Patent Publication Number: US-2015083543-A1

Title: Release mechanism for a friction clutch

Description:
The present invention claims the benefit of Japanese Patent Application No. 2013-199986 filed on Sep. 26, 2013 with the Japanese Patent Office, the disclosure of which is incorporated herein by reference in its entirety 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to the art of a release mechanism for disengaging a friction clutch engaged by a diaphragm spring by applying a load to the diaphragm spring according to a hydraulic pressure delivered to an actuator. 
     2. Discussion of the Related Art 
     One example of the release mechanism is disclosed in JP-A-2001-50295. According to the teachings of JP-A-2001-50295, a hydraulic actuator is arranged around an input shaft of a transmission, and a piston of the actuator is fitted onto the input shaft while being allowed to move in an axial direction of the input shaft. In order to push the piston toward a diaphragm spring, an oil pressure is delivered to a hydraulic chamber formed on an opposite side of the diaphragm spring across the piston. To this end, the piston is connected with an inner race of a relief bearing, and an inner circumferential portion of the diaphragm spring is connected with an outer race of the relief bearing. In order to push the piston and the return spring away from the diaphragm spring, a return spring for establishing a pushing force in the axial direction is arranged around the hydraulic actuator. Accordingly, when the oil pressure is delivered to the hydraulic chamber, the piston and the relief bearing are moved toward the diaphragm spring against the pushing force of the return spring, thereby pushing the inner circumferential portion of the diaphragm spring. 
     Meanwhile, JP-A-9-303423 discloses a spring retainer comprising a bottomed-cylindrical piston bore formed between a shaft and a housing. In the bore, a piston is arranged in the bottom side while being allowed to move in an axial direction of the shaft. A cancel plate is arranged to be opposed to the piston, and a backward movement of the cancel plate is restricted. In addition, a plurality of coil springs are arranged between the piston and the cancel plate, and a multiple plate clutch is arranged in the opposite side of the piston across the cancel. According to the teachings of JP-A-9-303423, therefore, the multiple plate clutch is engaged by delivering fluid between the bore and the piston thereby moving the piston toward the cancel plate against the elastic forces of the coil springs. 
     In turn, JP-A-2010-112529 discloses an automatic transmission in which a plurality of brakes are arranged in an axial direction of an input shaft of the transmission, and in which a plurality of return springs are arranged in an outer circumferential side of friction plates of the brakes. Each brake is individually provided with a piston situated between the friction plates and a casing of the transmission, and a hydraulic chamber to which fluid is delivered. According to the teachings of JP-A-2010-112529, therefore, the brake is engaged by delivering fluid to a hydraulic chamber from a diametrically inner side, thereby moving the piston in the axial direction of the input shaft toward the friction plates against elastic forces of the return springs. 
     Thus, in the release mechanism taught by JP-A-2001-50295, the return spring is arranged around the hydraulic actuator, and an oil passage for delivering the fluid to the hydraulic chamber is formed while detouring the return spring. Therefore, a length of the release mechanism has to be elongated in the axial direction. In addition, since the return spring is arranged around the hydraulic actuator, a diametrical dimension of the release mechanism may also be increased. 
     The diametrical dimension of the release mechanism taught by JP-A-2001-50295 may be reduced by arranging the coil springs taught by JP-A-9-303423 or JP-A-2010-112529 around the hydraulic actuator instead of the return spring. However, if those coil springs are arranged in the same axial position as the return spring, the oil passage for delivering the fluid to the hydraulic chamber is still has to be formed in a manner to detour those coil springs. Therefore, the axial length of the release mechanism may not be shortened, 
     SUMMARY OF THE INVENTION 
     The present invention has been conceived noting the above-mentioned technical problems, and it is therefore an object of the present invention is to provide a release mechanism for a friction clutch in which an axial length is shortened. 
     The release mechanism according to the present invention is applied to a friction clutch that is constantly pushed in an axial direction to be engaged to transmit a torque between a rotary output member and a rotary input member. In the release mechanism, a hydraulic actuator that is formed to establish a hydraulic pressure in the axial direction in a manner such that a pushing force applied to the friction clutch is reduced, and a plurality of elastic members are arranged annularly around a rotational center axis in a manner to establish an elastic force in the axial direction. In order to achieve the above-explained object, according to the release mechanism of the present invention, a predetermined interval is maintained between the adjacent elastic members, and an oil passage is formed to be communicated with the hydraulic actuator while passing through the interval. 
