Abstract:
A torque transmission device which may be used to transmit torque, as inputted from an automotive engine through a belt, to an accessory such as an alternator and is designed to have enhanced ability to absorb a change in rotation of a torque input member relative to an torque output member. The torque transmission device includes a slider sensitive to a given change in rotation of the input member relative to the output member to experience sliding motion and an elastic absorber which suppresses the sliding motion elastically to absorb the change in rotation of the input member. The elastic absorber is mechanically retained by the output member and inertially independent of the input member, so that the moment of inertia of the input member will be small, thus resulting in enhanced ability to absorb the change in rotation of the input member.

Description:
CROSS REFERENCE TO RELATED DOCUMENT 
       [0001]    The present application claims the benefit of Japanese Patent Application No. 2007-10456 filed on Jan. 19, 2007, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1 Technical Field of the Invention 
         [0003]    The present invention relates generally to a torque transmission device which may be used to transmit torque, as inputted from an automotive engine through a belt, to an automotive accessory such as an alternator, and more particularly to an improved structure of such a torque transmission device with enhanced ability to absorb a change in rotation between torque input and output members. 
         [0004]    2 Background Art 
         [0005]    In recent years, the consciousness of the global environmental challenges has been heightened. In order to save the fuel, automotive engines have been decreased in idle speed and friction thereof. In contrast, most automobiles install electric devices such as EPS (Electric Power Steering) devices, thus resulting in an increase in size of alternators, which leads to an increase in consumption of engine power. This facilitates the instability in rotation of the engine arising from a change in combustion of fuel in the engine, and the slippage or flutter of a belt extending from a crankshaft of the engine to an accessory, which results in a decrease in service life of the belt. In order to these drawbacks, Japanese Patent First Publication No. 2006-9899 teaches a torque transmission device equipped with a compression coil spring which connects between a pulley joined to the crankshaft of the engine through a belt and a rotor to which an accessory such as an alternator is attached to absorb a rapid change in rotation of the pulley. 
         [0006]    The above type of torque transmission device may be designed to decrease the moment of inertia of an torque input part including the pulley that is to idle for enhancing the efficiency in absorbing torque pulsations. However, the compression coil spring or a movable annular race is secured to the pulley, so that it is difficult to decrease the moment of inertia of the torque input part desirably. The torque transmission device is, therefore, not good at absorbing the change in rotation of the engine. 
       SUMMARY OF THE INVENTION 
       [0007]    It is therefore a principal object of the invention to avoid the disadvantages of the prior art. 
         [0008]    It is another object of the invention to provide an improved structure of a torque transmission device with enhanced ability to absorb a change in rotation between torque input and output members. 
         [0009]    According to one aspect of the invention, there is provided a torque transmission device which may be employed to transmit torque, as inputted from an automotive engine through a belt, to an automotive accessory such as an alternator and is designed to have enhanced ability to absorb a rapid change in speed of a torque input part. The torque transmission device comprises: (a) an input member to which torque is inputted; (b) an output member from which the torque, as transmitted from the input member through a transmission path, is outputted; (c) a bearing mechanism supporting the input member and the output member to be rotatable relative to each other; (d) a slider sensitive to a given change in rotation of the input member relative to the output member to experience sliding motion along a given path; and (e) an elastic absorber working to suppress the sliding motion elastically to absorb the change in rotation of the input member relative to the output member. The elastic absorber is mechanically retained by the output member and inertially independent of the input member. 
         [0010]    Specifically, the elastic absorber is not joined fixedly to the input member. In other words, when the speed of the input member changes, at least the elastic absorber does not follow the changed speed of the input member, so that the moment of inertia of the input member will be smaller than that in a conventional structure, like the one discussed above, thus resulting in enhanced ability to absorb the change in rotation of the input member. 
         [0011]    In the preferred mode of the invention, the slider may also be mechanically joined to the elastic absorber and disposed to slide independently of the input member when being subjected to the change in rotation of the input member relative to the output member. 
