Variable displacement fluid translating device

Abstract

An axial piston type fluid energy translating device has fluid passages in the pistons, shoes, swash plate and cam member to conduct fluid from the high pressure port to pockets on the back of the cam member adjacent the cam support. The high pressure fluid in the pockets counter-balances the force applied to the swash plate and cam member by the pistons exposed to high pressure fluid.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The instant invention relates to a variable displacement fluid translating device of the axial piston type and more particularly to a means for counterbalancing and lubricating a cam member of such a device.

2. Description of the Prior Art

A common type of variable displacement fluid translating device such as an axial piston pump or motor has a rotating barrel with a plurality of axially aligned cylinders. A piston connected at one end by a shoe to an angled swash plate is received in each of the cylinders and reciprocates as the barrel is rotated and the shoe slides along the swash plate. The stroke of the piston is controlled by the angularity of the swash plate and determines the volume of fluid displaced by the fluid translating device hereinafter referred to as a pump.

In one form of pump, the swash plate is supported by a cam member which pivots in a cam support about an axis perpendicular to the rotational axis of the barrel in order to change the volume of fluid displaced by the pump. During operation of the pump the cylinders and pistons in the barrel alternately connect with low and high pressure ports. As the pistons are exposed to the high pressure port, a large force is transmitted from the fluid through the pistons and shoes to the cam member. This large force becomes troublesome when it is desired to pivot the cam member in order to change the displacement of the pump. The force causes a great deal of friction between the cam member and the cam support which must be overcome before the cam member can pivot.

One means of reducing the friction between the cam member and its support is to coat one of the surfaces with a material which has a low coefficient of friction. However, it has been found that coated surfaces erode after heavy use and friction between the cam member and the cam support begins to increase.

Another means for reducing the friction between the cam member and the cam support is to connect fluid in the high pressure port with the space between the cam member and the cam support. In U.S. Pat. No. 3,682,044, an exterior hydraulic line is run from the high pressure port to a compensator block which connects the high pressure fluid with passages leading to holes in the cam support which holes communicate with the cam member. The effect of this high pressure fluid is to counterbalance some of the force applied by the pistons on the cam member and to lubricate the contacting cam member and cam support surfaces to reduce the friction between the surfaces and thereby reduce the force required to pivot the cam member relative to the cam support.

A disadvantage of connecting high pressure fluid to apertures positioned in the cam support is that the fluid is not always communicated to the point of maximum force between the cam member and the cam support. This is because the forces applied to the cam member act along a line perpendicular to the face of the cam member which receives the pistons. As the cam member is pivoted in the cam support, this force is directed through the cam member and acts on different portions of the cam support.

It is desirable to supply high pressure counterbalancing fluid between the cam member and the cam support which is at all times opposite the force applied to the cam member by the pistons.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a variable displacement fluid translating device in which the forces applied by the pistons on the cam member are counterbalanced by a pocket of high pressure fluid acting opposite the position at which the pistons are applying the force to the cam member.

It is another object of the instant invention to provide a variable displacement fluid translating device in which high pressure fluid is conducted through the pistons, the shoes and the cam member to pockets on the surface of the cam member adjacent the cam support in order to reduce the friction between the cam member and support.

Other objects of the invention will appear hereinafter, the novel features and combinations being set forth in the claims.

In the variable displacement axial piston pump of the instant invention, high pressure fluid in the barrel cylinders is fed through the pistons, the shoes, the swash plate and the cam member to pockets formed on the back surface of the cam member adjacent the cam support. The pockets are substantially aligned with the surfaces on the cam member to which the forces are applied by the pistons. Feeding the high pressure fluid through passages formed in the piston, the shoes, the swash plate and the rocker cam eliminates external hydraulic lines from the high pressure port to the cam support.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an axial section of a portion of the fluid energy translating device of this invention.

FIG. 2 is a view of the flat surface of the cam member adjacent the swash plate.

FIG. 3 is view of the back surface of the cam member which pivots within the cam member.

