch, where in the event of an error in the engaging mechanism, in particular the second actuating element, the auxiliary power take-off clutch immediately disengages and the uncoupled auxiliary power take-off is able to come to a stop. The auxiliary power take-off may be designed in the form of a directly driven engine power take-off shaft, which is directly connectable to the motor vehicle engine through the second friction clutch, and thus may be operated at the rotational speed of the motor vehicle engine, independent of any movement of the motor vehicle. Auxiliary power take-off clutches are usually referred to by the abbreviation of the English term power take-off as PTOs, while a safety auxiliary power take-off clutch or safety clutch, may be referred to as a safety PTO. 
     The first friction clutch is operable by an at least partially axial movement of the first actuating elements, where the first contact plate may be moved in the axial direction toward the first counter-pressure plate, while through frictionally pressing the first clutch plate between the first contact plate and the first counter plate a frictional lock may be produced between the motor vehicle engine and the transmission shaft in order to propel the motor vehicle. The first actuating element may be designed, for example, in the form of a first lever spring, which is operable independently of a second actuating element. Independent of the first friction clutch, the second friction clutch is engageable by means of the second actuating element and a thrust ring, in order to produce a frictional lock between the motor vehicle engine and the auxiliary power take-off, for example a power take-off shaft. 
     In this case, the second actuating element has at least one additional spring element, the spring element being situated pre-tensioned in an assembled state on the second actuating element and being situated between the second actuating element and the thrust ring. In an assembled state, the at least one spring element, preferably a plurality of spring elements, act like a pre-tensioned pressure three accumulator which is situated between the second actuating element and the thrust ring, while a movement of the second actuating element is transferable through the at least one spring element to the thrust ring, whereby a movement of the thrust ring may be achieved. The thrust ring may be of pot-like or pot-shaped design and may externally surround the first friction clutch, which may be situated inside a clutch cover, while the clutch cover may be at least partially surrounded by the thrust ring. The thrust ring is connected in this case to the second contact plate of the second friction clutch. The second clutch may be disengaged in a non-activated state. The engagement of the thrust ring, and thus of the second contact plate, takes place through actuation of the second actuating element, which may be situated on the face of the clutch cover of the first friction clutch, on a side facing away from the first and the second contact plates. The second actuating element may be mounted movably, preferably tiltably, by means of bolts attached to the clutch cover of the first friction clutch and a wire support ring positioned on the bolt. The wire support ring may be tightened radially around the outside of the bolts, while the second actuating element in the form of a second lever spring contacts the wire support ring and/or the bolts in such a way that the lever spring is mounted tiltably on the wire support ring and/or the bolt, in particular when the second actuating element is actuated. 
     The at least one spring element may be designed and situated on the second actuating element in such a way that, in an assembled state, in particular due to an elastic deformation of the spring element, a pre-stressing force may be caused in the spring element, while the pre-stressing force may counteract a displacement during an engagement and actuation of the second actuating element. Preferably, the pre-stressing force of the spring element may act in an axial direction, in particular in an axial direction facing away from the second contact plate. An engaging force to actuate the second friction clutch, which may act radially on the inside of the second actuating element, is able, due to the spacing at intervals of the support and/or contacting of the second actuating element on the wire support ring as well as the support of the spring element on the thrust ring, to bring about a transmission of the engaging force that is transferred to the thrust ring. 
