AXIAL PISTON HYDROSTATIC MACHINE WITH VARIABLE DISPLACEMENT

Abstract: An axial piston hydrostatic machine (1), which can be a pump or a motor, comprises a pair of cams (7, 8) formed like inclined discs in axially opposing relationship to control the reciprocating motion of the discs, one of the cams (8) being rotatable relative to the other (7) under the action of an elastic system which opposes a relative angular shift thereof tending to modify the displacement volume of the machine (1), so that the displacement is automatically modified as a univocal function of the torque applied to the shaft.

Description: "AXIAL PISTON HYDROSTATIC MACHINE WITH VARIABLE DISPLACEMENT" Technical Field The present invention relates to an axial piston hydrostatic machine capable of modifying its displacement volume in response to a variation of the torque moment applied to the rotating shaft of the machine, by virtue of an inherently automatic self-suf icient control reaction. Background Art Positive displacement hydrostatic machines with axially reciprocating pistons are known in the prior art, in which a variation of the displacement volume is obtained by acting on the stroke of the pistons. Two main technical solutions have been developed in the prior art. According to one solution, a swashplate inclined to the machine axis controls the pistons, and the other solution involves an oscillation of the whole cylinder block. It should be noted that both the above control systems involve a variation (oscillation) of the angle of inclination of the swashplate or the oscillating block. The variation of the swashplate or block inclination is carried out by varying its inclination about an axis which is perpendicular to the axis of the machine. In other words, the member controlling the setting of displacement does so by rotating about an axis perpendicular to the axis of the machine. In Italian patent No. 1189306 a control device is described for automatically modifying the displacement in positive displacement hydrostatic machines having radially reciprocating pistons, so-called radial piston hydrostatic machines. This solution has been further developped in a theoretical study, as illustrated in - 2 - articles of "Fluid" magazine number 228, April 1983, page 39; n. 234, October 1983, page 49; n. 286, February 1988, page 60. This theoretical development, which has been denominated "theory of elastic shaft" states that the control of the displacement is carried out by a rotation about the axis itself of the machine of the member—(or- members) controlling the displacement setting. In harmonization with the rotational motion of the machine, this produces a phase shift between the mechanisms which give rise to the machine displacement. This phase shift sets into an angular position of stable balance, when the torque moment due to an external load is balanced by a torsional moment produced by an elastic system coaxial to the machine shaft. According to the present invention, among axial piston hydrostatic machines, those which are best suited to an inherently automatic control (intrinsical control) are the axial machines having inclined planes or discs forming opposed cams controlling the reciprocating motion of pairs of pistons moving in cylinders which are circumferentially disposed in the rotary member of the machine. Conventional machines with opposed cams generally have both the two cams fixed and integral with the stationary member of the machine. They are therefore in this case fixed displacement machines. The axial piston machines having only one inclined disc or plate can have a fixed displacement when the disc inclination is fixed. Otherwise the disc or plate is oscillating and the displacement can be varied by a suitable servomechanism controlled by a signal driven as a function of the applied load. In the following description and in the claims the rotary member and the stationary member of an axial piston hydrostatic machine will be briefly denominated - 3 - rotor and stator respectively. At the present state of the art, machines of this type having an inherent control of the displacement setting are not known. By the term inherent control it is intended that the mechanism is completely self-sufficient to modify - he displacement as a function of the applied load. Disclosure of the Invention To obtain a inherently self-setting variable displacement machine with axially reciprocating pistons, according to the present invention, a machine is provided in which each cylinder houses a pair of axially aligned pistons in opposing relationship and having opposed inclined cams and in which a rotational relative movement between the cams is produced, so as to obtain an increase or decrease of the volume in the room generated by the reciprocating pistons in each cylinder. In particular the volume will be null when the directions of the cam inclination are parallel one to the other and the