Abstract:
The invention relates to a hydraulic apparatus ( 1 ) including: a casing ( 6 ) defining a first assembly; a shaft ( 2 ) defining a second assembly; said first and second assemblies being freely rotatable relative to one another; a multi-lobe cam ( 3 ) rotatably connected to one of said first or second assembly; a dispenser ( 51 ) and a dispenser cover ( 52 ); a cylinder block ( 4 ) mounted such as to be freely rotatable relative to said first and second assemblies and including a means for rotatably engaging relative to the other of said assemblies, characterized in that the hydraulic apparatus ( 1 ) includes a return means for moving the cylinder block ( 4 ) in order to rotatably disengage same from said first or second assembly, and in that the dispenser ( 5 ) is configured, when applying pressure, to rotatably engage the cylinder block ( 4 ) relative to the other of said first or second assembly.

Description:
GENERAL TECHNICAL FIELD 
       [0001]    The present invention relates to the field of hydraulic apparatuses and associated means for the commissioning of such hydraulic apparatuses. 
       PRIOR ART 
       [0002]    Throughout the present text, hydraulic apparatus will denote an apparatus which can function as a motor or hydraulic pump, and being for example controlled by means of the inclination of a control panel of said hydraulic apparatus. A hydraulic apparatus conventionally comprises a plurality of pistons arranged in housings, and describing out-and-back movements in contact with a cam. 
         [0003]    Hydraulic apparatuses having a free wheel configuration are known, that is, a configuration in which the hydraulic apparatus operates without pressure and without fluid flow and more particularly where the pistons are not in contact with the cam, such a configuration being for example advantageous on engines having mixed work conditions. Opposed to this free wheel configuration is the work configuration in which the pistons are in contact with the cam, and the hydraulic apparatus operates with a pressure and fluid flow. 
         [0004]    A hydraulic transmission typically comprises a hydraulic apparatus which is in free wheel configuration when the hydraulic transmission is not solicited, and shifts to the work configuration when solicited. 
         [0005]    This shift from the free wheel configuration to the work configuration is done by extending of the pistons which are put into contact with the cam, or more precisely their free ends are placed in contact with the cam. This shift from the free wheel configuration to the work configuration is called commissioning of the hydraulic apparatus, and corresponds to the application of a flow to the hydraulic apparatus. 
         [0006]    Commissioning can be done statically or dynamically. 
         [0007]    Static commissioning involves the immobilisation of the different components, and is therefore highly restrictive in terms of use. 
         [0008]    In dynamic mode, commissioning causes instantaneous flow, which the flow feed sources cannot provide. This involves the pistons not all being in contact instantaneously against the cam, and therefore causes noise. 
         [0009]    Also, commissioning such as commonly performed in such hydraulic apparatuses causes shocks during contacting of pistons on the cam, which has a direct impact on the service life of the motor, as well as noise, which is unpleasant for the user, whether the commissioning is carried out statically or dynamically. Also, commissioning of the hydraulic apparatus is not always instantaneous, as it needs to bring out enough oil to extend all the pistons. 
       PRESENTATION OF THE INVENTION 
       [0010]    The aim of the present invention is to propose a system having no such disadvantages. 
         [0011]    For this purpose, the invention proposes a hydraulic apparatus with radial pistons, comprising:
       a casing defining a first assembly,   a shaft defining a second assembly, said first and second assemblies being free in rotation one relative to the other,   a multilobe cam connected in rotation to one of said first or second assemblies,   a distributor and a distributor lid,   a cylinder block mounted in free rotation relative to said first and second assemblies, and comprising a plurality of cylinders in which pistons are arranged guided to slide radially in respective cylinders of the cylinder block and supported on the lobes of the cam, said apparatus comprises support springs arranged in said cylinders so as to keep the pistons supported against the cam,   characterized in that said hydraulic apparatus also comprises rotation engagement means adapted, during the application of a command, to immobilise the cylinder block relative to the other of said first or second assemblies.       
 
         [0018]    Said hydraulic apparatus typically has one or more of the following characteristics, taken independently or in combination:
       said engagement means are adapted to provide engagement by friction of said cylinder block with said first or second assembly;   one of said support springs has stiffness greater than the stiffness of said other support springs so as to define indexation of the cylinder block relative to the cam when the cylinder block is free in rotation relative to said first and second assemblies.       
 
         [0021]    The cam is typically:
       either connected in rotation to the shaft, the engagement means being adapted to connect in rotation the cylinder block to the casing;   or connected in rotation to the casing, the engagement means being adapted to connect in rotation the cylinder block to the shaft.       
 
         [0024]    With the hydraulic apparatus further comprising advantageously a synchronisation piston connected in rotation to the other of said first or second assemblies and adapted to cooperate with a housing arranged in the cylinder block, said housing forming a angular sector so as to define two stop positions of the synchronisation piston in said housing, said two stop positions defining two configurations of internal conduits of the distributor and of the cylinder block corresponding to the two directions of rotation of the mobile assembly of said hydraulic apparatus, said synchronisation piston being configured so as to position the cylinder block in the stop position adapted prior to the immobilisation by the engagement means of the cylinder block relative to the other of said first or second assemblies. 
         [0025]    According to a particular embodiment, said engagement means comprise
       return means tending to displace the cylinder block to disengage it in rotation relative to the other of said first or second assemblies, and   internal conduits of the distributor configured, during the application of pressure, to engage the cylinder block in rotation relative to the other of said first or second assemblies.       
 
         [0028]    According to another particular embodiment said engagement means comprise an electro-magnet, configured, during its activation, so as to generate a magnetic field which causes an attraction force between the cylinder block and a friction element so as to immobilise the cylinder block relative to said first or second assembly. 
         [0029]    According to a particular embodiment said engagement means comprise a device with teeth and grooves arranged on the engagement means adapted to cooperate with teeth and grooves arranged on the cylinder block or on said first or second assembly so as to immobilise the cylinder block relative to said first or second assembly. 
         [0030]    According to a particular embodiment, said engagement means comprise
       a sealed chamber filled with oil and housing discs connected in rotation to the cylinder block and to the other of said first or second assemblies, and   a control chamber adapted to control the pressure inside the sealed chamber, so as to provide engagement by visco-coupling.       
 
