Abstract:
A torque varying mechanism comprises an accommodation hole that has a relief hole formed in its bottom and that receives a check ball therein. The accommodation hole is configured so as to present a relationship in which the area of flow passage between the accommodation hole and the check ball increases linearly as a function of the amount of displacement of the check ball after opening of the relief hole, after which the flow passage area keeps a unvarying value. A hysteresis characteristic is imposed on a switching point where switching is made between a high torque transmission characteristic and a low torque transmission characteristic relative to an increase and a decrease of the vehicle velocity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a hydraulic power transmission joint for use in 4 wheel-drive motor vehicles for the distribution of driving forces between front and rear wheels, and more particularly to a hydraulic power transmission joint designed to suppress possible sudden torque variations upon the switching of torque transmission characteristics by use of a centrifugal torque varying mechanism. 
     2. Description of the Related Arts 
     Such a conventional hydraulic power transmission joint is known from U.S. Pat. Nos. 5,706,658 and 5,983,635. 
     This hydraulic power transmission joint comprises a housing coupled to one of input and output shafts that are capable of relative rotations and having a cam face formed on its inner side surface; a rotor coupled to the other of said input and output shaft and being rotatably accommodated in the housing, the rotor having a plurality of axially extending plunger chambers; a plurality of plungers each being reciprocatively accommodated in each of the plurality of plungers under a biasing force of return springs, the plurality of plungers being operated by the cam face upon the relative rotations of the input and output shafts; a discharge hole formed in the rotor and leading to the plurality of plunger chambers; and an orifice having a high-pressure chamber that leads to the discharge hole, the orifice generating a flow resistance under the action of flow of oil discharged by operations of the plurality of plungers. 
     In the hydraulic power transmission joint being currently developed by the present inventors, a valve block is coupled to the rotor for rotations jointly therewith and is provided with a centrifugal torque varying mechanism. The centrifugal torque varying mechanism has a weight which when the vehicle velocity exceeds a predetermined level, pivots around a weight fulcrum by a centrifugal force to open a relief hole that has been blocked by a check valve so far, thereby achieving a release of the high-pressure oil. Switching is thus made to a lower torque transmission characteristic than the torque transmission characteristic used for the duration in which the relief hole is closed, to thereby prevent the temperature of the joint from raising due to the increased vehicle velocity. 
     Referring to FIG. 1, there is shown by way of example a centrifugal torque varying mechanism comprising a valve block  101  that includes a relief hole  102  leading to a high-pressure chamber. When the vehicle velocity V exceeds a predetermined vehicle velocity Vt as seen in FIG. 3, a weight can pivot by a centrifugal force so that a member  104  for receiving a check ball  103  is displaced to the direction of an arrow E, allowing the check ball  103  to open the relief hole  102 . At that time, oil is released through the relief hole  102  as indicated by an arrow F. A large variation occurs in the area of flow passage when the relief hole  102  is opened by the check ball  103 , and as indicated by an arrow J 1  of FIG. 3 the torque transmission characteristic is switched from a first torque transmission characteristic H to a lower second torque transmission characteristic I, resulting in a large torque reduction rate. When the vehicle velocity V drops to below the predetermined vehicle velocity Vt and the weight returns to its original position after the switching to the second torque transmission characteristic I the member  104  receiving the check ball  103  is displaced to the direction of an arrow G as seen in FIG. 2, allowing the check ball  103  to block the relief hole  102 . The torque transmission characteristic upon this switching as indicated by an arrow J 2  of FIG. 3 is switched from the second torque transmission characteristic I to the initial first torque transmission characteristic H, resulting in a large torque increase rate. 
     In case of such a hydraulic power transmission joint, however, the area of flow passage changes to a large extent when the relief hole is opened or closed by the check ball, with the result that the torque transmission characteristics are liable to be influenced by the variation of flow rate and the torque transmission characteristics may vary in a brief period of time, thus disadvantageously affecting the vehicle behaviors. Furthermore, the switching of the torque transmission characteristics is effected using the predetermined vehicle velocity Vt as the reference value, and hence if the vehicle velocity is in the vicinity of the vehicle velocity Vt, then even a slight variation may induce a switching of the torque transmission characteristics in spite of the travelling at a fixed velocity, which will also affect the vehicle behaviors. 
     SUMMARY OF THE INVENTION 
     The present invention provides a hydraulic power transmission joint capable of reducing the torque variation rate upon the switching of torque transmission characteristics, preventing frequent switching of the torque transmission characteristics when travelling at an unvarying velocity, and alleviating influences on the vehicle behaviors. 