volume will be maximum when the directions of the opposed cams are in phase opposition. In any other intermediate position a displacement volume will be generated which is determined in an univocal way by the angular phase shift of the two cams. The function which expresses this relation is illustrated in the theory described in the above cited articles of "Fluid" magazine. An object of the present invention is to provide a connection of the cams which makes any relative position of the cams to correspond to a reaction of elastic type proportional to the angular phase shift of a cam with respect to the other. As the control mechanism only requires a relative rotational movement of the cams, it is absolutely irrelevant in practical applications to make a choice between a solution providing cams integral with the stationary member of the machine, or a solution - 4 - providing rotating cams driven by the shaft of the machine. One of the cams therefore in the following of the present description and in the claims will be denominated "balancing member", in that upon carrying out a relative rotational movement with respect to the other cam, which hereinafter will be simply denominated "cam", it will effect the automatic setting of the displacement. Therefore an object of the present invention is an axial piston hydrostatic machine, of the type with opposed cams, characterized by the fact that: a) each cylinder houses two pistons axially reciprocating in opposed directions, so that the cylinder block is provided with two arrays of circumferentially disposed pistons on each of its two opposite faces, this cam controlling the pistons on one first face of the cylinder block and the stroke of each pair of opposed pistons relatively one to the other defining the displacement volume of the machine; b) a balancing member having a surface inclined with respect to the axis of rotation of the shaft, controls the pistons on the other face of the cylinder block, the balancing member being rotatably mounted around the axis of rotation, relatively to both the cam and the cylinder block; and c) an elastic system, adapted to produce a torsional moment which opposes to a variation of the angular relative position around the axis of rotation, of the balancing member and the cam, the elastic system being into engagement with the balancing member on the one side, and with the cam on the other side, so that the angle of relative rotation between the balancing member and the cam is a univocal function of the moment applied to the rotating shaft of the machine and a variation of the machine displacement 'is a function of the variation of the above mentioned applied moment. An important feature of the machine according to the invention resides in the fact that a synchronization of the rotary control valve can be carried out in simple manner, in spite of the higher degree of complexity of this machine with respect to the conventional fixed displacement machines. - Brief Description of the Drawings The present invention will be illustrated in further detail with reference to some embodiments, as shown in the accompanying drawings, in which: figure 1 diagramatically shows an axial piston hydrostatic pump, the driving shaft of which is integral with the cylinder block; figure 2 diagramatically shows an axial piston hydrostatic motor in which the cylinder block is integral with the stator and the output shaft is integral with the cam; figure 3 diagramatically shows a pump in which the driving shaft is integral with the cam and the elastic system is a spiral spring; figure 4 diagramatically shows a pump in which the driving shaft is integral with the cam and the elastic system comprises a coil spring acting on a movable body; figure 5 diagramatically shows a pump having a cylinder block integral with the driving shaft and an elastic system comprising a hydraulic gas accumulator acting on a screw-threaded movable body; and figure 6 diagramatically shows a motor having an output shaft integral with the cam and an elastic system comprising a hollow movable body with a double screw thread and stressed by a hydraulic gas accumulator. In the drawings the graphic convention has been adopted of indicating with a continuous line the parts of the machine which are free to rotate or move in a relative motion, whereas all the stationary parts of the machine, that is those integral with the stator of the machine, are indicated with dotted lines. A dash and dot line is used for lines indicating an axis. Modes for carrying out the invention The machine according to the invention comprises, similarly to conventional machines-,-- a stator 1 (stationary member) , a rotating shaft 2 and a cylinder block 3 in which cylinder bores 4 are machined which open to the one or the other face of the cylinder block 3. Opposed pistons 5, 6 are housed in pairs into each cylinder bore 4. According to the conventional art, the pistons have