Patent Publication Number: US-2023147694-A1

Title: Hydraulic machine comprising a stack of discs acted on by a push rod

Description:
FIELD OF THE INVENTION 
     The invention relates to hydraulic machines, in particular, but not exclusively hydraulic machines with radial pistons. 
     PRIOR ART 
     Numerous types of hydraulic machines have already been proposed. One embodiment of a machine of this type will be found reproduced in  FIGS.  1  and  2   . It comprises essentially five assemblies: a housing  10 , a shaft  20 , an assembly  30  forming a motor or a pump located between the housing and the shaft, bearings or rolling element bearings  42 ,  43  for guiding the shaft in rotation with respect to the housing, and a brake  49 . 
     The housing  10  is intended to be attached to the chassis of a machine or of a vehicle. It comprises a cam  12  with multiple lobes sandwiched between two side elements of the housing. 
     The shaft  20  supports a power take-off  28  suitable for carrying an accessory which must be driven in rotation, for example the rim of a wheel or any other machinery, in the case where the machine constitutes a motor and delivers a driving torque to the accessory. In the case where it constitutes a pump, the power take-off receives a driving mechanical motor torque applied to the inlet of the machine. 
     The assembly  30  forming a motor or a pump has radial pistons. It comprises a distributor  32 , a cylinder block  34  which includes a plurality of radial cylinders  35  each housing a piston  36  each carrying a roller  38  in abutment against the cam  12  integral with the housing. When the distributor  32  applies cyclically a fluid under pressure inside the cylinders  35 , the force acting on the pistons  36  and the associated rollers  38  on the cam  12  drives the cylinder block  34  in rotation with respect to the cam and consequently with respect to the housing  10 . 
     The cylinder block  43  being linked in rotation with the central element  22  of the shaft, in this case the shaft is driven in rotation by the pressure of the fluid applied by the distributor. The hydraulic machine then constitutes a motor. 
     When, on the contrary, a mechanical force is applied by the power take-off on the shaft  20  in the direction of driving in rotation with respect to the housing  10 , the movement of the rollers and of the pistons with respect to the lobes of the cam induces a variation of the volume of the cylinders and consequently applies a fluid pressure on the distributor. The machine then functions as a pump. 
     This machine is reversible, and therefore operates equally as a pump or motor, and can also turn in both directions of rotation. A hydraulic machine of this type can therefore be found in four different situations, which define the four value quadrants of pressure and direction of flow: forward motion, in traction or holding, and rearward motion, in traction or holding. 
     The brake  49  is formed from a stack of disks linked alternatively in rotation, some to the shaft and the others to the housing. The disks are acted upon in mutual abutment, hence in the braking position, by an elastic element  56  such as a Belleville washer. 
     This member  56  is arranged between, on the one hand, an element of the housing, and on the other hand a longitudinal piston  51  which is in abutment on the stack of disks. A force opposing the elastic member can be applied in a control chamber  58 , placed between the stack of disks and the piston, in order to place the brake in a de-braking position. 
     Considered here necessarily are explanations which follow the rubbing surface, or friction surface, of the disks. This is the surface of the disks by which they are in mutual contact and which generates a friction producing braking. It must therefore be distinguished from the surface of the disks not providing this friction. Also considered is the thrust surface, which is the surface of the longitudinal piston  51  and of the first disk of the stack by which these two members are in mutual contact for the transmission of the thrust force. 
     In many machines, the friction surface and the thrust surface do not coincide when they are considered in the respective planes perpendicular to the axis. In other words, they are not locally centered with respect to one another (although the longitudinal piston and the disks are coaxial). This is for example a situation in which the friction surface is offset in the radial direction opposite to the shaft with respect to the thrust surface. In certain cases, it is the opposite that is observed. In other words, this is a situation in which the respective radial positions of the friction surface and of the thrust surface do not coincide, the friction surface being able to be located radially higher than the thrust surface and vice versa. 
