US3776102A

US3776102A - Variable-displacement type fluid pump or motor

Info

Publication number: US3776102A
Application number: US00231172A
Authority: US
Inventors: M Katayama
Original assignee: Tokico Ltd
Current assignee: Tokico Ltd
Priority date: 1971-03-03
Filing date: 1972-03-02
Publication date: 1973-12-04
Anticipated expiration: 1990-12-04
Also published as: FR2127908A5; DE2209996B2; DE2209996A1; SU452676A1; JPS5124722B1; DE2209996C3

Abstract

A variable-displacement type fluid pump or motor comprises, essentially, a rotor having ball pistons movable in the axial direction, a pair of cam members against which the ball pistons abut, ports for supplying hydraulic fluid into and discharging the same out of spaces between the ball pistons within cylindrical bores, a valve sleeve having ports for determining the timing of supply and discharge of the hydraulic fluid, and means for transmitting the rotation of the cam member to the valve sleeve. One of the pair of cam members is fixed, while the other is rotatably supported to a casing. The valve sleeve is also rotatably supported and is rotated by an angle equal to onehalf of the rotational angle of the movable cam member in the same direction by the rotation of the movable cam member transmitted by way of the rotation transmitting means.

Description

O Umted States Patent 1 [111 3,776,102 Katayama Dec. 4, 1973 [5 VARIABLE-DISPLACEMENT TYPE FLUID 3,030,933 4/1962 Bergstedt 91 501 PUMP MOTOR 3,249,020 5/1966 Albertson 91 /483 [75] Inventor: .lgaialyoshl Katayama, Yokohama, Primary Examiner wmiam L Freeh p Attorney-Eric H. Waters et al.

[73] Assignee: Tokico Ltd., Kanagawa-ken, Japan [22] Filed: Mar. 2, 1972 [57] ABSTRACT [30] Foreign Application Priority Data against which the ball pistons abut, ports for supplying Mar. 3, Japan hydraulic into and discharging the ame out of spaces between the ball pistons within cylindrical US. Clbores a valve sleeve having for determining the t of supply and discharge of the hydraulic fluid Fleld of Search and means for transmitting the rotation of the cam member to the valve sleeve. One of the pair of cam. References Clted members is fixed, while the other is rotatably sup- UNITED STATES PATENTS ported to a casing. The valve sleeve is also rotatably 3,601,013 8/1971 Albertson 91 501 pp and is rotated y an angle equill to One-half 3,166,016 1/1965 Thoma 91 4 3 of the rotational angle of the movable cam member in 3,662,551 8/1970 Denker 91/501 the same direction by the rotation of the movable cam 2,865,304 1953 Fallon 9 3 member transmitted by way of the rotation transmit- 3,603,211 9/1971 Firth 91 501 ting means 1,669,976 5/1928 Dc Lecuw 91/4 83 2,617,360 11/1952 Barker ,91/483 7 Claims, Drawing Figures H 5 27b 23 274 0 0 /3b /6 /9 29 v 2/ b J 0;; I71; I f5 5/ /a-. d 55 i i ,7, J i .1 W 2/ a a ,2 23/; I 26a 37 W1) 261;

App]. 190.; 231,172

A variable-displacement type fluid pump or motor comprises, essentially, a rotor having ball pistons movable in the axial direction, a pair of cam members VARIABLE-DISPLACEMENT TYPE FLUID PUMP OR MOTOR BACKGROUND OF THE INVENTION This invention relates to a fluid pump or motor of variable-displacement type and more particularly to'a new and advanced fluid pump or motor of this type wherein the relative opposed positions of two cam plates can be varied to vary the displacement volume of pistons and thereby to vary the capacity of the fluid pump or motor.

In general, a fluid pump or motor (hereinafter re-' ferred to simply as -fluid*device) has been developed as a fluid device of the bent-axis type'or the cam-plate type. In a fluid device of this type, plunger type pistons slide over cam plates and undergo reciprocating motion. For this reason, the piston motion is limited to a harmonic motion of one stroke per'onerevolution of a rotor. Consequently, conventional'fluid devices of the axial-piston type have had 'the disadvantage of substantially large fluctuations of discharge pressure when used as pumps or of output torque when used'as motors. Accordingly, in order to reduce the period of these fluctuations, ithas beennecessary to-use these fluid devices at rotational'speeds within the range ofmedium and high speeds. Furthermore, since the'reaction force of the hydraulic fluid cannot be. fully balanced, quantities such as the rigidity of the rotary shaft and the capacity of the bearings are required to be greater than those of fluid devices of other types. This requirement has givenrise to various difficulties such as an adverse effect on the serviceable life and performance of these fluid devices.