     While engaging as a result of an engaging force, the second actuating element with the at least one spring element, preferably the group of the second actuating element with the at least one spring element, initially has high rigidity, until the translated engaging force exceeds the pre-stressing force caused by the spring element. Until the pre-stressing force is exceeded, at least one spring element is able to preserve its position, and in essence its form. When the pre-stressing force is exceeded, the at least one pre-stressed spring element may be further deformed elastically, for example straightened, so that upon further engagement of the second actuating element a deformation occurs essentially in the at least one spring element, whereby the contact force transferred through the at least one spring element to the thrust ring, and thus to the second contact plate, is able to run out in a flat force characteristic curve after the pre-stressing has been surpassed. By means of the at least one pre-stressed spring element of the second actuating element, two rigidities of differing values may be integrated in the second actuating element. As a result, the contact force, corresponding to the chosen rigidities, may have two regions with different progressions of the force characteristic, for example in the form of an angular characteristic progression, with an essentially linear, steeper rise of the contact force in a first section and an essentially linear, flatter-progressing second section. This makes it possible to ensure in the second region a flatter progression of the characteristic of the contact force over the engagement distance that is relevant in operation, both in a new state and in a worn state of the second friction clutch. Thus a reduction of the contact force can be avoided in the event of wear, and the tolerance dependency of the contact force and/or engaging force can be decreased. Through the integration of two rigidities into the pre-stressed second actuating element, with little construction space and little use of materials, an initially high rigidity when engaging may be combined with a lesser rigidity in the engaged state, which enables the stresses in the actuating system when the second friction clutch is actuated to be reduced. 
     In a preferred embodiment, the second actuating element is a lever spring with lever spring tongues directed radially inward, there being at least one spring element situated on a lever spring tongue. On the lever spring, in particular the lever spring tongues, a plurality of spring elements, preferably in each case at least one, may be situated. Preferably, each of the lever spring tongues of the lever spring has a separate spring element. The spring elements may be situated, pre-stressed, on an underside of the lever spring tongue facing the thrust ring. The spring elements may be elastically deformed in the assembled state, whereby a pre-stressing force in an axial direction, preferably directed away from the thrust ring, may be produced. 
     During the engaging as a result of the engaging force, the translated engaging force may initially be less than the pre-tensioning of the spring elements, which enables the spring elements to retain their position and substantially their form. In this position, the lever spring together with the at least one spring element is able to have a high rigidity. If the translated engaging force continues to rise, the at least one spring element may be further elastically deformed, for example straightened. After the pre-stressing force has been surpassed, the at least one spring element may have little rigidity, so that even in the worn state of the second clutch an essentially constant contact force level can be maintained. 
     By particular preference, the lever spring tongues have an arched curvature. The arch may be formed in the area of the installed spring elements in the lever spring tongues, preferably every lever spring tongue. The arch may be formed away from the thrust ring and the spring element, while the arch may be bow-shaped. As a result, the spring element may be at least partially spaced apart from the lever spring tongue on the lever spring. Due to the curvature, in the pre-stressed state the spring element may fit closely against the lever spring tongue and in contact with it in a radially inner region and a radially outer region of the lever spring tongue, for example radially inside and outside of the arch. Due to the curvature in the lever spring tongue, when the spring element is bent space may be made available for axial movement of the spring element as the second friction clutch is engaged, whereby the required construction space may be reduced. 
     In a preferred embodiment, at least one fastening element is provided for fastening a spring element to a lever spring tongue. Each spring element can be fastened, pre-stressed, to a lever spring tongue by at least one fastening element in each case. The at least one fastening element may be situated in the area of the arch and/or radially inward and/or outward from the arch. The fastening element may be designed in the form of a materially bonded, frictional and/or form-locked connection. Preferably, the at least one fastening element makes a separable attachment of the spring element to the lever spring tongue possible, whereby greater variability in the adjustability of the rigidity may made possible, for example by exchanging one or more spring elements. 
     In an especially preferred embodiment, the fastening element is designed in the form of a clip connection with at least two holding clips. By means of the clip connection, a disconnectable attachment of the spring element to the lever spring tongues can be enabled. The at least two holding clips may be hook-shaped, and essentially centered on the longitudinal sides of the spring element. By means of the holding clips, the pre-stressed spring element can be fastened to the lever spring tongues in an assembled state, using a pre-stressing force. By means of the holding clips, the spring element can be fixed in the circumferential direction on the lever spring tongue, whereby the spring element can be positioned and secured for example against slipping sideways, in particular during engagement. 