     In both cases, it signifies that the thrust of the piston is not exerted in the alignment of the largest part of the friction surface or is not suitably centered with respect the latter locally. It follows that the thrust is not transmitted optimally to the disks, resulting a heterogeneous contact pressure in the friction surfaces. It follows that the thrust is poorly distributed in the disks, and in particular is too high in certain places. The possible result is a tendency for the appearance of an over-torque or a seizure. The over-torque is defined as the temporary provision of an additional torque, the intensity of which cannot be predicted. Considerable torque differences can then occur on two brakes mounted on the same axle for example, resulting in instability of the machine during braking. It is also possible to see vibrations appear. Compensating for it leads to over-dimensioning the set of components through which the braking torque passes. For example, the disks forming the brake would need to be thicker in order to allow the passage of this additional torque in the splined connections of these disks with their respective opposing parts. But these two situations are to be avoided. By way of an example, brakes with nitrided disks are particular subject to over-torque. 
     One object of the invention is to avoid the over-dimensioning of the components of the transmission chain of the braking torque (in particular, to avoid increasing the thickness of the disks, their diameter and the number of their splines) and reducing the risks of occurrence of an over-torque or a seizure of the machine. 
     DISCLOSURE OF THE INVENTION 
     To this end, a hydraulic machine is provided comprising:—a fixed part, —a part rotatably mounted about an axis with respect to the fixed part, —a stack of disks forming a brake or a clutch, 
     the disks being able to be in abutment against one another by way of friction surfaces having a mean friction radius measured from the axis, —a push rod able to push the disks into abutment against one another, in a direction parallel to the axis, over a thrust surface having a mean thrust radius measured from the axis and which extends within or beyond the mean friction radius, the push rod having a radial annular notch indenting the push rod from a side of the push rod opposite to the axis when the mean thrust radius extends beyond the mean friction radius, and otherwise from a side of the push rod closest to the axis. 
     In other words, the notch is located on the side of the gap between the place where the thrust is exerted and the line of the mean radius of the friction surface of the disks. Thus, in the presence of an offset between the friction surfaces and the thrust surface, the notch allows the push rod to bend in order to re-center the thrust force on the friction surfaces. Consequently, the thrust force is exerted more perpendicularly to the friction surfaces. For example, if the pressure is too great on the side closest to the outside of the stack of disks, the presence of the notch allows relieving this part. The pressure is thus distributed more homogeneously. The absence of a strong contact pressure thus avoids the appearance of a lubrication regime called “dry,” a regime in which the over-torque appears. Braking is better and more regular. Moreover, it is no longer necessary to over-dimension the transmission components of the braking torque chain. In summary, the invention allows better distribution of the thrust force on the stack of disks in order to avoid the phenomena of seizure and over-torque. In addition, this solution implements a single part, and not several parts movable with respect to one another, to simplify assembly and manufacture, in particular to reduce the risk of forgetting a part during the assembly of the machine. 
     It is possible to observe that the notch extends in a part of the push rod which transmits thrust to the disks. 
     It is possible to provide that the bottom of the notch extends adjacent to the thrust surface. In other words, a radius of the notch at the bottom of the notch is comprised between a smaller radius of the thrust surface and a larger radius of the thrust surface. 
     It is possible to provide that the bottom of the notch extends adjacent to the friction surfaces. In other words, a radius of the notch at the bottom of the notch is comprised between a smaller radius of the friction surfaces and a larger radius of the friction surfaces. 
     It is possible to provide that the machine is configured so that a member exerts a braking force on the push rod over a reception zone having a mean radius of reception measured from the axis, and which extends beyond the mean thrust radius when the mean thrust radius extends beyond the mean friction radius, and within it otherwise. 
     It is possible to provide that the reception zone has a smaller radius which extends beyond a larger radius of the thrust surface or of the friction surface when the means thrust radius extends beyond the mean friction radius, and within it otherwise. 
     It is possible to provide that the reception zone has a smaller radius which extends beyond a smaller radius of the notch when the mean thrust radius extends beyond the mean friction radius, and within it otherwise. 
     It is possible to provide that the push rod has a greater radius than a larger radius of the friction surfaces. 
     In this configuration, the push rod has a dimension with exceeds the friction surfaces of the stack of disks in the axial view of the machine. The push rod has a radius located outside the friction surface. 