As an axial-piston type fluid-device of a new type wherein these difficulties can be overcome, a fluid device of a so-called'axial ball-pistontype has'beenproposed. A fluid device of this type has a plurality of-pairs of ball pistons, which pairs successively undergo stroke motion.

In some cases, however, there arises the necessity of varying the discharge rate when the fluid device is operating as a pump or of varying the rotational torque when it is operating as a motor. These variations can be effected by varying the mutually relative positions of the two opposed cam plates thereby to vary the displacements of the stroke motions of the ball pistons.

Axial ball-piston fluid devices of this variabledisplacement type have been proposed, one example being that disclosed in the specification of US Pat. No. 3,508,465. In this proposed fluid device, two cam plates are simultaneously rotatedv in mutuallyopposite directions by one bevel gear thereby to vary the relative opposed positions of the two cam plates. However, since the two cam plates are being continually sub jected to reaction forces due to the ball pistons and have a movable construction, errors easily develop in the positions of the cam plates. This hasbeen a difficult problem in this type of fluid device. For this reason, it is desirable that the cam plates havea fixed'construction as much as possible.

SUMMARY OF THE INVENTION Accordingly, it is a general object of th'isinvention to pump or motor wherein the above describeddifficulties accompanying the proposed fluid device are overcome.

. 1 6 provide a novel and useful variable-displacement fluid 5 error arising in the relative opposed positions of the two cam plates, whereby the displacement volume can be varied and adjusted with high accuracy. Futhermore, it is possible to use, in th'efluid device of this invention, parts which are common to fluid devices of I the fixed capacity type.

ing a back pressure to. the discharge outlet of the leak-' age fluid, .the rotor can be caused'toa free wheeling;

Further objects and features of this invention will be apparent from the following detailed description with reference to the accompanying drawings, in which like partsare designated by like reference numerals.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 isa side elevation, in vertical section, showing essential parts of one embodiment of a variabledisplacement type fluid-device according to this invention;

FIG. 2 is a crosssection taken along the plane indicated by line II II in FIG. 1;

FIG. 3 is an exploded'perspective view showing essential parts of a mechanism for adjustably varying capacity in the fluid device shown in FIGS. 1 and'2;

FIGS. 4A through 4C are graphical representations fora description of the variations of the strokes and displacement volume of a pair of ball pistons with respect to the rotational angle of the rotor in the case where one cam plate is rotated by an angle 2 0 from the other cam plate, and, at the same time, the valve sleeve is rotated through anangle 0 FIGS. 5A through 5C are graphical representations similar to FIGS. 4A through 4C in the case where the valve sleeve isfixe'd, and one cam plate is rotated by an angle 2 0 from the other cam plate; and

FIG. 6 is a fragmentary side elevation, in vertical section, showing another embodiment of the ball piston part of the fluid deviceillustrated in FIG. 1.

DETAILED DESCRIPTION wall of the closed end of the housing 11 and the end cap 12 on their'inner sides respectively support bearings which are'coaxially aligned on an axis perpendicular to the end wall andthe end cap. These bearings rotatably support a rotor shaftl5 integrally and coaxially formed with a cylindrical rotor block or rotor cylinder 14 to constitute a rotor 13. Accordingly, the rotor cylinder 14 can rotate freely within the casing. This rotor cylinder 14 is provided therein with a plurality (six in the instant embodiment) of cylindrical bores 16 of circular cross-section bored completely therethrough with axes parallel to and equidistant from the axis of the shaft 15 with equal circumferential spacing therearound and constituting cylinders for operation therein of ball pistons as described hereinafter.

While the rotor cylinder 14 and the rotor shaft 15 of the rotor 13 in the instant embodiment are formed integrally from the same material stock, the rotor may also be made by fabricating the rotor cylinder and the rotor shaft separately but with mutually mating spline grooves and ridges and then assembling the rotor by fitting the shaft into the rotor cylinder.