their head provided with a sphere or a slipper pad, which for the sake of simplicity of representation are not shown in the figures. The piston heads are diagramatically represented in the figure with a simple half-circle. The pistons, by sliding on a track transmit the force produced in the cylinders by pressure oil, to a cam indicated with 7 in figure 1, which is here denominated fixed cam, and to a balancing member respectively, indicated with 8 in figure 1. The balancing member 8 has a surface inclined with respect to the axis of rotation of the shaft 2 and, according to the invention, it can angularly shift around this axis relatively to the cam 7. As the cam 7 and the balacing member 8 have a profile which is inclined with respect to the plane perpendicular to the axis of rotation, the linear motion of the pistons is tranformed into a rotational motion of the shaft and vice versa. It is also obvious that to this rotational motion a driving moment transmitted by the machine is associated, in that the axial force of the pistons produces a reaction perpendicular to the inclined track. The tangential component of this reaction force, multiplied by the average radius of the track, gives the above mentioned driving moment. It is to be observed that the total - 7 - moment is consequently divided into two halves," that i one half of the moment is transmitted through th reaction of the fixed cam 7, whereas the other hal through the reaction of the movable balancing member 8. It is now evident that the moment developed by th reaction on the movable balancing member—8-will make th balancing member rotate also with respect to the fixe cam 7, unless this is contrasted by an equal and contrar moment. To this purpose, in the simplest embodiment of th present invention, the balancing member 8 is connected t the fixed cam 7 by means of an elastic torsional shaft formed as a rod 11 of elastic material. This will enabl the balancing member 8 to rotate until the reaction of the torsional moment by the elastic shaft balances th moment acting on the balancing member 8. According to the general theory illustrated in the above cite articles, it is demonstrated that the balance of the moments is stable and the angular position of th balancing member 8 with respect to the fixed cam 7 is univocal. However, it can be appreciated that at th present state of the art the embodiment of a real elastic shaft made of steel is not realizable and therefore it is necessary to resort to systems which approach as well as possible the behaviour of an elastic torsional shaft. In a not to far future it is not to be excluded tha materials will be available which could be able t transmit a large torsional moment and at the same time t undergo a large angular strain, such as elastomeric o composite materials. Since within the scope of the present invention th machine can be realized in different embodiments, in th various figures of the drawings the basic elements of th machine which have been 'previously described, will b indicated with the same reference numerals. This permits the recognition of the consistency of the inventive concept, even in widely differentiated embodiments. As already mentioned hereinbefore, the machine can be realized with a rotating shaft 2 integral with the cylinder block 3 (figures 1, 5) , whereas the fixed cam 7 is integral with the stator 1 of-the--machine. Otherwise the driving shaft 2 can be integral with the cam 7 (figures 2, 3, 4, 6) whereas the cylinder block 3 is integral with the stator 1. However, in all these embodiments, and this is a basic feature of the present invention, the balancing member 8 is rotatably mounted both with respect to the elements integral with the stator 1, and the elements integral with the rotating shaft 2, by means of an elastic system capable of producing a reaction. Since the general lay-out of the machine comprises in all cases the above-mentioned basic elements, the following description is limited to some typical embodiments concerning the coupling of the elastic shaft. In figure 1 the cylinder block 3 is driven into rotation by the shaft 2 and is supported by a bearing 10 on the stator 1. The fixed cam 7 is integral with the stator 1 and the elastic coupling of the fixed cam 7 to the balancing member 8 is carried out by a rod 11 having a suitable capability of elastic torsion. Rod 11 is interposed between the balancing member 8 and the stator 1 with which, as stated above, the fixed cam 7 is integral. The balancing member 8 is provided at its center with an extended hub 12 forming a hollow shaft and it is supported by bearings 13 and 14 on the cylinder block 3 and the stator 1 respectively. One end 15 of the rod 11 is forceably fitted into a bore 16 of the hollow shaft 12 and the other end 17 is forceably fitted in a seat 18 provided in the stator or pump housing. The balancing member 8 therefore can rotate by a certain - 9 - angle about