US3366072A

US3366072A - Pump or motor device

Info

Publication number: US3366072A
Application number: US409130A
Authority: US
Inventors: Stephen S Baits; Walter J Iseman
Original assignee: Sundstrand Corp
Current assignee: Sundstrand Corp
Priority date: 1964-11-05
Filing date: 1964-11-05
Publication date: 1968-01-30
Anticipated expiration: 1985-01-30
Also published as: DE1528549B1; DE1528549C2; JPS4811521B1; GB1123375A

Description

Jan. 30, 1968 s. s. BAITS ET AL 3,366,072

PUMP QR MO TOR DEVICE Filed Nov. 1964 2 Sheets-Sheet l Jan. 30, 1968 s. s. BAITS ET AL 3,366,072

PUMP (JR-MOTOR DEVICE Filed Nov. 5, 1964 2 Sheets-Sheet 2 if H United States Patent O 3,366,072 PUMP R MOTOR DEVICE Stephen S. Baits and Walter J. Iseman, Rockford, Ill., illSlS lgIlPl'S to Sundstrand Corporation, a corporation of Filed Nov. 5, 1964, Ser. No. 409,130 12 Claims. (Cl. 103162) The present invention relates to hydraulic energy translating devices and more specifically to axial piston pumps and motors wherein valving is accomplished by rotation of a cylinder block against a stationary valve member.

In axial piston units of the type described, an angularly related cam member reciprocates the pistons in a rotating cylinder block and the pistons receive and discharge fluid through arcuately shaped ports in a stationary valve plate engaging the cylinder block. In existing units, optimum speeds for obtaining high poWer-to-weight ratios with good efflciency are about 120 inches per second average piston velocity. It is desirable that the unit be able to run faster than this in order to withstand overspeeds or to obtain higher power-to-Weight ratios.

So-called slipper units, with individual slippers on the pistons engaging the cam plate, can attain 120 inches per second, but only with very careful development and heavy spring bias on the pistons and cylinder block urging the former toward the cam and the latter toward the valve plate. One difliculty lies in the tendency of the slippers to tip off the cam plate due to centrifugal force. Slippers suffer immediate permanent damage if tipping occurs. Another difliculty lies in the high tipping moment on the block resulting from the high centrifugal forces of the relatively heavy pistons. Further, the inherent overhang of the pistons in slipper units is distinctly detrimental to performance because of the side loads on the pistons.

So-called bent units perform well but are bulky and awkward in variable stroke designs where the entire cylinder block and valve plate are pivotal to vary stroke.

It is therefore a primary object of the present invention to provide a new and improved axial piston pump with no overhung loads on the pistons and stroke adjustability by tilting the cam plate.

It is also an object of the present invention to provide a new and improved axial piston pump with articulated pistons connected to the swashplate through a unitary annular ring having only axial forces thereon to readily permit high speed operation.

Another object of the present invention is to provide a new and improved axial piston pump with connecting rods between the swashplate and the pistons with the connecting rods substantially perpendicular to the swashplate in all adjusted positions thereof so that the forces on the swashplate are substantially perpendicular only,

A further object of the present invention is to provide a compact variable stroke pump with lightweight pistons and low inherent clearance volume allowing eflicient operation for long life at very high speeds.

Another object of the present invention is to provide a new and improved axial piston pump with a cylinder block having a length substantially shorter than its diameter and the piston forces acting laterally on the cylinder block closely adjacent to the valve plate.

A more specific object of the present invention is to provide a new and improved axial piston fluid pump having a housing with a valve plate mounted therein with inlet and outlet passages and having a porting surface thereon, a cylinder block rotatably mounted in the housing having a plurality of axial cylinders and a face communicating with the cylinders and engaging the valve plate surface, the block having an axial length approximately the same as its radius, an input shaft rotatably mounted in the housing and splined to the cylinder block for rotating the same,

pistons slidably mounted in the cylinders adapted to receive and expel fluid through the passages, the pistons having semispherical sockets therein, a swashplate pivotally mounted in the housing and having a flat camming surface, a unitary annular ring rotatably mounted on the camming surface with a plurality of semispherical sockets therein corresponding in number to the pistons, a plurality of connecting rods between the ring and the pistons each having semispherical balls at both ends thereof seated respectively in one of said piston sockets and one of the ring sockets, means to restrain the connecting rods to stay approximately perpendicular to the cam plate in all adjusted positions of the cam member, and a resiliently biased retaining ring for urging the connecting rods and the annular ring against the camming surface and for urging the cylinder block against the valve plate.