     According to an aspect of the present invention there is provided a hydraulic power transmission joint adapted to be interposed between an input shaft and an output shaft that are rotatable relative to each other, to transmit torque as a function of the rotational-speed difference between the input and output shafts, the hydraulic power transmission joint comprising a housing coupled to one of the input and output shafts and having a cam face formed on its inner side surface; a rotor coupled to the other of the input and output shafts and being rotatably accommodated in the housing, the rotor having a plurality of axially extending plunger chambers; a plurality of plungers each being reciprocatively accommodated in each of the plurality of plungers under a biasing force of return springs, the plurality of plungers being operated by the cam face upon the relative rotations of the input and output shafts; an orifice formed in a valve block coupled to the rotor, for generating a flow resistance under the action of flow of oil discharged by operations of the plurality of plungers; and a torque varying mechanism provided in the valve block and having a weight that is pivoted by a centrifugal force to allow a check ball to move to open a relief hole, for the relief of hydraulic pressure, thereby switching a first torque transmission characteristic to a second torque transmission characteristic lower than the first one. 
     Such a hydraulic power transmission joint of the present invention is characterized in that the torque varying mechanism includes an accommodation hole for receiving the check ball therein, the relief hole being formed in the bottom of the accommodation hole, the accommodation hole being configured such that the area of flow passage between the accommodation hole and the check ball increases linearly as a function of the amount of displacement of the check ball after opening of the relief hole, after which the area of flow passage keeps an unvarying value. By virtue of this, a hysteresis characteristic is obtained in which when a predetermined first vehicle velocity is reached, the weight pivots to allow the check ball to move to open the relief hole, thereby effecting a switching from the first torque transmission characteristic to the second torque transmission characteristic lower than the first one, and in which when the vehicle velocity drops from the first vehicle velocity to the second vehicle velocity lower than the first one, the weight returns to its original position to allow the check ball to close the relief hole, thereby effecting a switching from the second torque transmission characteristic to the first torque transmission characteristic. 
     The accommodation hole may be flared in section with a plurality of steps from an opening of the relief hole toward an opening of the accommodation hole for example. The accommodation hole may have in section a flared portion with a plurality of steps from the opening of the relief hole toward the opening of the accommodation hole, for example, the accommodation hole further having a straight portion contiguous with the flared portion in the direction of flare. 
     According to the thus constructed hydraulic power transmission joint of the present invention, a gentler variation in the area of flow passage is achieved relative to the amount of displacement of the check ball induced by the weight provided in the torque varying mechanism, thereby making gentler the variation of the torque transmission characteristic relative to the variation of flow rate. This results in a reduced torque variation rate upon the switching of the torque transmission characteristics, contributing to stabilized vehicle behaviors. Due to the provision of the hysteresis characteristic in which the switching velocity from the first torque transmission characteristic to the second torque transmission characteristic is different from the switching velocity from the second torque transmission characteristic to the first torque transmission characteristic, frequent switching of the torque transmission characteristics upon the low-velocity travelling can be prevented to diminish the influences on the vehicle behaviors. 
     The above and other objects, aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an explanatory diagram of the relief-open state of a torque varying mechanism provided in a joint being currently developed by the present inventors; 
     FIG. 2 is an explanatory diagram of the relief-closed state of FIG. 1; 
     FIG. 3 is a graphic representation of torque transmission characteristics relative to vehicle velocities of FIGS. 1 and 2; 
     FIG. 4 is a sectional view of an embodiment of the present invention; 
     FIG. 5 is a perspective view of a valve block; 
     FIG. 6 is an exploded view of the valve block; 
     FIG. 7 is an enlarged sectional view of a centrifugal torque varying mechanism; 
     FIG. 8 is an explanatory diagram of the relief-open state of FIG. 7; 
     FIG. 9 is an explanatory diagram of the relief-closed state of FIG. 7; 
     FIG. 10 is a graphic representation showing the relationship between the amount of displacement of a check ball of FIG.  7  and the area of fluid passage; 
     FIG. 11 is an explanatory diagram showing the major part of the torque varying mechanism of FIGS. 1 and 2; 
     FIG. 12 is an explanatory diagram showing the major part of the torque varying mechanism in accordance with the present invention; 
     FIG. 13 is an explanatory diagram of the flow velocity acting on the check ball; and 
