Abstract:
A drive unit with a continuously variable cone-pulley transmission as used particularly in motor vehicles comprises a pressure valve and an OR-gate valve with slide pistons sharing a common valve bore. An interposed push member serves to reduce or eliminate transverse forces between the slide pistons.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a drive unit, particularly for motor vehicles, with a continuously variable cone-pulley transmission, i.e., a transmission that has pairs of conical disks at the input and output sides, respectively, and a chain-belt to transmit torque from one pair of conical disks to the other. The gripping force by which each pair of conical disks holds the chain-belt is applied through belt-tightener control members that are subjected to a pressure which is supplied at least in part by a torque sensor. At least one of the pairs of conical disks has, in addition, a ratio-setting control member. While the belt-tightener control members provide the gripping force, the at least one ratio-setting control member serves to set or change the transmission ratio, i.e., to shift the transmission. The at least one ratio-setting control member receives an amount of pressure that is regulated by a transmission-ratio valve as required to set the ratio that is called for at each point in time. In state-of-the-art arrangements of the kind that the invention relates to, the pressure provided by the transmission-ratio valve can be influenced by a pressure valve that works together with an OR-gate valve. A drive unit of this kind is known from DE 195 46 293 A1. 
     In the known drive unit of the publication just mentioned, the pressure valve cooperates with the OR-gate valve in such a manner that, if a quick shift of transmission ratios and thus a quick delivery of pressure to the ratio setting servo control member is required and if this pressure delivery cannot be effected to a sufficient extent by the torque sensor because the latter is transmitting only a low level of torque at the particular moment, the pressure valve supplies the transmission-ratio valve with a sufficient level of pressure for a quick position change of the pairs of conical disks and thus a quick shift of transmission ratios. The OR-gate valve in this arrangement receives two different return pressures from the transmission-shifting pressure circuit and automatically takes on a position where only the higher of the two pressures can have an effect on the pressure valve. The OR-gate valve as well as the pressure valve are configured as slide-piston valves and are combined in an arrangement where the respective slide pistons of the OR-gate valve and the pressure valve push against each other. 
     This known drive unit has an excellent performance record from actual practical use, but a possibility of problems has been recognized in the area of the cooperation between the OR-gate valve and the pressure valve, due to transverse forces occurring at the point where the respective slide pistons of the pressure valve and the OR-gate valve are in contact with each other. The two slide pistons of the known drive unit are arranged in a common cylinder bore hole and push against each other at the place where they are in mutual contact. When a pressure is applied to the slide piston of the OR-gate valve, a normal force in the axial direction of the cylinder bore hole is transferred to the slide piston of the pressure valve. Extended use of the drive unit may cause wear on the valve bore hole, so that the two slide pistons could settle into an angled position in relation to each other. Also, the valve bore receiving the two pistons may already have an initial out-of-straight condition because of production tolerances, which creates another situation where the two slide pistons are not in flush contact against each other. 
     These problems will have the effect that the force transfer from one valve piston to the other will not be free from transverse forces, so that there is a risk of a valve piston taking on a skewed position inside the valve bore due to the transverse force component. As a natural consequence, the skewed position, in turn, will lead to an increased rate of wear on the valve. Consequently, the condition of the combined system of pressure valve and OR-gate valve can deteriorate to the point where the pressure required by the ratio-shifting valve can no longer be supplied at the prescribed level, so that the desired quick shifting of the ratio of the cone-pulley transmission can no longer be delivered. 
     OBJECT OF THE INVENTION 
     It is therefore the object of the present invention to further develop the known drive unit so that a required capability for quick ratio-shifting of the cone-pulley transmission is maintained even after long-term use or in the presence of imperfections in the valve bore. 