     Preferably, the notch is dimensioned so that the mean friction radius is comprised between a larger radius of the notch and a smaller radius of the notch. 
     Thus the notch extends until the perpendicular to the mean radius of the friction surfaces. 
     It is possible to provide that a larger radius of the friction surfaces is greater than the smaller radius of the notch or that a smaller radius of the friction surfaces is less than the larger radius of the notch. 
     It is possible to provide that the machine is arranged so that at least one part of the push rod bends increasingly as thrust of the push rod against the stack increases. 
     Thus, this bending allows reducing the contact pressure in the zone of the thrust surface that is not suitable centered with respect to the friction surface. This part of the push rod can be the closest to the stack or that which is farthest away from it. 
     In one embodiment, a part of the push rod has an axial dimension which decreases in the direction of a free end of this part. 
     This configuration allows controlling the deformation of this deformable part of the push rod and therefore controlling the distribution of the contact pressure. This part of the push rod can be the closest to the stack or that which is farthest away from it. 
     It is possible to provide that, the notch having two main faces facing one another, one of these faces is inclined oriented in the direction of the exterior of the push rod. 
     This inclined face favors the bending of the push rod. Moreover, by simplifying access to the bottom of the notch it facilitates its machining if needed. 
     Advantageously the inclined face extends from one side of the notch closest to the stack. 
     This arrangement constitutes a means of producing the deformable part of the push rod while still controlling the deformation and the evolution of the contact pressure. 
     It is possible to provide that the push rod has a push rod guide face in the axial direction with respect to a support. 
     Preferably, the guide face extends entirely from one side of the notch farthest away from the stack. 
     Thus there is no interference within the push rod between the zone assigned to be deformed when thrusting and that serving as a guide. 
     It is possible to provide that the machine comprises a calibrated spring capable of pushing the push rod against the stack. 
     What is meant by that is that the spring supplies a constant braking load in the absence of other loading. It is then possible to dimension the notch depending on this intensity to obtain the desired distribution of stress. Thus, if desired, it is possible to dimension the bending of the part. 
     The machine can further have any one at least of the following features:
         it comprises a spring able to push the push rod against the stack and a de-braking chamber arranged so that hydraulic pressure in the chamber applies a force opposing braking to the spring;   the stack is in an oil bath;   the disks are made of nitrided steel;   the disks comprise a lining of a friction material;   the lining has grooves; and   the stack forms a brake or a clutch.       

     The grooves can thus serve to pass oil, if necessary. 
     It is possible to provide that the brake, if necessary, provides at least one of the functions of a motor, of an emergency brake, of a safety brake (so that, in the event of a breakdown of the machine, the brake immobilizes it) and of a parking brake. 
    
    
     
       DESCRIPTION OF THE FIGURES 
       We will now present an embodiment of the invention by way of a non-limiting example, with reference to the drawings in which: 
         FIG.  1    is an axial section view of a machine according to the prior art; 
         FIG.  2    is a transverse section view of the machine along the plane II-II of  FIG.  1   ; 
         FIG.  3    is a view of a machine according to one embodiment of the invention; 
         FIGS.  4  and  5    are different schematics in axial view showing mainly the push rod of the machine of  FIG.  3    and illustrating the characteristics of the latter; and 
         FIGS.  6  and  7    illustrate an installation for digitally simulating the forces in the push rod and the stack of disks of the machine of  FIG.  3   , and the pressure values observed. 
     
    
    
     Just like the machine of  FIGS.  1  and  2   , the hydraulic machine  2  according to the present embodiment of the machine, illustrated in  FIGS.  3  and  4   , is centered on an axis O-O, and comprises essentially five complementary assemblies: a housing  10 , a shaft  20 , an assembly forming a motor or a pump, not illustrated and positioned between the housing  10  and the shaft  20 , non-illustrated means forming bearings for guiding in relative rotation the shaft  20  with respect to the housing  10 , and a brake  49 . 
     The housing  10  is intended to be attached to the chassis of a machine or of a vehicle. It comprises a cam with multiple lobes, not illustrated, and sandwiched between two side elements of the housing. 
