GB2286858A

GB2286858A - A control system for a hydrostatic work machine

Info

Publication number: GB2286858A
Application number: GB9503157A
Authority: GB
Inventors: Helmut Bosselmann
Original assignee: G Duesterloh GmbH
Current assignee: G Duesterloh GmbH
Priority date: 1994-02-23
Filing date: 1995-02-17
Publication date: 1995-08-30
Anticipated expiration: 2015-02-17
Also published as: GB9503157D0; ITRM950102A0; DE4405802A1; GB2286858B; DE4405802C2; IT1278024B1; ITRM950102A1

Description

1 - -.

DESCRIPTION OF INVENTION

2286858 Title: "A control system for a hydrostatic work machineU THIS INVENTION relates to a control system f or a reversible hydrostatic work machine having work pistons, there being provided between a plane control surface having ports and a companion surface which is parallel to the control surface a control member which can be displaced by way of the machine shaft transversely to the rotational axis, has ducts f or the work f luid and isolates such ducts f rom an annular chamber on the peripheral side adapted to be supplied with the work fluid, the end faces of the control member being brought by means of a resilient biasing pressure, boosted by the pressure of the work fluid, into sliding contact with the control surf ace and companion surface. Such a control system is herein referred to as being "of the kind specified".

DE-PS1801541 discloses a control system of the kind specified. It is of use in radial piston and axial piston machines. The characteristic feature of this control system is the rigid connection between the machine shaf t and an eccentric disc which is f orced into a permanent rotating sliding contact with the control surf ace by the pressure of the work f luid and by a resilient biasing pressure. on the peripheral side of the eccentric disc there are disposed with the interposition of a rolling bearing an inner ring which also slides on the control surf ace and an outer ring which extends around the inner ring and which slides on a companion surface in the cover of the machine casing. Engagement of the inner ring with the control surface and of the outer ring with a companion surface is produced by the pressure of the work fluid and IF 1 2 by cup springs which are received between the inner ring Z" and the outer ring and act as expanding means.

The control member formed by the outer ring and inner ring therefore provides pressure-tight radial isolation of the chamber which is defined axially by the control surface and the companion surface. Consequently, chambers isolated from one another so as to be pressuretight are formed radially peripherally of the outer ring and radially inside the inner ring and are connected to or isolated from the ports in the control surface during the radial translational movement, which depends upon the angle of machine shaft rotation.

Because of the inclusion of the rolling bearing, the inner and outer rings perform substantially only radial translational movements as the machine shaft rotates, but the eccentric disc engages with the control surface in a continuous rotational contact on which no translational movement is superimposed. Consequently, the sliding contact surfaces of the eccentric disc and control surface experience substantial friction and therefore heavy wear. Associated with this is the evolution of considerable heat of friction, the removal of which is bound to be unsatisfactory since the sealing gap is re-wetted unsatisfactorily due to the pure rotational contact between the eccentric disc and the control surface.

The use of the known control system in association with work fluids which are poor lubricants is therefore possible, if at all, only with limitations.

It is therefore the object of the invention to provide an improved control system of the kind specified, t 1 3 which makes possible the use of work fluids which are poor lubricants.

According to the invention there is provided a control system for a reversible hydrostatic work machine having work pistons, there being provided between a plane control surface having ports and a companion surface which is parallel to the control surface a control member which can be displaced by way of the machine shaft transversely to the rotational axis, has ducts for the work fluid and isolates such ducts from an annular chamber on the peripheral side adapted to be supplied with the work fluid, the end faces of the control member being brought by means of a resilient biasing pressure, boosted by the pressure of the work fluid, into sliding contact with the control surface and companion surface, characterised in that an eccentric having a leakage fluid duct is provided on the machine shaft, which eccentric engages in the control member and is coupled, in the central pressureless leakage zone, with the control member for rotation relative thereto.