     SUMMARY OF THE INVENTION 
     To meet the foregoing objective, the invention provides a drive unit, particularly for motor vehicles, with a continuously variable cone-pulley transmission, i.e., a transmission that has pairs of conical disks at the input and output sides, respectively, with a chain-belt to transmit torque from one pair of conical disks to the other. The gripping force by which each pair of conical disks holds the chain-belt is applied through belt-tightener control members that are subjected to a pressure which is supplied at least in part by a torque sensor. At least one of the pairs of conical disks has in addition a ratio-shifting control member. While the belt-tightener control members provide the gripping force, the at least one ratio-shifting control member serves to set or change the transmission ratio, i.e., to shift the transmission. The at least one ratio-shifting control member receives an amount of pressure that is regulated by a transmission-ratio valve as required to set the ratio that is called for at each point in time. The pressure provided by the transmission-ratio valve can be influenced by a pressure valve that works together with an OR-gate valve. The force acting between the pressure valve and the OR-gate valve is conducted through an interposed push member which, to a large extent, eliminates transverse force components. 
     Advantageous embodiments of the invention have, individually or in combination, the features that 
     the OR-gate valve has a slide piston movable in a valve housing, 
     the pressure valve has a slide piston movable in a valve housing, 
     the respective slide pistons of the OR-gate valve and the pressure valve are arranged in a common bore hole of the valve housing. 
     It is further of advantage if the interposed push member through which the force is transmitted between the slide piston of the OR-gate valve and the slide piston of the pressure valve is at least in part received inside bore holes of the respective slide pistons of the OR-gate valve and the pressure valve in such a way that the push member has freedom of axial movement inside the piston bore holes. 
     In advantageous embodiments of the invention, the interposed push member is seated in the bore holes of the slide pistons with radial play and axial mobility. Thus, the push member can be moved axially in the slide-piston bores of the OR-gate valve and the pressure valve while the outside circumference of the push member ha s radial clearance from the interior wall of the slide-piston bore. It is advantageous if the radial play is large enough so that the outside circumference of the push member maintains radial clearance from the inside wall of the slide-piston bore in case that the valve bore in which the two slide pistons move does not run true or the two slide pistons are not in alignment with each other. The clearance of the push member inside the slide-piston bore is designed to be wide enough that the outside of the push member will not touch the wall of the slide-piston bore as long as the maximum straightness errors of the valve bore are within the given production tolerance. 
     According to a further developed embodiment of the invention, the end portions of the interposed push member and the bottom ends of the slide-piston bores are designed as an articulated link connection. The mobility of the link allows the longitudinal axes of the push member, the slide piston of the OR-gate valve, and the slide piston of the pressure valve to position themselves at an angle to each other. The radial play between the push member and the walls of the bore holes in the slide pistons of the OR-gate valve and the pressure valve is wide enough so that even with non-alignment between the interposed push member and the slide pistons of the OR-gate valve and the pressure valve, there is no contact between the outside circumference of the push member and the walls of the bore holes in the respective slide pistons of the OR-gate valve and the pressure valve. 
     In the arrangement just described, it is advantageous if the end portions of the push member are shaped as spheres or spherical segments and the ends of the slide-piston bore holes are shaped as cones or spherical segments. Thus, when the spherical or spherical segment-shaped ends of the push member are in contact with the conical or spherical segment-shaped ends of the slide-piston bore holes, the contact is a linear contact along a circle, as opposed to a contact between abutting surfaces. Due to the advantageous contacting arrangement, the longitudinal axes of the push member and the slide-piston bore holes of the OR-gate valve and the pressure valve can position themselves at angles to each other as described above. 
     It has proven to be advantageous if the slide-piston bore hole of the OR-gate valve reaches from the open end of the hole all the way into the opposite axial end portion of the OR-gate slide piston. In the case of the pressure valve, the slide-piston bore hole can extend from an open end to an axial location near a shutter edge of the slide piston of the pressure valve. To add length to the slide-piston bore of the pressure valve, the slide piston can have a tubular lengthwise extension radially surrounding the push member on the outside. 
     Thus, the invention removes the problem of the slide pistons of the OR-gate valve and the pressure valve being pushed into skewed positions due to the occurrence of transverse forces. When a force is transmitted between the slide pistons, the force is introduced in each of the slide pistons at the deeply recessed end of the respective slide-piston bore, where the longitudinal axes of the interposed push member and the slide pistons are allowed to position themselves at an angle to each other, thereby providing the mobility of a link that reduces the skewing moments acting on the slide pistons as a result of the transverse forces. Thus, if the precise-fitting shape of the valve bore has deteriorated as a result of wear, or if the two slide pistons are not in true alignment, the slide pistons can no longer fall into a twisted or skewed position inside the valve bore due to transverse forces. 