     A width of the oil passage in a circumferential direction is identical to or slightly shorter than the interval between the adjacent elastic members. 
     The friction clutch is comprised of a pushing member for applying a pushing force constantly to the friction clutch in the axial direction toward the output rotary member. Specifically, the plurality of elastic members are arranged in a manner such that a net force of the elastic forces thereof is applied homogeneously or equally to the pushing member around the rotational center. 
     For example, a diaphragm spring is employed as the pushing member. According to the present invention, both of a load resulting from the hydraulic pressure established by the hydraulic actuator, and the net force of the elastic forces of the elastic members are applied to an inner circumferential portion of the diaphragm spring. 
     More specifically, the plurality of elastic members are arranged in a symmetric manner with respect to a predetermined line extending perpendicular to the rotational center axis. 
     Thus, according to the present invention, the predetermined interval, that is a predetermined clearance is maintained between the adjacent elastic members, and the oil passage is formed in a manner to be communicated with the hydraulic actuator while passing through the interval. Specifically, the oil passage is formed in a manner to be overlapped at least partially with the elastic member in the axial direction. Therefore, an axial length of the release mechanism can be shortened, in other words, a thickness of the release mechanism can be thinned. 
     Since a plurality of the elastic members are arranged annularly around a rotational center axis, a diameter of each elastic member can be reduced in comparison with a case of using one elastic member. In addition, a width of the oil passage in a circumferential direction is identical to or slightly shorter than the interval between the adjacent elastic members. Therefore, the oil passage is allowed to be formed without detouring unnecessarily around the elastic member while passing through the interval between the elastic members. For this reason, the axial length of the release mechanism can be shortened, that is, the thickness of the releasing mechanism can be reduced. 
     In addition, since a plurality of the elastic members are thus arranged annularly around a rotational center axis, the net force of the elastic forces thereof can be applied homogeneously to the pushing member around the rotational center. Therefore, members forming the release mechanism will not be inclined or collide with each other due to imbalance of the elastic forces. 
     As described, the diaphragm spring is used as the pushing member to which the load and the elastic forces are applied for pushing the friction clutch. Therefore, a thickness of the release mechanism in the axial direction can be reduced. 
     As also described, specifically, the plurality of elastic members are arranged in a symmetric manner with respect to a predetermined line extending perpendicular to the rotational center axis. Therefore, the elastic forces of the elastic members can be applied homogeneously to the friction clutch around the rotational center. In addition, the members forming the release mechanism can be prevented from being inclined or collide with each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, aspects, and advantages of exemplary embodiments of the present invention will become better understood with reference to the following description and accompanying drawings, which should not limit the invention in any way. 
         FIG. 1  is a sectional side view showing one example of the release mechanism according to the present invention; 
         FIG. 2  is a cross-sectional view along the line II-II shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view along the line III-III shown in  FIG. 2 ; 
         FIG. 4  is a view showing another example of the release mechanism according to the present invention; 
         FIG. 5  is a view showing an example to partially modify the release mechanism shown in  FIG. 4 ; 
         FIG. 6  is a view showing still another example of the release mechanism according to the present invention; 
         FIG. 7  is a view showing an example to partially modify the release mechanism shown in  FIG. 6 ; 
         FIG. 8  is a view showing an example of a power train of the vehicle to which the release mechanism of the present invention is applied; 
         FIG. 9  is a view showing another example of a power train of the vehicle to which the release mechanism of the present invention is applied; and 
         FIG. 10  is a view showing still another example of a power train of the vehicle to which the release mechanism of the present invention is applied. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     Next, the present invention will be explained in more detail with reference to the accompanying drawings.  FIG. 8  is a view showing an example of a power train of the vehicle to which the release mechanism of the present invention is applied. As can be seen from  FIG. 8 , a clutch  5  is interposed between a crankshaft  2  serving as an output shaft of an engine  1  and an input shaft  4  of a transmission  3 . Therefore, a torque is allowed to be transmitted between the engine  1  and the transmission  3  by engaging the clutch  5 . In addition, a pair of driving wheels  7  is connected with an output side of the transmission  3  through a differential gear unit  6 . 