The essence of the invention is the feature that all the control member surfaces which are in sliding sealing contact with the control surface and companion surface move solely in radial translation. To this end, the eccentric which engages in the control member and is so coupled therewith as to be rotatable relatively thereto is provided on the machine shaft itself. Consequently, as the machine shaft rotates the eccentric can rotate in the control member, this rotatable mounting being disposed in the central pressureless leakage zone of the control system. To produce this pressureless state the two end faces of the rotational mounting are interconnected by way of the leakage fluid duct in the eccentric.

v 4 The translational movement of the control member per revolution of the machine shaft corresponds to twice the eccentric throw. As a result of this radial sliding movement of the control member, therefore, the sealing gaps at the control surface and companion surface are continuously re-wetted by work fluid. Consequently, not only is a fresh film of lubricant produced continuously in the sealing gaps but also the re-wetting provides additional cooling. Friction and therefore wear are reduced.

The sliding-contact zones of the control surface and companion surface of the control member are pressed together by the work fluid boosted by the resilient pressure so that only the biassing pressures required to seal the sealing gaps are applied to the surfaces sliding on one another. Consequently, leakage and friction are reduced and pressure and temperature effects are compensated for automatically. Due to the radial translational movement coupled with the freedom of the control member simultaneously to rotate, in dependence upon frictional resistance factors, around the axial displacement as a deviation, the system is relatively insensitive to soiling.

Those surfaces of the control member, the control surface and the companion surface which slide on one another in translational movements can be made of hardened materials. Also, wear-reducing coatings, more particularly ceramic coatings, are feasible. Another possibility is for the control member itself to be made of ceramic.

Preferably, the eccentric is coupled with the control member by way of a plain bearing. one possibility is for the cylindrical eccentric to slide directly in a 1 1 bore in the control member, such bore being adapted to the eccentric. Another possibility is for the eccentric to engage slidingly in a bearing bush disposed in the control member.

Instead of a plain bearing it may be advantageous to couple the eccentric with the control member by a rolling bearing. To minimise dimensions in this case, the rolling bearing can be more particularly a needle bearing.

In one embodiment of the invention, the control member is a one-piece control ring and the companion surface is disposed on a pressure plate, the same being received between the control ring and the machine casing and being biased by a resilient biasing pressure towards the control ring. The hydrostatic pressing of the control member on to the control surface and companion surface is produced by the work fluid and, for example, by a helical compression spring which is disposed coaxially of the machine shaft in the cover of the machine casing and acts on the back of the pressure plate. Conveniently, the diameter thereof corresponds to the diameter of the annular chamber on the peripheral side of the control member. Also, the pressure plate can have flow and return ports for the work fluid whereby, depending on the direction of rotation of the machine shaft, the work fluid can either be directed through the annular chamber or flow ducts to the piston chambers of the work pistons and the work fluid returning therefrom can be removed through the ducts in the control member or through the annular chamber.

In another embodiment of the invention, the control member is received in an inner ring which is coupled with the eccentric for displacement relatively thereto, the inner ring bearing on the control surface, and in an outer 6 ring whi ch extends stepwise around the inner ring and bears on the companion surface. the inner and outer rings being so engaged with one another by a resilient expanding force as to be displaceable relatively to one another, the ducts being disposed in both of the rings. In this case the control member is in two parts. The inner ring is so coupled with the eccentric as to be displaceable relatively thereto and is in sliding contact with the control surface. The outer ring which extends stepwise around the inner ring is engaged therewith by an expanding force and is pressed thereby as well as by the work f luid pressure on to the companion surface. The work f luid ducts in the control member are present in both the inner ring and outer ring. The inner and outer ring zones adjacent the flow ducts are sealed. The expanding force is provided, for example, by a corrugated spring disposed near the eccentric between the inner and outer rings and by another corrugated spring received between the inner and outer rings on the edge adjacent the periphery.

In yet another embodiment of the invention the control member is in three parts. In this embodiment, the control member is received in an inner ring coupled with the eccentric f or movement relatively thereto, the inner ring bearing on the control surface, and in an outer ring which is in sliding engagement with the companion surface and extends around the inner ring, an expanding chamber having a resilient expanding force is disposed between the inner ring and the outer ring, a pressure ring is disposed between the inner ring and the companion surface with the interposition of a resilient expanding f orce, the ducts are disposed in the inner ring and the expanding chamber is connected by way of a changeover valve in the inner ring to the chamber on the peripheral side and to the ports. Thus, the inner ring is coupled with the eccentric for relative 1 7 rotation and the outer ring extends around the inner ring. The inner ring bears only on the control surf ace, the ducts being provided solely in the inner ring. On the side facing the companion surface the inner ring is formed with an annular groove in which the sleeve-like pressure ring engages. The same is in contact with the companion surface and is spaced apart from the inner ring by a corrugated spring. The relative movement between the inner ring and the pressure ring, in association with a seal between the inner ring and the pressure ring, also ensures that satisfactory sealing tightness and automatic take-up of play relatively to the central pressureless leakage zone is provided.