     The shaft  20  supports a power take-off suitable for carrying an accessory which must be driven in rotation, for example the rim of a wheel or any other machinery, in the case where the machine constitutes a motor. In the case where it constitutes a pump, the power take-off receives a mechanical motor torque applied to the inlet of the machine. 
     The assembly forming a motor or a pump has radial pistons. It essentially comprises a distributor and a cylinder block which includes radial cylinders housing the respective pistons and each carrying a roller in abutment against the cam integral with the housing. The number of cylinders and therefore of pistons of the cylinder block differs from the number of lobes of the cam. As in the machine of  FIGS.  1  and  2   , the cam is radially external and the cylinder block, located radially inside the cam, has its cylinders which open radially outward, facing the cam. The distributor is supplied via an element which forms a cover and connector for feeding the distributor. 
     In a manner known per se, when the distributor cyclically applies a fluid under pressure inside the cylinders, the loading of the pistons and of the associated rollers on the cam drives the cylinder block in rotation with respect to the cam and consequently with respect to the housing. The cylinder block being linked in rotation with the central element of the shaft  20  by a system of longitudinal splines, the shaft is driven in rotation by the pressure of the fluid. In this case, the machine constitutes a motor. When, on the contrary, a mechanical force is applied by the power take-off on the shaft  20  in the direction of driving in rotation with respect to the housing  10 , the movement of the rollers and of the pistons with respect to the lobes of the cam induces a variation of volume of the cylinders and consequently applies a fluid pressure to the distributor. The machine then functions as pump. This machine is thus reversible, and therefore functions equally as a pump or as a motor, and can also turn in two directions of rotation. 
     To limit the axial bulk of the machine, the power take-off can be provided on the outside of a radially external element as is the case with  FIG.  3   . According to a variant embodiment, the power take-off is placed axially on a disk transverse to the axis O-O, as was the case in  FIG.  1   . 
     Indicated in  FIG.  3    is the fixed part associated with the housing  10  and the rotating part comprising the shaft  20 . The means forming bearings, not illustrated, comprise two rolling element bearings with rollers interleaved between the shaft  20  and the housing  10 . 
     The shaft  20  has a central zone or hub  22  intercepted by the axis O-O and a cylindrical extension  24  remote from the axis, forming a ring gear and connected to the central zone by a connecting disk  16 . 
     The housing  10  has a cylindrical axial extension  11 . 
     The brake  49  is of the disk brake type. It is formed from a stack of disks  52 ,  54  connected alternatively in rotation, some to the shaft  20  and the others to the housing  10 . The disks are therefore connected to the fixed part  10  and to the rotating part  20  with a spatial alternation along the longitudinal axis O-O. 
     Each of the disks  52 ,  54  is formed by a washer. Their connection in rotation to the housing and to the shaft respectively can be formed for example by the engagement of ribs provided in the radially outer or alternatively radially inner periphery of the disks  52 ,  54  into the complementary splines or grooves formed facing them on the ring gear  24  of the shaft and the extension  11  of the housing respectively associated. As a variant, it is possible to provide an intermediate part for connection in rotation interleaved between the disks  52  or  54  and one of the housing  10  or the shaft  20 . 
     The machine has a brake chamber  55  placed opposite to the cylinder block in the axial direction with respect to the stack of disks. It is thus situated in  FIG.  3    to the right of this stack. In this case, the brake  49  is arranged at the axial end of the machine adjacent to the power take-off, i.e. near the power take-off, opposite to the distributor with respect to the cylinder block. 
     Qualified here as “proximal” is the part of a brake part  49  oriented toward the assembly forming a motor or a pump, hence to the left in  FIG.  3   , and “distal” its part oriented opposite, remotely from this assembly, toward the outside of the machine, to the right in the figure. 
     Moreover, the brake chamber  55 , just like the axial extension  11  of the housing  10 , are in this case interleaved in the radial direction between the central zone  22  and the ring gear  24  of the shaft  20 . 
     The stack  50  of disks is again placed here between two axial abutments. One is similar to the abutment  17  of  FIG.  1    and is not illustrated in  FIG.  3   . The other is the push rod  80  which will be described below. The alternating disks are acted on in mutual abutment, hence in the braking position, by an elastic element  56 , for example of the Belleville washer type. The Belleville washer has its concavity directed toward the stack  50  in this case. It abuts a longitudinal annular piston  51  acting on the stack  50  during braking, preferably at its radially outer periphery according to the embodiment illustrated. 