     The novel features that are considered as characteristic of the invention are set forth in particular in the appended claims. The improved apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain presently preferred specific embodiments with reference to the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description of the invention is based on the attached drawing in which 
     FIG. 1 represents an overall view of a drive unit according to the invention; 
     FIG. 2 represents an enlarged view of the slide pistons of the OR-gate valve and the pressure valve that are arranged in a common valve bore hole; 
     FIG. 3 represents a similar view as FIG. 2, but without the helix spring; 
     FIG. 4, in a similar view as FIG. 3, shows the misalignment of the two slide pistons in relation to each other; 
     FIG. 5, in a similar view as FIG. 3, illustrates an out-of-straight error of the valve bore; and 
     FIG. 6 represents an enlarged view of the end portion of the interposed push member and the slide-piston bore hole. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 of the drawing gives an overall view of the parts of a drive unit according to the invention. On the input side, a pair of conical disks  1  is mounted in a rotationally locked connection on a shaft A. Analogous to the input pair of disks  1 , another pair of conical disks  2 , associated with the output side of the drive unit, is mounted in a rotationally locked connection on a shaft B. Each of the pairs of conical disks  1  and  2  has an axially fixed conical disk  1   b ,  2   b , respectively, and an axially movable disk  1   a ,  2   a . An endless flexible torque-transmitting device, in this case a chain  3 , loops around the disk pairs  1 ,  2  to transmit torque from one to the other. 
     The disk pair  1  can be axially tightened by means of a piston/cylinder unit  4  which functions in such a way that a pressure introduced into piston/cylinder unit  4  causes an axial displacement of the axially movable conical disk  1   a . In analogous manner, the axially movable conical disk  2   a  can be axially displaced on the shaft B by means of a piston/cylinder unit  5  in order to tighten the chain  3  against the axially fixed conical disk  2   b.    
     In addition to the piston/cylinder units  4 ,  5 , further piston/cylinder units  6 ,  7  are provided for shifting the transmission ratio. The shifting function is performed by injecting or removing pressure medium, e.g., oil, in the pressure compartments  6   a ,  7   a  of the piston/cylinder units  6 ,  7  in accordance with the required transmission ratio. The pressure medium can be supplied by a pump  8  which can be followed downstream by a volume-flow limiting valve  24 . To remove pressure medium, the pressure compartments  6   a ,  7   a  can be connected to a drain conduit that leads to a sump. To effect a ratio change of the transmission, one of the two pressure compartments  6   a ,  7   a  is pressurized by an inflow of more pressure medium while the volume of the other of the two compartments is at least part-way reduced by letting pressure medium escape through the drain conduit  9 . A valve  10  serves to direct the inflow and outflow of pressure medium. 
     A torque sensor  11  is mounted on the shaft A and serves to generate a pressure that depends at least in part on the torque that needs to be transmitted. The torque sensor  11  transfers the torque entering the shaft A to the input-receiving pair of conical disks  1 . In the torque sensor, the torque is transmitted through a ramp disk  12  that is axially fixed on the shaft A but has a limited amount of rotational mobility. Facing the ramp disk  12  is an axially movable ramp disk  13  separated from disk  12  by ramp roller bodies such a spherical balls  14 . A rotation of the ramp disk  12  in relation to ramp disk  13  causes the balls  14  to run up on the ramps so that a rotary displacement of the ramp disk  12  causes an axial displacement of the ramp disk  13 . 