the axis of the machine and it will take the angular-position which corresponds to the balance between the moment acting on the balancing member 8 and the torsional elastic reaction of the rod 11. Figure 2 diagramatically shows a hydrostatic motor having axial pistons 5, 6 and a cam 7 opposed to a balancing member 8, in which the cylinder block 3 is integral with the stator 1 of the motor and the cam 7 and the balancing member 8 rotate along with the shaft 2. The elastic coupling between the cam 7 and the balancing member 8 comprises a rod 21 having a high torsional elasticity, which besides end enlargements 22 and 23 has an enlargement 24 in the middle. Cam 7 is integral with the shaft 2 and it is supported by bearings 25 and 26 on the stator and the cylinder block respectively and is provided in the middle with a blind end bore coaxial to the shaft axis. The balancing member 8 has a coaxial hollow shaft extended hub and it is supported by bearings 27 and 28 on the cylinder block 3 and the stator 1 respectively. The end enlargement 23 is forceably fitted within a blind end bore in the cam 7, and the end enlargement 22 is forceably fitted into the end of the coaxial bore provided in the balancing member 8. According to the invention, the middle enlargement 24 drives into rotation a distributor 29 coaxially placed within the cylinder block 3. In operation, the moments of the forces produced by the pistons which act on the cam 7, make the cam 7 and the balancing member 8 to rotate along with the machine shaft 2. As the balancing member 8 is able to rotate also respect to the cam 7, the moment acting on this cam will make the balancing member 8 rotate with respect to the cam 7. In this way the torsional shaft is twisted so as to take, during rotation, an angular phase shift θ in correspondence with which a balance is established between the moment acting on the movable cam and the elHstic- torsional reaction of the elastic rod. It is to be noted that the distributor has to rotate by a phase delay of θ/2 with respect to the rotation θ of the balancing member 8 relatively to the cam 7, in accordance with what is provided in the theory exposed in the above mentioned articles. According to the invention, this,condition is satisfied in a surprisingly efficient and simple manner by placing the distributor in the mid position and driven by the middle enlargement 24. In fact, this automatically ensures the phase delay θ/2, as the twist angle at the middle point of the rod is exactly one half of the rotation at the ends. Figure 3 diagramatically shows a hydrostatic pump having axial pistons 5 and 6 as well as a cam 7 and a balancing member 8 in opposing relationship, in which the cylinder block 3 is integral with the stator 1 and the cam 7 and the balancing member 8 are supported by bearings 30 and 31 and they rotate integrally with the shaft 2 which drives both of them into rotation. The cam 7 is fixedly mounted on the machine shaft 2 and it is rigidly driven by a key 32, while the balancing member 8 can not rotate about the shaft being forceably fitted thereon. The balancing member 8 in fact is provided with a hollow shaft central hub 33 to be forceably fitted on the machine shaft 2 and an extension to the opposite direction, in which a cylindrical cavity 34 is formed. Within the annular space between the machine shaft and the cylindrical cavity 34, a wide band flat spiral archimedes ** spring 35 is housed, one end of which is fixed to the balancing member 8 by a stop screw 36 and the other end to the machine shaft by a stop screw 37. The balancing member 8 consequently is able to rotate, during the rotation of the shaft 2, by a certain angle also relatively to the cam 7 and it will assume the angular position of balance between the moment acting on the * ba-l-ancing member 8 and the reaction due to the elastic stress of the spiral spring 35. Figure 4 diagramatically shows a hydrostatic pump having axial pistons 5, 6 as well as a cam 7 and a balancing member 8 in axially opposed relationship, in which the cylinder block 3 is integral with the pump housing or stator 1, while the cam 7 and the balancing member 8 rotate with the shaft which drives them into rotation. The cam 7 integral with the shaft 2, is provided with a coaxial hollow shaft hub 41 and it is supported by bearings 42 and 43 on the cylinder block 3 and the stator 1 respectively. The balancing member 8 is provided with a coaxial hollow shaft hub 40 on the side of the cylinder block 3 and, on the opposite side, with a cylindrical cavity 44. The balancing member 8 is supported by bearings 45 and 46 on the cylinder block 3 and the stator 1 respectively. The elastic coupling of the cam 7 to the balancing member 8 is provided by a cylindrical movable body 47 shaped as a coaxial spindle. The surface of the section of the movable body 47 facing the cam 7 is externally splined and it is engaged in a complementary internally