Each of the above mentioned cylindrical bores 16 at its two ends accomodates

respective ball pistons

17a and 17b slidably disposed therein to constitute a pair with outer parts projecting out of the cylindrical bore. These

ball pistons

17a and 17b in each cylindrical bore 16 are seated and held in the seats of

respective seal rings

18a and 18b disposed slidable within the cylindrical bore 16 inward of the ball pistons. The pair of

seal rings

18a and 18b are continually urged in axial directions outward from the cylindrical bore 16 by the elastic force of a compression coil spring 19 disposed within the bore 16 between the

seal rings

18a and 18b. Accordingly, the outward force of the spring 19 is transmitted through the

seal ring

18a and 18b to the

ball pistons

17a and 17b to urge them in their outward projecting directions in each cylindrical bore 16. The

ballpistons

17a and 17b at their outermost parts abut against the cam surfaces of a pair of mutually opposed

cam plates

20 and 21 and are thereby limited in their outward displacement.

The cam plate 20 is fixed to the inner end surface of the housing .11 by a dowel pin 22, while the cam plate 21 is rotatably supported on the inner side of the end cap 12. The

cam plates

20 and 21 respectively have circular cam surfaces, each having a plurality (nine in the instant embodiment) of

cam crests

20a and 21a, against which the outer parts of the

ball pistons

17a and 17b abut, and over which they roll along the cam paths of the cam plates as the rotor 13 rotates. As a result, the

ball pistons

17a and 17b, in following the cam surface, are displaced in the axial direction of the cylindrical bore 16 in accordance with the projection of the

cam crests

20a and 21a from the cam valleys 20b and 21b. The cam curves of the cam surfaces of the

cam plates

20 and 21 are so determined that, in the case where the fluid device 10 is used as a motor, the total sum of the driving torques of all cylindrical bores 16 becomes a constant value. In the case where the fluid device 10 is used as a pump, the cam curve is so determined that the pump discharge pressure becomes constant. Furthermore, the cam surface is so designed that, ordinarily, in one half cycle of the cam curve, there exist three constant and continuous periods, namely, an acceleration period, a constant velocity period, and a deceleration period of the torque. The displacement volume of the fluid device 10 is determined by the distances of projections of the

cam crests

20a and 21a. Furthermore, the number of the cam crests may be selected as desired. Specifications of the fluid device 10 such as the range of operational rotational speed and output torque can be changed by suitably interchanging the

cam plates

20 and 21 and a valve sleeve 24 described hereinafter.

The rotor cylinder 14 is further provided between its peripheral cylindrical surface and the cylindrical bores 16 with hydraulic fluid supply (or suction) ports 23a and exhaust (or discharge) ports 23b extending outwardly in radial directions. A valve sleeve 24 having the shape of a hollow cylinder with open ends is fitted rotatably within and in sliding contact with the inner cylindrical surface of the housing 11 and rotatably accommodates therewithin the rotor cylinder 14. For oiltight sealing, O-rings 25 are provided between the housing 1 1 and the valve sleeve 24. The valve sleeve 24 is provided with ports 26a and 26b formed therein in positions to confront the

ports

23a and 23b, respectively, as indicated in FIG. 1. These ports 26a and 26b are each of a number equal to the number (nine in the instant embodiment) of the crests of the

cam plates

20 and 21, and each of these ports has a certain angular width as viewed in the axial direction as in FIG. 2. Although not shown in FIG. 2, the ports 26a and 26b are formed in mutually staggered or offset positions as viewed in the axial direction so that the both ports 26a and 26b will not simultaneously confront the

ports

23a and 23b of the rotating rotor 13 as described hereinafter.

The inner cylindrical surface of the housing 1 1 is provided with

annular grooves

27a and 27b formed therein around the entire inner circumference at positions to confront the ports 26a and 26b of the valve sleeve 24. The housing 11 is further provided with a hydraulic fluid supply (or suction) port 28a and an exhaust(or discharge) port 28b communicating the

annular grooves

27a and 27b respectively with the outside.