     The torque sensor  11  has a pressure compartment  15  that is connected to the pump  8  by way of the conduits  18 ,  19 ,  20 . The pressure compartment  15  serves to generate a variable pressure of a magnitude that depends on the torque that is to be transmitted. The conduit  20  has a branch  21  through which the pressure compartment  7   a  of the piston/cylinder unit  7  can be connected to the pump  8 . The pressure compartment  4   a  of the piston/cylinder unit  4  communicates with the pressure compartment  15  of the torque sensor  11  by way of a conduit that is not specifically indicated in the drawing. A drainage channel  22  running through shaft A can be connected to the pressure compartment  15  of the torque sensor  11 . With the axial displacement of the ramp disk  13  of the torque sensor  11 , as described above, an opening  23  in the shaft A functioning as a valve can allow pressure medium to exit through the drainage channel  22 . The valve  23  in combination with the drainage channel  22  forms a throttle. The ramp disk  13  by virtue of its axial displacement functions as a regulating piston that closes or opens the valve opening  23  to a degree that depends on the torque to be transmitted, so that a pressure originating from the pump  8  can be built up in the pressure compartment  15  of the torque sensor  11 . An analogous amount of pressure is also communicated through the aforementioned but not specifically illustrated connection from the pressure compartment  15  to the pressure compartment  4   a . Furthermore, a corresponding pressure is also propagated through the conduits  20  and  21  to the pressure compartment  5   a  of the second piston/cylinder unit  5 . The pressure for setting or changing the transmission ratio of the cone-pulley transmission is built up in the pressure compartments  6   a  and  7   a . Due to the parallel arrangement of the piston/cylinder units  4 ,  5  and the piston/cylinder units  6 ,  7 , the forces produced by the ratio-setting pressure are additively superimposed on the torque-dependent belt-tightening forces that are generated in the pressure compartments  4   a ,  5   a.    
     In addition to the previously mentioned pressure compartment  15 , the torque sensor has a second pressure compartment  16  that can be connected to the pressure compartment at an operating point that depends on the transmission ratio of the cone-pulley transmission, so that the hydraulic combination of the two pressure compartments  15  and  16  provides an increase in the effective axially oriented surface. The two pressure compartments are connected or disconnected as a function of the axial displacement of the axially movable conical disk  1   a . To this effect, the conical disk can be used as a valve part in cooperation with a suitable arrangement of connecting channels or bore holes in the components of the disk pair  1  and the torque sensor  11 . It can be of advantage, if only the first pressure compartment  15  is pressurized over most or all of the underdrive part of the range of the cone-pulley transmission. The merging of the two pressure chambers  15  and  16  can occur at the point where the transmission changes from a speed-reducing mode to a speed-amplifying mode, i.e., at the point where the transmission ratio is approximately 1:1. As a result of this arrangement, the pressure can be modulated as a function of the torque and also as a function of the transmission ratio, with the ratio-dependent modulation being super-imposed on the torque-dependent modulation. The switch-over from pressurizing the first pressure compartment  15  alone to pressurizing both pressure compartments  15 ,  16  at a transmission ratio of 1:1 has the effect that in the underdrive range of the transmission, i.e., in a speed-reducing mode, only the first pressure compartment  15  is pressurized, while in the overdrive range of the transmission, i.e., in a speed-amplifying mode, both of the pressure compartments  15  and  16  are pressurized. Thus, with a given amount of input torque at the input of the torque sensor  11 , the pressure generated by the torque sensor is greater when the transmission is in an underdrive mode, because the pressure is acting only against the effective surface of compartment  15 , in contrast to an overdrive mode of the transmission where the combined hydraulically effective surface of compartments  15  and  16  is available to convert the pressure into an axial force. Consequently, the pressure generated by the torque sensor is smaller in the high-speed range than in the low-speed range. 