     FIG. 14 is a graphic representation showing the torque transmission characteristics relative to the vehicle velocities, of the torque varying mechanism of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 4 is a sectional view of an embodiment of a hydraulic power transmission joint in accordance with the present invention. A companion flange  1  is coupled to a propeller shaft associated with a front wheel driving shaft. A cam housing shank  2  is inserted into the companion flange  1  for spline coupling. The cam housing shank  2  has on its outer periphery a front bearing  3  by way of which the cam housing shank  2  is supported on a differential gear case  4 . Between the differential gear case  4  and the companion flange  1  there are provided a seal member  5  and a cover  6  that prevent in cooperation entrance of foreign particles and outflow of differential gear oil. A housing  8  is secured at a weld  7  to the right-hand end of the cam housing shank  2 . On its inner side surface, the cam housing shank  2  is provided with a cam face  9  having two or more raised portions. The cam housing shank  2  serves as a cam by way of this cam face  9 . Plugs  10  and  11  are inserted into the cam housing shank  2 , for allowing oil to be injected into the interior of the joint or discharged therefrom. A rotor  12  is rotatably accommodated in the housing  8  and is engaged with a main shaft  13  for integral rotation therewith. A drive pinion gear  14  associated with a rear differential gear is fixedly inserted into the interior of the main shaft  13  so that the main shaft  13  rotates jointly with the drive pinion gear  14 . The rotor  12  is formed with a plurality of axially extending plunger chambers  15 , each of which houses a plunger  16  slidably by way of a return spring  17 . An intake passage  18  is formed in the plunger  16  at its head side that communicates with a low-pressure chamber  19 . The intake passage  18  opens to the plunger chamber  15  by way of a communication hole  20  that is opened or closed by a one way valve block  21  for intake in the form of a ball. The interior of the plunger chamber  15  is formed with a valve seat  22  on which the one way valve block  21  is seated. A check plug  23  is disposed at the stepped portion of the valve seat  22 . Between the check plug  23  and the one way valve block  21  there is interposed a check spring not shown that serves to urge the one way valve block  21  for positioning. The return spring  17  intervenes between the check plug  23  and the bottom of the rotor  12 . A discharge hole  24  is formed in the rotor  12  so as to open to the plunger chamber  15 . A one way valve block  25  for discharge in the form of a ball is disposed in the discharge hole  24 . The discharge hole  24  is formed with a valve seat  26  on which the one way valve block  25  is seated. 
     The rotor  12  is followed by a valve block  27  which is provided with a high-pressure chamber  28  that communicates with the discharge hole  24  of the rotor  12 . A restriction member  29  projects into the high-pressure chamber  28  of the valve block  27  for positioning the one way valve block  25  at a predetermined location. The valve block  27  is provided with an orifice member  31  having an orifice that opens to the high-pressure chamber  28 . The valve block  27  and the rotor  12  are positioned relative to each other by a pin  32  and are rigidly fastened together by a bolt  33 . When the plunger  16  is in its intake stroke, the one way valve block  21  for intake at the head of the plunger  16  is opened allowing oil to flow through the low-pressure chamber  19 , intake passage  18  and the communication hole  20  into the plunger chamber  15 . At that time, the one way valve block  25  for discharge at the discharge hole  24  of the rotor  12  is closed uniting a back flow of oil from the high-pressure chamber  28 . On the contrary, when the plunger  16  is in its discharge stroke, the one way valve block  25  at discharge side is opened allowing oil within the plunger chamber  15  to flow through the discharge hole  24  and high-pressure chamber  28  into the orifice  30 . At that time, the one way valve block  21  for intake is closed to prevent oil from leaking through the communication hole  20  and intake passage  18  into the low-pressure chamber  19 . The bearing retainer  34  is rigidly press fitted into the housing  8  and is positioned by a snap ring  35 . The bearing retainer  34  is formed with a through-hole  36  that communicates with the low-pressure chamber  19 . Needle bearings  37  and  38  are interposed between the bearing retainer  34  and the valve block  27  and between the bearing retainer  34  and the main shaft  13 , respectively. An seal ring  39  is also provided between the bearing retainer  34  and the main shaft  13  for the prevention of an outflow of oil. Outside the bearing retainer  34  there is slidably provided an accumulator piston  40  for absorbing oil thermal expansion and contraction, the accumulator piston  40  defining an accumulator chamber  41  that communicates with the low-pressure chamber  19  by way of the through-hole  36  in the bearing retainer  34 . O-rings  42  and  43  are interposed between the accumulator piston  40  and the housing  8  and between the accumulator piston  40  and the bearing retainer  34 , respectively. Return springs  45  and  46  are disposed between an accumulator retainer  44  and the bottom of the accumulator piston  40 . The extended portion of the bearing retainer  34  has on its outer periphery a rear bearing  47  by way of which the bearing retainer  34  is supported by the differential gear case  4 . A lubricant groove  48  and a seal member  49  are provided in the left-hand opening of the main shaft  13 . 