The rotor cylinder 14 is further provided with through holes 29 formed therethrough parallel to the cylindrical bores 16. In the instant embodiment, the through holes 29 are of the same number (six) as the cylindrical bores 16. The two ends of each of the through holes 29 are communicative with spaces 30 formed between the housing 11 and the two ends of the rotor cylinder 14. The end cap 12 has on its inner side a central cavity 31 accommodating one end of the rotor shaft 15 and a fluid passage hole 32 communicating the cavity 31 to one of the above mentioned spaces 30. The end cap 12 is further provided with an aperture 33 for controlling leakage fluid which communicates the cavity 31 with the outside. In the case where, during the operation of the fluid device, hydraulic fluid leaks out of the rotor cylinder 14 through interstices such as gaps between the seal rings 18 and

ball pistons

17a and 17b and the cylindrical bores 16, the-fluid which has thus leaked collects in the spaces 30 and it then discharged out of the fluid device through the through holes 29, fluid passage hole 32, the cavity 31, and the fluid control aperture 33.

When hydraulic fluid at a pressure higher than the normal fluid supply pressure is supplied through the control aperture 33, the

ball pistons

17a and 17b are subjected to this pressure from the side of the spaces 30 and are pushed inward in the cylindrical bores 16, overcoming the force of the springs 19. Consequently, the

ball pistons

17a and 17b are released from contacting with the

cam plates

20 and 21, and the rotor 13 assumes a state wherein it can rotate freely, that is, a state wherein it can freewheel. This state corresponds to that of the fluid device in cases such as that wherein it is used, for example, as the motor of a winch and is caused to lower a load abruptly after the load has been hoisted.

The above described through holes 29 have, in addition to being passage holes for fluid which has leaked, the function of also reducing the mass of the rotor cylinder 14 thereby to decrease the moment of inertia thereof and to improve the rotational responsiveness. In the case where this fluid device is used as a servomotor operated in cooperation with a servo-valve, its control performance, particularly, is remarkably improved. The shape of the cross section of the through holes 29 is not limited to a circle but may be any suitable shape such as a circular sector or an ellipse. By selecting the optimum cross-sectional shape in each case, the moment of inertia of the rotor 13 can be reduced even more effectively.

An essential part of the fluid device 10 according to this invention is the capacity or displacement'varying machanism, which will now be described with respect to one embodiment thereof. The cam plate 21 has a cam surface which is identical to that of the cam plate and, in assembled state, is in coaxial opposition to the cam surface of the cam plate 20. The cam plate 21 further has a cylindrical hollow part 34, as indicated in FIG. 3, which is rotatably fitted onto a hollow boss part of the end cap 12 on the inner side thereof. The cam plate 21 further has an arm extending downward and having a lower end on which a partial (sector) worm gear 36 is formed. This worm gear 36 is meshed with a worm 37 supported on and fixed to a rotatable shaft, which extends out of the casing, and a turning knob 38 for adjustably varying the capacity or displacement volume is fixed to the outer end of this shaft. The rotational angle of the knob 38 is indicated by a calibration scale 39. By turning the knob 38, the worm 37 is rotated, and the cam plate 21 is rotated about the boss part of the end cap 12.

The cam plate 21 is still further provided at its upper peripheral part with a partial (sector) gear 40 formed integrally therewith and meshed with a pinion or gear wheel 41. The gear wheel 41 is supported on and fixed to a shaft 42, which is fitted in a hole 43 in the end surface of the valve sleeve 24 in a freely rotatable manner and, moreover, in a manner such as to be rotatable integrally together with the valve sleeve without any play relative to the center of rotation of the valve sleeve. The gear wheel 41 is further meshed with a partial internal gear 44 formed on the inner side of the housing 11 at its end to which the end cap 12 is secured.

The radii and gear ratios of the cam plate 21 and the gear wheel 41 are so selected that, when the cam plate 21 is rotated in one direction by an angle of 2 0 by the worm 37, the valve sleeve 24 will be rotated in the same direction by an angle of 09.

The fluid device 10 and the capacity varying mechanism of the above described construction according to this invention operate in the manner described below with respect to the case where the fluid device is used as a fluid motor.