     Preferably, the spring element has a cutout to rest on a bulge of the thrust ring. The cutout may be designed in the form of a groove situated transversely to the longitudinal extent of the spring element. The cutout may be essentially matched to the shape of the ring-shaped bulge formed on the face of the thrust ring. Due to the resting of the cutout on the bulge in an assembled state, a positioning of the spring element in the radial direction and a securing against displacement, in particular during engagement of the second clutch, can be guaranteed. 
     The invention also relates to a gear train for a motor vehicle, having at least one dual clutch, which may be designed and refined as described above, for coupling a drive shaft of a motor vehicle engine to a transmission shaft of a motor vehicle transmission and/or an auxiliary power take-off of the motor vehicle. By that means a reduction in the contact force can be avoided in the event of wear, and the tolerance dependency of the contact force and/or actuating force can be decreased. 
     The invention is explained below by way of example with reference to the accompanying drawings, on the basis of preferred exemplary embodiments; the features depicted below can each depict an aspect of the invention, both individually and in combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The figures show the following: 
         FIG. 1 : a schematic sectional view of a dual clutch according to the invention; 
         FIG. 2 : a general view of a dual clutch according to the invention; 
         FIG. 3 : a second actuating element with spring elements; 
         FIG. 4 a   : the second actuating element from  FIG. 3  with assembled spring elements; 
         FIG. 4 b   : a sectional view of the pre-stressed second actuating element from  FIG. 4   a;    
         FIG. 5 : a thrust ring with a pre-assembled second actuating element and bolts with a wire support ring; 
         FIG. 6 : a diagram to depict the force characteristic curves of a contact force and of an engaging force. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  depicts a dual clutch  10  with a first friction clutch  12  and a second friction clutch  14 , the first friction clutch  12  and the second friction clutch  14  being depicted in a non-engaged state. The first friction clutch  12  is designed as a sheet metal cover single clutch with a first clutch plate  16  to be frictionally clamped between a first contact plate  18  and a first counter plate  20 , where the first contact plate  18  is movable in an axial direction by actuating a first actuating element  22  in the form of a first lever spring. Underlying the first friction clutch  12  is the second friction clutch  14  in the form of a safety auxiliary power take-off clutch with a second clutch plate  24  to be clamped between a second contact plate  26  and a second counter plate  28 , where the second contact plate  26  is movable in an axial direction independent of the first friction clutch  12  when a second actuating element  30  is actuated. The second friction clutch  14  is depicted in a non-actuated, disengaged state. The second actuating element  30  is designed in the form of a second lever spring with lever spring tongues  32  extending radially inward. The second actuating element  30  has a plurality of axially pre-stressed spring elements  34 , there being a spring element  34  situated on the contact plate side of every lever spring tongue  32  of the lever spring. The lever spring tongues  32  each have an arch  33  formed essentially in the center, which is formed away from the second contact plate  26  and the spring element  34 . The spring elements  34  are each situated in the area of the arch  33  in such a way that the spring elements  34  contact the respective lever spring  32  in a bowlike or bridge-like manner, radially inside and radially outside of the arch  33 . The transmission of an engaging motion during actuation of the second actuating element  30  in the form of the second lever spring pre-stressed by the spring elements  34  takes place through a thrust ring  36  onto the second contact plate  26 , which is connected to the thrust ring  36 , where the spring elements  34  of the second actuating element  30  contact the thrust ring  36  to transmit the engaging motion. The thrust ring  36  is secured against release by a threaded connection  38  in the radial direction with the second contact plate  26 , the thrust ring  36  being received in a groove  40  in the second contact plate  26 . The groove  40  is formed radially outside in a surface of the second contact plate  26  facing away from the second clutch plate  24 . 