         [0033]    The invention also relates to a system comprising at least two hydraulic apparatuses such as defined previously, in which a first of said hydraulic apparatuses is connected to a principal motor via a clutch so as to function as a pump, and feed via a hydraulic circuit a second of said hydraulic apparatuses operating as a motor to provide assistance on a drive shaft of a vehicle wheel, said first and second hydraulic apparatuses comprising a common drain line for their respective casings connected to said hydraulic circuit via check valves. 
         [0034]    The invention also relates to a system comprising at least one hydraulic apparatus such as defined previously operating as a motor and each mounted on a driven axle of a vehicle, and a hydraulic pump with variable displacement, said system further comprising a calculator adapted to control displacement of the hydraulic pump such that displacement of the hydraulic pump multiplied by its drive speed in rotation is equal to displacement of the total of said hydraulic motors multiplied by the drive speed of the hydraulic driven motors connected to the axles plus Epsilon, where Epsilon represents sliding which produces a traction effect. 
         [0035]    The invention also relates to a vehicle comprising a primary motor driving in rotation a primary axle defining the motorised wheels of the vehicle, and a secondary axle, 
         [0036]    said vehicle comprising a primary hydraulic apparatus such as defined previously mounted on said primary axle so as to be driven in rotation at the same speed as said motorised wheels, and a secondary hydraulic apparatus such as defined previously mounted on the secondary axle so as to be driven in rotation at the same speed as the wheel(s) mounted on said secondary axle, said primary and secondary hydraulic apparatuses being connected so as to enable driving of the secondary axle by the secondary hydraulic apparatus during the commissioning of said hydraulic apparatuses. 
         [0037]    The invention also relates to a method for displacement of a hydraulic apparatus comprising a casing defining a first assembly, a shaft defining a second assembly, said first and second assemblies being free in rotation one relative to the other, a multilobe cam connected in rotation to one of said first or second assemblies, a cylinder block mounted free in rotation relative to said first and second assemblies, and comprising a plurality of cylinders in which are arranged pistons guided to slide radially in respective cylinders of the cylinder block, supported on the lobes of the cam and being held supported against the cam, 
         [0038]    said method being characterized in that immobilisation of the cylinder block relative to the other of said first or second assemblies is controlled so as to cause displacement of the hydraulic apparatus. 
         [0039]    Immobilisation of the cylinder block relative to the other of said first or second assemblies is advantageously carried out by application of pressure in a distributor of said hydraulic apparatus driving the displacement of said distributor so that it acts on the cylinder block to immobilise it relative to the other of said first or second assemblies. 
     
    
     
       PRESENTATION OF FIGURES 
         [0040]    Other characteristics, aims and advantages of the invention will emerge from the following description which is purely illustrative and non-limiting, and which must be considered with respect to the attached diagrams, in which: 
           [0041]      FIG. 1  shows a partial sectional view of a hydraulic apparatus according to an aspect of the invention, 
           [0042]      FIG. 2  shows a partial sectional view of a variant of a hydraulic apparatus according to an aspect of the invention, 
           [0043]      FIGS. 3 to 10  show partial sectional views of other variants of hydraulic apparatuses according to an aspect of the invention, 
           [0044]      FIGS. 11 and 12  show two views of an example of integration of a variant of a hydraulic apparatus according to an aspect of the invention on a vehicle axle, 
           [0045]      FIG. 13  shows a view of another variant of a hydraulic apparatus according to an aspect of the invention coupled with a differential for integration for example on a vehicle axle, 
           [0046]      FIGS. 14 to 16  show several examples of assemblies executing the hydraulic apparatuses according to an aspect of the invention. 
       
    
    
       [0047]    In all figures, common elements are denoted by identical reference numerals. 
       DETAILED DESCRIPTION 
       [0048]      FIG. 1  shows a partial sectional view of a hydraulic apparatus according to an aspect of the invention. This figure shows the axis of rotation X-X of the hydraulic apparatus  1 . 
         [0049]    The hydraulic apparatus  1  comprises a shaft  2 , a cam  3  typically a multilobe cam, a cylinder block  4 , a distributor  51 , a distributor lid  52  and a casing  6 . 
         [0050]    The casing  6  and the shaft  2  are mounted turning relative to the other, typically by means of bearings, in this case two conical bearings  21 . 
         [0051]    In the following description, it will be considered that the shaft  2  is fixed and that the casing  6  is turning to illustrate an embodiment of the invention. But it is of course understood that the inverse configuration, that is, with a fixed casing  6  and a turning shaft  2  is also possible. 
         [0052]    The cam  3  is typically formed by a ring adjacent to the axially internal face of the casing  6 , and comprises on its radially internal surface a series of lobes equi-distributed about the axis X-X. Each of the lobes typically has an overall appearance of sinusoidal type. In the embodiment illustrated, the cam  3  is connected in rotationally fixed manner with the casing  6 . 
         [0053]    The cylinder block  4  is placed inside the ring forming the cam  3 . It defines a plurality of cylinders  41  oriented radially relative to the axis X-X and terminating on the external peripheral face of the cylinder block  4  opposite the cam  3 . 
         [0054]    A piston  42  is mounted to slide radially respectively in each of the cylinders  41 . Each piston  42  is supported on the radially internal surface of the cam  3 . 
         [0055]    The cylinder block  4  has a central bore by which it is engaged on the end of the shaft  2  of the hydraulic apparatus  1 . 
         [0056]    The distributor  51  and the distributor lid  52  are adapted to apply in a controlled manner pressurised fluid successively to each of the pistons  42 , more precisely in the internal chamber of the cylinders  41  adjacent to the pistons, such that the successive support of the pistons  42  on the lobes of the cam  3  causes relative rotation of the cylinder block  4  and elements which are connected to it relative to the cam  3  and therefore to the casing  6  or inversely. For this purpose, there is dissymmetry between the number of lobes formed on the cam  3  and the number of connected pistons  42  located in the cylinder block  4 . 
         [0057]    A distribution plan  53  is defined between the distributor  51  and the cylinder block  4 , corresponding to the faces of the distributor  51  and of the cylinder block  4  adapted to be put in contact with each other. 
         [0058]    In the embodiment illustrated, the distributor lid  52  is connected in rotation to the shaft  2 , while the distributor  51  is connected in rotation to the casing  6  by means not illustrated in detail. 
         [0059]    The cylinder block  4  also comprises a spring  43  arranged in each of the cylinders  41  so as to be supported on the radially internal face of each of the pistons  42 , and keep it supported against the cam  3  even in the absence of hydraulic pressure. 
         [0060]    The hydraulic apparatus  1  according to an aspect of the invention also comprises engagement means  7  adapted, during application of a command, to immobilise the cylinder block  4  relative to the shaft  2 . 
         [0061]    In the embodiment illustrated, the engagement means  7  are mounted fixed about an end of the shaft  2  so as to be partially arranged between the cylinder block  4  and the shaft  2 . 
         [0062]    The engagement means  7  also have a support surface  71  adapted to engage the cylinder block  4 . In the embodiment illustrated, the support surface is a radial collar, arranged so as to engage a lateral surface of the cylinder block  4  by friction. 
         [0063]    Therefore, the engagement means  7  can be controlled so as to engage the cylinder block  4 , and via application of a sufficient contact force, immobilise the cylinder block  4  relative to the engagement means  7 , and therefore relative to the shaft  2 . This immobilisation of the cylinder block  4  relative to the shaft  2  carries out displacement of the hydraulic apparatus  1 , that is, its commissioning, as defined previously. 
         [0064]    When the hydraulic apparatus  1  is not put into service, that is, when the engagement means do not connect the cylinder block  4  in rotation, the springs  43  keep the pistons  42  in contact with the cam  3 . 
         [0065]    The cylinder block  4  is therefore synchronised relative to the cam  3 , that is, connected in rotation to the cam  3 . 
         [0066]    When the hydraulic apparatus  1  is put into service, that is, when the engagement means are activated, the cylinder block  4  is connected in rotation relative to the shaft  2 , and therefore turning relative to the cam. 
         [0067]    The springs  43  keep the pistons  42  in contact with the cam  3  and therefore force them to follow the cam  3  in the case of a driving hydraulic apparatus which shows a pump operation. 
         [0068]    The movement of the pistons  42  involves auto-aspiration; and a hydraulic flow will be set up in the conduits of the associated hydraulic circuit. 
         [0069]    In the case of a driven hydraulic apparatus, the application of pressure in the conduits of the associated circuit actuates the engagement means  7  so as to engage the cylinder block  4 , while the cylinders  41  of the pistons  42  are fed, which operates the hydraulic apparatus  1 . 
         [0070]    The springs  43  have an active role when the engagement means are not actuated, in that they keep the pistons  42  in contact with the cam  3 . 
         [0071]    The cylinder block  4  is therefore synchronised relative to the cam  3 , that is, connected in rotation to the cam  3 . 
         [0072]    Several variants can be envisaged for providing this immobilisation of the cylinder block  4  relative to the engagement means  7 . 
         [0073]    According to a first variant, an electro magnet is placed on the engagement means  7  or on the cylinder block  4 , which produces a controllable control and immobilisation element. 
         [0074]    According to another embodiment, a friction track is placed on the engagement means  7  and/or on the cylinder block  4 . An example of such an embodiment to be described hereinbelow in reference to FIG.  6  is the use of brake discs connected respectively to the engagement means  7  and the cylinder block  4 , where its being put into contact will allow progressive immobilisation of the cylinder block  4  relative to the engagement means  7  and therefore relative to the shaft  2 . 
         [0075]    According to another variant, a dog-clutch device is used, that is, a device with teeth and grooves arranged on the engagement means  7  and on the cylinder block  4 , for immobilising it in rotation when these teeth and grooves are engaged. 
         [0076]      FIG. 2  shows a partial sectional view of a variant of a hydraulic apparatus according to an aspect of the invention. 
         [0077]    In this variant, the shaft  2  is connected in rotation to the cam  3 . 
         [0078]    The distributor  51  is mounted fixed in rotation on the shaft  2 , around the shaft  2 , typically by means of grooves arranged at the surface of the shaft  2  cooperating with grooves made in a bore of the distributor  51 . 
         [0079]    The distributor lid  52  is mounted turning around the external periphery of the distributor  51 , and is connected in rotation to the casing  6 . 
         [0080]    The shaft  2  comprises several sliding elements  24  arranged so as to allow rotation of the cylinder block  4  relative to the shaft  2 . These sliding elements are for example pads, or ball, roller or needle bearings. 
         [0081]    An elastic return element such as a spring  22  ensures the support of the distributor  51  against the cylinder block  4 , with sufficient force so as to create sealing between these two elements  51  and  4 . 
         [0082]    Two mobile assemblies in rotation one relative to the other are defined:
       a first assembly defined by the casing  6  and the distributor lid  52 , and   a second assembly defined by the shaft  2 , the cam  3  and the distributor  51 , said first and second assemblies being free in rotation one relative to the other.       
 