High-pressure hydraulic fluid supplied into the supply port 28 a flows successibely through the annular groove 27a and ports 26a. The position of the valve sleeve 24 is initially so set that, when a pair of

ball pistons

17a and 17b, abutting against the cam surfaces of the

cam plates

20 and 21, assume a state wherein they are pressed into their innermost positions within their cylindrical bore l6(i.e., the positional state indicated by the lower pair of ball pistons in FIG. 1), a port 26a of the valve sleeve 24 and the pertinent port 23a of the rotor 13 become communicative. Furthermore, the relative angular positions of the

cam plates

20 and 21 are initially so set that, in order to obtain the maximum displacement volume, the crests of their respective cam surfaces are in exactly opposed alignment.

When the port 26a and the pertinent port 23a become communicative as mentioned above, the highpressure hydraulic fluid supplied through the port 28a flows through the port 23a and is thus supplied into the space between the seal rings 18a and 18b within the cylindrical bore 16. As a result, the pressure of the hydraulic fluid thus supplied is transmitted through the seal rings 18a and 18b to the

ball pistons

17a and 17b to impart outwardly directly forces to the

ball pistons

17a and 17b, that is, a rightward force to the ball piston 17a and a left-ward force to the ball piston 17b as viewed in FIG. 1. Consequently, the

ball pistons

17a and 17b are respectively forced against the cam surfaces of the

cam plates

20 and 21 and at the same time are forced to rol down inclined cam surfaces toward the bottoms of the valleys between one pair of corresponding cam crests 20a and 21a to the succeeding pair of cam crests. Since the forces due to the pressure of the hydraulic fluid at this time is divided into respective components in the rotational direction of the rotor 13 and components in the axial direction of the cylindrical bore 16, the

ball pistons

17a and 17b roll down their respective inclined cam surfaces and, at the same time, impart a torque to the rotor 13 whereby the rotor 13 rotates in one direction.

The angular widths and relative positional relationship of the ports 26a and 26b are so set that. during the interval from the state when the

ball pistons

17a and 17b contact the cam crests 20a and 21a to the state when they reach the bottoms of the valleys 20b and 21b of the cam surfaces after rolling down the inclined surfaces (the latter state corresponding to that of the upper pair of ball pistons in FIG. 1), the

ports

26a and 23a are in a communicative state, while, at the same time, the ports 26b and 23b are in a noncommunicative state. When the

ball pistons

17a and 17b are at the bottoms of the cam valleys 20b, the

ports

20a and 23a become non-communicative, while ports 26b and 23b become communicative.

At this time, the above described operation is started with respect to the

ball pistons

17a and 17b in another cylindrical bore 16, whereby the rotor 13 receives a further torque urging it to rotate in the same direction.

The rotation of the rotor 13 causes the ball pistons 17a"- and 17b which have reached the bottoms of their respective cam valleys to roll up the inclined surfaces leading to the succeeding cam crests and are thereby displaced inward (i.e., respectively leftward and rightward as viewed in FIG. 1) in their cylindrical bore 16.

Consequently, the hydraulic fluid in the cylindrical bore 16 between the seal rings 18a and 18b is discharged by way of ports 23b and 26b, annular groove 27b, and port 28b to a drain side outside of the fluid device 10. Thereafter, the above described operation is repeated successively with respect to the remainder of the pairs of ball pistons, whereby the rotor 13 continues to rotate smoothly. The torque thus imparted to the rotor 13 is transmitted to the outside through the rotor shaft 15, the fluid device 10 thereby operating as a motor.

In the case where the rotational direction of the rotor 13 is to be reversed relative to that described above, the port 28b is used as the supply port of the highpressure hydraulic fluid, and the port 28a is used as the discharge or exhaust port. Furthermore, in the case where the fluid device 10 is to be used as a pump, the

rotor shaft 15 is rotated by mechanical motive power supplied from the outside, and the

port

28a or 28b is used as the suction port, while the

port

28b or 280 is used as the discharge port.

The displacement volume between the

ball pistons

17a and 17b in this fluid device is determined by a relative movement stroke wherein the respective movement directions of the ball pistons are also taken into consideration. The relative movement stroke of the

ball pistons

17a and 17b is determined by the relative opposed positions of the cam crests and valleys of the mutually

opposed cam plates

20 and 21. Furthermore, the normal operation of this fluid device 10 as a motor or as a pump is determined by the variation of the above mentioned displacement volume and by the valve timing of the communicative and non-communicative states of the ports 26a and 26b of the valve sleeve 24 and the

ports

23a and 23b of the rotor cylinder 14 of the rotor 13.