     The thrust ring  36  encircles the first friction clutch  12  essentially on the outside, the thrust ring  36  at least partially penetrating a clutch cover  42  of the first friction clutch  12 , whereby passage of the thrust ring through the clutch cover  42  of the first friction clutch  12  can be guaranteed. On the face of the pot-like or pot-shaped thrust ring  36 , on a face turned toward the second actuating element  30 , a ring-shaped encircling bulge  37  is formed, which contacts the spring elements  34  of the pre-stressed second actuating element  30 . The bulge  37  contacts the spring elements  34  of the second actuating element  30  radially on the inside, which is tiltably supported radially on the outside of the contact point of the spring elements  34  with the bulge  37 , on a wire support ring  44 . The wire support ring  44  is situated radially outside around a plurality of bolts  46 , which are connected to the clutch cover  42  of the first friction clutch  12 . By means of the distance between the wire support ring  44  and the bulge  37 , as well as the point of introduction of the engaging force located radially on the inside, transmission of the engaging force acting on the thrust ring  36  can be enabled. The spring elements  34  are each fastened to the lever spring tongues  32  by means of a fastening element  35  in the form of a clip connection with two hook-shaped holding clips  39 , the holding clips  39  being formed essentially centered and laterally on the spring elements  34 . The spring elements  34  have a cutout  41  to rest on the bulge  37 , which is matched to the shape of the bulge  37 . 
       FIG. 2  depicts the dual clutch  10  in an assembled state, the lever spring tongues  32  with the spring elements  34  of the pre-stressed second actuating element  30  assembled pre-stressed thereon being depicted in a non-actuated state. The spring elements  34  contact the bulge  37  of the thrust ring  36 . The second actuating element  30  fits closely against, the thrust ring  36  radially on the outside. The thrust ring  36  protrudes at several places through the clutch cover  42  of the first friction clutch  12 , and is detachably connected to the second contact plate  26  of the second friction clutch  14  by radial threaded connections  38 . 
       FIG. 3  depicts the second actuating element  30  in an unassembled state in the form of a lever spring with lever spring tongues  32 , and spring elements  34  spaced at a distance from them with fastening elements  35  in the form of hook-shaped holding clips  39 . The second actuating element  30  has nine lever spring tongues  32  arranged radially inside and nine spring elements  34 . The lever spring tongues  34  each have an arch  33 . The spring elements  34  have a cutout  41  to rest on the bulge of the thrust ring (not shown), which is formed transversely to the longitudinal extension of the spring elements  34 . The cutouts may be formed arcuately, transversely in the spring elements  34 , in order to be able to rest against the ring-shaped bulge without a gap. The fastening elements  35  are designed in the form of clip connections, each having two hook-shaped holding clips  39  situated centered and laterally on the spring elements  34 . 
       FIG. 4 a    depicts the second actuating element  30  in an assembled state with the spring elements  34  fastened to the lever spring tongues  32 . The second fastening element  30  is pre-stressed in the axial direction by the spring elements  34 , which are fastened pre-stressed to the lever spring tongues  32  using the fastening elements  35 . A sectional view along the line A-A of a lever spring tongue  32  with a spring element  34  situated thereon is depicted in  FIG. 4 b   . The spring element  34  is held pre-stressed on the lever spring tongue  32  by the holding clips  39  in the area of the arch  33 . 
       FIG. 5  depicts the thrust ring  36  with the rest of the dual clutch  10 , and a second actuating element  30  preassembled at an interval and pre-stressed. Shown spaced apart from the thrust ring  36  and the pre-stressed second actuating element  30  with the spring elements  34  assembled on the lever spring tongues  32  is the wire support ring  44  with the necessary bolts  46  for fastening to the clutch cover  42  of the first friction clutch  12 . 