         [0012]    The slider may be made of a ball. The input member may be made of a hollow cylinder which has a first groove formed in an inner periphery thereof. Similarly, the output member may be made of a hollow cylinder which has a second groove formed in an outer periphery thereof and is disposed inside the input member to define between the first and second grooves the given path along which the slider slides when the slider is subjected to the given change in rotation of the input member relative to the output member. 
         [0013]    The slider has a diameter A. The depth H 1  of the first groove is selected to meet a relation of (A/2)&gt;H 1 . Similarly, the depth H 2  of the second groove is selected to meet a relation of (A/2)&gt;H 2 . This ensures holding of the slider between the first and second grooves while allowing the first and second grooves to rotate relative to each other. 
         [0014]    The first groove may be designed to extend in the form of a spiral in an axial direction of the input member to restrict movement of the slider in the axial direction. The second groove may be designed to extend straight in the axial direction of the output member to restrict movement of the slider in a circumferential direction of the input member. This defines the path along which the slider slides. 
         [0015]    The elastic absorber is implemented by a spring. The torque transmission device further includes a slider retainer disposed between the slider and the spring to retain the slider in abutment therewith. The use of the slider retainer ensures the desired motion of the slider regardless of the configuration of an end of the spring with which the slider is placed in abutment. For example, the slider is made of a hollow cylinder. 
         [0016]    The bearing mechanism is made up of two bearings disposed across the slider between the input and output members. 
         [0017]    The input member may be designed to have a first annular member which protrudes inwardly from an inner periphery thereof and has the first groove formed in an end surface facing in an axial direction of the input member. The output member may also be designed to have a second annular member which protrudes outwardly from an outer periphery thereof and has the second groove formed on an end surface facing the end surface of the first annular member. The slider is disposed between the first and second grooves. The first and second annular members are sensitive to a change in angular position between the input and output members to change in relative positional relation between the first and second grooves in a radial direction of the first and second annular members to move the slider. 
         [0018]    The input member may alternatively be designed to have a first annular member which protrudes inwardly from an inner periphery thereof and has the first groove formed in an end surface thereof facing in an axial direction of the input member to define a first ridge thereon. Similarly, the output member may be designed to have a second annular member which protrudes outwardly from an outer periphery thereof and has the second groove formed on an end surface thereof facing in an axial direction of the output member to define a second ridge thereon. The first ridge is fit in the second groove. The second ridge is fit in the first groove. The first and second annular members are sensitive to a change in angular position between the input and output members to change in relative positional relation between the first ridge and the second groove and between the second ridge and the first groove in a radial direction of the first and second annular members to move the slider. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
           [0020]    In the drawings: 
           [0021]      FIG. 1  is a partial longitudinal sectional view which shows a torque transmission device according to the first embodiment of the invention; 
           [0022]      FIG. 2  is a perspective view which shows a hollow cylinder installed in the torque transmission device of  FIG. 1 ; 
           [0023]      FIG. 3  is a longitudinal sectional view which shows the structure of a torque pulsation absorbing mechanism installed in the torque transmission device of  FIG. 1 ; 
           [0024]      FIG. 4  is a partial longitudinal sectional view which shows a torque transmission device according to the second embodiment of the invention; 
           [0025]      FIG. 5(   a ) is a partially sectional view which illustrates for the case where a slider is nipped between grooves without sliding along a path defined between the grooves in the second embodiment; 
           [0026]      FIG. 5(   b ) is a partially sectional view which illustrates for the case where a slider slides along a path defined between grooves when a change in rotation of a pulley relative to a rotor shaft occurs in the second embodiment; 
           [0027]      FIG. 6  is a partial longitudinal sectional view which shows a torque transmission device according to the third embodiment of the invention; 
           [0028]      FIG. 7(   a ) is a partially sectional view which illustrates for the case where a slider is nipped between grooves without sliding along a path defined between the grooves in the third embodiment; and 
           [0029]      FIG. 7(   b ) is a partially sectional view which illustrates for the case where a slider slides along a path defined between grooves when a change in rotation of a pulley relative to a rotor shaft occurs in the third embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to  FIG. 1 , there is shown a torque transmission device according to the first embodiment of the invention which will be discussed as being designed as a pulley unit for automotive alternators, but may be employed as that for another type of accessories such as automotive air conditioners. 