For example, in an extreme case wherein the relative positions of the

cam plates

20 and 21 are so set that the cam crests of one cam plate are mutually opposite the cam valleys of the other, and the cam valleys of the former cam plate are mutually opposite the cam crests of the latter, the

ball pistons

17a and 17b merely move simultaneously to the right or to the left, while maintaining a constant spacing mutually therebetween, as the rotor 13 rotates. In this case, accordingly, there is no variation of the displacement volume, and suction of hydraulic fluid and discharge thereof are not accomplished when the rotor 13 is forced to rotate compulsorily by power applied from the outside.

In varying the displacement volume, the aforedescribed knob 38 is rotated by a suitable angle, whereupon the worm 37 rotates to rotate the worm gear 36, whereby the cam plate 21 is rotated. Simultaneously, the rotation of the cam plate 21 causes the gear wheel 41 meshed with

gear teeth

40 and 44 to undergo a planetary gear motion whereby it rotates about its own axis and simultaneously revolves about an orbital path. The orbital revolution of the gear wheel 41 causes an integral rotation of the valve sleeve 24. In this operation, as mentioned hereinbefore, the rotation of the cam plate 21 by an angle 2 6 causes the valve sleeve 24 to rotate in the same direction by an angle 0.

The case where only the cam plate 21 is rotated by an angle 2 0, and the valve sleeve 24 is held fixed as it is will now be considered. For this purpose, reference is made to FIG. 5A, which indicates the variation of the rightward displacement stroke of the ball piston 17a on the right side in abutment with the cam plate 20 with respect to the rotational angle of the rotor 13. FIG. 58 indicates the variation of the leftward displacement stroke of the left ball piston 17b abutting against the cam plate 21 with respect to the rotational angle of the rotor 13. As is apparent from a comparison of FIGS. 5A and 5B, the angular position of the peak of curve shown in FIG. 5B is offset by the angle 2 0 from the angular position of the peak of the curve shown in FIG.

5A. However, the valve sleeve 24 is not being rotated relative to the cam plate 20, and the valve timing determined by the positional relationships of the ports 26a and 26b of the valve sleeve 24 and the

ports

23a and 23b of the rotor 13 are the same as those prior to the rotation of the cam plate 21. More specifically, the valve timing is as follows. With respect to the curve of FIG. 5A, in the intervals from the bottoms of the valleys to the peaks of the curve (i.e., 0 to 20, 40 to 60 etc., in the instant embodiment), the hydraulic fluid supplying port 28a is communicating by way of the

ports

26a and 23a with the cylindrical bore 16, whereby the mechanism is in the valve-open state, while the discharge port 28b is not communicative with the cylindrical bore 16, and the mechanism is in the valve-closed state, the pressure within the cylindrical bore 16 being at a high value. Furthermore, during the intervals from the peaks to the bottoms of the valleys (i.e., 20 to 40, 60 to etc., in the instant embodiment), the states are reversed. That is, the hydraulic fluid supplying port 28a is not communicative with the cylindrical bore 16, whereby the mechanism is in the valve-closed state, while the discharge port 28b is communicative by way of ports 26b and 23b with the cylindrical bore 16, whereby the mechanism is in the valve open state, and the interior of the cylindrical bore 16 is at a low pressure.

At this time, the curve of variation of the displacement volume determined by the spacing between the right and left

ball pistons

17a and 17b and by the respective displacement directions of these ball pistons becomes as indicated in FIG. 5C. As is apparent from this figure, the relationship between the variation of the ball pistons and the above described valve timing is such that a negative part in the displacement volume is produced. Moreover, this negative part is produced abruptly at the valve timing changeover point as is best indicated on the right-hand side of the curve shown in FIG. 5C. This negative part of the displacement volume means that while the fluid device 10 is being operated normally as a motor, for example, it suddenly operates as a pump. This phenomenon is undesirable.