Other and further objects and advantages of the present invention will be readily apparent from the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a longitudinal sectional view of the hydraulic unit of the present invention;

FIG. 2 is a cross-sectional view of the hydraulic unit taken generally along line 22 of FIG. 1;

FIG. 3 is a cross-sectional view taken generally along line 3-3 of FIG. 1 with the swashplate and connecting rods removed.

While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail an embodiment of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiment illustrated. The scope of the invention will be pointed out in the appended claims.

It should be understood that, while the hydraulic unit described in this specification is referred to as a pump, the principles of the invention are equally applicable to a hydraulic unit acting as a motor.

Referring now to FIG. 1, a pump housing 10 may c0mprise a unitary met-a1 casting having a bushing 11 pressfitted within a counterbore in one end of the housing 10 for seating the outer race of bearing 12. The bearing 12 supports a pump input shaft 13 Within the housing 10. One end of shaft 13 has splines 14 thereon which engage drive splines in a central bore 16 in a pump cylinder block 17.

The cylinder block 17 is a cylindrical metal body having, for example, seven cylinders 18 formed therein opening directly to end face 20 of the cylinder block and communicating with the cylinder block port face 21 through cylinder block ports 22. Viewing FIG. 3, radially inwardly of cylinders 18 and angularly disposed intermediate the cylinders, closed end bores 25 are formed in an annular pattern in the cylinder block 17 opening at cylinder block face 20 for receiving springs for a purpose described hereinafter. As shown in FIG. 1, the axial length of the cylinder block 17 along the axis of rotation Otf shaft 13 is less than the radius of the block in a plane perpendicular to the axis of shaft 13.

Slidably mounted in each of the cylinders 18 are pistons 26 each having a semispherical socket 27 which surrounds and captures a spherical ball 28 of one of a pinrality of connecting rods 29. Spherical balls 31 are integrally formed on the other ends of the connecting rods 29 respectively, and are seated within semispherical sockets 32 in a unitary annular ring 33. The connecting rods 29 are therefore pivotally connected to both the pistons 26 and the unitary annular ring 33.

The annular ring 33 serves to transfer the camming force from the swashplate or cam member 35 to the pistons 26 and has the plurality of the sockets 32 formed in one face thereon a circle drawn about the axis or center of the annular ring 33. The swashplate or cam member 35 is pivotally mounted in the housing by rollers 40 and 41 rotatably mounted on shafts 42 and 43 fixed in the housing 10, there being two rollers on each of the shafts 42 and 43. Parallel arouate surfaces 45 are formed on the back of the swashplate 35 to receive rollers 40 and 41 so that the swashplate pivots about point 46. As shown, a counterbore is formed in the face of the swashplate 35 loosely receiving annular ring 33 and defining a camming surface 48 which serves to earn the unitary annular ring 33 and :the pistons 26 to drive the pistons to the right and expel fluid from the cylinders 18.

In this specific construction illustrated, the swashplate 35 is pivotable from the maximum stroke position illustrated in FIG. 1 to a neutral, no-stroke position in which the cam face 48 is parallel to the end of the cylinder block, but it will be understood that in other embodiments, the swashplate may be fixed or pivotable in opposite directions from a neutral, no-stroke position. The center 46 about which the swashplate pivots is usually located near the point where the shaft axis pierces the plane containing the spherical connections of rod ends 28 with pistons 26. The pivot axis 46 may be located directly at such point, but it is preferably displaced from such point herein, axially and radially, in order to modify the nature of the forces required to control the swashplate angle, and such relocation of the pivot is facilitated by the present constnuction including the free mounting of ring 33 in the swashplate.

The cylinder block 17 is supported by a bearing 49 in the housing 10, and the point where the shaft axis pierces the plane containing the spherical rod ends 28 preferably bisects the axial length of the bearing 49 supporting the cylinder block in the housing 10, or, in the case of a shaft supported block, bisects the radially locating connection between block and shaft.

In order to cause rotation of the annular ring 33, with the cylinder block 17, the connecting rods 29 are maintained approximately perpendicular to the annular ring 33 by means of a retaining ring 50 fixed on the annular ring 33 and disposed inside the annular array of connecting rods 29. The retaining ring 50 is formed with guide channels as at 50a respectively, receiving the reduced central portions of the connecting rods 29 and maintaining the latter generally perpendicular to the ring 33 but permitting limited pivotal movement relative to the ring 33 as will appear.

