US3813188A - Pump assembly for power steering system - Google Patents

Abstract

A hydraulic power steering system includes a main pump assembly which consists of two variable displacement reversible pumps of different sizes. These pumps are coupled to a common drive shaft so that they turn together and their displacements are controlled so that the output of the smaller is always a fixed proportion of the larger. A small fixed displacement hand pump is operated through means of the steering wheel and provides a flow to control the displacements of the two variable displacement pumps to place the latter in stroke. Output flow from the larger pump is connected to and provides the operating fluid for the steering motor. Output flow from the smaller pump is fed back to counteract the effects of the hand pump and return the two variable displacement pumps to their zero displacement positions once the turning of the steering wheel ceases. The amount of fluid pumped to the steering motor is thus proportional to the amount of fluid delivered by the hand pump.

Description

Nelson 1 May 28, 1974 1 1 PUMP ASSEMBLY FOR POWER STEERING SYSTEM [75] Inventor: Roger John Nelson, Cedar Falls,

[62] Division of Ser. No. 166,202, July 26, .1971, Pat. No.

[52] U.S. Cl 417/202, 417/204, 417/216, 418/18,4l8/37 [51] Int. Cl. F04b 23/10 [58] Field of Search ..4l8/3, 18, 33, 37, 38, 418/131, 241; 91/496, 497; 417/202, 204,

[56] References Cited 7 FOREIGN PATENTS OR APPLlCATlONS 444,706 3/1936 Great Britain 418/18 1,040,567 10/1958 Germany 418/37 Primary Examiner william L. Freeh [57] ABSTRACT A hydraulic power steering system includes a main pump assembly which consists of two variable displacement reversible pumps of different sizes. These pumps are coupled to a common drive shaft so that they turn together and their displacements are con trolled so that the output of the smaller is always a fixed proportion of the larger. A small fixed displace ment. hand pump is operated through means of the steering wheel and provides a flow to control the displacements of the two variable displacement pumps to place the latter in stroke. Output flow from the larger pump is connected to and provides the operating fluid for the steering motor. Output flow from the smaller pump is fed back to counteract the effects of the hand pump and return the two variable displacement pumps to their zero displacement positions once the turning of the steering wheel ceases. The amount of fluid pumped to the steering motor is thus proportional to the amount of fluid delivered by the hand pump.

3 Claims, 5 Drawing Figures PATENTEDMAY 28 m4 SHIEU 1 BF 4 FIG. I

PATENTEDmze @914 3813188 sum 3 nr 4 I PATENTEUmes mm 3.813; 188

' sum u UF 4 CROSS REFERENCE TO RELATED APPLICATION This is a division of application Ser. No. 166,202,

filed July 26, 1971 now US. Pat. No. 3705493.

BACKGROUND OF THE INVENTION The present invention relates to a hydrostatic steering system and more particularly relates to a main pump assembly for such a system.

In known hydraulic power steering systems, valving of various types is actuated either directly or indirectly by steering wheel motion to connect a source of fluid pressure to one or the other of the work ports of a double-acting steering motor and to connect the remaining work port to exhaust. A hydraulic or mechanical feedback from the steering motor is used to return the valving to a non-actuatedcondition once the steering wheel motion ceases. Successful operation of these systems is primarily dependent on the responsiveness of the valve members to input and feedback signals and this responsiveness is in turn dependent on keeping. tolerances such that the valve members will seat and unseat properly-and suchthat the feedback linkages are free from slack or play." It is often very difficult and/or expensive to design, manufacture and maintain these tolerances.

Another disadvantage of these sytems is that responsiveness is often lost due to the necessity. caused by the SUMMARY OF THE INVENTION According to the present invention, there is provided a main pump'assembly for a hydraulic power steering system. Specifically, thereis provided a main pump assembly including a pair of variable displacement reversible pumps mounted as a-unit in one housing and having a common drive shaft, one of the pair of pumps delivering operating fluid to a reversible steering motor in response to beingjplaced in stroke; by a steering wheel operated hand pump and the other of the pair of pumps delivering feedback fluid to the stroke control means to place the one pump out of stroke once the operation of the hand pump has ceased.