The expanding chamber between the inner and outer rings is communicated by way of a changeover valve with the pressurised work fluid. The expanding force is boosted by a corrugated spring in the expanding chamber. Consequently, not only are the sealing surfaces of the inner and outer rings in sliding contact with the control surface and companion surface respectively but also the necessary biasing pressure is created to produce a sealing effect in order to isolate the annular chamber adjacent the periphery from the flow ducts. Another result is that only the axial forces required for sealing purposes between the inner ring and the control surface, on the one hand, and between the outer ring and the companion surface, on the other hand, are produced in order tq minimise friction and wear.

In a still further embodiment, the control member has an inner ring bearing on the control surface and an outer ring which engages the companion surface and extends around the inner ring, an expanding chamber having a resilient expanding force is disposed between the inner and 8 outer rings, the expanding chamber communicates by way of a changeover valve in the inner ring with the annular chamber on the peripheral side and with the ducts, and a support ring is received between the inner ring and the eccentric, is coupled for relative movement with both the inner ring and the eccentric and is pressed on to the control surface by way of a pressure ring in contact with the companion surface and of a resilient expanding force disposed between the support ring and the pressure ring, the ducts extending between the inner ring and the support ring. In this case, in addition to a relatively displaceable coupling between the eccentric and the control member, a relative displaceability is provided in the control member itself. An additional degree of freedom is therefore provided for the control member. Also, incorporating a support ring in the inner ring with provision for relative displacement provides the advantage that only the pressure operative in the flow ducts is required to ensure satisfactory sealing of the gaps, on the one hand, between the support ring and the control surface and, on the other hand, between the pressure ring and the companion surface. Consequently, when the flow ducts are used to convey the return fluid, much lower biasing pressures are produced so that the friction between the support ring and the control surface and between the pressure ring and the companion surface and, therefore, wear are reduced.

The relative displaceability between the support ring and the inner ring can be provided by a plain bearing, the inner ring being formed with a continuous axial bore in which the support ring engages slidingly by way of a central collar on the peripheral side. The flow ducts can take the form of perforations in the collar or edge recesses therein.

t 9 Another possibility is f or the support ring and the inner ring to be coupled by a rolling bearing. The rolling bearing is preferably more particularly a roller bearing disposed on the peripheral side of the support ring in the central longitudinal part thereof. The rollers are retained on the support ring and roll on the inner cylindrical surface of the inner ring. In this case the flow ducts are formed by the roller bearing.

Preferably, the control surface and/or the companion surf ace are disposed on wearing discs disposed between the control member and the machine casing.

By virtue of these features, readily replaceable long-life wearing elements can be prepared without subsequent work having to be carried out on the machine casing itself. In this arrangement, the wearing disc on the control surface side has the ports communicating with the chambers of the work pistons whereas the wearing disc on the other side of the control member has the flow and return ducts or apertures enabling the work f luid to f low to and return from the piston chambers. Conveniently, the wearing discs are secured non-rotatably to the machine casing.

The wearing discs can be made of a hardened material or ceramic. They can have coatings of hardened materials or of ceramic.

Preferably the ports 4 in the control surface are of reniform shape and advantageously each fluid duct extending to the piston chambers terminates in such a port. The exit cross-section of the ports is such that even at reduced eccentricity the control member ensures an adequately large opening cross-section. conveniently, the outer lands of the ports are adapted to the outer contour of the control member and the inner lands are adapted to the radially outer contour of the flow ducts.

Embodiments of the invention are described below by way of example with reference to embodiments shown in the drawings wherein:

FIGURE 1 is a fragmentary view in vertical longitudinal section on the line A-A of Figure 7 through a part of a reversible hydrostatic work machine, showing a control system therefor, FIGURES 2 to 6 are views similar to Figure 1 but showing variant control systems, and FIGURE 7 is a view in cross-section on the line VII-VII of Figure 1.