At a full displacement position of the swash-plate member 35 the connecting rods have a substantial angular relationship to the pistons 26 and therefore exert side loads on the pistons through the spherical balls 28, but the connecting rods 29 remain substantially perpendicular to the camming surface 48 in all adjusted positions of the swashplate 35 and therefore exert no side loads, or loads parallel to face surface 48 on the cam member or swashplate 35. As the input shaft 13 rotates the cylinder block 17 together with the pistons 26 and the annular ring 33, the pistons 26 and the universal connections 28 therein will travel in a circular path about the axis of the shaft 13, and the universal connections 31 in the annular ring 33 will travel in a circular path about the axis of the ring 33, but when viewed from the plane of the annular ring 33, the connections 28 will travel in an elliptical path relative to the path of the connections 31 so that while the connecting rods remain effectively normal ot the swashplate, some limited pivotal movement must be provided at the ball joints 31 and the fit of the connecting rods in the guide channels 50a permits the necessary movement. As the annular ring 33 holding the connecting rods in place against the swashplate 35 is a one piece construction, the centrifugal forces on the rods due to rotation of the ring with the cylinder block 17 are restrained and there is no danger of the rods being thrown radially outward as in separate slipper units.

Each of the connecting rods 29 exerts a lateral force on the pistons 27, the centroid of which substantially bisects the axial length of the cylinder block bearing 49 as described above, and lies close to the port face 21 a distance therefrom less than the radius of the block. In this manner the side forces on the block 17 are located close to the porting face 21 of the cylinder block and are effectively opposed by bearing 49, the proper location of which is facilitated by the present construction.

The retaining ring 50 is mounted within a central opening 51 of the annular ring 33 and engages the annular ring and the spherical balls 31 on the connecting rods 29 thereby serving as a piston return mechanism holding the connecting rods and the annular ring 33 against the swashplate 35 during intake strokes of the piston, i.e., when the pistons are moving to the left in FIG. 1. The retaining ring 50 is resiliently urged toward the annular ring 33 by springs 52 mounted in each of the closed end bores 25 of the cylinder block acting through means including an annular spring seat member 54a of angular cross section slidably fitted on shaft 13 tightly enough to prevent objectionable tipping and carrying an annular semispherical ball member 54b loosely mounted on the member 54a. The spherically shaped outer surface of member 5412 engages a complementary inner surface of retainer 50, both preferably concentric about point 46. In this manner the springs 52 also serve to urge the cylinder block 17 against a porting and seating surface 60 on valve plate 61 which is bolted to housing 10 as at 62. The cylinder block 17, connecting rods 29, and annular ring 33 rotate as a unit with the input shaft 13.

The port plate 61 has arcuate ports 64 and 65 opening to the seating surface 60 that serve as either high or low pressure ports depending upon the direction of rotation of input shaft 13 and operation as a pump or motor. Communicating with the ports 64 and 65 are inlet or

outlet passages

66 and 67 in the port plate 61 adapted to be connected respectively to a suitable Source of fluid (not shown) and the hydraulic load (not shown).

The angle of the swashplate may be controlled in any suitable manner. As illustrated in FIG. 2, a constant pressure valve 70 is mounted within the port plate 61 and serves to maintain a constant outlet pump pressure. As the details of the constant pressure valve form no part of the present invention, they will be described only generally. A chamber 71 communicates with the high pressure passage of the pump normally balancing spool 72 against the force of spring 73 in operation. As the pressure in high pressure passage increases above the desired constant level, the spool 72 will move to the right against the force of the spring, closing port 74 connected to the tank (not shown) and opening port 75 which is also connected to the high pressure passage (or a source of control fluid) thereby admitting pressure fluid to passage 76. Passage 76 communicates with a chamber 78 behind the swashplate control piston 79 shown in FIG. 1. In this manner as the pressure increases the piston 79 will move to the left decreasing the angle of the swashplate 35 and reducing the piston stroke and thereby lowering the pressure in the high pressure passage. When the pressure in the high pressure passage again reaches the desired level, the pressure in chamber 71 will balance the force of the spring 73 on the spool valve and hold the piston 79 at a position to deliver fluid at the desired pressure.