splined axial bore in the cam 7. The other surface section of the movable body 47 is provided with a long pitch helicoidal screw thread 49 which is threadedly engaged with a screw threaded coaxial bore in the hollow shaft hub 40 of the balancing member 8. It should be noted that the spline engagement can also be embodied as a screw thread engagement, in which case a shorter stroke of the movable body 47 will be obtained. The movable body 47 is able to axially move to and fro by virtue of the balancing member 8 and the cam 7 being rotating relatively one to the other. A cylindrical coil spring 50 is housed in the cavity 44 of the balancing member and it is mounted on the movable body 47 by means of-a—pl-ate 51 and a fixing nut 52. The spring 50 elastically counteracts the axial movement of the movable body 47 in the direction which produces a decrease of the pump displacement. The -balancing member 8 therefor can rotate by a certain angle during the rotation of the shaft 2, also relatively to the cam 7, so causing the movable body 47 to move axially until the balancing member 8 assumes its set of balance between the moment acting on it and the elastic compression reaction of the cylindrical coil spring 50, in view of the axial shift of the movable body 47. Figure 5 diagramatically illustrates a hydrostatic pump having axial pistons 5, 6, cam 7 and balancing member 8, in which the cylinder block 3 rotates as driven into rotation by the shaft 2 which is supported by a bearing 60 on the stator 1. The cam 7 is integral with the stator or casing 1 of the pump and the elastic coupling of the cam 7 and the balancing member 8 is provided by a long pitch concentric screw thread engagement. An inner screw thread 61 is integral with the stator and an additional screw thread 69 is provided internally to a hollow cylindrical hub 62 of the balancing member 8. The balancing member 8 is suitably supported on bearings 63 and 64 in the cylinder block 3 and the stator 1 respectively. Between the screw threads 61 and 69 a threadedly engaged movable body 65 is inserted, which shifts in the axial direction as a consequence of the relative rotation of the balancing body 8 and the screw thread 61 integral with the stator. The axial displacement of the movable body 65 is elastically counteracted by a hydraulic piston actuator 66 fitted in a cylinder 67 which is an integral part of the stator and in which pressure oil is acting in communication with a hydraulic gas accumulator 68. Figure 6 diagramatically illustrates a hydrostatic motor having axial pistons 5, 6, -cam— - and a balancing member 8 aligned in opposing relationship, in which the cylinder block 3 is integral with the housing or stator 1 of the motor, while the cam 7 and the balancing member 8 rotate integrally with the output shaft 2. The fixed cam 7 is rigidly mounted on the output shaft 2 and being driven into rotation by a key 70 and moreover being supported on bearings 71 and 72 on the stator 1 and the cylinder block 3 respectively. The balancing member 8 is provided, on the side facing the cylinder block 3, with a coaxial hollow shaft hub 74 and on the opposite side with a cylindrical cavity 75. The balancing member is supported in bearings 76 and 77 on the cylinder block 3 and the stator 1 respectively. The elastic coupling between the cam 7 and the balancing member 8 is provided by a sleeve-like movable cylindrical body 78, which is internally threadedly engaged with the shaft 2 through a long pitch screw thread 79. The sleeve 78 moreover is externally threadedly engaged by a long pitch screw thread 80 with a bore in the balancing member 8 provided in the hollow shaft hub 74. A rotation of the cam 7 relative to the balancing member 8 produces an axial shift of the movable body or sleeve 78, which shift is opposed by a hydraulic piston actuator 81 acting inside the cylindrical cavity 75 formed integral with the balancing member 8. The hydraulic actuator receives pressure oil being connected to a hydraulic gas accumulator. The movable body 78 is provided with an extension 84 which drives a distributor 83 into rotation by means of a splined engagement. It should be noted that in accordance with what is provided by the theory, the distributor has to rotate with a phase delay of θ/2 with respect to the angle of rotation θ of the balancing—member- 8- relatively to the cam 7. Since an axial shift of the movable body 78 automatically involves also an angular rotation of the movable body, which is a function of the angle of -inclination of the engaged screw threads, it is sufficient to calculate the thread pitches so as to obtain the desired phase delay of θ/2. It is self-evident that a great number of combinations of mechanical elements are possible to realize an elastic shaft. However, all these embodiments fall within the scope of the present invention, in that the embodiments illustrated in the drawings are purely exemplificative.