     FIG. 5 is a perspective view of the valve block  27  of FIG.  4 . The valve block  27  is coupled to the rotor  12  for rotating jointly therewith. The valve block  27  is provided with a pair of centrifugal torque varying mechanisms generally designated at  50  and  51 , respectively. The outer periphery of the valve block  27  is formed with a couple of accommodation recesses  52  and  53  which receive weights  54  and  55 , respectively, in a pivotal manner. The weights  54  and  55  can pivot outwardly around weight fulcrums  56  and  57  by a centrifugal force when the vehicle velocity exceeds a predetermine value. Opposite to the weight fulcrums  56  and  57  of the weights  54  and  55  in the accommodation recesses there are interposed return springs  58  and  59  between the weights and the valve block  27 . More specifically, as is clear from an exploded view of FIG. 6, the weights  54  and  55  are provided with three spring accommodation holes  60  and  61 , respectively, for receiving the springs  58  and  59 , respectively. The spring accommodation holes  60  and  61  receive one ends of three return springs  58  and  59 , with the other ends thereof being retained by retainers  62  and  63 , respectively, provided on the valve block  27 . 
     FIG. 7 is an enlarged sectional view of the centrifugal torque varying mechanism  50  of FIG.  5 . The accommodation recess  52  is formed in the outer periphery of the valve block  27  so as to receive the weight  54  pivotally around the weight fulcrum  56 . At the end opposite to the weight fulcrum  56 , the weight  54  is formed with the spring accommodation holes  60  for receiving one ends of the return springs  58 . The other ends of the return springs  58  are retained by the retainer  62  provided on the valve block  27  so that the return springs  58  can urge the weight  54  inward. A pin insertion hole  64  is formed in the inside of the weight  54 , with a pin  66  being press-fitted thereinto. The pin  66  is press-fitted in such a manner as to project into a recessed portion formed in the weight  54 . The pin insertion hole  64  is formed with a through-hole  69  that opens to the exterior of the weight  54 . The valve block  27  is formed with a high-pressure chamber  71  and a relief hole  72  in communication with the high-pressure chamber  71 . The valve block  27  is further formed with an accommodation hole  74  serving to receive a check ball  73  and communicating with the relief hole  72 . The check ball  73  is pressed by the pin  66  press-fitted into the weight  54  urged by the return spring  58 , to thereby block the relief valve  72 . Once the vehicle velocity exceeds a predetermined value, the weight  54  pivots outward around the weight fulcrum  56  by a centrifugal force against the biasing force of the return spring  58 , allowing the pin  66  to be disengaged from the check ball  73 . This eliminates the force pressing the check ball  73  so that the check ball  73  can open the relief hole  72  by the hydraulic pressure from the high-pressure chamber  71 , whereupon the hydraulic pressure within the high-pressure chamber  71  enters the recessed portion  68  for release to the low pressure side. The torque transmission characteristic is thus switched to a lower torque transmission characteristic whereby the temperature of the joint is prevented from becoming higher with the increased vehicle velocity. 
     FIG. 8 depicts the relief-open state of the relief portion of the centrifugal torque varying mechanisms  50  and  51 , and FIG. 9 depicts the relief-closed state of the same. The relief hole  72  leading to the high-pressure chamber  71  is formed in the valve block  27 . The accommodation hole  74  leading to the relief valve  72  is also formed in the valve block  27  for receiving the check ball  73 . The accommodation hole  74  includes a first-step flared portion  75  contiguous with the opening of the relief hole  72 , a second-step flared portion  76  contiguous with the first flared portion  75 , and a straight portion  77  continuous with the second step flared portion  76 . The first-step and second-step flared portions  75  and  76  are designed to have their respective predetermined cone angles b and a. The straight portion  77  is designed to have a predetermined diameter c. Herein, let the amount of displacement of the check ball  73  and the area of flow passage in the opened state be x and A, respectively, in FIG.  8 . Then, the cone angle b of the first-step flared portion  75 , the cone angle a of the second-step flared portion  76  and the diameter c of the straight portion  77  are defined such that with respect to the amount of displacement x, the area of flow passage A continues to take a certain value L after linear increase as shown in FIG.  10 . 
     On the contrary, in the relief-closed state of FIG. 9, let a hydraulic pressure be ΔP, the area of contact over which the hydraulic pressure ΔP acts on the check ball  73  be e, and a pressing force with which the return spring  58  presses the check ball  73  be F. The pressing force F is a load obtained by leverage comprised of the weight  54  acting as a lever, the weight fulcrum  56 , the point where force is applied of the return spring  58 , and the point on which the pin  66  acts. The pressing force F is a load that is amplified by an amplification ratio in the form of a certain lever ratio of the distance between the fulcrum  56  and the return spring  58  to the distance between the fulcrum  56  and the pin  66  in FIG. 5, the load being applied to the check ball  73  by way of the weight  54  and the pin  66 . Therefore, if 
     