     A flow-volume limiting valve  24  is arranged downstream of the pump  8  and the channel  17  of FIG.  1 . The flow-volume limiting valve  24  serves to limit the rate of volume flow of the constant volume stream that is delivered by the pump  8 . The pump  8  also pressurizes the torque sensor  11  in addition to the piston/cylinder units  4 ,  5  and  6 ,  7 . A pressure valve  25 , which cooperates with an OR-gate control element or OR-gate valve  28 , serves to increase the pressure upstream of the valve  10  that serves to set or change the ratio of the cone-pulley transmission. Thus, the pressure valve  25  in combination with the OR-gate valve  28  ensures that the pressure in conduit  18 , i.e., upstream of the valve  10 , is higher than the pressure required in the conduits  26 ,  27  that supply the piston/cylinder units for the setting or changing of the transmission ratio. As can be seen in FIG. 1, the pressure valve  25  also has a connection to the torque sensor  11  and the piston/cylinder unit  4  by way of the conduit  20 . There is further a connection between the pressure valve  25  and the piston/cylinder unit  5  by way of the conduit  21 . As discussed previously, the torque-dependent pressure in the pressure compartments  4   a ,  5   a  depends on the pressure delivered by the torque sensor  11  and thus on the magnitude of the torque received by the torque sensor  11 . As explained above, it is possible to use the torque sensor to generate a ratio-dependent pressure modulation that is superimposed on the torque-dependent pressure modulation. In a case where the torque sensor is receiving only a small amount of torque and, accordingly, the pressure generated by the torque sensor is small, it is possible that in a critical situation the ratio-dependent pressure could be inadequate for a desired quick change of the transmission ratio. This kind of condition will occur, e.g., with a strong deceleration of the vehicle at low engine torque, where a rapid change of the transmission ratio is required. To ensure an adequate pressure level upstream of the valve  10  and thus a sufficient amount of pressure in the conduits  26 ,  27  for pressurizing the ratio-actuating piston-cylinder units  6 ,  7  for a rapid change of transmission ratio in this kind of situation, the pressure valve  25  is provided as a part of the hydraulic transmission-control system. The pressure valve  25  in combination with the OR-gate valve  28  causes the conduit  20  to receive a lower amount of pressure which, in turn, causes the pressure in conduits  18 ,  19 , i.e., upstream of the ratio-actuating valve  10 , to rise. Consequently, the pressure existing ahead of the valve  10  will be higher than the pressure in conduits  26 ,  27 . The pressure levels existing in the conduits  26 ,  27  are returned by way of conduits  29 ,  30  to a functional unit consisting of the pressure valve  25  and a valve of the type of the OR-gate valve  28 . The pressure valve  25  has an axially movable slide piston  31  contained in a valve cylinder bore  45 . Also arranged for axial movement within the same valve cylinder bore  45  is a slide piston  32  of the OR-gate valve  28 .  20  The two slide pistons  31 ,  32  are moveable independently of each other along the axial direction of the valve cylinder bore  45 . The details of the arrangement of the pressure valve  25  and OR-gate valve  28  are shown in FIG.  2 . 
     The two slide pistons  31 ,  32  push against each other through an interposed push member  46 . The return conduit  29  is connected to a pressure compartment  34 , and the return conduit  30  is connected to a pressure compartment  35  located axially between the slide pistons  31  and  32 . If the pressure in the conduit  27  and thus also in the return conduit  30  is higher than in the conduit  26  and return conduit  29 , then the higher pressure will fill the pressure compartment  35  and act directly on the slide piston  31  of the pressure valve  25 . In the opposite case, i.e., if the pressure in the conduit  26  and thus also in the return conduit  29  is higher than in the conduit  27  and return conduit  30 , the higher pressure will fill the pressure compartment  34  and act against the slide piston  32  which, through the push member  46 , pushes the slide piston  31  in the direction where the pressure valve  25  will become closed. This explains how the valve  28  functions as an OR-gate valve, where in each case the higher of the respective pressures in the return conduits  29 ,  30  is applied to the slide piston  31  of the pressure valve  25 . A compression spring  36  is pre-tensioned, bearing at one end against a holding ring  37  seated in the valve housing containing the valve bore  45  and at the other end against the slide piston  31 . The spring  36  is pre-tensioned with an appropriate force, so that a minimum amount of pressure required for shifting the transmission ratio exists in the conduit  19  upstream of the valve  10 . On the opposite side from the spring  36 , the slide piston  31  faces a pressure compartment  38  and is thereby exposed to a pressure that corresponds to the pressure in conduit  18 , i.e., the pressure that exists upstream of the ratio-shifting valve  10 . If the pressure in the conduits  18  and  19  exceeds a certain minimum, the pressurization of the slide piston  31  by way of the pressure compartment  38  causes the slide piston  31  to open a passage to the conduits  20 ,  21  and to the torque sensor  11 . Thus, the compression spring  36  and the combination of the pressure valve  25  and OR-gate valve  28  determine the minimum pressure in the conduits  18  and  19 . Also, as a result of the pressures acting on both sides of the slide piston  31 , i.e., from the pressure compartment  38  and from whichever of the conduits  26 ,  27  carries the higher pressure, the same arrangement provides the desired pressure differential between the higher of the pressures in the conduits  26  or  27  on the one hand and the pressure upstream of the ratio-shifting valve  10  on the other. 