         [0085]    The cylinder block  4  is mounted turning relative to these two assemblies. 
         [0086]    The engagement means  7  are here mounted on the distributor lid  52 ; these are a dog-clutch device such as described previously, that is, a device with teeth and grooves arranged on the engagement means  7  cooperating with teeth and grooves arranged on the cylinder block  4  and on the distributor lid  52 . 
         [0087]    The engagement means  7  can be shifted in translation according to the axis X-X so as to be arranged only on one or the other of the cylinder block  4  or of the distributor lid  52  and leave these elements  4  and  52  free in rotation one relative to the other, or so as to be astride between these two elements  4  and  52  and connect them in rotation. 
         [0088]    The engagement means  7  therefore connect the cylinder block  4  in rotation with one of the assemblies defined previously, in this case the first assembly, that is, the assembly not comprising the cam  3 . 
         [0089]    Also, the distributor  51  such as illustrated comprises grooves  511  and  512  for conveying the fluid, which cooperate with grooves  521  and  522  arranged in the distributor lid  52  to form channels. 
         [0090]    Grooves  523  enclosing the resulting channels are arranged in the distributor lid  52 , these grooves forming housings adapted to receive dynamic sealing elements. The grooves  511  and  512  arranged in the distributor  51  are advantageously made so that each has two lateral walls  511   a,    511   b,    512   a  and  512   b,  these lateral walls being made such that for each of the grooves  511  and  512 , the lateral wall which is the closest to the cylinder block  4  has a surface greater than that which is further away. 
         [0091]    In reference to the embodiment presented in  FIG. 2 , the lateral walls  511   a  and  512   a  have a surface greater than the surfaces  511   b  and  512   b.    
         [0092]    Therefore, the pressurised fluid feed will tend to displace the distributor  51  towards the cylinder block  4  under the effect of the force resulting from the pressure applied to these walls  511   a  and  512   a.    
         [0093]    This particular configuration of the grooves  511  and  512  ensures auto-maintenance of the sealing between the cylinder block  4  the distributor  51 . A spline  513  immobilises the distributor in rotation  51  relative to the shaft  2 ; the distributor  51  is rotationally fixed with the cam  3  to the extent where the latter is rotationally fixed with the shaft  2 , for synchronising the feed of the pistons on the cam profile. 
         [0094]      FIG. 3  presents a variant of the embodiment of the  FIG. 2 , in which the engagement means  7  combine a dog-clutch structure with friction surfaces adapted to produce synchronisation of the cylinder block  4  relative to the distributor lid  52  on which the engagement means  7  are mounted prior to the engagement of these two pieces via the dog-clutch. 
         [0095]    The engagement means  7  are constituted by two mobile parts  72  and  73 . 
         [0096]    The first mobile part  72  is mounted sliding on the distributor lid  52 , typically by means of grooves and complementary grooves, and has a friction surface  74  adapted to come into contact with a friction surface  48  of the cylinder block  4 , the contacting of these surfaces  48  and  74  causing friction to create synchronisation of the cylinder block  4  with the distributor lid  52 . 
         [0097]    The second mobile part  73  is mounted sliding on the first mobile part  72 , for example also by means of grooves and complementary grooves, and is adapted to engage with grooves and complementary grooves arranged on the cylinder block  4  once the latter is synchronised with the distributor lid  52 . 
         [0098]    A ball stop  75  releases the second mobile part  73  in translation only once the synchronisation between the distributor lid  52  and the cylinder block  4  is created due to engagement of the friction surfaces  74  and  48 . In fact, engagement of dog-clutch type such as produced by this second mobile part  73  is advantageously created statically, that is, when the two relevant elements are immobilised one relative to the other. 
         [0099]    During application of a command, the engagement means  7  connect the cylinder block  4  and the distributor lid  52  in rotation with the first assembly, that is, the assembly not comprising the cam  3 . 
         [0100]    The engagement means  7  can also comprise a peripheral groove  741  allowing its displacement via a lever or any other system engaging it. 
         [0101]      FIG. 4  shows another variant of a hydraulic apparatus  1  according to the invention, in which the engagement means  7  are a source of pressure feed adapted to apply pressure to an adapted surface of the distributor lid  52 , and apply pressure to the cylinder block  4  and connect the cylinder block  4  with the piece  6  in rotation. 
         [0102]    The source of pressure feed is illustrated here schematically as a hydraulic pump  711 , which can for example be a boost pump, adapted to feed a localised volume  712  with pressure between the casing  6  and the distributor lid  52 , and apply pressure on the latter so as to shift it towards the cylinder block  4 , which is now supported against a friction pad  64  connected to the casing  6 , the pressure applied in the volume  712  being adapted to produce immobilisation of the cylinder block  4  relative to the casing  6  due to friction between the cylinder block  4  and the friction pad  64 . 
         [0103]    The distributor lid  52  and/or the casing  6  advantageously comprise sealing means  713  ensuring sealing of the volume  712  in which the pressure is applied. 
         [0104]    The engagement means  7  connect the cylinder block  4  in rotation with the first assembly during application of a command, that is, the assembly not comprising the cam  3 . 
         [0105]    According to another embodiment of this variant, the friction pad is arranged at the interface between the cylinder block  4  and the distributor lid  52 , while the cylinder block  4  is supported on a sliding pad arranged on the shaft  2 . The application of the pressure inside the volume  712  causes immobilisation of the cylinder block  4  relative to the distributor lid  52 , and therefore relative to the casing  6  due to friction between the cylinder block  4  and the friction pad of the distributor lid  52 . 
         [0106]      FIG. 5  shows another variant of a hydraulic apparatus  1  according to the invention such as presented previously in  FIG. 2 . 
         [0107]    In this variant, the distributor lid  52  is connected in rotation to the cylinder block  4  via an indexation element, for example a screw or a bolt, and mounted free in rotation relative to the casing  6 . 
         [0108]    Two mobile assemblies in rotation one relative to the other are defined: 
         [0109]    a first assembly defined by the casing  6 , and
       a second assembly defined by the shaft  2 , the cam  3  and the distributor  51 , said first and second assemblies being free in rotation one relative to the other.       
 