       FIG. 1  is a sectional side view of the release mechanism of the present invention and the clutch  5 . As can be seen from  FIG. 1 , the clutch  5  is arranged between the engine  1  and a housing  8  of the transmission  3 . A protrusion  9  is formed on the housing  8  while protruding toward the engine  1  in the axial direction of the input shaft  4  of the transmission  3 , and the after mentioned release mechanism  25  is fitted onto the protrusion  9 . Here, the broken line I in  FIG. 1  represents an axial center of the input shaft  4  of the transmission  3 . For example, a dry-type clutch in which oil is not interposed between engagement surfaces is used as the clutch  5 , and the crankshaft  2  of the engine  1  is selectively connected with the input shaft  4  of the transmission  3  to transmit the torque therebetween by engaging the clutch  5 . Specifically, a flywheel  11  is attached to the crankshaft  2  by a bolt  10 , and an annular pressure plate  12  is opposed to a face of the flywheel  11  of the transmission  3  side. A clutch disc  13  is interposed between the flywheel  11  and the pressure plate  12 . As can be seen, the clutch disc  13  is comprised of a first friction member  14  facing toward the flywheel  11 , and a second friction member facing toward the pressure plate  12 . 
     The clutch disc  13  is connected with the input shaft  4  through a torsional damper  13  to transmit a torque. The torsional damper  13  is a conventional damper adapted to damp torque pulses caused by firing impulse of the engine  1 . Accordingly, the crankshaft  2  and the flywheel  11  serve as the input side rotary member of the present invention, and the input shaft  4  of the transmission  3  serves as the output side rotary member of the present invention. 
     In the clutch  5  thus structured, a friction acting between the first friction member  14  and the flywheel  11 , and a friction acting between the second friction member  15  and the pressure plate  12  are increased by increasing a pressure to clamp the clutch disc  13  by the pressure plate  12  and the flywheel  11 . Consequently, the clutch  5  is engaged so that the crankshaft  2  is connected with the input shaft  4  in a manner to transmit torque. By contrast, the friction acting between the first friction member  14  and the flywheel  11 , and the friction acting between the second friction member  15  and the pressure plate  12  are reduced by lowering a pressure to clamp the clutch disc  13  by the pressure plate  12  and the flywheel  11 . Consequently; the crankshaft  2  is disconnected from the input shaft  4  so that the clutch  5  is disengaged. 
     The pressure plate  12  is covered by a clutch cover  17  attached to the flywheel  11  by a not shown bolt or the like. Specifically, the clutch cover  17  is adapted to cover the pressure plate  12  from the transmission  13  side and from the outer circumferential side, and a plurality of hole  18  are formed on the clutch cover  17  at predetermined intervals in the circumferential direction. In order to retain the after-mentioned diaphragm spring  22 , a retainer member  20  is interposed between the clutch cover  17  and the pressure plate  12 . The retainer member  20  and one of the end portions of a strap plate  19  are fixed to a face of the pressure plate  12  facing to the transmission  3  by a rivet  21 . Likewise, the other end portion of the strap plate  19  is fixed to an inner face of the clutch cover  17  by the rivet  21 . Thus, the pressure plate  12  and the clutch covert  17  are connected with each other through the strap plate  19 . Therefore, an elastic force of the strap plate  19  is applied to the pressure plate  12  in a direction to isolate the pressure plate  12  away from the clutch disc  13 . 
     Specifically, an outer circumferential edge of the diaphragm spring  22  is retained by an inner circumferential edge of the retainer member  20  and the pressure plate  12 . Therefore, the portion of the diaphragm spring  22  of outer circumferential side of the after-mentioned pivot ring  24  is moved integrally with the pressure plate  12  in the axial direction of the input shaft  4 . 
     A plurality of hook portions  23  are formed by bending an inner circumferential portion of the clutch cover  17  toward the engine  1  in a manner to orient the leading end portion to the outer circumferential side. As can be seen, two pivot rings  24  individually having circular cross-section are held in an inner space of the hook portion  23  across the diaphragm spring  22 . 