An object of the invention is to provide a steering system utilizing a minimum of valvingand having a very responsive feedback signal-producing and delivering means. v

More specifically it is an object to provide in such a system the aforementioned main pump assembly, the assembly being compact and having no intermediate valving between the pair of pumps.

' A further object is to control the displacements of the pair of pumps such that the displacements of one is al- 1 ways a fixed direct proportion of the other.

Another object is to provide a design wherein the amount of fluid delivered by the one variable displacement pump to the steering motor is directly proportional to the amount of input signal fluid delivered by the hand pump to, the one pump.

These and other objects-will be apparent from the ensuing description andappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematical view showing the hydraulic steering system embodying the'pump assembly of the present invention.

FIG. 2 is a transverse sectional view of the pump assembly showing the pistons for controlling the stroke or displacement of the two pumps.

FIG. 3 is a longitudinal sectional view of the pump assembly taken along the line 3-3 of FIG. 2.

FIG. 4 is a transverse sectional view of the pump assembly taken along the line 44 of FIG. 3.

F IG. 5 is a transverse sectional view of the right-hand end cap of the pump assembly taken along the line 5-5 of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, there is schematically shown a hydraulic power steering system indicated in its entirety by the reference numeral 10. The system 10 is illustrated as being a closed system, however, it should be understood that a source of fluid for making up leakage may be connected to the system in the usual manner.

The system 10 includes a main pump assembly 12 comprising a steering fluid delivery pump 14 and a feedback fluid delivery pump 16, the pump 14 being the larger of the two pumps and shown to the right of the'pump 16 in FIG. 1. The pumps 14 and 16 are reversible flow, variable displacement pumps and are continuously driven at the same speed preferably by the engine of the prime mover being steered, here represented, at 18, in block form.

The steering fluid delivery pump 14 includes a pair of intake- exhaust ports 20 and 21 which are respectively connected to the service or work ports 22 and 23 of a reversible hydraulic steering motor 24 by a pair of feed lines 26 and 27. As will be apparent from the description below, the pumps 14 and 16 are normally at zero stroke or displacement positions when no steering is taking place and may be placed into stroke by selectively pressurizing the top and bottom ends respectively of upper and lowerbores 28 and 29 in which are respectively located stroke or displacement control pistons 30 and '32. Such pressurization is accomplished through means of a reversible flow, fixed displacement hand pump 33 driven through means of a steering wheel 34 and having a pair of intake- exhaust ports 35 and 36 respectively connected by lines 37 and 38 to the bores 28 and 29. Pressurized fluid is respectively delivered through the ports 35 and 36 in response to turning the steering wheel clockwise for making a right-hand turn and counterclockwise for making a left-hand turn. The feedback pump 16 also has a pair of intake- exhaust ports 39 and 40 respectively connected via passages 41 and 42, to the bores 28 and'29 for a purpose explained below.

The main pump assembly 12' appears in detail in FIGS. 2-5 and includes a box-like housing or casing 44 defining a substantially rectangular cavity 46 and including left and right side walls 48 and 50 (FIG. 3) in which cylindrical openings ,52 and 54 are respectively located in coaxial relationship to a horizontal axis X-X. A drive shaft 56 lines along the axis X-X and extends axially through a bearing end cap' 58 inserted in the opening 52 and terminates in the cavity 46 short of a bearing end cap 60 inserted in the opening 54. A pair of circular pumping piston mounting plates 62 and 64 are splined at axially-spaced locations on the shaft 56 for rotation with the latter and include respective hubs 66 and 68 on which the inner races of a pair of tapered roller bearing assemblies 70 and 72 are respectively mounted, the outer races of the bearing assemblies 70 and 72 respectively being received in a pair of thrust rings 74 and 76, which in turn are respectively received in the housing openings 52 and 54.