FIG. 4 is a view of the swash plate taken along the line 4--4 on FIG. 1.

FIG. 5 is a view similar to that of FIG. 4 with the number of apertures in the swash plate changed.

FIG. 6 is a view similar to that of FIG. 4 with the number of apertures in the swash plate changed.

FIG. 7 is a view similar to that of FIG. 4 with the number of apertures in the creep plate changed.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The axial piston pump 10 of the instant invention has a casing which includes an outer cylindrical body 12 and an end cap 13. The end cap 13 is attached to the body 12 by fastening means not shown. The casing forms an internal cavity 15 which houses the operating mechanism of the pump 10.

A barrel 16 is rotatably mounted in a roller bearing 17 in body 12. Bearing 17 is located in body 12 by a shoulder 18 and a retaining ring 19 which is urged against bearing 17 by end cap 13. An oil seal 20 prevents fluid seepage at the joint between body 12 and end cap 13.

A prime mover not shown drives barrel 16. The prime mover is attached to one end of a shaft 22 which is coupled to barrel 16 by splines 23. Shaft 22 is restrained from moving axially relative to barrel 16 by a bolt 24 which passes through a spring 25, a retainer 26 and is threaded into shaft 22. Clearance between bolt 24 and retainer 26 permits limited axial movement of shaft 22 relative to barrel 16.

Within barrel 16 are formed a plurality of cylinders 28 which are aligned parallel with the axis of rotation of barrel 16. The cylinders 28 are equally spaced from each other within barrel 16 and are displaced radially from the axis of rotation an equal distance. Positioned within each cylinder 28 is a hollow piston 29.

At one end of each piston 29 is a shoe 30 which is attached to a swash plate 31 by a means of a shoe retainer assembly 32. When barrel 16 is rotated, the shoes 30 slide over swash plate 31 and reciprocate the pistons 29 in the cylinders 28 to thereby pump fluid.

The shoe retainer assembly 32 maintains shoes 30 in contact with swash plate 31. The assembly 32 comprises a shoe retainer plate 34 which has a plurality of openings 35 which are of sufficient diameter to pass over a piston 29 and engage a shoulder 36 on each shoe 30. A bolt 38 draws retainer plate 34 towards swash plate 31 to clamp the shoes 30 therebetween. Bolt 38 passes through a retainer washer 39, a socket member 41, cam member 37, a spring 44, a spring retainer 45, and has a castle nut 46 turned on the end thereof. Spring 44 prevents spring retainer 45 from contacting cam member 37 and enables retainer plate 34 to move slightly in response to a high axial force on bolt 38. This allows piston shoes 30 to move away from swash plate 31 slightly and ride on a fluid cushion therebetween.

Cam member 37 is pivotally mounted in a cam support 50. When cam member 37 is moved relative to cam support 50, the stroke of the pistons 29 is changed and therefore the displacement of pump 10 is changed. One means of pivoting cam member 37 is a double headed piston 55 which is connected to cam member 37 by means of a bifurcated arm 52.

When rocker cam 37 is pivoted such that swash plate 31 is perpendicular to shaft 22, there is no reciprocating movements of pistons 29 when barrel 16 is rotated and pump 10 is operating at its minimum fluid displacement when barrel 16 is rotated. When rocker cam 37 is pivoted such that swash plate 31 is angled its greatest amount, the pistons 29 move through their greatest stroke and pump 10 is operating at its maximum fluid displacement when barrel 16 is rotated.

When barrel 16 is rotated and pump 10 is displacing fluid, the pistons 29 are alternately exposed to high and low pressure ports in a port plate 58. The pressure in the high pressure port may be on the order of several thousand pounds per square inch. This force in transmitted through the piston 29 and its shoe 30 to the swash plate 31. The force on the swash plate 31 acts to urge the cam member 37 against the cam support 50. This creates a large amount of friction which must be overcome when cam member 37 pivots relative to cam support 50.