         [0031]      FIG. 1  is a partially longitudinal sectional view which shows a half of the pulley unit  100  for the sake of simplicity of illustration. The pulley unit  100  includes a pulley  10 , a rotor shaft  20 , sliders  30 , a hollow cylinder  40 , a compression coil spring  50 , a slider retainer tube  52 , and bearings  60  and  62 . 
         [0032]    The pulley  10  is made of a hollow cylinder with V-grooves  12  around which a belt (not shown) is wound from a crankshaft of an automotive internal combustion engine (not shown). The pulley  10 , as illustrated  FIGS. 1 and 3 , also has a spiral V-groove  14  (will also be referred to as a first groove below) formed in an inner periphery thereof. The spiral V-groove  14  extends in a lengthwise direction, like one turn of a typical coil spring, but may extend in the form of two or more of the coil spring or less than one turn of the coil spring. The rotor shaft  20  is made of a hollow cylinder working as an output shaft joined to an input shaft (not shown) of the alternator. The rotor shaft  20  has formed in an inner periphery thereof an internal thread  22   a  which makes a joint with an external thread formed in the shaft of the alternator firmly. The hollow cylinder  40  is fit on the outer periphery of the rotor shaft  20 . The hollow cylinder  40  is, as clearly illustrated in  FIG. 2 , made up of a cylindrical body  43  and a flange  44  extending perpendicular to a length of the cylindrical body  43 . The cylindrical body  43  has formed in the outer periphery thereof a plurality of grooves  42  (will also be referred to as second grooves below) which extend straight in parallel to a longitudinal center line thereof. The flange  44  is made of a disc and works as a stopper with which the compression coil spring  50  is placed in abutting contact. The rotor shaft  20  and the hollow cylinder  40  may alternatively be formed integrally with each other. 
         [0033]    The compression coil spring  50  is, as clearly illustrated in  FIG. 3 , wound on the outer periphery of the hollow cylinder  40  coaxially therewith and has one of opposed ends joined or welded to the flange  44 . The compression coil spring  50  is also joined or welded at the other end to an end of the slider retainer tube  52 . The slider retainer tube  52  also has the other end with which the sliders  30  are placed in abutment. The slider retainer tube  52  may be joined firmly or welded to the sliders  30 . 
         [0034]    The sliders  30  are balls placed one in each of the second grooves  42  of the hollow cylinder  40 . The sliders  30  move along the second grooves  42  upon a change in relative angular position between the pulley  10  and the hollow cylinder  40 . Each of the sliders  30  is in slidable contact with the inner periphery (i.e., the first groove  14 ) of the pulley  10 , so that it does not follow the rotation of the pulley  10 . In  FIG. 3 , some of the sliders  30  are omitted for the ease of visibility, but there are as many sliders  30  as the second grooves  42  of the hollow cylinder  40 . The end of the slider retainer tube  52  with which the sliders  30  abut is inclined to the axis (i.e., the longitudinal center line) of the slider retainer tube  52  in the form of a spiral. Each of the sliders  30  is fit in both the first groove  14  formed in the inner periphery of the pulley  10  and one of the second grooves  42  formed in the outer periphery of the hollow cylinder  40 . The first groove  14  is, as described above, in the form of a spiral. The second grooves  42  extend, as described above, straight parallel to each other. Therefore, each of sliders  30  is restricted by the first groove  14  from moving in the longitudinal direction of the hollow cylinder  40  and by a corresponding one of the second grooves  42  from moving in the circumferential direction of the hollow cylinder  40 . When the pulley  10  rotates relative to the hollow cylinder  40 , in other words, a change in rotation of the pulley  10  relative to the rotor shaft  20  occurs, it will cause each of the sliders  30  to move in the longitudinal direction of the hollow cylinder  40 . 