         [0111]    The cylinder block  4  and the distributor lid  52  are mounted turning relative to these two assemblies. 
         [0112]    The engagement means  7  are here a dog-clutch device such as described previously, that is, a device with teeth and grooves arranged on the engagement means  7  cooperating with teeth and grooves arranged on the casing  6  and on the distributor lid  52 . 
         [0113]    The engagement means  7  connect the cylinder block  4  and the distributor lid  52  in rotation with the first assembly, that is, the assembly not comprising the cam  3  during application of a command. 
         [0114]      FIG. 6  shows a partial sectional view of a variant other than a hydraulic apparatus according to an aspect of the invention. 
         [0115]    In this variant, the shaft  2  is connected in rotation to the cam  3 . 
         [0116]    The distributor  51  is mounted fixed in rotation on the shaft  2 , around the shaft  2 , typically by means of splines arranged at the surface of the shaft  2  cooperating with grooves made in a bore of the distributor  51 . 
         [0117]    The distributor lid  52  is as such mounted turning around the distributor  51 , and is connected in rotation to the casing  6 . 
         [0118]    As in the preceding embodiments, the cylinder block  4  is arranged so that the pistons  42  are in the ring forming the cam  3 , and are kept in contact with the cam  3  by means of the spring  43 . 
         [0119]    The cylinder block  4  is associated here with a disc tray  44  to which it is connected in rotation, this disc tray  44  comprising a plurality of discs  45  uniformly spaced. 
         [0120]    The distributor lid  52  also comprises a plurality of complementary discs  54  which are arranged alternating with the discs  45  of the disc tray  44 . 
         [0121]    These two groups of discs  45  and  54  have a function similar to conventional brake discs; they are configured to alternate between a configuration in which they are not in contact and leave the distributor lid  52  and the cylinder block  4  free in rotation one relative to the other, and a configuration in which they are in contact and immobilise the distributor lid  52  and the cylinder block  4  one relative to the other under the effect of friction. 
         [0122]    A support element  8  is here arranged around the shaft  2 , between the shaft  2  and the distributor  51 . The engagement means are connected in rotation to the shaft  2  and the distributor  51 , typically by means of grooves and splines, and is mobile in translation according to the axis X-X. 
         [0123]    The support element  8  comprises a compression spring  82  positioned so as to produce compression force of the distributor  51  against the cylinder block  4 , supported tightly between the distributor  51  and the cylinder block  4 . 
         [0124]    An elastic washer  83  is positioned supported on the shaft  2  and acts on the support element  8  in opposition to the compression force exerted by the compression spring  82 , producing its return to the disengaged position of the discs  45  and  54 , that is, the position of the system in which the cylinder block  4  is free in rotation relative to the distributor  51 , and therefore relative to the casing  6 . The washer  83  and the compression spring  82  are configured so as to maintain the close support between the distributor  51  and the cylinder block  4 , whether the hydraulic apparatus is fed by pressure or not. 
         [0125]    It is understood that the washer  83  and the compression spring  82  can be replaced by other adapted elastic means. 
         [0126]    When operating, the pressure feed will tend to displace the distributor  51  towards the cylinder block  4  under the effect of the force resulting from the pressure applied to these walls  511   a  and  512   a  as described previously. With the distributor  51  being supported against the cylinder block  4 , the latter will also be displaced in the same direction, that is, in the direction of moving away relative to the distributor lid  52 . This relative displacement of the cylinder block  4  relative to the distributor lid  52  will cause engagement of the discs  45  and  54 , and connect in rotation the cylinder block  4  relative to the assembly comprising the casing  6  and the distributor lid  52 , causing commissioning of the hydraulic apparatus  1 . 
         [0127]    To disengage the hydraulic apparatus, that is, put it in a configuration in which the flow is zero, it suffices therefore to stop supplying it with pressure; when the pressure applied on the walls  511   a  and  512   a  of the distributor  51  reaches a given threshold value, the discs  54  and  45  disengage, releasing the cylinder block  4  in rotation relative to the distributor lid  52  and the casing  6 . 
         [0128]    The engagement means are therefore formed here by the combination of the discs  45  and  54  and of the support element  8 , ensuring engagement or not in rotation of the cylinder block  4  relative to the casing  6  and the distributor lid  52 . 
         [0129]      FIG. 7  presents another variant of a hydraulic apparatus according to an aspect of the invention. 
         [0130]    This variant has a function similar to that presented in  FIG. 6 , in which the discs  45  and  54  have been replaced by two conical half-shells  46  and  47  connected in rotation respectively to the cylinder block  4  and the distributor lid  52 . 
         [0131]    These two conical half-shells  46  and  47  are configured so that when the cylinder block  4  and the distributor lid  52  are shifted in the direction of moving away one relative to the other, the surfaces of these two conical half-shells  46  and  47  engage by friction, and connect the cylinder block  4  and the distributor lid  52  in rotation. 
         [0132]    As for the embodiment presented in  FIG. 6 , the pressure feed will tend to displace the distributor  51  towards the cylinder block  4  under the effect of the force resulting from the pressure applied to these walls  511   a  and  512   a,  as described previously. With the distributor  51  being supported against the cylinder block  4 , the latter will also be displaced in the same direction, that is, in the direction of moving away relative to the distributor lid  52 . This relative displacement of the cylinder block  4  relative to the distributor lid  52  will cause engagement of the two conical half-shells  46  and  47 , and connect the cylinder block  4  in rotation relative to the assembly comprising the casing  6  and the distributor lid  52 , causing commissioning of the hydraulic apparatus  1 . 
         [0133]    Disengaging the hydraulic apparatus therefore means stopping to supply it with pressure; so when the pressure applied to the walls  511   a  and  512   a  of the distributor  51  reaches a given threshold value, the two conical half-shells  46  and  47  disengage, releasing the cylinder block  4  in rotation relative to the distributor lid  52  and the casing  6 . 
         [0134]    As in the previous embodiment, the engagement means are therefore formed here by the combination of the two conical half-shells  46  and  47  and the support element  8 , ensuring engagement or not in rotation of the cylinder block  4  relative to the casing  6  and the distributor lid  52 . 
         [0135]      FIGS. 8 and 9  illustrate two other variants having a function similar to that presented in reference to  FIGS. 6 and 7 . 
         [0136]      FIG. 8  shows a variant of  FIG. 6  in which the support element  8 , the compression spring  82  and the washer  83  have been omitted. 
         [0137]    Return means  9  are arranged so as to tend to displace the cylinder block  4  to disengage it in rotation relative to the distributor lid  52  and the casing  6 , by disengaging the discs  45  of the cylinder block of the complementary discs  54  of the distributor lid  52 , that is, by disengaging the clutch existing between the cylinder block  4  and the casing  6  formed by the discs  45  and  54 . 
         [0138]    In the embodiment illustrated, the return means  9  is a compression spring of washer type, commonly designated by the name Belleville washer. 
         [0139]    Also, the return means  9  ensure holding of the cylinder block  4  in contact with the distributor  51 , and therefore produce sealing between these two pieces to perform the operation of the hydraulic apparatus  1  by supplying the pistons  42  of the cylinder block  4 . 
         [0140]    As already shown in reference to the preceding figures, the distributor  51  such as illustrated comprises grooves  511  and  512  for conveying fluid, which cooperate with grooves  521  and  522  arranged in the distributor lid  52  to form channels. 
         [0141]    Grooves  523  enclosing the channels formed in this way are arranged in the distributor lid  52 , these grooves forming housings adapted to receive dynamic sealing elements. 
         [0142]    The grooves  511  and  512  arranged in the distributor  51  are advantageously made so as to each present two lateral walls  511   a ,  511   b,    512   a  and  512   b,  these lateral walls being made such that for each of the grooves  511  and  512 , the lateral wall which is the closest to the cylinder block  4  has a surface greater than that which is further away from it. 
         [0143]    In reference to the embodiment shown in  FIG. 8 , the lateral walls  511   a  and  512   a  have a surface greater than the surfaces  511   b  and  512   b.  The diameter of the distributor  51  increases as it approaches its contact surface with the cylinder block  4 . The distributor  51  can be qualified as stepped distributor. 
         [0144]    Therefore, the pressurised fluid feed will tend to shift the distributor  51  towards the cylinder block  4  under the effect of the force resulting from the pressure applied to these walls  511   a  and  512   a.    
         [0145]    This particular configuration of the grooves  511  and  512  ensures auto-maintenance of the sealing between the cylinder block  4  and the distributor  51 . The distributor  51  is connected in rotation to the shaft  2 ; the distributor  51  is rotationally fixed with the cam  3  to the extent where the latter is rotationally fixed with the shaft  2  to synchronise the feed of the pistons on the cam profile. 
         [0146]    A similar function can be achieved by arranging a thrust chamber, in the distributor  51 , arranged so that its pressure feed causes displacement of the distributor  51  towards the cylinder block  4 . 
         [0147]    During fluid feed, displacement of the distributor  51  towards the cylinder block  4  under the effect of the force resulting from the pressure applied to the walls  511   a  and  512   a  causes engagement of the discs  45  and  54 , connecting the cylinder block  4  to the casing  6  in rotation and causing displacement of the hydraulic apparatus  1 . 
         [0148]    The engagement means  7  are therefore formed here by the combination of the discs  45  and  54  forming a clutch between the cylinder block  4  by means of the disc tray  44  and the casing  6  by means of the distributor lid  52 ),
       the return means  9 , and   the configuration of the distributor  51  causing displacement of the distributor  51  towards the cylinder block  4  during the application of pressure.       
 