     Specifically, the diaphragm spring  22  is a conventional disc spring member having a plurality of radially inwardly directed spring fingers, and an inner circumferential portion of the diaphragm spring  22  is contacted with a bearing  28  of a release mechanism  25  of the present invention. Therefore, the pressure plate  12  is pushed by an elastic force of the diaphragm spring  22  toward the clutch disc  13  so that the clutch disc  13  is clamped by the pressure plate  12  and the flywheel  11 . That is, the clutch  5  is engaged by the elastic force of the diaphragm spring  22 . Accordingly, the diaphragm spring  22  serves as the pushing member of the present invention, and the elastic force of the diaphragm spring  22  corresponds to the pushing force of the present invention. 
     The release mechanism  25  is adapted to apply a load for disengaging the clutch  5  to the inner circumferential portion of the diaphragm spring  22 . As can be seen from  FIG. 1 , the release mechanism  25  is comprised of a hydraulic actuator  26 , a return spring  27  and the bearing  28 . The hydraulic actuator  26  is comprised of an inner body  29  fitted onto the protrusion  9 , and an outer body  30  situated around the inner body  29 . Specifically, the inner body  29  is comprised of an inner cylinder  29   a  extending coaxially with the input shaft  4 , and an annular plate  29   b  extending radially from an end portion of the inner cylinder  29   a  of the transmission  3  side while being contacted with the housing  8 . Meanwhile, the outer body  30  is comprised of an outer cylinder  30   a  extending coaxially with the input shaft  4  in an outer circumferential side of the inner cylinder  29   a , and an annular plate  30   b  extending radially from an end portion of the outer cylinder  30   a  of the transmission  3  side, in order to retain the return spring  27 , an outer circumferential portion of the annular plate  30   b  of the outer body  30  is flexed to a substantially right angle. An arrangement of the return springs  27  will be explained later. 
     A piston  31  is inserted into a cylindrical space created between the inner cylinder  29   a  and the outer cylinder  30   a  while being allowed to reciprocate in the axial direction of the input shaft  4 . That is, the cylindrical space serves as a hydraulic chamber  32 , and an oil passage  33  is connected with the hydraulic chamber  32 . Therefore, the piston  31  is hydraulically moved toward the flywheel  11  by delivering the fluid to the hydraulic chamber  32  from a not, shown hydraulic source via the oil passage  33 . Specifically, the oil passage  33  is formed to radially penetrate the annular plate  30   b  in a manner to be situated between the return springs  27  adjacent to each other. Alternatively, the oil passage  33  may also be formed between the inner body  29  and the annular plate  30   b  in the above-explained manner. In addition, in order to avoid an oil leakage, a sealing member may be disposed on an end portion of the piston  31  of a pressure receiving face side. 
     The other end portion of the piston  31  is bent radially outwardly, and the above-explained bearing  28  is attached to the bent portion in a manner to be contacted with the inner circumferential portion of the diaphragm spring  22 . That is, the piston  31  and the diaphragm spring  22  are allowed to be rotated relatively with each other. Therefore, when the fluid is delivered to the hydraulic chamber  32  so that the piston  31  is moved toward the flywheel  11 , a load is applied to the inner circumferential portion of the diaphragm spring  22  according to the hydraulic pressure through the bearing  28  thereby resiliently deforming the spring fingers of the diaphragm spring  22 . As a result, the pushing force of the diaphragm spring  22  is reduced so that the clutch  5  is disengaged. 
     The above-mentioned return springs  27  are arranged on the annular plate  30   b  of the outer body  30  annularly around the center axis I of the input shaft  4  at predetermined intervals, in a manner to extend in parallel with the center axis I. Specifically, a coil spring is employed as the return spring  27 , and each coil springs  27  has a same elastic force, length, wire diameter, outer diameter and etc. That is, one of the end portions of each return spring  27  is individually contacted with the annular plate  30   b  of the outer body  30 , and the other end portion of each return spring  27  is individually contacted with the above-explained bent portion of the piston  31 . Therefore, a net force of the elastic forces of the return springs  27  is applied homogeneously or equally around the center axis I to the piston  31  and the bearing  28 . 