       
         F&gt;Δ P·e   
       
     
     then, the check ball  73  blocks the relief hole  72 . If with the vehicle velocity V exceeding a predetermined first velocity V 1  the weight  54  works by the centrifugal force and 
     
       
         (Δ P·e )+(centrifugal force)&gt; F   
       
     
     results, then the check ball  73  opens the relief valve  72 . When the check ball  73  opens the relief valve  72  as shown in FIG. 8, the variations of the area of flow passage A relative to the amount of displacement x of the check ball  73  and thus the variations of the torque transmission characteristics relative to the variance of flow rate will become gentler since the accommodation hole  74  for the check ball  73  has the two-step flared portions consisting of the first-step flared portion  75  and the second-step flared portion  76 . In case of FIGS. 1 and 2, the area of flow passage A becomes infinite when the check ball  103  opens the relief hole  102  as in FIG.  11 . As opposed to this, in the embodiment of the present invention, the area of flow passage A after relief-opening is restricted to keep a certain value L irrespective of the amount of displacement x of the check ball  73  as shown in FIG.  12 . Thus, in FIG. 11 the check ball  103  is subjected to a smaller drag as a result of outflow of oil through the relief hole  102 , whilst in FIG. 12 the check ball  73  can experience a larger drag by the restriction of the area of flow passage A. 
     The drag D which the check ball  73  undergoes is given as              D   =       C   D          1   2        ρ                   V   2        S             (   1   )                                
     where S: check ball project area, 
     V: flow velocity, 
     ρ: fluid density, and 
     C D : resistance coefficient 
     The project area S of the check ball  73  can be expressed as              S   =       π   4          d   2               (   2   )                                
     where d: diameter of the check ball  73   
     FIG. 13 illustrates exclusively the check ball  73  of FIG. 12 subjected to oil from the orifice. The flow velocity V at that time is given as        V   =     Q   A                            
     where Q: flow rate, and 
     A: the area of flow passage 
     Furthermore, the resistance coefficient C D  is set to 0.34 for example. By restricting the area of flow passage A after relief opening in this manner, it is possible to increase the drag D to which the check ball  73  is subjected. For this reason, a hysteresis as in the torque transmission characteristic relative to the vehicle velocity in FIG. 14 can be provided in which a vehicle velocity V 1  (first vehicle velocity) where a high torque transmission characteristic (first torque transmission characteristic) M is switched to a low torque transmission characteristic (second torque transmission characteristic) N is different from a vehicle velocity V 2  (second vehicle velocity) where the low torque transmission characteristic N is switched to the high torque transmission characteristic M. This prevents frequent switchings of the torque transmission characteristics upon the travel at a certain velocity. The operative function will then be described. For the duration in which the vehicle velocity V does not reach a predetermined vehicle velocity V 1  of FIG. 14, the centrifugal force can not overcome the pressing force F, and hence 
     