A hollow rectangular stroke or displacement control member 78 is located for vertical movement between and has left and right sides 80 and 82 (FlG. 3) in engagement with the opposed faces of the plates 62 and 64. A pair of coaxial circular openings 84 and 86 respectively in the control member sides 80 and 82 are concentric to and cooperate with a cylindrical tube 88 loosely received on the drive shaft 56 to define a circular path 90 (FIG. 2) and confine seven pumping pistons or elements 92, of the pump 14, having the shapes of cylindrical ring segments. Respectively located centrally in the seven pistons 92 are seven horizontal pins 94 having slippers 96 and 98 respectively pivotally received on their leftand right-hand ends (FIG. 3), the slippers 96 being respectively slidably received in seven radially extending equiangularly-spaced slots 100 in the inner face of the plate 62 and the slippers 98 being respectively slidably received in seven radial slots 102 in the inner face of the plate 64 in opposed relationship to the slots 100,. It is here noted that the slippers 96 also serve as the pumping pistons or elements of the feedback pump 16 as is further described below. Forming continuations of and extending centrally from the upper and lower ends of the stroke control member 78 are lands 104 and 106 respectively, which are positioned on vertically opposite sides of the circular path 90 and have respective curved surfaces 108 and 110, each of which scalingly engage the outer surface of one or more of the pistons 92 at every position of the pistons 92 along the path 90 and divide the hollow interior of the stroke control member 78 into leftand righthand working chambers 112 and 114 (FIG. 2). The intake and exhaust ports and 21 of the pump 14 extend vertically through the upper wall of the member 78 from the chambers 112 and 114 respectively. The lower portions of a pair of tubular fittings or conduits 116 and 118 are respectively sealingly and slidably received in the ports 20 and 21 and the upper portions of the fittings 116 and 118 are threaded into bores 120 and 122 extending vertically through the upper wall of the housing 44.

In a manner described below, the strokes or displacements of the pistons 92 are adjusted by moving the stroke control member 78 vertically in the housing 44. For this purpose, the bores 28 and 29 are respectively located in vertical axial alignment in the upper and lower walls of the housing 44 and the inner ends of the pistons 30 and 32 project into the cavity 46 and are respectively in engagement with flat bearing surfaces 128 and 130 on the upper and lower walls of the control member 78. Thus, the pistons 30 and 32 act to shift the member 78 vertically when the bores 28 and 29 are selectively pressurized by operation of the hand pump 33 as described above. The control member 78 is guided in its vertical movement by a pair of spaced, parallel generally rectangular plates 132 and 134 having their outer portions fixed in the walls of the housing 44 and having inner portions respectively in engagement with the leftand righthand sides and 82 of the control member 78.

In addition to guiding the member 78, the plates 132 and 134 also serve to form portions of working chambers for the pump 16. For this purpose, the plates 132 and 134 respectively have central substantially circular openings 136 and 138 arranged concentrically to the axis X-X and being of diameters slightly smaller than those of the plates 62 and 64. The openings 136 and 138 are respectively disposed about annular shoulders 140 and 142, respectively on the opposed axial ends of the plates 62 and 64, the openings 136 and 138 each including a pair of diametrically opposite, upper and lower lands 144 and 146 forming the only areas of contact between the openings and the shoulders. Thus, with reference to FIGS. 3 and 4, it can be seen that the openings 136 and 138 respectively of the guide plates 132 and 134 respectively cooperate with the shoulders 140 and 142 of the circular plates 62 and 64 and with the left and right sides 80 and 82 of the control member 78 to form a left-hand pair of substantially fluid-tight annular working chambers 148 and a similar right-hand pair of annular working chambers 150, the right and left chambers associated with the plate 62 being in fluid communication with the radially outer portions of the slots 100 and the right and left chambers associated with the plate 64 being in fluid communication with the radially outer portions of the slots 102. The intakeexhaust port 39 of the feedback pump 16 includes a pair of passages 152 (only one shown) extending vertically in the upper portions of the guide plates 132 and 134 from the left-hand working chambers 148 to a cross passage 154 in the upper wall of the housing 44. The passage 42 is also in the upper wall of the housing 44 and interconnects the cross passage 154 with the piston bore 28. Similarly the intake-exhaust port 40 of the pump 16 includes a pair of passages 156 extending vertically in the lower portions of the plates 132 and 134 from the right-hand working chambers to a cross passage 158 in the lower wall of the housing 44, the passage 41 interconnecting the cross passage 158 with the piston bore 29.