     Thus, according to the example shown in  FIG. 1 , the clutch  5  is disposed between the engine  1  and the transmission  3  in the torque transmitting direction, and the elastic forces of the return springs  27  are applied to the bearing  28  in the same direction as the hydraulic pressure applied to the bearing  28  from the hydraulic actuator  26 , thereby pushing the piston  31  and the bearing  28  constantly toward the flywheel  11 . Alternatively, the bearing  28  may also be retained by applying the elastic forces of the return springs  27  to the bearing  28  from the opposite direction to the hydraulic pressure applied from the hydraulic actuator  26 . Here, provided that the release mechanism of the present invention is applied to a hybrid vehicle which is allowed to be driven by a motor torque while disconnecting the engine from the power train, the elastic forces of the return springs  27  are applied to the bearing  28  in the same direction as the example shown in  FIG. 1 . 
     As described, the return springs  27  are arranged in a manner such that the elastic forces thereof are applied homogeneously around the center axis I to the piston  31 , the bearing  28  and the diaphragm spring  22 . To this end, a unit of springs formed by combining a plurality of springs may also be used as the return spring  27 . In this case, the unit of the springs will also be arranged annularly at predetermined intervals. Alternatively, the return springs  27  may also be arranged in a symmetric manner with respect to a predetermined line extending perpendicular to the center axis I. Accordingly, the return spring  27  serves as the elastic member of the present invention. 
     Referring now to  FIG. 2 , there is shown a cross-sectional view along the line II-II shown in  FIG. 1 . In the example shown in  FIG. 2 , nine return springs  27  are arranged in total on the annular plate  30   h  of the outer body  30  around the outer cylinder  30   a  at regular intervals, that is, while keeping predetermined intervals. Each return spring  27  is erected in a manner to extend in parallel with the center axis I. In addition, those return springs  27  are arranged in a symmetric manner with respect to a predetermined line extending perpendicular to the center axis I. Therefore, a net force of the elastic forces of the return springs  27  is applied homogeneously around the center axis I to the inner circumferential portion of the diaphragm spring  22 . The oil passage  33  is extended while passing through an interval between the return springs  27  adjacent to each other, and a width of the oil passage  33  in a circumferential direction of the annular plate  30   b  is identical to or slightly shorter than the interval between outer circumferences of the adjacent return springs  27 . Referring now to  FIG. 3 , there is shown a cross-sectional view along the line shown in  FIG. 2 . As can be seen from  FIG. 3 , the oil passage  33  is formed in a manner to radially penetrate the annular plate  30   b  of the outer body  30  at a level within a length of the return springs  27  in the direction of the center axis I. 
     Thus, in the release mechanism  25 , a plurality of the return springs  27  is arranged annularly while keeping predetermined intervals so that the oil passage  33  is allowed to be formed between the adjacent return springs  27 . In addition, the oil passage  33  is formed to radially penetrate the annular plate  30   b  at the level within a length of the return springs  27  in the direction of the center axis I. Therefore, an axial length of the hydraulic actuator  26  can be shortened without detouring the return springs  27  so that a thickness of the release mechanism  25  is reduced. In addition, since the return springs  27  are arranged annularly around the outer cylinder  30   a , that is, around the center axis I at predetermined intervals, the net force of the elastic forces created by the return springs  27  is applied homogeneously around the center axis I to the diaphragm spring  22  via the piston  31  and the bearing  28 . Therefore, the piston  31  can be prevented from being contacted with the inner cylinder  29   a  of the inner body  29 . Specifically, the bent portion of the piston  31  can be prevented from being inclined to be contacted with the annular plate  30   b  of the outer body  30 . That is, the diaphragm spring  22  can be prevented from being inclined with respect to the center axis I of the input shaft  4  of the transmission  3 . 
     Referring now to  FIG. 4 , there is shown a cross-section of another example of the release mechanism  25  in which a unit U comprising three return springs  27  is employed. Specifically, in the unit U, the return springs  27  are arranged while keeping predetermined intervals in a manner to extend in parallel with the center axis I. As can be seen from  FIG. 4 , a pair of units U is arranged on the annular plate  30   b  around the outer cylinder  30   a  while keeping predetermined intervals, in a manner such that the return springs  27  are situated in a symmetric manner with respect to a predetermined line extending perpendicular to the center axis I. That is, six return springs  27  are arranged annularly around the center axis I of the input shaft  4 . In this example, the oil passage  33  is extended while passing through an interval between the adjacent units U, and a width of the oil passage  33  in a circumferential direction of the annular plate  30   b  is identical to or slightly shorter than the interval between outer circumferences of the outermost return springs  27  of each unit U adjacent to each other  FIG. 5  shows an example to partially modify the release mechanism  25  shown in  FIG. 4 . As can be seen from  FIG. 5 , the oil passage  33  may be situated closer to one of the units U arranged in a manner such that the return springs  27  are situated in a symmetric manner with respect to a predetermined line extending perpendicular to the center axis I. 