     The embodiment of the drive unit as shown in FIG. 1 includes a proportional valve  40  which, by way of a conduit  42 , pressurizes a pressure compartment  41  of the ratio-shifting valve, where a pre-tensioned spring  43  is arranged to counteract the force exerted by the pressure in the pressure compartment  41 . When the pressure compartment  41  is not pressurized, the pre-tensioned spring  43  pushes the slide piston  44  of the ratio-shifting valve  10  into a position where a passage is opened between the conduit  27  and the drain conduit  9  and another passage is also opened between the conduit  26  and the conduit  18 ,  19  upstream of the ratio-shifting valve  10  by way of corresponding shutter edges of the slide piston  44 . Thus, the connection to the drain conduit  9  keeps the conduit  27  essentially pressure-free, while the conduit  26  is pressurized by the full strength of the pressure supplied by the pump  8 . This condition causes the transmission to shift towards a high speed. 
     When the pressure compartment  41  of the ratio-shifting valve  10  is pressurized by the proportional valve  40 , the slide piston  44  is moved to the right, opposed by the force of the spring  43 , so that the conduit  27  can be supplied with pressure from the conduit  18  on the input side of the ratio-shifting valve and the conduit  26  is connected to the drain conduit  9 . This condition causes the transmission to shift towards a slow speed. The pressure in the pressure compartment  41  can be set through an appropriate control input into the proportional valve  40 , whereby the respective pressure levels in conduits  26  and  27  can be set at any point between the full strength of the supply pressure and essentially zero strength, i.e., the pressure-free condition that occurs when one of the conduits is connected to the drain conduit  9 . The control input into the proportional valve  40  can be provided by a suitable electronic control unit. 
     As is immediately evident from FIG. 2 of the drawing, the push member  46  is arranged to be axially movable in a piston bore hole  47  of the slide piston  32  of the OR-gate valve and in a piston bore hole  48  of the slide piston  31  of the pressure valve  25 . The valve bore  45  containing the two slide pistons  31 ,  32  can be closed by a stopper plug  48  with a ring seal  49 , as shown in the left-hand part of the drawing FIG.  2 . 
     The slide piston  31  of the pressure valve  25  has a tube-shaped extension  50  on the side of the slide piston  31  facing towards the holding ring  37 , serving to extend the piston bore hole  48  and also providing on the outside a centering collar for the compression spring  36 . 
     The holding ring  37 , which serves as a seat for the spring  36 , is supported on one side by the housing, more specifically, by a shoulder of a recess  99  of the housing. The spring  36  pushes axially against the holding ring  37 , so that the latter, in turn, is held against the housing. At the same time, the holding ring  37  serves as a centering device for the spring  36 . Preferably, the holding ring  37  is arranged in a through hole of the housing, and the outside diameter of the spring  36  is less than the diameter of the long bore of the valve. The holding ring  37  has two centering diameters  37   a  and  37   b , the larger of which ( 37   a ) is on a shoulder by which the holding ring is centered in the valve bore, and the smaller is the inside diameter  37   b  of a recess on the opposite side where the spring  36  is centered in the holding ring. The centering diameter  37   a  corresponds to the bore diameter  100  of the valve bore. 
     It is advantageous if the holding ring  37  and the recess  99  are of a shape allowing the holding ring to be mounted only in its correct orientation. This can be achieved, e.g., by providing a taper  98 . 
     The holding ring  37  is a metal part, e.g., formed out of solid metal or sheet metal. However, in another embodiment, the holding ring  37  can also be made of a polymer material. 
     FIG. 3 shows an analogous representation of the inventive arrangement as FIG. 2, but without the compression spring  36 . 
     FIG. 3 represents an “ideal” position of the two slide pistons  31 ,  32  inside the valve bore  45 , i.e., a position in which there is neither a misalignment of the two slide pistons  31 ,  32  in relation to each other nor an imperfection in the shape of the valve bore  45 . 