Claims: CLAIMS 1. A hydrostatic machine compri-s-ing: a shaft rotating about a longitudinal axis of rotation, a stationary stator, a cylinder block having a circumferentially disposed array of cylinders with axes —parallel to said axis of rotation, piston reciprocating in said cylinders, a cam having a surface inclined with respect to said axis of rotation and being rotatably mounted relatively to said cylinder block about said axis for controlling the movement of said pistons, said rotating shaft being integral with a single one of said cam and said cylinder block, the other one being stationary, characterized in that: a) each cylinder houses two pistons axially reciprocating in opposed directions, so that said cylinder block is provided with two circumferentially disposed arrays of pistons on each of its two opposite faces, said cam controlling the pistons on one first face of the cylinder block and the stroke of each pair of opposed pistons relatigely one to the other defining the displacement of the machine; b) a balancing member having a surface inclined with respect to said axis of rotation, which controls the pistons on the other face of the cylinder block, said balancing member being rotatably mounted about said axis of rotation relatively to both said cam and said cylinder block; and c) an elastic system, adapted to produce a torsional moment which opposes to a variation of the angular relative position on said axis of said balancing member and said cam, said elastic system being in engagement with said balancing member on the one side and with the cam on the other side, so that the angle of relative rotation between the balancing member and the cam is a univocal function of the moment applied to the rotating shaft of the machine and a variation of the machine displacement is a function- of the variation of said applied moment. 2. A machine according to claim 1, in which the machine shaft is integral with the cylinder block, characterized in that the cam is integral with the stator, and the balancing member is pivotally mounted about said axis and rotates relatively to the cylinder block and the stator, * so that said elastic reaction system acts between said stator and said balancing member. 3. A machine according to claim 1, in which the shaft is integral with said cam and the cylinder block is integral with said stator, characterized in that the balancing member is pivotally mounted about said axis and rotates relatively to said cam and said stator, so that the elastic reaction system acts between said balancing member and said cam. 4. A machine according to any of claims 1, 2 or 3, in which the elastic reaction system is a cylindrical rod of elastic material coaxial to said axis of rotation. 5. A machine according to claim 4, further comprising a distributor driven into rotation by said cylindrical rod. 6. A machine according to any of claims 1, 2 or 3, in which the elastic, reaction system is a flat spiral spring. 7. A machine according to claim 6, in which said spiral spring is fixed at one end to said cam through said shaft and at the other end to the balancing member. 8. A machine according to any of claims 1, 2 or 3, in which the elastic reaction system comprises: a body axially movable along said axis of rotation, engaged through a double 'long pitch threaded engagement with said cam and said balancing member respectively, and an elastic means able to produce the reaction force which opposes to the axial movement- of said movable body resulting from a relative rotation of the cam and the balancing body. 9. A machine according to claim 8, in which the movable body is substantially cylindrical and it is engaged with said cam and said balancing member by means of a long pitch threaded engagement and a linear axial sliding engagement respectively, or vice versa. 10. A machine according to claim 8, in which said moveable body is a substantially cylindrical sleeve having concentric long pitch outer and inner threads, said threads being engaged with respective threads provided on the cam and the balancing member. 11. The machine according to claim 9 or 10, in which a helicoidal coil spring stresses said movable body to produce the elastic reaction to the movement thereof. 12. A machine according to claim 1 or 10, in which a hydraulic actuator stresses said movable body to produce an elastic reaction to the movement thereof. 13. A machine according to claim 9 or 10, in which a gas hydraulic actuator stresses said movable body to produce an elastic reaction to the movement thereof. 14. A machine according to claim 8, further comprising a distributor driven into rotation by the axially movable body. . 15. A machine according to claim 12, in which said hydraulic actuator engaged with the movable body can be driven from the exterior of the machine. 16. A machine according to claim 12, in which a gas hydraulic accumulator is connected to the hydraulic actuator controlling said movable body.

Inventors: MAURER GUALTIERO, MAURER, GUALTIERO
Applicants: MAURER GUALTIERO, MAURER, GUALTIERO
Application year: 1989
Main classification: F04B1
IPC codes: F04B1