       
         
           F&gt;ΔP·e 
         
       
     
     results and the weight does not work. In consequence, as in FIG. 9, the relief hole  72  remain blocked by the check ball  73 . The torque characteristic at that time results in a high torque characteristic as indicated at M in FIG.  14 . When the vehicle velocity V exceeds the vehicle velocity V 1 , the centrifugal force allows the weight  54  to pivot outward. When the weight  54  pivots, the pin  66  moves as shown in FIG. 8 to allow the check ball  73  to open the relief hole  72 . At that time, a switching occurs from the high torque transmission characteristic M to the low torque transmission characteristic N as indicated by an arrow O of FIG. 14. A switching point where the high torque transmission characteristic M is switched to the low torque transmission characteristic N is given as 
     
       
           F=ΔP·e+ (centrifugal force) 
       
     
     In this case, the variations of the area of flow passage A relative to the amount of displacement x of the check ball  73  and thus the variations of the torque transmission characteristics relative to the variance of flow rate will become gentler since the accommodation hole  74  for the check ball  73  has the two-step flared portions consisting of the first-step flared portion  75  and the second-step flared portion  76 , thereby reducing the influences on the vehicle behaviors. 
     When the vehicle velocity V lowers from the vehicle velocity V 1  to the vehicle velocity V 2  after the switching to the low torque transmission characteristic M, the centrifugal force decreases allowing the weight  54  to return to its original position. For this reason, as shown in FIG. 9, the check ball  73  is pressed by the pin  66  to close the relief valve  72 . This allows the low torque transmission characteristic N of FIG. 14 to be gently switched to the high torque transmission characteristic M. The vehicle velocity V 2  at which switching is made from the low torque transmission characteristic N to the high torque transmission characteristic M is given as 
     
       
           F=ΔP·e+D+ (centrifugal force) 
       
     
     That is, by restricting the area of flow passage A after relief opening to be a certain value L irrespective of the amount of displacement x of the check ball  73 , it is possible to increase the drag D to which the check ball  73  is subjected, whereupon as in FIG. 14 a hysteresis of velocity width R can be provided at the switching point V 1  where the high torque transmission characteristic M is switched to the low torque transmission characteristic N and at the switching point V 2  where the low torque transmission characteristic N is switched to the high torque transmission characteristic M. It is therefore possible to prevent the torque transmission characteristics from being frequently switched even though variances in velocity occur during the travel at a certain velocity, thereby reducing the influences on the vehicle behaviors. 
     According to the present invention as set forth hereinabove, the influences on the vehicle behaviors can be reduced by diminishing the torque variance speeds by making gentler the variations of the area of flow passage relative to the amount of displacement of the check ball in the torque varying mechanism and thus the variations of the torque transmission characteristics relative to the variances of the flow rate. The hysteresis can be provided at the switching point where the high torque transmission characteristic is switched to the low torque transmission characteristic when the vehicle velocity increases and at the switching point where the low torque transmission characteristic is switched to the high torque transmission characteristic when the vehicle velocity decreases, thereby making it possible to prevent the torque transmission characteristics from being frequently switched upon the constant velocity travel and thus to-diminish the influences on the vehicle behaviors. 
     It will be appreciated that the present invention is not intended to be limited to the above embodiments and that it covers any appropriate variants without impairing its objects and advantages. The present invention is not restricted by the numerical values shown in the above embodiments.