Accordingly, the fluid device of this invention is so adapted that the cam plate 21 is rotated by the angle 2 0, while, at the same time, the valve sleeve 24 is rotated by the angle 0. FIGS. 4A and 4B, similarly as in FIGS. 5A and 5B, respectively, indicate curves of variations of strokes in the rightward and leftward strokes of the right side and

leftside ball pistons

17a and 17b. Since the cam plate 21 is rotated by the angle 2 6 relative to the cam plate 20, the peaks of the curves of FIGS. 4A and 4B are mutually offset by the angle 2 0.

Furthermore, since the valve sleeve 24 is rotated by the angle 6 in the same direction as the cam plate 21, the changeover point of the valve timing is offset by the angle 0 from that in the case described above in conjunction with FIGS. 5A, 5B, and 5C. More specifically, the interval wherein the hydraulic fluid supplying port 28a is in communicative (valve-open) state with the cylindrical bore 16, and the discharge port 28b is in non communicative (valve-closed) state with the cylindrical bore 16 corresponds to the range wherein the rotational angle of the rotor 13 is 0 to (20 0), (40 0) to (60 0), etc. in FIG. 4A. The interior of the cylindrical bore 16 at this time is at a high pressure. F urthermore, the interval wherein the hydraulic fluid supplying port 28a is in non-communicative (valveclosed) state with the cylindrical bore 16, and the discharge port 28b is in communicative (valve-open) state with the cylindrical bore 16 corresponds to the range wherein the rotational angle of the rotor 13 is (20 9) to (40 (60 0) to (80 0), etc. in FIG. 4A. The interior of the cylindrical bore 16 at this time is at a low pressure.

The curve of variation at this time of the displacement volume determined by the spacing between the right and left

ball pistons

17a and 17b and by their respective stroke directions is as shown in FIG. 4C. The strokes of the

ball pistons

17a and 17b at the point of angle 0 in FIGS. 4A and 4B are equal. At this point, furthermore, changeover of the valve timing is carried out. For this reason, as indicated in FIG. 4C, the curve of variation of the displacement volume is always a continuous curve of a normal hump or peak shape which is on the positive side with respect to the vertical coordinate, and there is no possibility of occurrence of negative parts and discontinuous parts in the curve as shown in FIG. 5C. Accordingly, the fluid device always operates as a normal motor or pump.

The displacement volume varies in accordance with the selection of the value of the rotational angle 2 0 of the cam plate 21, and, with increase in the angle 2 0, the displacement volume decreases in the manner indicated by the dashed lines in FIG. 4C. In the instant embodiment, the displacement volume becomes zero when the angle 2 0 is 40.

While the

cam plates

20 and 21 both receive reaction forces due to the

ball pistons

17a and 17b, there is no occurrence of deviation in the position one of the cam plates (cam plate 20) since this cam plate 20 is fixed. Accordingly, by merely constructing only the movable cam plate 21 so that it will not be subject to positional deviation due to reaction force, it is always possible to select accurately the mutual positions of the

cam plates

20 and 21 and to accomplish accurate variation and adjustment of the capacity. Furthermore, since the valve sleeve 24 is never subjected to a reaction force due to the rotation of the rotor 13, there is no necessity of strict specifications in the construction of parts such as the gear wheel 41 and the shaft 42.

Another embodiment of the mechanism for transmitting the rotation of a cam plate to the valve sleeve is illustrated in FIG. 6. A cam plate 50 of this embodiment corresponds to the movable cam plate 21 of the preceding embodiment, and has gear teeth 51 on the inner side of its end. These gear teeth 51 are meshed with a gear wheel 52 supported rotatably on a vertical shaft 53 supported on the outer peripheral surface of the valve sleeve 24 in the vicinity of the end thereof. The gear wheel 52 is further meshed with a gear teeth 54 formed in the housing 11. Rotation of the cam plate 50 causes the gear wheel 52 to rotate about its axis and, at the same time, to revolve about the axis of the valve sleeve 24, which is caused to rotate. The mechanism of this embodiment is advantageous in that the radius of the gear wheel 52 relative to the radius of the cam plate 50 can be selected at will.

Further, this invention is not limited to these embodiments but various variations and modifications may be made without departing from the scope and spirit of the invention.