         [0035]    The first groove  14  may be in the form of an oval, in other words, may be designed to extend in the form of a sine-wave when developed on a plane. The first groove  14  may also be in the form of a circular or semi-circular wave in the case where a change in angular position between the pulley  10  and the rotor shaft  20  is restively small. Specifically, the first groove  14  may be so shaped that the sliders  30  move in the lengthwise direction of the compression coil spring  50  when the pulley  10  and the hollow cylinder  40  (i.e., the rotor shaft  20 ) rotate relative to each other. 
         [0036]    The end of the slider retainer tube  52  with which the sliders  30  abut is so shaped as to geometrically coincide with the first groove  14  in order to distribute the elastic pressure, as exerted by the compression coil spring  50 , into uniform fractions acting on the sliders  30 , respectively. In other words, the end of the slider retainer tube  52  and the first groove  14  extend spirally at the same angle to the longitudinal center line of the hollow cylinder  40 . If the diameter of each of the sliders  30  is defined as A, the depth H 1  of the first groove  14  is selected to meet a relation of (A/2)&gt;H 1 . Similarly, the depth H 2  of the second grooves  42  is selected to meet a relation of (A/2)&gt;H 2 . 
         [0037]    The bearings  60  and  62  are located away from each other in alignment with an axis of rotation of the pulley unit  100  across the sliders  30  to secure the rotor shaft  20  to be rotatable relative to the pulley  10 . Each of the bearings  60  and  62  is, as clearly illustrated in  FIG. 1 , fit on the rotor shaft  20  and held at an inner ring thereof by a shoulder formed on the outer periphery of the rotor shaft  20  from moving in the axial direction of the rotor shaft  20 , thereby positioning the pulley  10  in the axial direction thereof. The compression coil spring  50  and the sliders  30  are disposed between the bearings  60  and  62 . 
         [0038]    The operation of the pulley unit  100  will be described below. 
         [0039]    When the pulley  10  is being rotated by the output torque from the engine in a steady state, the torque, as inputted to the pulley  10 , is transmitted to the rotor shaft  20  along a torque transmission path extending from the pulley  10  to the rotor shaft  20  through the sliders  30 , the slider retainer tube  52 , the compression coil spring  50 , and the hollow cylinder  40 . 
         [0040]    When the pulley  10  is accelerated from the steady state speed, it will result in a lag in rotation of the rotor shaft  20  because the rotor shaft  20  is joined firmly to a rotor of the automotive alternator, so that it has a great inertia. This results in a change in rotation (i.e. speed) of the pulley  10  relative to the rotor shaft  20  (i.e., the hollow cylinder  40 ), thus causing the first groove  14  of the pulley  10  to move relative to the second grooves  42  of the hollow cylinder  40  (i.e., the rotor shaft  20 ) to push the sliders  30  in a direction in which the compression coil spring  50  is compressed. The orientation of the end of the slider retainer tube  52  with which the sliders  30  abut and the first groove  14  relative to the axis of rotation of the pulley unit  100  are, as described above, selected to move the sliders  30  with movement of the pulley  10  relative to the rotor shaft  20 . Accordingly, the part of the torque of the pulley  10  arising from a change in rotation thereof is absorbed by the compression coil spring  50 . The rotor shaft  20  starts to accelerate behind the pulley  10 . This ensures the transmission of torque from the pulley  10  to the rotor shaft  20  without torque pulsation. Conversely, when the pulley  10  is decelerated, it will result in a lag in deceleration of the rotor shaft  20 . This causes the first groove  14  of the pulley  10  to rotate relative to the rotor shaft  20  to move the sliders  30  in a direction in which the compression coil spring  50  is stretched, thereby absorbing the torque of the pulley  10  partially. The rotor shaft  20  starts to decelerate behind the pulley  10 . This ensures the transmission of torque from the pulley  10  to the rotor shaft  20  without torque pulsation. 