The axial forces acting on the circular plates 62 and 64 during pumping operation, due to the pressure existing in the left and right working chambers 112 and 114 of the pump 14, are balanced by the action of axially opposed left-hand groups and axially opposed righthand groups of small pistons 160 and 162 respectively located in bores arranged in substantially semi-circular paths in the axial inner ends of the right and left sides of each of the bearing end caps 58 and 60, as viewed from the left end of shaft 56 in FIG. 3. The left and right groups of pistons 160 and 162 respectively have their inner axial ends disposed for respectively engaging the left and right sides of the outer axial ends of the thrust rings 74 and 76. Thus, the left groups of pistons 160 are in axial alignment with and on the opposite sides of the chamber 112 and the right groups of pistons 162 are in axial alignment with and on the opposite sides of the chamber 114. A fluid passage 163, shown branched from linev27, interconnects the left working chamber 112 with grooves 164 located in the end caps 58 and 60 on opposite sides of the chamber 112 and being in fluid communication with the outer, axial endsof the pistons 160. A further fluid passage 165, shown branched from line 26, interconnects the t .5 right working chamber 114 with grooves 166 located in the end caps 58 and 60 on opposite sides of the chamber 114 and being in fluid communication with the outer axial ends of the pistons 162.

A description of the operation of the power steering system It) follows and it is to be noted that in this description it is assumed that the system 10 is completely charged with hydraulic fluid and that right and left steering is respectively effected by extension and retraction of the steering motor 24.

The drive shaft 56 of the steering and feedback fluid control pumps 14 and 16, respectively, is driven continuously, in the direction of the arrows (FIGS. 2 and 4), during operation of the vehicle engine 18. When no steering is taking place, the hand pump 33 is, of course,

not being operated and fluid is blocked in the lines 37 and 38 and holds the stroke control pistons 30 and 32 in central positions wherein the pistons support the stroke con-trol member 78 centrally in the housing 44. The pumps ,14 and 16 are then in neutral, zerodisplacement conditions wherein the pumping elements 92 of the pump 14 and the pumping'elements 96 and 98 of the pump 16 trace respective circular paths about an axis concentric with that of the drive shaft 56. When the paths are so positioned, there is no relative movement between adjacent pumping elements 92, between the pumping elements 96 and the slots 100, nor

between the pumping elements 98 and the slots 102.

Thus, no fluid is displaced by the pumps 14 and 16.

If it is desired to steer the vehicle to the right, the pump 14 is placed in stroke in one ofa first set of active conditions wherein fluid is pumped from the line 26 to the line27 to cause extension of the steering motor 24. This is accomplished by turning the steering wheel 34 clockwise to drive the hand pump 33 to displace fluid from the line 38 to the line 37 to cause the stroke control pistons 30 and 32 to shift downwardly and position the stroke control member 78 below its central position a distance determined by the amount of fluid displaced by the pump 33, the member 78 being illustrated in its lowermost position. The pumping elements 92, 96 and 98 now are positioned to trace-respectivecircular paths which are eccentric toand below the drive shaft 56. As adjacent elements-92 move downwardly in the right chamber 114 the spaces therebetween open and as adjacent elements 92 move upwardly in the left chamber 112, the spaces therebetween close. Thus, fluid is pumped from the right to the left chamber and hence from the line 26 to the line 27 to cause extension of the steering cylinder 24. It is to be noted that the amount of fluid displaced by the pump 14 depends upon the eccentricity of the. axis of the path of the elements 92, the fluctuations of the spaces between adjacent elements 92 increasing with increasing eccentricity.