     Referring now to  FIG. 6 , there is shown a cross-section of still another example of the release mechanism  25  in which a unit U comprising two return springs  27  is employed. In this example, the return springs  27  are arranged at a predetermined interval in a manner to extend in parallel with the center axis I. Here, a distance of the interval between the return springs  27  is same in each unit U. As can be seen from  FIG. 6 , three units U are also arranged on the annular plate  30   b  around the outer cylinder  30   a  while keeping predetermined intervals, in a manner such that the return springs  27  are situated in a symmetric manner with respect to a predetermined line extending perpendicular to the center axis I. That is, six return springs  27  are also arranged annularly around the center axis I of the input shaft  4 . In this example, the oil passage  33  is also extended while passing through an interval between the adjacent units U, and a width of the oil passage  33  in a circumferential direction of the annular plate  30   b  is identical to or slightly shorter than the interval between outer circumferences of the return springs  27  of each unit U adjacent to each other,  FIG. 7  shows an example to partially modify the release mechanism  25  shown in  FIG. 6 . As can be seen from  FIG. 7 , the oil passage  33  may also be situated closer to one of the units U arranged in a manner such that the return springs  27  are situated in a symmetric manner with respect to a predetermined line extending perpendicular to the center axis I. 
     In the examples shown in  FIGS. 4 to 7 , the oil passage  33  may be formed in the annular plate  30   b  at the level within the length of the return springs  27  in the direction of the center axis I. Therefore, the axial length of the hydraulic actuator  26  can be shortened so that the thickness of the release mechanism is reduced, In addition, since the return springs  27  are thus arranged annularly around the outer cylinder  30   a , that is, around the center axis I at predetermined intervals, the net force of the elastic forces created by the return springs  27  is also applied homogeneously around the center axis I to the diaphragm spring  22  via the piston  31  and the bearing  28 . Therefore, the piston  31  can be prevented from being inclined to be contacted with the inner cylinder  29   a  of the inner body  29 . In addition, the bent portion of the piston  31  can also be prevented from being contacted with the annular plate  30   b  of the outer body  30 . That is, the diaphragm spring  22  can be prevented from being inclined with respect to the center axis I of the input shaft  4  of the transmission  3 . 
     Here will be explained another example of a power train of the vehicle to which the release mechanism  25  of the present invention can be applied, with reference to  FIGS. 9 and 10 . As can be seen from  FIG. 9 , a motor-generator  34  is connected with the crankshaft  2  of the engine  1  through the clutch  5 . An output shaft  35  of the motor-generator  34  is connected with the input shaft  4  of the transmission  3  through another clutch  36 , and the driving wheels  7  are connected with the output side of the transmission  3  through a differential gear unit  6 .  FIG. 10  shows still another example of a power train of the vehicle to which the release mechanism of the present invention is applied. In the example shown in  FIG. 10 , a dual-clutch transmission  37  is connected with the crankshaft  2  of the engine  1  through the clutch  5 , and the driving wheels  7  are connected with the output side of the dual-clutch transmission  37  through a differential gear unit  6 . In addition, another motor-generator  38  is also connected with the dual-clutch transmission  37 . Therefore, the hybrid vehicle shown in  FIG. 9  or  10  can be driven by a torque of the motor-generator  34  or  38 . In this case, a power loss resulting from a concurrent rotation of the engine  1  can be reduced by disengaging the clutch  5 , and in this situation, the engine  5  is allowed to be halted. In addition, the hybrid vehicle shown in  FIG. 9  or  10  may also be driven by torques of the engine  1  and the motor-generator  34  or  38 . In the hybrid vehicle shown in  FIG. 9  or  10 , a cranking of the engine  1  is carried out by the motor-generator  34  or  38  while engaging the clutch  5 . 
     Although the above exemplary embodiment of the present invention have been described, it will be understood by those skilled in the art that the present invention should not be limited to the described exemplary embodiments, but that various changes and modifications can be made within the spirit and scope of the present invention.