         [0151]      FIG. 9  shows another embodiment in which the cylinder block  4  is adapted to be connected in rotation to the shaft  2  during application of a command; the cylinder block  4  comprises a plurality of discs  45  adapted to cooperate with a plurality of other discs  25  connected to the shaft  2 . 
         [0152]    The function is similar to that of the embodiment shown in  FIG. 8 . 
         [0153]    In the absence of pressure feed by the distributor  51 , the return means  9 , illustrated here as a Belleville washer, keep the cylinder block  4  supported against the distributor  51 , and disengages the discs  45  and  25  forming the clutch between the cylinder block  4  and the shaft  2 . 
         [0154]    During the fluid feed, the stepped structure of the distributor  51  causes displacement of the distributor  51  towards the cylinder block  4 , and engagement of the discs  45  and  25 , connecting the cylinder block  4  in rotation to the shaft  2  and causing displacement of the hydraulic apparatus  1 . 
         [0155]    Such a fixed cam structure  3  is advantageous in terms of wear; when the hydraulic apparatus  1  is not operating, the cam  3 , the cylinder block  4 , the distributor  51  and the distributor lid  52  are immobile, preventing especially wear on the sealing elements between these pieces. 
         [0156]    Several other types of return means  9  can be utilised. Particular examples are the use of a friction pad with one or more compression springs, a pulling element associated with a compression spring mounted on the distributor  5 . The return means  9  can also be mounted supported on another element of the hydraulic apparatus  1 , as it holds the cylinder block  4  against the distributor  51 . 
         [0157]      FIG. 10  shows another embodiment in which the cylinder block  4  is adapted to be connected in rotation to the shaft  2  during the application of a command. 
         [0158]    In this embodiment, the link in rotation between the cylinder block  4  and the shaft  2  is created by a synchronisation element in rotation and by dentures. 
         [0159]    The synchronisation element such as presented is a synchronisation piston  91 , connected in rotation to the shaft  2  for example by means of splines. The synchronisation piston  91  is mounted sliding according to the axis X-X, and is coupled to a washer  92  arranged supported on the shaft  2  performing a spring function between the shaft  2  and the synchronisation piston  91 . 
         [0160]    The cylinder block  4  and the shaft  2  each have splines, respectively  94  and  95 , arranged so as to engage when the cylinder block is displaced according to the axis X-X so as to stop on the shaft  2 . 
         [0161]    Displacement of the cylinder block  4  according to the axis X-X is typically done under the effect of the fluid feed of the distributor  51  which causes a compression force of the distributor  51  on the cylinder block  4 . 
         [0162]    The embodiment illustrated also comprises an actuating piston  96  adapted to create compression force on the cylinder block  4  and ensure displacement conjointly with the effect of the fluid feed of the distributor  51 . 
         [0163]    The cylinder block  4  comprises one or more housings in the form of angular sectors arranged opposite the synchronisation piston  91 . 
         [0164]    Contact between the piston  91  and the cylinder block is made by friction such that contact between the cylinder block  4  and the synchronisation piston  91  causes engagement in rotation of the cylinder block  4  relative to the synchronisation piston  91  and therefore relative to the shaft  2 . 
         [0165]    The form of angular sectors of the housings enables engagement in rotation of the cylinder block  4  relative to the synchronisation piston  91  and enables angular lateral play between these two elements. 
         [0166]    In this way, two stop positions of the synchronisation piston  91  on the cylinder block  4  against two different walls of the housings are defined, as a function of the directions of rotation of the turning part of the hydraulic apparatus  1 . 
         [0167]    These two stop positions align the internal conduits of the cylinder block  4  and of the distributor  51  in their appropriate configuration according to the direction of operation of the hydraulic apparatus  1 , that is, the directions of rotation of its turning part. 
         [0168]    Once this first engagement in rotation is completed, the cylinder block  4  continues its displacement according to the axis X-X until the splines  94  and  95  of the cylinder block  4  and of the shaft  2  respectively engage, forming dog-clutch engagement between the cylinder block  4  and the shaft  2 . The washer  92  enables translation of the synchronisation piston  91  with the cylinder block  4  according to the axis X-X. 
         [0169]    Displacement is therefore done twice, during displacement of the cylinder block  4  according to the axis X-X;
       first, floating engagement of the cylinder block  4  is achieved so as to position its internal conduits in the planned configuration and to create speed synchronisation which prevents the noise of engagement of the teeth of the dog-clutch links, and then   second, this floating engagement of the cylinder block  4  is locked by engagement of dog-clutch type with the shaft  2 .       
 