Referring to Figures 1 to 6 there can be seen the casing 1 of a reversible hydrostatic radial piston motor 2. A number of piston chambers (not shown in greater detail) in which work pistons are operative are disposed in casing 1. The piston chambers communicate by way of fluid ducts 3 with ports 4 which are reniform at least in the delivery zone and which are provided in a plane control surf ace 5 on the casing 1.

As can be gathered from Figure 7, a total of five ports 4 are provided in the embodiments shown in Figures 1 - 6 and are disposed at a peripherally uniformly divided spacing on a pitch circle 6. Each port 4 has a straight inner land 9 which extends tangentially to rotational axis 7 of shaft a of motor 2 and which merges by way of arcuate end edges 10 into two outer lands 11 disposed at a reduced inclination to one another.

1 11 As can also be gathered from Figures 1 to 7, cylindrical end 12 of shaft 8 overhangs the control surf ace 5 and is offset eccentrically by an amount A from the axis 7 of the main part of shaft 8. The eccentric 12 and a reduced-diameter part 13 of the shaft 8 into which the eccentric 12 merges have extending through them a leakage f luid duct 14. The duct 14 extends from the end face of the eccentric 12 and opens, on the periphery of the reduced part 13 into a stepped bore 15 in the casing 1, through which bore 15 the shaft 8 extends.

The eccentric 12 is coupled for relative movement with a control member 16, 16a-16e which will be described in greater detail hereinafter and which is disposed in a casing cover 18 releasably connected by screwthreaded pins 17 to casing 1. To provide sealing tightness between the cover 18 and the casing 1 the cover 18 engages over an axial projection 19 of the casing 1. Also, a ring seal 21 is provided at the transition from the projection 19 to the radial annular contact surface 20 for the cover 18. An annular chamber 23 adapted to be supplied with work fluid is provided between casing cover wall 22 and control member 16, 16a-16e.

As further common features, the control systems 24, 24a-24e according to Figures 1 to 6 have connections 25, 26 of which, for example, the connection 25 is supplied with the pressurised work fluid, hereinafter called pressure fluid, whereas the fluid returning from the piston chambers and hereinafter called return fluid is removed through the other connection 26. Changing over the connections 25, 26 reverses the direction of rotation of the motor 2.

The control member 16 of the control system 24 of Figure 1 is a unitary control ring.The same engages 12 slidingly with the contrcl surface 5 and with a companion surface 27 which extends parallel to the surface 5 and is disposed on a pressure disc or plate 28. The plate 28 is stepped towards base 29 of cover.18 and is engaged in a correspondingly stepped recess 30 in the cover 18.

In its central zone the plate 28 has an axial central pin 31 sealed off from the base 29 by a ring seal 32 on the peripheral side. Disposed in association with the end face of the pin 31 is a helical compression spring 33 which extends into a recess 34 in the base 29 and tends to move the plate 28 towards the control ring 16.

In the step 35 which extends concentrically around the pin 31 the plate 28 is formed with a number of axial bores 36; the same communicate with an annular groove 37 in the base 29, the groove 37 being connected to the connection 26. A ring seal 38 on the peripheral side seals off the step 35 from the base 29 of the casing cover 18.

The pressure plate edge 39 on the peripheral side is formed with a number of peripherally distributed edge recesses 40 extending into an annular groove 41 in the cover 18, the groove 41 being connected to the connection 25.

The ring 16 has, both on the side frontally opposite the surface 5 and on the side frontally opposite the companion surface 27, one annular groove 42, 43 in each case. The two grooves 42, 43 are interconnected by a number of bores 44 distributed around the periphery of the ring 16.

Consequently, the pressure f luid f lows from the connection 25 through the groove 41, recesses 40, chamber 1 13 2 3 and ducts 3 to the -piston chambers whereas the return f luid f lows theref rom through the ducts 3, groove 42, bores 44, groove 43, bores 36 and groove 37 back to the connection 26.

The ring 16 is formed with a central bore 45 receiving a bearing bush 46 in which the eccentric 12 engages slidingly.