In order to reduce the friction between the cam member 37 and the cam support 50, high pressure fluid is routed to a pocket 71 in the rear face 59 of cam member 37 to counterbalance the force applied to the swash plate 31. Pocket 71 is best seen in FIG. 3.

High pressure fluid flows through bore 61 in piston 29, a bore 62 in shoe 30 and into a cavity 63 adjacent swash plate 31. Swash plate 31 has a plurality of circumferentially arranged passages 65 extending therethrough as seen in FIGS. 1 and 4 through 7. Each passage 65 is in communication with a shallow metering groove 66 formed on the bottom side of swash plate 31 adjacent cam member 37.

As seen in FIG. 2, a pair of ports 67 are formed in rocker cam 37 to receive fluid which feeds from the grooves 66. One port 67 is aligned with the grooves 66 and passages 65 which receive fluid from pistons exposed to the high pressure port and the other port 67 is aligned with the grooves at 66 and passages 65 which receive fluid from the pistons exposed to the low pressure port of the pump.

Since only one pump port at a time will conduct high pressure fluid, only one port 67 at a time will receive high pressure fluid.

A bore 70 extending from the top surface 68 of rocker cam 37 to the rear face 59 conducts fluid in the port 67 to the pocket 71. When the high pressure fluid collects in pocket 71 it resists the force applied to swash plate 31 by pistons 29 as they are exposed to the high pressure port. The fluid in pocket 71 also lubricates the face 57 on cam support 50 and face 59 on cam member 37 to reduce the friction therebetween.

Since pocket 71 is formed in face 59 on the movable cam member 37, the high pressure fluid is acting at a point directly opposite where force from the high pressure fluid is being applied by pistons 29 to the swash plate 31 or any angular position of cam member 37 in cam support 50.

High pressure fluid is normally fed from the shoes 30 to the passages 65 in grooves 66 to a port 67. However, when a passage 65 in the swash plate 31 is uncovered, by rotation of the shoes 30 fluid under pressure in the port 67 will leak back through grooves 66 to the uncovered passages 65. Since the passages 65 are only uncovered for a short time, only a small amount of fluid will flow back therethrough. The amount of fluid flowing back through is further reduced because the cross sectional area of a groove 66 is much smaller than that of a passage 65. The small grooves 66 also prevent a large amount of fluid from flowing from the cavities 63 under shoes 30 which would drain the fluid required to provide a fluid film between the shoes 30 and the creep plate 31.

It is important that high fluid pressure in port 67 be maintained in order to maintain the pressure of the counterbalancing fluid in the pocket 71. This is only possible if at any moment the majority of the passages 65 which connect to a port 67 are being fed fluid at high pressure or are blocked. Consequently, spacing of the passages 65 is critical.

Refering to FIG. 4, it has been found that it is undesirable to have the number of passages 65 equal to the number of pistons 29 in the pump 10 or equal to any multiple thereof. With this number of passages 65 it is possible that all of the passages 65 could be uncovered at one time and consequently no high pressure fluid would be supplied to port 67. FIG. 5 shows that with a number of passages 65 equal to double the number of pistons 29 that at times half of the passages 65 would be uncovered, and consequently, a majority of passages 65 would not be feeding high pressure fluid to the port 67.

FIG. 6 shows a creep plate 31 which has eight passages 65 and FIG. 7 shows a creep plate 31 which has twelve passages 65. When these creep plates 31 are used with a pump having seven pistons 29, and shoes 30 there are more passages 65 feeding high pressure fluid or blocking the escape of this fluid from port 67 than there are passages open and providing a means of escape to the high pressure fluid.

In FIG. 7, it can be seen that passages 65 are equally spaced from each other and distance along the arc of the circle on which all passages 65 lie between the farthest outside edges of two adjacent passages 65 is less than the outside diameter of a shoe 30. Consequently, a single shoe 30 may overlie two passages 65.