The feedback fluid control pump 16 is placed into stroke simultaneously with the pump 14 since the pumping elements 96 and 98 respectively move radially outwardly in the slots 100 and 102 concurrently with the downward movement of the pumping elements 92 and move radially inwardly in the slots 100 and 102, concurrently with the upward movement of the pumping elements 92 thus causing fluid to be pumped from the working chambers 148 to the working chambers 150 and hence from the line 37 to the line 38 to partially nullify the effect of the hand pump 33 on the stroke control pistons 30 and 32. When the right turn is completed, the steering wheel 34 is again held stationary, thus deactivating the pump 33. The pump 16 continues to displace fluid from the line 37 to the line 38 until the stroke control pistons 30 and 32 again positionthe stroke control member 78 centrally in the housing44 to dispose the paths of the pumping elements 92, 96 and 98 concentrically with the shaft 56 and once again establish the neutral, zero-displacement conditions in the pumps 14 and 16.

The vehicle may be steered to the left by placing the pump 14 in stroke in one of a second set of active conditions wherein fluid is transferred from the line 27 to the line 26 to cause retraction of the steering motor 24. This is accomplished in much the same manner as the operation for steering to the right, however, now the steering wheel 34 is turned counterclockwise to drive the pump 33 to transfer fluid from the line 37 to the line 38 and cause the pistons 30 and 32 to position the stroke control member 78 above its control position. The pumping elements 92, 96 and 98 again trace paths which are eccentric to the shaft 56, however, now the space between adjacent elements 92 progressively close and open respectively as the elements 92 move downwardly in the right chamber 114 and upwardly in the left chamber 112. Thus, fluid is pumped from the line 26'to the line 27 to cause retraction of the steering motor 24. The pumping elements 96 and 98 move with the elements 92 and act in the slots 100 and 102 to transfer fluid from the working chamber to the working chamber 148, and hence from the line 38 to the line 37 in opposition to the action of the hand pump 33. Thus, as described relative to right-hand steering, the feedback pump 16 acts to restore itself and the pump 14 to their neutral conditions once the driving of the hand pump 33 has stopped.

The axial fluid pressure loads transferred to the bearing assemblies 70 and 72, during rightand'left-hand turning operations when the leftand right- hand working chambers 112 and 114 are respectively pressurized, are balanced by fluid pressure delivered from the chamber 112 to the groups of pistons 162 via the line 163 and the grooves 166 and by fluid pressure delivered from the chamber 114 to the groups ofpistons via the line 165 and the grooves 164.

Thus, it can be seen that since the pumps 14 and 16 are driven at the same speed and their strokes are adjusted simultaneously by proportional amounts, the displacement of the smaller pump 16 is always a fixed proportion, less than one, of the larger pump 14 when the pumps are in one or the other of their sets of active conditions. Further, it can be seen that since the amount of fluid displaced by thehand pump 33 determines the position of the stroke control member 78, the displacements of the pumps 14 and 16 are a direct proportion of the amount of fluid pumped by the hand pump 33. Additionally, it can be seen that a steering system is provided wherein no valving is used. These features are important in that they give stability and responsiveness to the system 10 as well as give the operator a feel" for the responsiveness of the system.