         [0172]    Displacement is adapted to the direction of operation of the hydraulic apparatus  1  due to floating engagement of the cylinder block  4  which enables positioning of the internal conduits adapted to the direction of rotation of the hydraulic apparatus  1 . 
         [0173]      FIGS. 11 and 12  illustrate a structure showing an operation such as described in reference to  FIG. 9 , integrated in an assembly comprising a differential connected to two half-shafts driving wheels. 
         [0174]    In this embodiment, the casing  6  is fixed, and the hydraulic apparatus  1  is associated with a differential  23  arranged in the casing  6 . 
         [0175]    The cam  3 , the distributor  51  and the distributor lid  52  are here connected in rotation to the casing  6 , the shaft  2  being a turning shaft. 
         [0176]    The differential  23  mounted on the shaft  2  of the hydraulic apparatus  1 , and is connected to two half-shafts  2   a  and  2   b  which can turn at different speeds. The body (commonly called a cage) of the differential  23  is connected in rotation to the shaft  2 . 
         [0177]    This variant applies for example to providing assistance on an axle of a vehicle, the two half-shafts  2   a  and  2   b  each being connected to a wheel, respectively  2   c  and  2   d.  It suffices therefore to start a pump to feed the hydraulic apparatus  1  so that the latter engages and operates as a motor to provide hydraulic assistance on the shaft  2 , in this case a vehicle axle. 
         [0178]    These different variants comprising a stepped distributor  51  and return means  8  or  9  ensuring close contact between the cylinder block  4  and the distributor  51  form a compact and robust system by limiting the use of dynamic sealing means. 
         [0179]    Engagement in rotation of the cylinder block  4  occurs progressively; the higher the pressure delivered by the distributor  51 , the greater the friction force between the discs  45  of the clutch of the cylinder block  4  and the complementary discs  54  or  25 . 
         [0180]    More generally, given a hydraulic apparatus  1  according to an aspect of the invention two mobile assemblies in rotation one relative to the other are defined:
       a first assembly defined by the casing  6 , and   a second assembly defined by the shaft  2 , said first and second assemblies being free in rotation one relative to the other.       
 