Obviously, those skilled in the art may make various changes in the details and arrangements of parts without departing from the spirit and sculp of the invention as it is defined by the claims hereto appended. Applicant, therefore, wishes not to be restricted to the precise construction hereindisclosed.

Claims

Having thus described and shown an embodiment of the invention, what is desired to secure by Letters Patent of the United States is:

1. A variable displacement fluid energy translating device comprising: a body; a barrel rotatably mounted within the body; a plurality of cylinders formed in the barrel; a piston within each cylinder; a cam support in the body; a cam member mounted on the cam support and pivotable relative to the support; a swash plate having a top surface and a bottom surface and mounted on the cam member; a shoe pivotably attached to each piston adapted to slide on the top surface of the swash plate and reciprocate the piston within a cylinder when the barrel is rotated; means for pivoting the cam member from a position causing maximum fluid flow in one direction through the translating device to a position causing maximum fluid flow in another direction; a bore in each piston for conducting fluid under pressure in the cylinder through each piston to its shoe; a second bore in each shoe which feeds fluid from the piston through the shoe to a space between the bottom of the shoe and the top surface of the swash plate to hydraulically balance the shoe; a plurality of apertures in the swash plate for conducting fluid from the bottom of each shoe through the swash plate; a first collector port for collecting fluid under pressure from some of the swash plate apertures when the device is causing fluid flow in one direction, a second collector port for collecting fluid under pressure from other of the swash plate apertures when the device is causing fluid flow in the other direction, the first and second collector ports being isolated from one another, a third bore in the cam member for conducting fluid from the first collector port through the cam member; a first pocket on the back of the cam member connected to the third bore for receiving the fluid from the first collector port; a fourth bore in the cam member for conducting fluid from the second collector port through the cam member; a second pocket on the back of the cam member connected to the fourth bore for receiving fluid from the second collector port; wherein the fluid under pressure in one of the first and second pockets substantially reduces the friction between the cam member and the cam support and applies a force on the cam member which resists the force applied by the pistons on the swash plate.

2. The variable displacement fluid energy translating device recited in claim 1, including restrictor means positioned between the swash plate apertures and the collector ports to limit the amount of pressure fluid that flows out of uncovered swash plate apertures from the collector ports and to maintain the hydraulic balance on the shoes.

3. A variable displacement fluid energy translating device comprising: a body; a barrel rotatably mounted within the body; a plurality of cylinders formed in the barrel; a piston within each cylinder; a cam support in the body; a cam member mounted on the cam support to form a bearing pivotable relative to the support; a swash plate having a top surface and a bottom surface and mounted on the cam member; a shoe pivotably attached to each piston and slideable on the top surface of the swash plate to reciprocate the pistons within the cylinders when the barrel is rotated; means for pivoting the cam member from a position of minimum displacement of the translating device to a position of maximum displacement of the translating device; a bore in each piston for conducting fluid under pressure in the cylinder through each piston to its shoe; a second bore in each shoe which feeds fluid from the piston through the shoe to a space between the bottom of the shoe and the top surface of the swash plate to provide pressure fluid beneath the shoe to substantially hydraulically balance the shoe in opposition to the thrust of the piston; a plurality of apertures in the swash plate for sequentially conducting fluid from the bottom of each shoe through the swash plate; a collector port; fluid restrictor means connecting the swash plate apertures and the collector port and limiting the rate of fluid flow from an uncovered aperture to maintain pressure fluid beneath each shoe; a third bore in the cam member for conducting fluid from the collector port through the cam member; a pocket on the back of the cam member for receiving the fluid from the third bore to lubricate the bearing surfaces between the cam support and cam member wherein the fluid in the pocket applies a counter-acting force on the cam member which resists the force applied by the pistons and shoes on the swash plate.

4. The variable displacement fluid translating device recited in claim 3 wherein the fluid restrictor means comprises a plurality of shallow grooves formed on the bottom surface of the swash plate and the shallow grooves are angled with respect to the apertures in the swash plate.