         [0041]    As apparent from the above discussion, when the speed of the pulley  10  has changed rapidly, and the pulley  10  rotates relative to the rotor shaft  20 , the sliders  30  and the compression coil spring  50  do not follow the changed rotation of the pulley  10 . In other words, the sliders  30 , the slider retainer tube  52 , and the compression coil spring  50  are not joined rigidly to the pulley  10 , but to the rotor shaft  20  through the hollow cylinder  40 . Therefore, as compared with the conventional structure, as discussed in the introductory part of this application, the moment of inertial of a torque input part (i.e., the pulley  10 ) is small, thus enhancing the absorption of a change in speed of the pulley  10  in the pulley unit  100 . 
         [0042]      FIG. 4  illustrates a pulley unit  100 A according to the second embodiment of the invention. 
         [0043]    The pulley unit  100 A includes a pulley  10 A, a rotor shaft  20 A, sliders  30 A, a compression coil spring  50 , a slider retainer  52 A, bearings  60  and  62 . The same reference numbers as employed in the first embodiment will refer to the same parts, and explanation thereof in detail will be omitted here. The reference numbers to which the letter “A” is affixed refer to modifications of the parts, as discussed in the first embodiment. 
         [0044]    The pulley  10 A has an annular ring  16  formed on an inner peripheral wall thereof. The annular ring  16  has a spiral V-groove  14 A (will also be referred to as a first groove below) formed in a flat end surface thereof extending perpendicular to the longitudinal center line of the pulley  10 A. The slider retainer  52 A is made of an annular disc and fit on an outer periphery of the rotor shaft  20 A. The slider retainer  52 A has a spiral V-groove  42 A (will also be referred to as a second groove below) formed in an end surface thereof facing the annular ring  16 . The first groove  14 A and the second groove  42 A are identical in configuration or mirror-image symmetrical, so that they coincide with each other when the pulley  10 A and the rotor shaft  20 A are in a given angular relation to each other. When the pulley  10 A and the rotor shaft  20 A are shifted in angular position from each other, it will cause the first groove  14 A and the second groove  42 A to be shifted radially of the pulley  10 A (i.e. the rotor shaft  20 A) from each other. 
         [0045]    In the case where a change in angular position between the pulley  10 A and the rotor shaft  20 A is restively small, one or both of the first groove  14 A and the second groove  42 A may alternatively be designed in the form of an oval, a circular or semi-circular wave, or a circle. In the case where both the first groove  14 A and the second groove  42 A are formed to be circular or circular arc, at least one of the first groove  14 A and the second groove  42 A need to be located eccentrically with the axis of rotation of the annular ring  16  and the annular disc  52 A. 
         [0046]    The rotor shaft  20 A has an annular stopper  44 A protruding outwardly. Instead of the annular stopper  44 A, the hollow cylinder  40 , as illustrated in  FIG. 40 , may be fit on the outer periphery of the rotor shaft  20 . The compression coil spring  50  is, like in the first embodiment, placed in abutting contact of an end thereof with an end of the annular stopper  44 A and secured firmly or welded thereto. The compression coil spring  50  is also joined or welded at the other end thereof to the slider retainer  52 A. The slider retainer  52 A is slidable on the rotor shaft  20 A in the axial direction of the rotor shaft  20 A, but held by the end of the compression coil spring  50  from rotating in the circumferential direction thereof. The degree to which the slider retainer  52 A is held from rotating in the circumferential direction of the rotor shaft  20 A may be enhanced by forming on the inner periphery of the slider retainer  52 A a plurality of ridges which extend in the lengthwise direction of the rotor shaft  20 A, machining in the outer periphery of the rotor shaft  20 A a plurality of grooves extending in the same direction as the ridges, and engaging them. 
         [0047]    The sliders  30 A are, like in the first embodiment, balls and retained between the first groove  14 A and the second groove  42 A at a regular interval away from each other in the circumferential direction of the rotor shaft  20 A. Such retaining of the sliders  30 A at the regular interval may be achieved by a ball-cage assembly, as used in typical ball bearings. 