     With the spring  56  are associated, on the side of the spring opposite to the piston in the axial direction, a stop ring  85  housed in an annular recess of the extension  11  and a wedge  89  interleaved between them in the axial direction. The ring and the wedge ensure the axial blockage of the edge of the spring closest to the axis O-O. 
     A force opposing the elastic member  56  can be applied in a control or de-braking chamber  58 , located between the stack of disks  50  and the longitudinal piston  51 , in order to place the brake  49  in the de-braking position. The piston  51 , which defines the de-braking chamber  58 , is movable by sliding along the axis O-O relative to the housing  10 . 
     It comprises a part forming a cylindrical sheath  62  interleaved in the axial direction between the stack of disks  50  and the spring  56 . The sheath  62  is extended by a part in the form of a washer  64  transverse to the axis O-O, associated with a gasket  68  with respect to the extension  11  to constitute a wall of the de-braking chamber  58 . The sheath  62  serves, at its proximal end, as a loading element in abutment with the stack of disks  52 ,  54 . The parts  62  and  64  are assembled to one another in a rigid and sealed manner, for example by screwing and by incorporating a static sealing means such as a seal, or are of a single block as illustrated. The piston  51  thus has an “L” shaped half-section, having a branch  64  transverse to the axis and a longitudinal branch parallel to this axis, which corresponds to the sheath  62 . The piston  51  is thus able to apply to the spring  56  a force opposing braking, when a pressure is applied in the de-braking chamber  58  to expand it. Other modalities of positioning and of cooperation between the piston  51  and the spring  56  are possible. 
     The machine comprises a ring  70 , transverse to the axis O-O, placed between the stack of disks  50  and the washer  64  of the piston. The ring  70  forms one of the transverse walls of the de-braking chamber  58 . It is housed in the radial direction between the sheath  62  and the extension  11  facing it and has a clearance for sliding with respect to the piston  51 , i.e. a radial dimension less than the radial gap existing between these parts, to allow at least a slight axial relative movement between the ring  70  and the piston  51 . The ring  70  is associated with two seals  72 ,  74  which cooperate respectively with the piston  51  and the housing element  11 . The sheath part  62  is able to slide relative to the ring  70 . 
     The de-braking chamber  58  thus has an annular shape and is delimited in the axial direction by the piston washer  64  and the ring  70  and radially by the sheath  62  and the extension  11 . 
     The de-braking chamber  58  is closed by the three seals  68 ,  72  and  74 . 
     The seal  68  associated with the piston  51  is of the type called “joint D ring” complying with standard DIN 11850 comprising a semicircular face and a flat face. It is housed in a groove of complementary width, formed preferably in the washer  64 , or as a variant in the element  11  of the housing placed facing the latter. The flat face of the seal  68  is placed in the bottom of the groove. Its semicircular face rests against the facing element capable of relative translation along the axis O-O. 
     The seal  72  providing the seal between the ring  70  and the piston  51  is of the same type, in this case. 
     The seal  74  providing the seal between the ring  70  and the housing  10  element  11  placed facing it is preferably an O ring seal, or a “D-ring” D-shaped ring. When it is placed on the washer  70  as illustrated, the use of a D-ring allows good handling of the seal during assembly. 
     The de-braking chamber  58  is supplied by means of at least on channel  13 , at least partly longitudinal, formed in the extension  11  of the housing to open into the chamber  58 . Without this being limiting, the channel  13  can for example open at a step formed on the periphery of the longitudinal extension  11  as illustrated in  FIG.  3   . The channel  13  is connected to a de-braking control line of the hydraulic machine. 
     The push rod  80  is interleaved in the axial direction between the stack of disks  50  and the ring  70 . 