As can be seen from considering Figures 1 and 7 together, the effect of coupling the shaft 8 with the ring 16 by way of the eccentric 12 which engages for relative movement in the ring 16, is that the same performs substantially merely a radial translational movement corresponding to twice the eccentric throw X. Consequently, the sealing gaps 47, 48 between the surface 5 and the annular sliding surfaces 49, 50 of the ring 16, on the one hand, and between the companion surface 27 of the plate 28 and the annular sliding surfaces 51, 52 of the ring 16, on the other hand, are continuously wetted by the pressure fluid and return fluid and thus cooled and continuously re-lubricated.

The main difference between the control system 24a shown in Figure 2 and the control system 24 of Figure 1 is that the control member 16a of the system 24a is received in an inner ring 53, the same being coupled with the eccentric 12 for relative movement and bearing on the control surface 5, and an outer ring 54 which engages stepwise around the inner ring 53 and engages slidingly with the companion surface 27 of the casing cover base 29.

The eccentric 12 engages slidingly and directly in a bore 55 of the inner ring 53. The central step 56 of the inner ring 53 is sealed off from the step 58 of the outer 1 14 ring 54 on the control surface side by a ring seal 57. A corrugated spring 60 is disposed between the outer ring step 58 and the inner ring step 59 and tends to press the annular sealing surfaces 61, 62 of the inner ring 53 on to the control surface 5 and the annular sealing surfaces 63, 64 of the outer ring 54 on to the companion surface 27, the same being directly disposed on the casing cover base 29.

As will also be apparent, a corrugated spring 67 is received between the step 65 of the outer ring 54 on the cover side and the step 66 of the inner ring 53 on the cover side and also serves to press the inner ring 53 on to the control surf ace 5 and the outer ring 54 on to the control surf ace- 27. A ring seal 69 is disposed between the inner ring step 66 on the cover side and the central step 68 of the outer ring 54.

On the side frontally opposite the control surface 5 the inner ring 53 is formed with an annular groove 70 which by way of a number of peripherally distributed bores 71 in the central step 56 of the inner ring 53 and by way of corresponding bores 72 in the central step 68 of the outer ring 54 is connected for fluid conveyance to an annular groove 73 disposed on that side of the outer ring 54 which is frontally opposite the companion surface 27.

In other respects, the control system 24a of Figure 2 corresponds to the control system of Figure 1. However, in the case of Figure 2 also, the eccentric 12 can engage in a bearing bush 46 in the inner ring 53 whereas in the embodiment of Figure 1 the eccentric 12 can directly engage slidingly, in the manner shown in Figure 2, in a bore 55 in the control ring 16.

1 Figure 3 shows a control system 24b wherein the control member 16b is received in an inner ring 74, the same being coupled with the eccentric 12 for relative movement and bearing on the control surface 5, and in an outer ring 75 which is in engagement with the companion surface 27 and extends around the inner ring 74. In this case too, the eccentric 12 directly engages slidingly in a bore 76 in the inner ring 74. A bearing bush 46 can of course be provided as in the embodiment of Figure 1.

Both the inner ring 74 and outer ring 75 have two steps, ring seals 81, 82 sealing off the inner ring step 77 on the cover side and the outer ring step 78 on the cover side and the inner ring step 79 on the control surf ace side from the outer ring step 80 on the control surface side.

An expanding chamber 83 receiving a corrugated spring 84 is disposed between the two ring seals 81, 82. As will also be apparent, the chamber 83 can communicate by way of bores 85, 86 in the inner ring step 79 on the control surface side and by way of a changeover valve 87 associated with the bores 85, 86, on the one hand, with the annular chamber 23 extending around the control member 16b and, on the other hand, with an annular groove 88 formed in the inner ring 74 on the control surface side.

on the side frontally opposite the companion surface 27 disposed directly on the casing cover base 29, the inner ring 74 is formed with another annular groove 89 which is connected to the annular groove 88 on the control surface side by way of peripherally distributed bores 90. A substantially hat-shaped pressure ring 91 engages in the annular groove 89 facing the companion surface 27 and is sealed off by a ring seal 93 from a central pin 92 of the inner ring 74. A corrugated spring 96 is disposed between 16 end face 94 of pin 92 and base 95 of pressure ring 91. The spring 96 and the spring 84 in the chamber 83 press the sealing surfaces 97, 98 of the pressure ring 91 and outer ring 75 on to the companion surface 27 and the annular sealing surfaces 99, 100 of the inner ring 74 on to the control surface 5.