         [0183]    The cylinder block  4  is mounted free in rotation relative to said first and second assemblies. 
         [0184]    The cam  3  is connected in rotation to one or the other of these assemblies; for example to the first assembly in the embodiment illustrated in  FIG. 1 , and to the second assembly in the embodiment illustrated in  FIG. 2 . 
         [0185]    The engagement means  7  provide immobilisation of the cylinder block  4  relative to the other of said first or second assemblies during application of a command, for example the pressure feed of the hydraulic apparatus  1  or the engagement of an actuator such that the cylinder block  4  and the cam  3  are each connected in rotation to a assembly different, creating displacement of the hydraulic apparatus. 
         [0186]    The engagement means  7  can act directly on the cylinder block  4  such as for example in the embodiment illustrated in  FIG. 1  where the engagement means come into direct contact with the cylinder block  4  to immobilise it in rotation, or indirectly via action exerted on another element, such as for example the distributor lid  52  in the embodiment illustrated in  FIG. 2  or the distributor  51  in the embodiment shown in  FIG. 1 . 
         [0187]    Immobilisation of the cylinder block  4  relative to one or the other of the assemblies is advantageously done progressively, for example by progressive application of contact force to produce and gradual progressive commissioning. 
         [0188]    The engagement means  7  can especially be actuated by a boost pump of a hydraulic circuit associated with the hydraulic apparatus. 
         [0189]    Also, the springs  43  applying the pistons  42  to the cam  3  imply that the hydraulic apparatus  1  is self-suctioning as already specified previously in reference to  FIG. 1 , that is, this hydraulic apparatus  1  can be used as a pump without needing to be coupled to a boost pump to avoid the risk of cavitation. 
         [0190]    In fact, since the cylinder block turns  4  relative to the cam  3 , aspiration occurs due to contact between the pistons  42  and the cam  3 , which cancels out the risk of cavitation. 
         [0191]    The invention provides commissioning of the hydraulic apparatus  1  at a non-zero speed and zero flow. The move is made from a disengaged configuration in which the flow is zero, to a configuration in which the relative rotation of both assemblies causes fluid flow. 
         [0192]    Contrary to hydraulic apparatuses according to the prior art, it is not necessary to carry out transitory phases of progressive pressure increase to perform the extension of pistons and pressurising of the closed loop of the associated hydraulic circuit or of the casings. 
         [0193]    In fact, commissioning can be done progressively to the point of immobilisation of the cylinder block  4  relative to one or the other of the assemblies, for example under the effect of friction, this immobilisation which in some variants can be doubled by means of a dog-clutch or any other adapted locking means. 
         [0194]    The boost pump is optional and minimised, if needed, its function being limited to compensation of any leaks. 
         [0195]    The invention has particular application on the hydraulic assistance circuits of vehicles, for example trucks, agricultural vehicles or construction equipment, or even on service vehicles, or even automobiles. In the case of a hydraulic apparatus  1  with revolving casing  6  and a shaft  2  fixed such as described previously, the hydraulic apparatus  1  is typically arranged so that its fixed part forms a wheel spindle, the revolving casing being connected to the wheel. 
         [0196]    Commissioning of the hydraulic apparatus  1  typically shifts from transmission of traction or propulsion type to transmission of four-wheel drive type. For example, in the case of a vehicle with four wheels having a principal motor driving its front wheels, the rear wheels can advantageously be equipped with such hydraulic apparatuses  1  and shift to four-wheel drive transmission during commissioning of these hydraulic apparatuses. 
         [0197]      FIG. 13  shows another variant of a use of a hydraulic apparatus according to an aspect of the invention. 
         [0198]    The discs  45  and  25  are arranged here in a sealed chamber  56  with pressure controlled by means of a control chamber  57 . 
         [0199]    The control chamber  57  is connected to the conduits of the distributor  51 . The sealed chamber  56  is filled with oil. 
         [0200]    The control chamber  57  is configured to control the pressure inside the sealed chamber  56 , modifying the viscosity of the oil in the sealed chamber  56 . The rise in viscosity of the oil in the sealed chamber  56  produces engagement of the discs  45  and  25  by visco-coupling. In the absence of the application of pressure by the control chamber  57 , the oil inside the sealed chamber  56  has low viscosity and therefore does not create engagement of the discs  45  and  25 . 
         [0201]    A free wheel control, for example a washer spring, is arranged to release pressure inside the sealed chamber  56  and shift the hydraulic apparatus  1  into free wheel configuration. 
         [0202]    This engagement by visco-coupling has several advantages. It performs a clutch function without friction, therefore reducing wear relative to a conventional clutch of dog-clutch type. The risk of noise during the clutch is also reduced. 
         [0203]    According to a particular application, there can be chain-mounting of several hydraulic apparatuses  1  according to the invention on a vehicle. 
         [0204]    A first hydraulic apparatus is mounted on a wheel axle, and a second hydraulic apparatus on a driving axle connected to a motor such as a thermal motor of the vehicle. When the aim is to activate hydraulic assistance, the two hydraulic apparatuses need to be put into service, the second hydraulic apparatus having a pump operation so as to feed the first hydraulic apparatus which operates as a motor and drives the associated wheel axle. 
         [0205]      FIG. 14  shows an embodiment of such mounting. 
         [0206]    This mounting comprises a hydraulic circuit C, comprising two hydraulic apparatuses  110  and  120  operating respectively as pump and motor. 
         [0207]    These two hydraulic apparatuses  110  and  120  are each coupled to a clutch,  112  and  122  respectively. 
         [0208]    The clutch  112  executes coupling of the pump  110  to a bloc motor M, typically a thermal motor which can be linked to a gearbox. 
         [0209]    The clutch  122  executes coupling of the hydraulic motor  120  to a shaft  130 , for example a vehicle axle. 
         [0210]    In the embodiment illustrated, the clutches  112  and  122  are controlled by hydraulic controls respectively  114  and  124 , adapted to initiate the associated clutch when control pressure is applied, and disengage it when the pressure applied is less than a given threshold value. 
         [0211]    These hydraulic controls  114  and  124  are each doubled by a primary control, respectively  115  and  125 , the clutches  112  and  122  which can each be triggered either by a single primary or hydraulic control, or by the combined effect of both controls. In the embodiment illustrated, the primary controls  115  and  125  are electric controls, given that other variants are possible, especially by means of pneumatic, mechanical or even hydraulic controls. 
         [0212]    The hydraulic circuit C has a line connecting the admission of the hydraulic pump  110  to the backflow of the hydraulic motor  120 , and a line connecting the backflow of the hydraulic pump  110  to the admission of the hydraulic motor  120 . To use an operation example, these lines will be designated respectively by BP and HP corresponding to the low-pressure line and the high-pressure line of the circuit C. 
         [0213]    The circuit C such as illustrated comprises two shuttle valves  116  and  126 , each connecting the line HP to the line BP, and delivering pressure to the hydraulic controls  114  and  124  respectively. 
         [0214]    These shuttle valves  116  and  126  are configured so as to take off the higher pressure of the two lines HP and BP, in this case the high-pressure line connected to the backflow of the hydraulic pump  110 , and convey it to their respective hydraulic controls  114  and  124 . 
         [0215]    The hydraulic circuit C also comprises overpressure valves  142  and  144  adapted for evacuation of fluid in the circuit C, these overpressure valves being connected respectively to the line BP and to the line HP, and being configured so as to be passing when the pressure in the line to which they are connected exceeds a threshold value. 
         [0216]    These two overpressure valves are connected to a distributor  146  and create an interrupter function to enable or not emptying of the lines HP and/or BP. 
         [0217]    Also, the hydraulic apparatuses  110  and  120  have their respective casings connected together by the same drain line  117 , in turn connected to the lines HP and BP respectively by check valves  118  and  119 . 
         [0218]    This common drain line  117  creates equilibrium between the pressure in the casings of the hydraulic apparatuses  110  and  120  with the high-pressure HP line to execute disengagement of the hydraulic apparatus. It also executes aspiration of leaks from the hydraulic apparatuses  110  and  120  towards the hydraulic circuit C; the check valves  118  and  119  perform a resupply function of the hydraulic circuit C, and perform a self-suctioning system not needing booster means. 
         [0219]    The drain line  117  typically also comprises a breather tube  127 , or evacuation system for oil vapour to the exterior. 
         [0220]    In a mounting also comprising a gearbox, the drain line  117  is advantageously also connected to the casing of this gearbox, which provides a system having a drain line typically fitted with a single breather tube for the gearbox and the hydraulic apparatuses. The drain creates a reserve for the oil feed to the two hydraulic apparatuses. 
         [0221]    In operation, there is an initial state where the two clutches  112  and  122  are disengaged; the hydraulic pump  110  and the hydraulic motor  120  are therefore disengaged and the flow into the hydraulic circuit is zero. 
         [0222]    To start up hydraulic assistance on the shaft  130 , the primary control  115  of the clutch  112  is actuated so as to couple the hydraulic pump  110  to the motor M, and set up a flow in the circuit C via the hydraulic pump  110 , which defines the high-pressure lines HP and low-pressure lines BP as a function of its direction of operation. 
         [0223]    Next, the primary control  125  is actuated so as to couple the hydraulic motor  120  with the shaft  130 . In the case of a hydraulic motor  120  with a stepped distributor as shown previously especially in reference to  FIGS. 6 ,  7 ,  8 , and  9 , the feed from the hydraulic motor  120  will cause engagement in rotation of the cylinder block  4  relative to one or the other of the assemblies such as defined previously, and will start the hydraulic motor  120  displacement. 
         [0224]    Also, setting pressure in the lines HP and BP will cause application of pressure via the controls  114  and  124  which take off pressure in the line HP as described previously, ensuring holding by clutches  112  and  122  in the clutch position. 
         [0225]    The system now engaged therefore drives the shaft  130  in rotation and provides hydraulic assistance, ensuring its operating maintenance. 
         [0226]    Disengagement of the assistance can be done either by ceasing to actuate the primary controls  115  and  125  or by applying inverse primary controls, going in the direction of disengagement of a clutch or the clutches  112  and/or  122 , and/or via the distributor  146  to empty the lines HP and BP by sending the fluid to the casing of the motor  120  and/or the pump  110 . The torque of the clutches  112  and  122  can also be decreased so as to lower it below the threshold value for driving the associated hydraulic apparatus. 
         [0227]    The mounting associated with the hydraulic apparatuses  1  according to an aspect of the invention is therefore simple to achieve, and needs no multiple switching elements to set up several pressure levels for commissioning the hydraulic apparatuses  1 . 
         [0228]    According to a particular embodiment, the hydraulic pump  110  is mounted on a primary vehicle axle, connected to one or more motorised wheels, while the hydraulic motor  120  is mounted on a secondary vehicle axle. The primary axle is driven by a primary motor, typically thermal. 
         [0229]    The hydraulic pump  110  therefore turns at the same speed as the motorised wheels of the primary axle of the vehicle, and the hydraulic motor  120  turns at the same speed as the secondary axle of the vehicle. 
         [0230]    When the hydraulic pump  110  and the hydraulic motor  120  are disengaged, only the primary motor ensures driving of the vehicle. 
         [0231]    When the hydraulic pump  110  and the hydraulic motor  120  are put into service, the hydraulic pump  110  takes off the torque of the primary axle, and transmits it to the secondary axle via the hydraulic motor  120  which ensures its driving. 
         [0232]    This provides integral transmission, with a 1:1 ratio between the rotation speeds of the hydraulic pump  110 , the hydraulic motor  120  and the wheels of the vehicle, close to losses and leaks of the hydraulic circuit. 
         [0233]    This type of mounting of hydraulic apparatuses on axles especially does without reduction ratios inherent in former mountings of hydraulic apparatuses on power take-offs of thermal motors or on speed shafts, and proposes a simplified structure for executing hydraulic assistance on a vehicle. 
         [0234]    As a variant, the hydraulic pump  110  can supply two hydraulic motors, each reconnected to a half-axis of an axle driving a wheel. 
         [0235]      FIG. 15  shows a variant of the previous mounting. 
         [0236]    This mounting also comprises two hydraulic apparatuses  1  such as shown previously, one  110  operating as a pump and the other  120  operating as a motor. This diagram shows the drain lines of the casings of these hydraulic apparatuses  110  and  120 , connected each to the tank R at ambient pressure. 
         [0237]    As earlier, this mounting carries out commissioning of both hydraulic apparatuses to provide hydraulic assistance on the shaft  130  coupled to the motor  120  via the clutch  122 . 
         [0238]    The overpressure valves  142  and  144  are arranged so as to discharge any pressure excess from the lines HP and BP into the circuit C or into the casings of the hydraulic apparatuses  110  and  120 . 
         [0239]    The distributor  146  here performs an emptying function of the lines HP and BP, by discharging the higher pressure of these two lines into the tank R, this higher pressure of the two lines HP and BP being taken off by means of a shuffle valve  136 . 
         [0240]    This mounting comprises a boost pump  150  which takes off fluid in the tank R, and feeds the circuit. It is linked to an overpressure valve  152  and discharges any pressure excess to the tank R. 
         [0241]    This boost pump  150  is also linked to an engagement distributor  156  for connecting the hydraulic controls  114  and  124  respectively controlling the clutches  112  and  122  either at the boost pump  150  or at a tank R at ambient pressure. Therefore, activation of the engagement distributor  156  executes engagement of both clutches  112  and  122 , and therefore the commissioning of both hydraulic apparatuses  110  and  120 . 
         [0242]      FIG. 16  shows another variant in which the hydraulic apparatus is fed by accumulators, which creates a power-recovery circuit. 
         [0243]    This diagram again shows a hydraulic apparatus  120  according to the invention, connected to a shaft  130 , said hydraulic apparatus here operating as a motor. The commissioning of this motor  120  is controlled by means of the clutch  122 , which is in turn controlled by the hydraulic control  124 . 
         [0244]    A shuffle valve  126  taps off the higher pressure of the two hydraulic branches connected to the motor  120 , and applies it to the control  124 . 
         [0245]    The pressure feed is achieved by means of two accumulators  171  and  172 . To illustrate an operating example, here the accumulator  171  is a high-pressure accumulator and the accumulator  172  is a low-pressure accumulator. 
         [0246]    A check valve  173  is arranged between these two accumulators to prevent the pressure being over the level of the low-pressure accumulator  172  relative to the pressure at the level of the high-pressure accumulator  171 . 
         [0247]    Two overpressure valves  174  and  175  discharge any excess pressure into a tank R at ambient pressure. 
         [0248]    The circuit such as shown also comprises a distributor  160  having five orifices:
       a first orifice  161  connected to the high-pressure accumulator  171 ,   a second orifice  162  connected to the tank R,   a third orifice  163  connected to the low-pressure accumulator  172 ,   a fourth orifice  164  and a fifth orifice  165  reconnected to the admission and the backflow of the motor  120 .       
 