     The machine therefore comprises here, in the axial longitudinal direction, from the proximal toward the distal, in particular:
         the stack of disks  50 ,   the push rod  80  in direct axial abutment on the first disk of the stack,   the ring  70  in direct axial abutment on the push rod  80 ,   the sheath  62 , itself also in direction axial abutment on the push rod  80  around the ring  70 ,   the de-braking chamber  58 ,   the washer  64  of the piston  51 ,   the spring  56 ,   the wedge  89 , and   the stop ring  85 .       

     A wedge  87  is interleaved in the axial direction between a shoulder of the extension  11  of the housing and the portion of the ring  70  closest to the axis. 
     At rest, the spring  56  acts on the sheath  62  and the push rod  80  in axial abutment on the stack of disks  52 ,  54  and thus acts on the disks in mutual abutment against one another. The brake  49  is then in the braking position. The shaft  20  is rigidly immobilized with respect to the housing  10 . 
     On the other hand, it is assumed that a fluid pressure is applied in the de-braking chamber  58  and generates an axial force opposing the spring  56  and greater than the braking force generated by the latter. Because the axial abutment  87  limits the movements of the ring  70 , the increase in volume of the de-braking chamber leads to a movement of the piston  51  in the direction of the spring  56  and therefore to a reduction in free space for the latter, between the piston  51  and the wedge  89 , and therefore nullifies the force exerted in the braking position by the spring  56  on the stack of disks  52 ,  54 . The latter no longer being acted upon in mutual abutment, the brake  49  is placed in the de-braking position. The shaft can rotate with respect to the housing. 
     Before describing the detail of the push rod  80 , it is important to highlight that the general organization of the machine which has been presented is given by way of an example, and that very many other configurations are possible. This configuration of the machine and its different features are therefore not linked to the invention. They only belong to the present embodiment. For example, the invention is compatible with the general configuration of the prior art machine of  FIGS.  1  and  2   . 
     With reference in particular to  FIGS.  3  and  4   , the wedge  80  has a generally annular shape. It has a flat front annular proximal face  82  perpendicular to the axis O-O and able to come into axial contact with the first disk  54  of the stack. Distinguished on this face on the one hand is an annular thrust zone  84  able to come effectively into contact with this disk and an inert or inactive zone  86  which is not in contact with this disk when the thrust zone is. The thrust zone  84  is found facing the disk, unlike the inert zone  86 . 
     The thrust zone  84  has a mean radius R p  measured with respect to the axis. This mean is calculated for example as the arithmetic mean do the extreme radii of the thrust zone, namely the smallest R m , and the largest R 1 . Other means of calculation are possible, for example by taking into account the integral of the radii of the thrust zone. 
     The disks  52 ,  54  are able to be in abutment against one another by means of friction surfaces. Thus, in the same manner, an annular friction zone  88  can be distinguished on each disk which is able to come effectively into contact with the contiguous disk in the braking position and an inert or inactive zone  79  which is not in contact with this disk in the braking position. The friction zone  88  is located facing the contiguous disk unlike the inert zone  79 . 
     The friction zone  88  has a mean radius R f  with reference to the axis. Here too, the mean is calculated for example as the arithmetic mean of the extreme radii of the friction zone, namely the smallest R 2  and the largest R 1 . Other modes of calculation are possible, for example by taking into account the integral of the radii of the zone. 
     The disks  52  in direct engagement with the ring gear  24  are identical with one another. Likewise, the disks  54  in direct engagement with the housing  10  are identical with one another. Because the friction zones  88  of all the disks coincide in the axial direction, the mean radius R f  is the same throughout the stack. 
     In the configuration illustrated in  FIG.  3   , the mean thrust radius R p  extends beyond the mean friction radius R f . In other words, the mean thrust radius is greater than the mean friction radius. This signifies that the thrust zone  84  of the push rod  80  is offset with respect to the friction zones  88  in the radial direction opposite to the axis O-O. 
     It is also possible to observe here that the push rod  80  has a larger radius R M , measured at its rear face in contact with the sheath and at its outer cylindrical face, greater than the larger radius R 1  of the friction surfaces  88 . But even the front face  82  of the push rod has here a greater radius R a  greater than the larger radius R 1  of the friction surfaces  88 . It further has a smaller radius R m , greater than the smaller radius R 2  of the friction surfaces  88 . 