A swashplate return mechanism 80 pivots the swashplate or cam member 35 to full displacement when the spool valve ports the chamber 78 to tank. The return mechanism consists of a piston 81 urged toward the swashplate by a spring 82 mounted within sleeve 83 fixed within a bore in a boss 85 formed on the upper portion of the housing 10.

An end cap 86 bolted to the left end of housing 10 as at 87 supports a shaft seal assembly generally designated by the numeral 88. The shaft seal consists of a first seal member 89 fitted on shaft 13 and a second seal member 90 mounted within a counterbore in end cap 86 and resiliently urged toward the first seal member 89 by springs 91.

Seal members

89 and 90 engage one another along

radial surfaces

92 and 93 forming a seal preventing the escape of any hydraulic fluid from the interior of the pump housing.

Suitable seals

95 and 96 prevent the escape of fluid along the housing and along the shaft 13 respectively, and a seal 97 prevents leakage between the end closure and the housing.

In order to hydrostatically balance the ring 33 and reduce friction between the ring and the swashplate face 48, thereby to reduce wear, the back face of the ring 33 is preferably provided with fluid chambers 100 respectively ported as at 101 to the sockets receiving the balls 31, and each of the connecting rods 29 includes a longitudinal passage 102 communicating with a port 103 through the end of the associated piston 26 and leading to the cylinder in which the piston reciprocates.

Summarizing the advantages of the construction described herein, it will be understood that use is made of relatively short pistons which are light in Weight and re- Sult in low block tipping moment from centrifugal force and low piston return inertia forces. Clearance volume is inherently low. As a result of the reduced tipping moment, a more uniform film thickness is maintained between the block and the port plate. The pistons are free from overhung loads, and the ball and socket connection of the rod to the piston allows the piston to conform to the bore thereby to utilize the entire piston area to absorb the side load. The centroid of the piston side loads is located less than 75 percent of the length of the block from the cylinder block port face and is effectively opposed by means mounting the cylinder block. Variable stroke and displacement are obtained without swinging the cylinder block. The angle of the connecting rods relative to the pistons is variable with variation in stroke, but the connecting rods remain essentially normal to the swashplate face regardless of stroke, and are restrained in a unitary ring allowing for very high speed operation and freedom in placement of the cam plate pivot point. The cylinder block may be short and lightweight. The compact construction results in savings in space, weight, and manufacturing costs. Piston speeds can be in excess of 120 inches per second easily, and this is useful in providing overspeed capability and in obtaining very exceptional power-to-weight and size ratios.

We claim:

1. An energy translating device comprising: a valve plate having inlet and outlet ports therein and having a seating surface, a rotatable cylinder block having a plurality of cylinders therein and having a face communicating with said cylinders, said face engaging said seating surface and rotatable with respect to said valve plate, said block having an axial length substantially less than its diametrical width, means locating said cylinder block radially, pistons slidably mounted in said cylinders adapted to receive and expel fluid through said passages, a cam member having an camming surface for reciprocating said pistons, and rigid rods pivotally connected at one end to said camming surface and pivotally connected at their other ends to said pistons to reciprocate said pistons, said rods being substantially perpendicular to said camming surface, said rods exerting a side force on each of said pistons, the centroid of the points of pivotal interconnections between said rods and said pistons lying in a plane perpendicular to the axis of said cylinder block and bisecting said locating means.

2. An axial piston fluid pump comprising: a housing, a valve plate mounted on said housing having inlet and outlet passages therein having a seating surface thereon, a cylinder block rotatably mounted on said housing and having a plurality of axial cylinders therein and having a face communicating with said cylinders and engaging said valve plate surface, said block having an axial length approximately the same as its radius, an input shaft ro- 6 tatably mounted in said housing and splined to said cylinder block for rotation thereof, pistons slidably mounted in said cylinders adapted to receive and expel fluid through said passages, said pistons having semisphen'cal sockets therein, a cam member pivotally mounted in said housing and having a flat camming surface, a unitary annular ring rotatably mounted on said camming surface, said ring having a plurality of semispherical sockets therein corresponding to said pistons, a plurality of rigid connecting rods between said ring and said pistons each having semispherical balls at both ends thereof seated respectively in one of said piston sockets and one of said ring sockets, said rods 'being substantially perpendicular to said camming surface in all adjusted positions of said cam member, said rods exerting through said semispherical balls lateral forces on said pistons, the centroid of the points of origin of the forces lying within said cylinder block relatively close to said seating surface, said cam member pivotal axis passing approximately through the centroid of said plane, and a resiliently biased retaining ring for urging the connecting rods and the annular ring against the camming surface and for urging the cylinder block against the valve plate said retaining ring including means for rotating said rods with the cylinder block.