1 claim:

1. A pump assembly comprising: a housing; a drive shaft rotatably mounted in said housing; pumping element mounting means including centrally-located means defining a cylindrical ring-shaped opening encircling said shaft and a pair of circular end plate means fixed for concentric rotation with said shaft at spaced locations thereon in closing relationship to the axially opposite ends of said ring-shaped opening; said centrally-located means being mounted in said housing for movement in a path crosswise to said shaft, the axis of said ring-shaped opening being located such that it lies on the axis of said shaft when the ring-shaped opening is at an intermediate position relative to said shaft; a plurality of equiangularly-spaced, radially extending guide slots arranged in opposed pairs in the opposed surfaces of said circular plates; a plurality of pumping elements having the shapes of cylindrical ring segments being located in said ring-shaped opening respectively between each pair of opposed guide slots; a plurality of guide slippers located one each in each of said guide slots, the pair of slippers in the opposed pairs of guide slots being pivotally connected to the opposite sides of the pumping element located therebetween; said centrally located means forming a pair of fluid chambers in fluid communication with the spaces between said pumping elements, the chambers being separated by diametrically-spaced lands located in line with said path; first and second intake-exhaust ports being in fluid communication respectively with said pair of fluid chambers; displacement control means operatively connected to said centrally located means for selectively shifting the latter along said path; said circular end plate means each including an annular shoulder at the radially outer ends of said guide slots; a pair of guide plate means fixed to said housing and respectively having substantially circular openings disposed concentrically about and spaced from said annular shoulders; said guide plate means embracing said end plate means and said centrally located means and cooperating therewith to form a pair of fluid chambers in fluid communication with the radially outer ends of said guide slots; and a pair ofland means in each of said circular openings extending radially inwardly from diametrically opposite locations in line with said path into sealing engagement with said shoulder thus dividing said pair of chambers into first and second halves and first and second intake-exhaust port means respectively being in fluid communication with the first and second halves of said pair of chambers.

2. A pump assemblycomprising: a housing; a drive shaft rotatably mounted in said housing; pumping element-mounting means including centrally-located means defining a cylindrical ring-shaped opening encircling said shaft, and a pair of circular end plate means fixed for concentric rotation with said shaft at spaced locations in closing relationship to the axially opposite ends of said ring-shaped openings; a plurality of opposed pairs of equiangularly-spaced guide slots extending radially in the axially opposed surfaces of said plates; a plurality of ring segment-like pumping elements located in said ring-shaped opening, one being between each opposed pair of guide slots; slipper means pivotally connected to each of said pumping elements and being slidably received in said guide slots for constraining said elements for rotation with said shaft and for radial movement in said slots; said centrallylocated means being mounted in said housing for movement along a straight path to either side of said shaft from an intermediate position wherein said ring-shaped opening is concentric with said shaft; said centrallylocated means forming a fluid chamber in fluid communication with the spaces between said pumping elements; said centrally-located means including land means at diametrically opposite locations relative to said ring-shaped opening in line with said path and cooperating with the radial outer portions of said pumping elements to divide said fluid chamber into first and second halves; first and second intake-exhaust ports being in fluid communication respectively with said first and second chamber halves; and displacement control means operatively connected to said centrallylocated means for selectively moving the latter along said path; said shaft being rotatably mounted in said housing through means of a pair of bearing assemblies respectively received in cylindrical openings in opposite sides of said housing and having opposed, axial inner ends in engagement with said circular end plate means; a pair of cap members respectively received in closing relation to said cylindrical housing openings and having axial inner ends respectively adjacent the axial outer ends of said bearing assemblies; a plurality of pressure-balancing pistons shiftably mounted in bores arranged circularly in each of said cap members in axial alignment with the axial outer ends of said bear ing members for movement against said bearing mem' bers for counteracting axial pressure loads imposed on said bearing members by forces acting on said circular end plates; the balancing pistons in each cap member being divided into first and second groups respectively in axial alignment with said pair of fluid chambers; and fluid passage means respectively connecting said pair of fluid chambers to said first and second groups of pistons.