     The push rod  80  has an annular radial notch  90  indenting the push rod from a circumferential side of the push rod opposite to the axis O-O considering that the mean thrust radius R p  extends here beyond the mean friction radius R f . This notch has a generally annular shape and generally extends along a plane perpendicular to the axis. The notch  90  extends in a part of the push rod which transmits a thrust to the disks. 
     Here the notch has a “U” shaped profile in the circumferential direction and has two main faces  91 ,  92  facing one another. The rear face  92  is flat and perpendicular to the axis. 
     The notch extends for example over more than half of the total thickness of the push rod measured in the radial direction. But is can also extend over two-thirds, even three-quarters of this dimension and take on any value comprised between these limits, including the limits. 
     The notch  90  is dimensioned in this case so that the mean friction radius R f  is comprised between a larger radius R a  of the notch and a smaller radius R e  of the notch. The bottom of the notch thus extends here until the end of the mean friction radius R f , and a little beyond it in the radial direction moving toward the axis. 
     Thus the notch  90  delimits a front portion  81  of the push rod  80  which can come into contact with the stack  50  and a rear portion  83  which cannot come into contact with the stack and may come into contact with the ring  70  in the present configuration. 
     The bottom of the notch  90  extends adjacent to the thrust zone  84  with reference to the axis. In other words, the radius R e  of the notch at the bottom of the notch is comprised between the smaller radius R m , of the thrust zone  84  and the larger radius R 1  of the thrust zone. 
     The bottom of the notch extends adjacent to the friction surfaces  88 . In other words, the radius R e  of the notch at the bottom of the notch is comprised between the smaller radius R 2  of the friction surfaces  88  and the larger radius R 1  of the friction surfaces. In particular, the larger radius R 1  of the friction surfaces  88  is greater than the smaller radius of the notch. 
     The radius R M  of the rear part  83  is in this case larger than that R a  of its front part  81 . Here in particular, the sheath  62  exerts an axial pressure in the portion of the rear part  83  which has the larger radius and which has no facing portion in the push rod at the front part  81 . 
     The machine is configured so that the sheath  62  exerts a braking force on the push rod  80  over a reception zone  78  of the push rod having a mean reception radius R r , shown in  FIG.  3    and measured from the axis. This radius is greater than the mean thrust radius R p  when the latter extends beyond the mean friction radius R f , as is the case here. 
     In this case, the reception zone  78  has a smaller radius R c  which is greater than a larger radius R 1  of the thrust surface or the friction surface. This smaller radius R c  is also greater than the smaller radius R e  of the notch measured at the bottom of the latter. 
     Like most parts, the push rod  80  is made of metal. The notch  90  confers upon it the possibility of bending in order to approach the free end of the front part  81  in the direction of the rear part  83  and/or approach the free end of the rear part in the direction of the front part. This bending occurs when the push rod is in abutment against the stack  50  in the braking position. This bending has been shown in a very exaggerated manner in  FIG.  5    in solid lines, in comparison with the non-bent configuration illustrated in dotted lines. This figure illustrates by way of an example the case in which it is the free end of the front part  81  which approaches the rear part  83 , it being understood that the reverse is also possible, as well as the combination of the two movements. 
     In the example of bending illustrated in  FIG.  5   , when the bending occurs, it is the portion of the front part  81  closest to its free end which bends first and more than the rest of this front part. This bending allows reducing the contact pressure on a part of the contact surface. It also allows distributing the contact pressure in a determined manner. The thrust surface  84  against the stack of disks  50  therefore tends to be reduced to bring it to its portion closest to the axis O-O. And especially the thrust exerted by the push rod  80  on the stack tends to be concentrated on the part of the thrust surface  84  closest to the axis in order to coincide more with the friction surfaces  88 . This distribution of loads therefore compensates the difference between the mean thrust R p  and friction R f  radii. 
     This bending illustrated the influence of the notch  90  which, producing a weakening of the push rod  80 , tends to bring back the eccentric force on RM around the radius R e . A diffusion of the force then occurs, concentrated at R e , toward the surface  84 . This diffusion is a function of the thickness of the front part  81 , denoted e. This thickness, correctly determined, ensures a diffusion of the pressing force F in the most homogeneous possible manner. 