         [0048]    The operation of the pulley unit  100 A will be described below. 
         [0049]    When the pulley  10 A is driven by the belt to accelerate the rotor shaft  20 A from a steady state speed, the rotor shaft  20 A will lag behind the pulley  10 A because the rotor shaft  20 A joined firmly to a rotor of the automotive alternator has a great inertia. This causes the first groove  14 A of the pulley  10 A to rotate relative to the second groove  42 A of the slider retainer  52 A, thereby moving the sliders  30 A radially and inwardly of the pulley  10 A to compress the compression coil spring  50  through the slider retainer  52 A. 
         [0050]      FIG. 5(   a ) illustrates for the case where the first groove  14 A of the pulley  10 A coincides with the second groove  42 A of the slider retainer  52 A in the longitudinal direction of the rotor shaft  20 A. The first groove  14 A and the second groove  42 A are, as described above, of a V-shape in cross section. When the pulley  10 A and the rotor shaft  20 A are rotating in the steady state, the first groove  14 A and the second groove  42 A are aligned with each other in the longitudinal direction of the rotor shaft  20 A, so that the distance between the annular ring  16  and the slider retainer  52 A across the sliders  30 A in the axial direction of the pulley unit  100 A is minimized, and the sliders  30 A are retained firmly between the annular ring  16  and the annular disc  52 A, thereby transmitting the torque from the pulley  10 A to the rotor shaft  20 A.  FIG. 5(   b ) illustrates for the case where the pulley  10 A (i.e., the annular ring  16 ) rotates relative to the rotor shaft  20 A (i.e., the slider retainer  52 A), so that the first groove  14 A of the pulley  10 A is shifted from the second groove  42 A in the circumferential direction of the pulley  10 A to exert the pressure on the sliders  30 A in the axial direction of the rotor shaft  20 A, thereby pushing the slider retainer  52 A to compress the compression coil spring  50 . Specifically, the slider retainer  52 A is moved apart from the annular ring  16  as a function of a shift in angular position of the first groove  14 A from the second groove  42 A, thereby compressing the compression coil spring  50 . This causes the torque of the pulley  10 A to be partially absorbed by the compression coil spring  50 . The rotor shaft  20 A starts to accelerate behind the pulley  10 A. This ensures the transmission of torque from the pulley  10  to the rotor shaft  20  without torque pulsation. 
         [0051]    Conversely, when the pulley  10 A is decelerated, it will result in a lag in deceleration of the rotor shaft  20 A. This causes, like the above, the slider retainer  52 A to be moved to compress the compression coil spring  50 , thereby absorbing the torque of the pulley  10 A partially. The rotor shaft  20 A starts to decelerate behind the pulley  10 A. 
         [0052]      FIG. 4  illustrates a pulley unit  100 B according to the third embodiment of the invention. 
         [0053]    The pulley unit  100 B includes a pulley  10 B, a rotor shaft  20 A, a slider  30 B, a compression coil spring  50 , bearings  60  and  62 . The same reference numbers as employed in the first and second embodiment will refer to the same parts, and explanation thereof in detail will be omitted here. The reference numbers to which the letter “B” is affixed refer to modifications of the parts, as discussed in the first or second embodiment. 
         [0054]    The pulley  10 B has an annular ring  16 B formed on an inner peripheral wall thereof. The annular ring  16 B has a spiral V-groove  14 B (will also be referred to as a first groove below) formed in an end surface thereof extending perpendicular to the longitudinal center line of the pulley  10 B to define a spiral barb-like ridge. 