3. A pump assembly comprising: a housing; a drive shaft rotatably mounted in said housing; pumping element-mounting means including centrally-located means defining a cylindrical ring-shaped opening encircling said shaft and a pair of circular end plate means fixed for concentric rotation with said shaft at spaced locations thereon in closing relationship to the axially opposite ends of said ring-shaped opening; said centrally-located means being mounted in said housing for movement in a path cross-wise to said shaft, the axis of said ring-shaped opening being located such that it lies on the axis of said shaft when the ring-shaped opening is at an intermediate position relative to said shaft; a plurality of .equiangularly-spaced, radially extending guide slots arranged in opposed pairs in the opposed surfaces of said circular plates; a plurality of pumping elements having the shapes of cylindrical ring segments being located in said ring-shaped opening respectively between each pair of opposed guide slots; a plurality of guide slippers located one each in each of said guide slots, the pair of slippers in the opposed pairs of guide slots being pivotally connected to the opposite sides of the pumping element located therebetween; said centrally located means forming a pair of fluid chambers in fluid communication with the spaces between said pumping elements, the chambers being separated by diametrically-shaped lands located in line with said path; first and second intake-exhaust ports being in fluid communication respectively with said pair of fluid chambers; displacement control means operatively connected to said centrally located means for selectively shifting the latter along said path; said shaft being rotatably mounted in said housing through means of a pair of bearing assemblies respectively received in cylindrical openings in opposite sides of said housing and having opposed, axial inner ends in engagement with said circular end plate means; a pair of cap members respectively received in closing relation to said cylin- 10 forces acting on said circular end plates; the balancing pistons in each cap member being divided into first and second groups respectively in axial alignment with said pair of fluid chambers; and fluid passage means respectively connecting said pair of fluid chambers to said first and second groups of pistons.

Claims (3)

1. A pump assembly comprising: a housing; a drive shaft rotatably mounted in said housing; pumping element mounting means including centrally-located means defining a cylindrical ringshaped opening encircling said shaft and a pair of circular end plate means fixed for concentric rotation with said shaft at spaced locations thereon in closing relationship to the axially opposite ends of said ring-shaped opening; said centrally-located means being mounted in said housing for movement in a path crosswise to said shaft, the axis of said ring-shaped opening being located such that it lies on the axis of said shaft when the ring-shaped opening is at an intermediate position relative to said shaft; a plurality of equiangularly-spaced, radially extending guide slots arranged in opposed pairs in the opposed surfaces of said circular plates; a plurality of pumping elements having the shapes of cylindrical ring segments being located in said ring-shaped opening respectively between each pair of opposed guide slots; a plurality of guide slippers located one each in each of said guide slots, the pair of slippers in the opposed pairs of guide slots being pivotally connected to the opposite sides of the pumping element located therebetween; said centrally located means forming a pair of fluid chambers in fluid communication with the spaces between said pumping elements, the chambers being separated by diametrically-spaced lands located in line with said path; first and second intake-exhaust ports being in fluid communication respectively with said pair of fluid chambers; displacement control means operatively connected to said centrally located means for selectively shifting the latter along said path; said circular end plate means each including an annular shoulder at the radially outer ends of said guide slots; a pair of guide plate means fixed to said housing and respectively having substantially circular openings disposed concentrically about and spaced from said annular shoulders; said guide plate means embracing said end plate means and said centrally located means and cooperating therewith to form a pair of fluid chambers in fluid communication with the radially outer ends of said guide slots; and a pair of land means in each of said circular openings extending radially inwardly from diametrically opposite locations in line with said path into sealing engagement with said shoulder thus dividing said pair of chambers into first and second halves and first and second intake-exhaust port means respectively being in fluid communication with the first and second halves of said pair of chambers.