         [0253]    This distributor is controlled by control means  166  comprising especially hydraulic, electric, pneumatic or electric controls and return means such as springs ensuring return to the equilibrium position of the distributor  160  in the absence of a control command. 
         [0254]    This distributor  160  can alternate between three configurations to be described hereinbelow. 
         [0255]    In a first configuration, which is the configuration illustrated in  FIG. 16 , the fourth and fifth orifices  164  and  165  are connected to the second orifice  162 , while the first and third orifices  161  and  163  are closed. This configuration therefore connects the admission and the backflow of the motor  120  to the tank R; there is no pressure to trigger the clutch  122 , and the motor  120  is therefore disengaged. 
         [0256]    In a second configuration, the first orifice  161  is connected to the fifth orifice  165 , the third orifice  163  is connected to the fourth orifice  164 , and the second orifice  162  is closed. 
         [0257]    This configuration therefore connects the motor  120  to the accumulators  171  and  172 , which therefore defines a high-pressure line and a low-pressure line, in this case respectively the line connected to the high-pressure accumulator  171  and the line connected to the low-pressure accumulator  172  triggers the clutch  122  and commissions the motor  120  so that it drives the shaft  130  in rotation. 
         [0258]    The third configuration is similar to the second configuration, but reverses the links of the first and third orifices  161  and  163  with the fourth and fifth orifices  164  and  165  reversing the directions of rotation of the motor  120 . 
         [0259]    Whether the first or third configuration, the motor  120  is fed with pressure by the high-pressure accumulator, and then discharges into the low-pressure accumulator which performs a power-recovery function. 
         [0260]    As for the circuits shown in the preceding figures, this circuit exploits the advantageous structure of the hydraulic apparatus according to the invention, in this case the motor  120 , performing simple commissioning, without wear or noise, which produces a simplified circuit which is therefore reduced in terms of bulk, cost and mass. 
         [0261]    In the particular case of application on a vehicle comprising
       a primary motor adapted to selectively drive a hydraulic pump with variable displacement equipped with a clutch or not, and   N driven axles with N a natural whole number greater than or equal to 1, each of said axles being fitted with a hydraulic apparatus such as described previously connected together so that the hydraulic apparatuses of the driven axles are mounted in parallel relative to the hydraulic apparatus of the driving axle.       
 
         [0264]    The vehicle is advantageously also equipped with a calculator which receives information characteristic of the speed of the driven motors connected to the wheels and information characteristic of the drive speed of the pump in rotation. 
         [0265]    The calculator runs displacement adjustment of the hydraulic pump as soon as it is started such that displacement of the pump multiplied by its drive speed in rotation is equal to displacement of all the N motors multiplied by the drive speed of the driven motors connected to the wheels plus Epsilon. 
         [0266]    The value Epsilon represents sliding which produces a traction effect. Epsilon can be zero or very low at the moment of clutching. 
         [0267]    The predicted displacement of the motors can take into account a rate of leak or losses of the circuit. 
         [0268]    When starting the pump, the clutch of the hydraulic apparatus or hydraulic apparatuses such as described previously and operating as a motor can be done concomitantly, or after a brief delay. When the motors are fed, but not clutched, they can turn without torque at a speed approaching that of the shaft on which they are placed, which is an advantageous situation favouring a clutch without shocks, particularly if the link system of the cylinder block to the casing is of dog-clutch type. At this moment, even though the pump flows and the motors turns, there is very little torque being supplied, and the difference in pressure between the hydraulic feed and return lines is very slight. 
         [0269]    When the pressure is set in the closed loop of the hydraulic circuit, creating a low-pressure return line and a high-pressure feed line, the calculator controls displacement of the pump as a function of the variations in pressure in the hydraulic circuit, for example by means of accurate pressure readings at different points of the circuit. 
         [0270]    Once the hydraulic motor or the hydraulic motors is clutched, a difference in pressure in the hydraulic lines proportional to the torque provided by the motors will be created, and control of the pump as a function of pressure becomes possible. The control displacement of the hydraulic pump can be varied, either as torque or speed, to provided traction or containment of the driven axles as well known to those skilled in the art. This control from pressure information can be activated from the moment when a difference in pressure greater than a predetermined threshold preferred by those skilled in the art appears between the hydraulic feed and return lines. It can also be activated after a delay following clutching or any other means for detecting that the clutch of the motors is effective. 
         [0271]    This particular embodiment enables more immediate driving by the driven motor, causing fewer skidding, wear or noise forces, and preventing shocks or pressure peaks. 
         [0272]    Coupled with the progressive effect of the clutch without shock of the hydraulic apparatus such as presented, and particularly if it is progressive, the result is particularly supple and silent engagement of the hydraulic assistance. IN THE CLAIMS