     Ideally, the contact surfaces  82  and  88  are congruent. In other words, R m =R 2 ; R a =R 1  and therefore R f =R p . 
     The push rod  80  has a guide face  96  for sliding the push rod in the axial direction with respect to a support, formed here by a shoulder  98  of the ring  70 . To this end, the latter has a cylindrical face  100  accomplishing a surface contact with the face  96 . The guide face  96  ensures centering of the push rod in the machine. 
     As illustrated in  FIG.  4   , the guide face  96  extends entirely from one side of the notch  90  farthest away from the stack, namely on the rear distal part  83 . Thus any risk of interference with the guide function during the thrust and the bending is avoided in the braking situation. In  FIGS.  3  and  5   , the push rod is illustrated without presenting this feature. 
     It will be observed that the contact pressure is transmitted from the piston  51  to the stack solely by means of the push rod  80 , the latter constituting a rigid assembly which can be formed as a single part. 
     By way of a variant, in the contrary situation, the thrust surface  84  has a means thrust radius R p  with respect to the axis which extends within the mean friction radius R f . In other words, the mean thrust radius is less than the mean friction radius. This signifies that the thrust zone of the push rod  80  is offset with respect to the friction zone  88  in the direction of the axis. 
     Moreover, and independently of this aspect, it is seen in  FIG.  4    that one 91 of the main faces of the notch  90  is inclined to be oriented in the direction of the exterior of the push rod. Here this is the front proximal face, therefore located on the side of the stack  50 , on the part  81 . This inclination facilitates the access to the bottom of the notch during the production of the latter, particularly by machining. 
     As regards the face which extends on the side of the notch closest to the stack and because the thrust face  84  is flat, it follows that the proximal part  81  of the push rod, closest to the stack, has an axial dimension e which decreases in the direction of a free end of this portion. This dimension has been illustrated in particular in  FIG.  4   , in an exaggerated manner. Due to this arrangement, in this example the part  81  bends increasingly as the pressure of the push rod against the stack increases. This arrangement therefore allows good control of the amplitude of bending as a function of the thrust force. 
     But a similar shape could be given to the other part  83  of the push rod to obtain the same property on this other part, in combination with this configuration of the proximate part  81 , or not. 
     The push rod  80  which forms a wedge therefore has an optimized shape for diffusion and distribution of the contact pressure into the stack of disks. 
     This is shown by  FIGS.  6  and  7   , which show a digital simulation of the distribution of forces in the machine. In this simulation, only the sheath  62 , the push rod  80 , the stack  50  which is shown here as a single block and the wedge  17  located on the other side of the stack have been taken into account. Here the push rod  80  has the same configuration as that of  FIG.  3   , the notch  90  indenting the push rod from its side farthest from the axis. 
     The two figures illustrate the distribution of forces in the stack of disks. It can be seen that this distribution is good. In fact, the pressure generated at the contact interface between the sheath  62  and the push rod  80  is near 6 MPa. This is the same pressure value that is found in the entire median portion of the push rod  80 , particularly at the bottom of the notch  90  and up to the contact interface with the stack  50 . On the other hand, inside the stack, the local pressure drops very rapidly when moving away from the push rod, to take on values comprised between 3 and 4 MPa, or even less. In this example, the pressure generated on the side of the sheath therefore drips by 2 MPa once transmitted into the stack. Moreover, the mean pressure measured inside the stack remains relatively constant. A good compromise is therefore achieved between the distribution of contact pressures in the stack of disks, on the one hand, and the mechanical resistance and machining constraints of the push rod on the other hand. 
     The stack of disks  50  is in a lubrication and cooling bath. The disks are made for example of nitrided steel. They can be equipped with a lining of a friction material, the lining being grooved for example to allow the passage of oil between the disks. The disks can thus include perforations or through grooves for the same reasons. 
     Of course, it is possible to apply numerous modifications to the invention without departing from its scope. 
     For example, the stack of disks can serve as a clutch in the rotating machine. 
     The guiding of the push rod  80  can occur by means of a face of the push rod oriented in the direction opposite to the axis.