3. An axial piston fluid pump having a housing, a valve plate mounted therein with inlet and outlet passages and having a porting surface thereon, a cylinder block rotatably mounted in the housing having a plurality of axial cylinders and a face communicating with the cylinders and engaging the valve plate porting surface, the block having an axial length approximately the same as its radius, an input shaft rotatably mounted in the housing and splined to the cylinder block for rotating the same, pistons slidably mounted in the cylinders adapted to receive and expel fluid through the passages, the pistons having semispherical sockets therein, a swashplate pivotally mounted in the housing and having a fiat camming surface, a unitary annular ring rotatably mounted on the camming surface with a plurality of hemispherical sockets therein corresponding in number to the pistons, a plurality of rigid connecting rods between the ring and the pistons each having semispherical balls at both ends thereof seated respectively in one of said piston sockets and one of the ring sockets, means constraining the connecting rods approximately perpendicular to the camming surface in all adjusted positions of the cam member, and a resiliently biased retaining ring for urging the connecting rods and the annular ring against the camming surface and for urging the cylinder block against the valve plate.

4. An axial piston pump comprising: a housing, a valve plate in the housing having inlet and outlet passages, a cylinder block rotatably mounted in the housing and having a plurality of axial cylinders communicable through one end of the block successively with said inlet and outlet passages on rotation of the cylinder 'block, an input shaft rotatably mounted in the housing and connected to the cylinder block for rotating the latter, pistons reciprocable in the cylinders respectively, a swashplate mounted in the housing at the opposite end of the cylinder block and having an inclined camming surface, annularly arranged bearing means rotatably mounted and acting as a unit on the camming surface, said annularly arranged bearing means being unrestrained peripherally, a plurality of rigid connecting rods having opposite ends respectively pivotally connected to the pistons and to the annularly arranged bearing means, means on the annularly arranged bearing means permitting limited pivotal movement of the rods relative to the ring and constraining the rods to remain approximately perpendicular to the camming surface, said last recited means laterally locating said annularly arranged bearing means.

5. An axial piston pump, comprising: a housing, a valve plate in the housing having inlet and outlet passages, a cylinder block rotatably mounted in the housing and having a plurality of axial cylinders communicable through one end of the block successively with said inlet and outlet passages on rotation of the cylinder block, an input shaft rotatably mounted in the housing and connected to the cylinder block for rotating the latter, pistons reciprocable in the cylinders respectively, a swashplate mounted in the housing at the opposite end of the cylinder block and having an inclined camming surface, an annular ring rotatably mounted on the camming surface with freedom to move laterally, a plurality of rigid connecting rods having opposite ends respectively pivotally connected to the pistons and to the annular ring, means on the annular ring permitting limited pivotal movement of the rods relative to the ring and constraining the rods to remain approximately perpendicularly to the camming surface, said last recited means laterally locating said annular ring, a spherically shaped inner surface on said annular ring, and a ball member on said input shaft having a spherically shaped outer surface engaging the spherically shaped inner surface on said annular ring, and spring means acting against said cylinder block and said last recited member, urging the cylinder block toward the valve plate and urging the annular ring toward the camming surface.

6. A combination as defined in claim 5, wherein the ball member is loosely mounted around said shaft for limited lateral movement relative thereto.

7. A combination as defined in claim including means mounting the swashplates for pivotal movement about an axis passing through the center of the spherical surface on said ball member.

8. A combination as defined in claim 5 including a spring seat member slidable on said shaft and having an angular cross section including a radially outwardly extending annular flange engaging said spring means and an axially extending annular portion on said shaft and loosely carrying said ball member.

9. A hydraulic energy translating device comprising: a valve plate having inlet and outlet ports therein and having a seating surface, a cylinder block having a plurality of cylinders therein and having a face slidably engaging said seating surface, pistons slidably mounted in said cylinders adapted to receive and expel fluid through said ports, a cam member having a camming surface for reciprocating said pistons, means pivotally mounting said cam member for varying the displacement of the device, annularly arranged bearing means rotatably mounted and acting as a unit on said camming surface with a plurality of semispherical sockets therein corresponding in number to the pistons, a plurality of rigid connecting rods between the bearing means and the pistons each having semispherical portions at both ends thereof seated respectively in one of said pistons and one of the bearing means sockets, and means engageable with said rods separate from said bearing means sockets for driving said bearing means whereby timing torque for the bearing means is transferred from the cylinder block through the rods to the bearing means.