2. A pump assembly comprisIng: a housing; a drive shaft rotatably mounted in said housing; pumping element-mounting means including centrally-located means defining a cylindrical ring-shaped opening encircling said shaft, and a pair of circular end plate means fixed for concentric rotation with said shaft at spaced locations in closing relationship to the axially opposite ends of said ring-shaped openings; a plurality of opposed pairs of equiangularly-spaced guide slots extending radially in the axially opposed surfaces of said plates; a plurality of ring segment-like pumping elements located in said ring-shaped opening, one being between each opposed pair of guide slots; slipper means pivotally connected to each of said pumping elements and being slidably received in said guide slots for constraining said elements for rotation with said shaft and for radial movement in said slots; said centrally-located means being mounted in said housing for movement along a straight path to either side of said shaft from an intermediate position wherein said ring-shaped opening is concentric with said shaft; said centrally-located means forming a fluid chamber in fluid communication with the spaces between said pumping elements; said centrally-located means including land means at diametrically opposite locations relative to said ring-shaped opening in line with said path and cooperating with the radial outer portions of said pumping elements to divide said fluid chamber into first and second halves; first and second intake-exhaust ports being in fluid communication respectively with said first and second chamber halves; and displacement control means operatively connected to said centrally-located means for selectively moving the latter along said path; said shaft being rotatably mounted in said housing through means of a pair of bearing assemblies respectively received in cylindrical openings in opposite sides of said housing and having opposed, axial inner ends in engagement with said circular end plate means; a pair of cap members respectively received in closing relation to said cylindrical housing openings and having axial inner ends respectively adjacent the axial outer ends of said bearing assemblies; a plurality of pressure-balancing pistons shiftably mounted in bores arranged circularly in each of said cap members in axial alignment with the axial outer ends of said bearing members for movement against said bearing members for counteracting axial pressure loads imposed on said bearing members by forces acting on said circular end plates; the balancing pistons in each cap member being divided into first and second groups respectively in axial alignment with said pair of fluid chambers; and fluid passage means respectively connecting said pair of fluid chambers to said first and second groups of pistons.

3. A pump assembly comprising: a housing; a drive shaft rotatably mounted in said housing; pumping element-mounting means including centrally-located means defining a cylindrical ring-shaped opening encircling said shaft and a pair of circular end plate means fixed for concentric rotation with said shaft at spaced locations thereon in closing relationship to the axially opposite ends of said ring-shaped opening; said centrally-located means being mounted in said housing for movement in a path cross-wise to said shaft, the axis of said ring-shaped opening being located such that it lies on the axis of said shaft when the ring-shaped opening is at an intermediate position relative to said shaft; a plurality of equiangularly-spaced, radially extending guide slots arranged in opposed pairs in the opposed surfaces of said circular plates; a plurality of pumping elements having the shapes of cylindrical ring segments being located in said ring-shaped opening respectively between each pair of opposed guide slots; a plurality of guide slippers located one each in each of said guide slots, the pair of slippers in the opposed pairs of guide slots being pivotally connected to the opposite sides of the pumping element locAted therebetween; said centrally located means forming a pair of fluid chambers in fluid communication with the spaces between said pumping elements, the chambers being separated by diametrically-shaped lands located in line with said path; first and second intake-exhaust ports being in fluid communication respectively with said pair of fluid chambers; displacement control means operatively connected to said centrally located means for selectively shifting the latter along said path; said shaft being rotatably mounted in said housing through means of a pair of bearing assemblies respectively received in cylindrical openings in opposite sides of said housing and having opposed, axial inner ends in engagement with said circular end plate means; a pair of cap members respectively received in closing relation to said cylindrical housing openings and having axial inner ends respectively adjacent the axial outer ends of said bearing assemblies; a plurality of pressure-balancing pistons shiftably mounted in bores arranged circularly in each of said cap members in axial alignment with the axial outer ends of said bearing members for movement against said bearing members for counteracting axial pressure loads imposed on said bearing members by forces acting on said circular end plates; the balancing pistons in each cap member being divided into first and second groups respectively in axial alignment with said pair of fluid chambers; and fluid passage means respectively connecting said pair of fluid chambers to said first and second groups of pistons.

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