By way of the changeover valve 87 the chamber 83 is in continuous communication with whichever connection 25, 26 is conveying the pressure fluid.

In the case of the control system 24c of Figure 4 the control member 16c comprises first an inner ring 102, the same bearing on the control surface 5 by way of an annular sealing surface 101, and an outer ring 104 which bears against the cover base 29 by way of an annular sealing surface 103 on the companion surface 27 and extends around the inner ring 102. The two rings 102, 104 have two steps. The inner ring step 105 f acing the surface 27 is sealed off from the outer ring step 106 facing the companion surface 27 by a ring seal 107 and the inner ring step 108 facing the control surf ace 5 is sealed of f from the outer ring step 110 facing the control surface 5 by a ring seal 109. An expanding chamber Ill is disposed between the two ring seals 107 and 109. A corrugated spring 112 is received in the chamber 111 and tends to press the inner ring 102 on to the surface 5 and the outer ring 104 on to the surface 27. By way of bores 113, 114 in inner ring step 108 and of a changeover valve 115 received in the bores 113, 114 the chamber 111 communicates, on the one hand, with the annular chamber 23 and, on the other hand, with a zone 116, in the f orm of an annular groove, between inner ring 102 and an inner support ring 118, the same being coupled with the eccentric 12 for relative movement by way of a needle bearing 117.

i 1 17 In its central longitudinal zone the support ring 118 carries a roller bearing 119 whose rollers 120 roll on the cylindrical inner surface 121 of the inner ring 102.

on the side facing the cover base 29 the support ring 118 has a connection piece 122 over which a hat-shaped pressure ring 123 engages. A ring seal 125 is received between pressure ring wall 124 and connection piece 122. Pressure ring base 126 is spaced apart from end face 127 of connection piece 122 by a corrugated spring 128. As will be apparent, the corrugated springs 121, 128 press the sealing surfaces 129, 103 of the pressure ring 123 and outer ring 104 on to the companion surf ace 27 and the sealing surfaces 101, 130 of the inner ring 102 and support ring 118 on to the control surface 5.

Instead of the needle bearing 117 the control system 24c of Figure 4 can have plain bearings 46, 55, 76 as in the embodiments of Figures 1 to 3.

The control system 24d shown in Figure 5 differs from control system ofFigure 4 only in that in Figure 5 a plain bearing 131 is provided instead of the'roller bearing 119 between the rings 118 and 102 and a bearing bush 46 is provided instead of the needle bearing 117 between the eccentric 12 and ring 118.

The annular groove zone 116 on the control surface side between the rings 118 and 102 communicates by way of edge recesses 132 in the support ring 118, which are uniformly distributed over the periphery, with an annular groove zone 133 which is also present between the rings 118 and 102 but which is frontally opposite the companion surface 27. The zone 133 communicates with the groove 41 and the same communicates with the connection 26.

18 The control system 24e of Figure 6 corresponds basically to the control system 24a of Figure 5. The only difference is that in Figure 6 the control surface 5 is disposed on a wearing disc 134 received between the control member 16a and the casing 1 and the companion surface 27 is disposed on a wearing disc 135 received between the control member 16e and the cover base 29. Both wearing discs 134, 135 are secured against rotation by pins 136. As will also be apparent, the ports 4 are devised in the wearing disc 134. The wearing disc 135 having the companion surface 27 is formed with bores 137 which correspond to the connections 25, 26.

The embodiments of Figures 1 to 4 can also have wearing discs 134, 135.

Rolling bearings 117, 119 in accordance with the embodiment shown in Figure 4 can be provided instead of the plain bearings 46, 131 of Figure 6.