     In a position of the two slide pistons  31 ,  32  as shown in FIG. 3, the ends  51 ,  52  of the interposed push member  46  are bearing against the respective ends of the piston bore holes  47 ,  48 , i.e., against the bottoms of the bore holes  47 ,  48 . 
     If the pressure compartment  34  is pressurized through the conduit  29  and the slide piston  32  is thereby urged leftwards in the plane of the drawing, the force that is generated in this process is transferred by way of the push member  46  to the slide piston  31 . To visualize the function of the push member  46 , FIG. 4 shows a misaligned condition of the slide pistons  31 ,  32 , and FIG. 5 shows an out-of-straight condition of the valve bore  45 , both of the drawings giving an exaggerated representation. 
     A misaligned condition that could occur in the two slide pistons  31 ,  32 , e.g., after long-term use of the drive unit, would lead to a transverse force component acting on the slide pistons  31 ,  32 . The transverse force component, in turn, could cause the slide pistons to settle into a skewed position that would further increase the wear on the valve bore  45 . 
     The interposed push member  46  provides an articulated connection so that, with a misaligned condition of the two slide pistons  31 ,  32 , the longitudinal axis  53  of the push member  46  will take an angled position to the respective longitudinal axis  54  of each slide piston  31 ,  32 . The push member  46  has sufficient clearance from the walls of the piston bore holes  47 ,  48  of the slide pistons  31 ,  32 , so that the angled position of the latter will not cause the push member  46  to touch the walls of the piston bore holes  47 ,  48 . The clearance gap can be of the order of 0.1 mm to 0.2 mm. 
     FIG. 5 of the drawing illustrates, in an exaggerated view, the case of an out-of-straight error of the valve bore  45 . In the representation shown, the valve bore  45  has a curvature that is particularly noticeable in the left half of the drawing, so that the slide piston  31  of the pressure valve  25  takes on an oblique position. Because the push member  46  functions as a swivel-jointed link between the areas where the ends  51 ,  52  meet the bottoms of the respective piston bore holes  47 ,  48 , the bottom of the piston bore hole  48  in relation to the end  52  of the push member  46  can tilt from the “ideal” alignment of FIG.  3 . As in the preceding case illustrated in FIG. 4, the radial play of the push member  46  inside the piston bore hole  48  prevents the push member  46  from touching the wall of the piston bore hole  48 . 
     FIG. 6 of the drawing represents an enlarged detail view of one of the end portions  51  or  52  of the push member  46  inside the respective piston bore hole  47  or  48 . 
     In the embodiment of the push member  46  as illustrated in FIGS. 4 and 5 as well as in the magnified detail drawing of FIG. 6, the end portions  51 ,  52  are shaped as spherical segments, so that each of the end portions  51 ,  52  is in linear contact along a circle with the concave-conical bottom  55  of the respective piston bore hole  47 ,  48 . If the two slide pistons  31 ,  32  become misaligned as illustrated in FIG. 4, or if the valve bore has an out-of-straight error as shown, e.g., in FIG. 5, the result will be a displacement of the end portions  51 ,  52  of the push member  46  in relation to the bottom  55  of the respective piston bore hole  47 ,  48 , as indicated by the arrow P in FIG.  6 . In other words, the central longitudinal axis  53  of the push member  46  can position itself at an angle to the respective longitudinal axes  54  of the slide pistons  31 ,  32 , so that a relative movement can take place at the swivel joints that are constituted by the end portions  51 ,  52  of the push member  46  and the bottoms  55  of the respective piston bore holes  47 ,  48 . As a result of this arrangement, no transverse forces are transmitted at the contact between the push member and the bottoms  55  of the bore holes in the slide pistons  31 ,  32 . 
     While in the preceding embodiment, the contact between the interposed push member and the bottoms of the slide piston bore holes has been described as a combination of the spherical ends of the push member with conical bottoms of the piston bore holes, the bottoms of the piston bore holes could also have the shape of spherical segments, which would likewise allow the contacts to function as swivel joints. 
     Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic and specific aspects of the aforedescribed contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the appended claims.