       FIG. 6  depicts a diagram to show the contact force F N  of the second friction clutch  14  of the dual clutch  10  according to the invention in comparison to a contact force F A  of a conventional dual clutch, as well as the engaging force F E  which is needed to engage the second friction clutch  14 . The engaging process of the second friction clutch  14  can be depicted in three phases. In the first phase the engaging force F E  acts radially inside on the pre-stressed second actuating element  30  with the spring elements  34 , in particular on the lever spring tongues  32  with the spring elements  34  assembled pre-stressed thereon. In the first phase, the free play between the second contact plate  26  and the second clutch plate  24  is first surmounted by the displacement of the second contact plate  26 ; the free play may amounts, for example, to 1.3 mm. Because of the free play no contact force F N,A  is produced in the first phase, and the pre-stressed second actuating element  30  tilts radially outside around the wire contact ring  44 , its pivot point. With the surpassing of the free play between the second contact plate  26  and the second clutch plate  24  the second phase of the engagement process begins, in which the second contact plate  26  presses the second clutch plate  24  against the second counter plate  28 , where the second counter plate  28  may be designed, for example, in the form of a flywheel. Because of the design, in particular the rigidities, of the pre-stressed second actuating element  30 , in particular of the lever spring tongues  32  with the spring elements  34  assembled pre-stressed thereon, the engagement force F E  acting on the thrust ring is transmitted. As long as the engaging force F E  is less than the pre-stressing force of the pre-stressed second actuating element  30 , the spring elements  34  remain in position on the lever spring tongues  32  and can remain essentially unshaped, while the group of lever spring tongues  32  and spring elements  34  possesses high rigidity. As a result, a steep, essentially linear rise of the contact force F N  can be enabled in the second phase. 
     The third phase of the engaging process begins essentially upon surpassing the pre-stressing force, in particular of the spring elements  34 , of the pre-stressed second actuating element  30 , by the engaging force F E , whereby with increasing engagement the spring elements  34  can be further deformed elastically, for example straightened. In this case, the elastic deformation takes place essentially in the spring elements  34 , due to the lower rigidity. Due to the integration of two rigidities into the pre-stressed second actuating element  30 , in particular into the lever spring tongues  32  with the spring elements  34  fastened to them pre-stressed, during the engagement an initially high rigidity can be combined with a lower rigidity in the engaged state. As a result, in comparison to the second phase, in the third phase the engagement zone which is relevant for operation, a lower linear increase of the contact force F N  can be enabled while the engagement travel increases. The curve of the contact force F N  flattens out significantly at the transition from the second to the third phase because of the two different integrated rigidities. The engagement travel which is relevant during the reliable operation of the second friction clutch  14  is made up of the difference in travel between the maximum and minimum engagement travel V 3  to V 2 , and the wear that occurs, V 2  to V 1 , where at V 2  there is no wear and at V 1  the maximum wear of, for example 0.8 mm, is present. Over the relevant engagement distance in the third phase a contact force F N  with a very slight linear rise can be guaranteed. After an essentially linear rise in the second phase, the engaging force F E  needed to actuate the second friction clutch  14  has in the third phase, over the relevant engagement path from V 3  through V 2  to V 1  essentially a very slight rise, which is virtually constant. In comparison to the contact force F N  of a dual clutch  10  according to the invention, the contact force F A  of a conventional dual clutch is depicted, which has an essentially linear progression of the contact force F A  in the third phase, whereby, for example, in the case of wear, V 2  to V 1 , a substantial decrease of the contact force F A  occurs. 
     REFERENCE LABELS 
     
         
           10  dual clutch 
           12  first friction clutch 
           14  second friction clutch 
           16  first clutch plate 
           18  first contact plate 
           20  first counter plate 
           22  first actuating element 
           24  second clutch plate 
           26  second contact plate 
           28  second counter plate 
           30  second actuating element 
           32  lever spring tongue 
           33  arch 
           34  spring element 
           35  fastening element 
           36  thrust ring 
           37  bulge 
           38  threaded connection 
           39  holding clips 
           40  groove 
           41  cutout 
           42  clutch cover 
           44  wire support ring 
           46  bolt 
         F N  contact force 
         F E  engaging force 
         F A  contact force 
         V 1  wear zone end 
         V 2  wear zone start, minimum engagement travel 
         V 3  maximum engagement travel