1 19

Claims

A control system for a reversible hydrostatic work machine (2) having work pistons. there being provided between a plane control surface (5) having ports (4) and a companion surf ace (27) which is parallel to the control surf ace (5) a control member (16, 16a-e) which can be displaced by way of the machine shaft (8) transversely to the rotational axis (7), has ducts (42, 43, 44; 70-73; 8890; 116, 119, 133; 116, 132, 133) for the work fluid and isolates such ducts from an annular chamber (23) on the peripheral side adapted to be supplied with the work fluid, the end faces (49-52; 61-64; 99, 100, 97f 98; 101, 130f 103, 129) of the control member (16. 16a-16e) being brought by means of a resilient biasing pressure (33; 60f 67; 84f 96; 112, 128), boosted by the pressure of the work fluid, into sliding contact with the control surface (5) and companion surface (27), characterised in that an eccentric (12) having a leakage fluid duct (14) is provided on the machine shaft (8), which eccentric (12) engages in the control member (16, 16a-16e) and is coupled, in the central pressureless leakage zone (15), with the control member (16, 16a-16e) for rotation relative thereto.
2. A system according to claim 1, characterised in that the eccentric (12) is coupled with the control member (16, 16a-16e) by a plain bearing (46; 55; 76).
3. A system according to claim 1, characterised in that the eccentric (12) is coupled with the control member (16, 16a-16e) by a rolling bearing (117).
4. A system according to any of claims 1 to 3, characterised in that the control member is a one-piece L a control ring (16) and the conpanion surf ace (27) is disposed on a pressure plate (28), the same being received between the control ring (16) and the machine casing (1) and being biased by a resilient biasing pressure (33) towards the control ring (16) (Fig. 1).
5. A system according to any of claims 1 to 3, characterised in that the control member (16a) is received in an inner ring (53) which is coupled with the eccentric (12) f or displacement relatively thereto, the inner ring bearing on the control surface (5). and in an outer ring (54) which extends stepwise around the inner ring (53) and bears on the companion surface (27), the inner and outer rings (53, 54) being so engaged with one another by a resilient expanding force (60, 67) as to be displaceable relatively to one another, the ducts (70-73) being disposed in both of the rings (Fig. 2).
6. A system according to any of claims 1 to 3, characterised in that the control member (16b) is received in an inner ring (74) coupled with the eccentric (12) for movement relatively thereto, the inner ring bearing on the control surface (5), and in an outer ring (75) which is in sliding engagement with the companion surface (27) and extends around the inner ring (74), an expanding chamber (83) having a resilient expanding force (84) is disposed between the inner ring (74) and the outer ring (75), a pressure ring (91) is disposed between the inner ring (74) and the companion surface (27) with the interposition of a resilient expanding force (96), the ducts (88-90) are disposed in the inner ring (74) and the expanding chamber (83) is connected by way of a changeover valve (87) in the inner ring (74) to the chamber (23) on the peripheral side and to the ports (88-90) (Fig. 3).

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7. A system according to any of claims 1 to 3, characterised in that the control member (16c-16e) has an inner ring (102) bearing on the control surface (5) and an outer ring (104) which engages the companion surface (27) and extends around the inner. ring (102), an expanding chamber (111) having a resilient expanding force (112) is disposed between the inner and outer rings (102, 104), the expanding chamber (111) communicates by way of a changeover valve (115) in the inner ring (102) with the annular chamber (23) on the peripheral side and with the ducts (116, 119, 133; 116, 132, 133), and a support ring (118) is received between the inner ring (102) and the eccentric (12), is coupled for relative movement with both the inner ring (102) and the eccentric (12) and is pressed on to the control surface (5) by way of a pressure ring (123) in contact with the companion surface (27) and of a resilient expanding force (128) disposed between the support ring (118) and the pressure ring (123), the ducts (116, 119, 133; 116, 132, 133) extending between the inner ring (102) and the support ring (118) (Figs. 4, 5 and 6).
8. A system according to claim 7, characterised in that the support ring (118) and the inner ring (102) are coupled by a plain bearing (131) (Figs. 5 and 6).
9. A system according to claim 7, characterised in that the support ring (118) and the inner ring (102) are coupled by a rolling bearing (119) (Fig. 4).
10. A system according to any of claims 1 to 9, characterised in that the control surface (5) andlor the companion surface (27) are disposed on wearing discs (134, 135) disposed between the control member (16, 16a16e) and the machine casing (1) (Fig. 6).

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11. A system according to any of claiMs 1 to 10, characterised in that the ports (4) are reniform (Fig. 7).
12. A control system for a reversible hydrostatic work machine, substantially as hereinbefore described with reference to any one of Figures 1 to 6 and 7 of the accompanying drawings.
13. A reversible hydrostatic work machine incorporating a control system according to any of claims 1 to 12.
14. Any novel feature or combination of features described herein.