         [0055]    The slider  30 B is an annular disc fit on an outer periphery of the rotor shaft  20 A and works as a combination of the sliders  30 A and the slider retainer  52 A illustrated in  FIG. 4 . The slider  30 B has a spiral V-groove  42 B (will also be referred to as a second groove below) formed in an end surface thereof facing the annular ring  16 B of the pulley  10 B to define a spiral barb-like ridge. The first groove  14 B and the second groove  42 B are so shaped,that the barb-like ridge, as defined by the first groove  14 B on the annular ring  16 B, engages or just fits in the second groove  42 B, while the barb-like ridge, as defined by the second groove  42 B on the slider  30 B, just fits in the first groove  14 B when the pulley  10 B and the rotor shaft  20 A are in a given angular position. When the pulley  10 B and the rotor shaft  20 A are shifted relative to each other from the initial angular position, it will cause the barb-like ridges on the annular ring  16 B and the slider  30 B to be shifted radially of the pulley  10 B (i.e. the rotor shaft  20 A) from each other. 
         [0056]    In the case where a change in angular position between the pulley  10 B and the rotor shaft  20 A is restively small, one or both of the first groove  14 B and the second groove  42 B may alternatively be designed in the form of an oval, a circular or semi-circular wave, or a circle. In the case where both the first groove  14 B and the second groove  42 B are formed to be circular or circular arc, at least one of the first groove  14 B and the second groove  42 B need to be located eccentrically with the axis of rotation of the annular ring  16 B and the slider  30 B. 
         [0057]    The compression coil spring  50  is, like in the first embodiment, joined or welded at an end thereof with an end of the annular stopper  44 A and also joined or welded at the other end thereof to the slider  30 B. The slider  30 B is slidable along the outer periphery of the rotor shaft  20 A in the axial direction of the rotor shaft  20 A, but held by the end of the compression coil spring  50  from rotating in the circumferential direction thereof. 
         [0058]    The operation of the pulley unit  100 B will be described below. 
         [0059]    When the pulley  10 B is driven by the belt to accelerate the rotor shaft  20 A from a steady state speed, the rotor shaft  20 A will lag behind the pulley  10 B because the rotor shaft  20 A joined firmly to a rotor of the automotive alternator has a great inertia. This causes the first groove  14 B of the pulley  10 A to rotate relative to the second groove  42 B of the slider  30 B, thereby causing the barb-like ridges on the annular ring  16 B and the slider  30 B to be shifted radially of the pulley  10 B to move the slider  30 B in the axial direction of the stator shaft  20 A to compress the compression coil spring  50 . 
         [0060]      FIG. 7(   a ) illustrates for the case where the spiral barb-like ridge, as defined by the first groove  14 B on the annular ring  16 B, is just fit in the second groove  42 B, while the spiral barb-like ridge, as defined by the second groove  42 B on the slider  30 B, is just fit in the first groove  14 B when the pulley  10 B and the rotor shaft  20 A are in the given angular position. Each of the barb-like ridges may engage a corresponding one of the first groove  14 B and the second groove  42 B with play or a gap. 
         [0061]      FIG. 7(   b ) illustrates for the case where the pulley  10 B and the rotor shaft  20 A are shifted relative to each other from the angular position in  FIG. 7(   a ), so that the barb-like ridges on the annular ring  16 B and the slider  30 B are shifted radially from each other to push the slider  30 B in the rightward direction, as viewed in the drawing. Specifically, the slider  30 B is moved apart from the annular ring  16 B as a function of a shift in angular position of the first groove  14 B from the second groove  42 B, thereby compressing the compression coil spring  50 . This causes the torque of the pulley  10 B to be partially absorbed by the compression coil spring  50 . The rotor shaft  20 A starts to accelerate behind the pulley  10 B. This ensures the transmission of torque from the pulley  10 B to the rotor shaft  20 A without torque pulsation. 
         [0062]    Conversely, when the pulley  10 B is decelerated, it will result in a lag in deceleration of the rotor shaft  20 A. This causes, like the above, the slider  30 B to be moved to compress the compression coil spring  50 , thereby absorbing the torque of the pulley  10 B partially. The rotor shaft  20 A starts to decelerate behind the pulley  10 B. This eliminates the pulsation of rotation of the pulley  10 B. 
         [0063]    While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments witch can be embodied without departing from the principle of the invention as set forth in the appended claims.