What I claim is:

l. A variable-displacement type fluid device operable as a pump and as a motor, comprising: a casing having a hollow space therein; a rotor for rotation in said space and provided with a plurality of cylindrical bores extending in directions parallel to its axis; a pair of ball pistons provided with a space therebetween within said bores; means for continually urging said pistons in opposite directions, outward from said bores; two cam members respectively disposed on opposed sides of said pistons and having respective cam surfaces against which said pistons abut, one of said cam members being supported rotatably in said casing and having first and second gear portions, the other cam member being fixed to said casing; a valve sleeve provided rotatably within said casing in coaxial confrontal relation to a rotational outer peripheral surface of said rotor; ports formed in said casing for selectively supplying and exhausting, as well as for suction and discharge of, hydraulic fluid for motor and for pump operation; ports formed in said sleeve and said rotor, operating in accordance with the rotation of the latter, selectively to place a specific one of said casing ports in communicative and non-communicative state with said space between said pistons; a first gear for meshing with said first gear portion and for rotating said one cam memher; a shaft extending through said casing for rotating said first gear; a second gear supported rotatably in said sleeve, for gear meshing with said second gear portion; and a fixed gear part provided on the inner side of said casing; said second gear being rotated about its own axis and revolved in orbit motion with said sleeve by the rotation of said one cam member, and said sleeve being rotated by means of said second gear by an angle which is one half of the rotational angle of said one cam member.

2. The fluid device as defined in claim 1, wherein said shaft is extended through said casing in the same plane as the rotating plane of said one cam member, and said shaft is provided with a knob at the outermost end thereof.

3. The fluid device as defined in claim 1, wherein said casing has therein a boss part on which said shaft is journaled, so that said one cam member rotates about said boss part within said casing.

4. The fluid device as defined in claim 1, wherein said second gear has a rotational shaft and is rotatably supported at the end surface of said sleeve so that said shaft is parallel to the axis of rotation of said rotor, and that said second gear rotates in a plane parallel to the plane of rotation of said one cam member.

5. The fluid device as defined in claim 1, wherein said second gear has a rotational shaft and is rotatably supported on the outer peripheral surface of said sleeve in the vicinity of one end thereof so that said shaft is perpendicular to the axis of rotation of said rotor, and that said second gear rotates in a plane perpendicular to the plane of rotation of said one cam member.

6. The fluid device as defined in claim 1, comprising a plurality of through holes formed through said rotor, parallel with said bores, and extending in directions parallel to the axial direction, said holes providing communication between cavity spaces within said casing at the two ends of said rotor for flow of the hydraulic fluid which has leaked, and to reduce the moment of inertia of said rotor.

7. The fluid device as defined in claim 6, further com-- prising an outlet for discharging leaked hydraulic fluid, communicating said cavity space on one side of said rotor with the outside of said casing.

Claims

1. A variable-displacement type fluid device operable as a pump and as a motor, comprising: a casing having a hollow space therein; a rotor for rotation in said space and provided with a plurality of cylindrical bores extending in directions parallel to its axis; a pair of ball pistons provided with a space therebetween within said bores; means for continually urging said pistons in opposite directions, outward from said bores; two cam members respectively disposed on opposed sides of said pistons and having respective cam surfaces against which said pistons abut, one of said cam members being supported rotatably in said casing and having first and second gear portions, the other cam member being fixed to said casing; a valve sleeve provided rotatably within said casing in coaxial confrontal relation to a rotational outer peripheral surface of said rotor; ports formed in said casing for selectively supplying and exhausting, as well as for suction and discharge of, hydraulic fluid for motor and for pump operation; ports formed in said sleeve and said rotor, operating in accordance with the rotation of the latter, selectively to place a specific one of said casing ports in communicative and non-communicative state with said space between said pistons; a first gear for meshing with said first gear portion and for rotating said one cam member; a shaft extending through said casing for rotating said first gear; a second gear supported rotatably in said sleeve, for gear meshing with said second gear portion; and a fixed gear part provided on the inner side of said casing; said second gear being rotated about its own axis and revolved in orbit motion with said sleeve by the rotation of said one cam member, and said sleeve being rotated by means of said second gear by an angle which is one half of the rotational angle of said one cam member.

7. The fluid device as defined in claim 6, further comprising an outlet for discharging leaked hydraulic fluid, communicating said cavity space on one side of said rotor with the outside of said casing.