10. A hydraulic energy translating device as defined in claim 9 wherein said means engageable with said rods permits limited pivotal movement of the rods relative to the bearing means and constrains the rods to remain approximately perpendicular to the camming surface, said last recited means laterally locating said bearing means, said cam member unrestraining said bearing means radially.

11. An axial piston pump as defined in claim 4 wherein said means on the bearing means permitting limited pivotal movement of the rods relative to the bearing means includes guide means engageable with said rods to assist in driving the bearing means in rotation.

12. A hydraulic energy translating device comprising: a housing member, a valve member having inlet and outlet ports therein, a cylinder block rotatably mounted in said housing member and having a plurality of cylinders therein, said block being in engagement with the valve member and rotatable relative to said valve member, a bearing supporting said cylinder block and aligned in a plane passing through said cylinders, said bearing comprising a roller bearing mounted in said housing member and having a portion thereof engaging the periphery of said cylinder block, pistons slidably mounted in said cylinders, a cam having a camming surface adjacent the other end of said block, and a plurality of rigid rods connecting the cam to said pistons, each having a universal connection with said cam and with one of said pistons, the centers of the universal connections with the pistons defining a plane the centroid of which substantially bisects said bearing.

References Cited UNITED STATES PATENTS 1,710,567 4/1929 Carey 103162 1,908,612 5/1933 Johnson 103-162 2,141,935 12/1938 Rose 103162 2,146,117 2/1939 Gros 103162 2,860,581 11/1958 Buchner 103162 2,967,491 1/1961 Wiggermann 103l62 3,036,434 5/1962 Mark 103-162 3,075,472 1/1963 Garnier 103162 3,096,723 7/1963 Puryear 103162 FOREIGN PATENTS 539,637 9/1941 Great Britain.

ROBERT A. OLEARY, Primary Examiner.

MARK M. NEWMAN, DONLEY J. STOCKING,

Examiners.

R. M. VARGO, W. L. FREEH, Assistant Examiners.

Patent No. 3,366,072 January 30, 1968 Stephens S. Baits et al.

ed that error appears in the above identified It is certifi e hereby corrected as patent and that said Letters Patent ar shown below:

In the heading to the printed specification, lines 4 and 5, "a corporation of Illinois" should read a corporation of Delaware Signed and sealed this lOth day of March 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

Claims

1. AN ENERGY TRANSLATING DEVICE COMPRISING: A VALVE PLATE HAVING INLET AND OUTLET PORTS THEREIN HAVING A SEATING SURFACE, A ROTATABLE CYLINDER BLOCK HAVING A PLURALITY OF CYLINDERS THEREIN AND HAVING A FACE COMMUNICATING WITH SAID CYLINDERS, SAID FACE ENGAGING SAID SEATING SURFACE AND ROTATABLE WITH RESPECT TO SAID VALVE PLATE, SAID BLOCK HAVING AN AXIAL LENGTH SUBSTANTIALLY LESS THAN ITS DIAMETRICAL WIDTH, MEANS LOCATING SAID CYLINDER BLOCK RADIALLY, PISTONS SLIDABLY MOUNTED IN SAID CYLINDERS ADAPTED TO RECEIVE AND EXPEL FLUID THROUGH SAID PASSAGES, A CAM MEMBER HAVING AN CAMMING SURFACE FOR RECIPROCATING SAID PISTONS, AND RIGID RODS PIVOTALLY CONNECTED AT ONE END TO SAID CAMMING SURFACE AND PIVOTALLY CONNECTED AT THEIR OTHER ENDS TO SAID PISTONS TO RECIPROCATE SAID PISTONS, SAID RODS BEING SUBSTANTIALLY PERPENDICULAR TO SAID CAMMING SURFACE, SAID RODS EXERTING A SIDE FORCE ON EACH OF SAID PISTONS, THE CENTROID OF THE POINTS OF PIVOTAL INTERCONNECTIONS BETWEEN SAID RODS AND SAID PISTONS LYING IN A PLANE PERPENDICULAR TO THE AXIS OF SAID CYLINDER BLOCK AND BISECTING SAID LOCATING MEANS.