US3921503A

US3921503A - Control system for a fluid system

Info

Publication number: US3921503A
Application number: US372696A
Authority: US
Inventors: Philip A Kubik
Original assignee: Individual
Current assignee: Individual
Priority date: 1972-03-21
Filing date: 1973-06-22
Publication date: 1975-11-25
Anticipated expiration: 1992-11-25

Abstract

A control system for a variable displacement fluid pump or motor device having a displacement control mechanism operable upon actuation to vary the amount of fluid displaced by the device. An actuating means is operatively coupled to the displacement mechanism and actuates the same when communicated to a source of fluid pressure, the actuating means comprising a housing having a pressure chamber with a piston member movable therein in response to pressure fluid. The piston has a restricted passage which communicates pressure in the pressure chamber directly to a source of low pressure to provide a constant limited flow of control fluid throughout the control system prior to initial operation thereof to insure that the temperature of the fluid in the control system is elevated to a normal operating temperature.

Description

nited States Patent Wk 14 1 Nov. 25, 1975 CONTROL SYSTEM FOR A FLUID SYSTEM [76] Inventor: Philip A. Kubik, 6809 Spruce Drive, Freeh Birmingham, Mich 48012 Asszstant Exammer-O. P. LaPomte Attorney, Agent, or Fzrm-Basrle and Wemtraub [22] Filed: June 22, 1973 [21] Appl. No.: 372,696

57 ABSTRACT Related US. Application Data 1 [62] Division of Ser. No. 236,736, March 21, 1972, A control system for a variable displacement fluid abandoned. pump or motor device having a displacement control mechanism operable upon actuation to vary the CL 6; 60/ amount of fluid displaced by the device. An actuating [51] Int. Cl.2 FOIB 3/00 means is operatively coupled to the displacement 1 Field of Search mechanism and actuates the same when communi- 417/2l7; 91/505, 506; 9 60/444 cated to a source of fluid pressure, the actuating means comprising a housing having a pressure cham- References Cited her with a piston member movable therein in response UNITED STATES PATENTS to pressure fluid. The piston has a restricted passage 2,286,358 6/1942 Geiger 60/381 Which communicates Pressure in the chamber 2197,33] 9/1942 wyligm 91/279 directly to a source of low pressure to provide a con- 2,588,522 3/1952 Harris 417/222 Stant limited flow of control fluid throughout the con- 2,659,204 11/1953 Conway et al... 60/329 trol system prior to initial operation thereof to insure ,447 10/1955 Hancock 60/431 that the temperature of the fluid in the control system 3,676,020 7/1972 Andreasen CI 31.. 91/506 i levated to a normal perating temperature 3,700,356 10/1972 Kubik 417/222 FOREIGN PATENTS OR APPLICATIONS 1 Claim, 4 Drawing Figures 41184 1970 Japan 417/217 US. Patent Nov. 25, 1975 Sheet 1 of 2 3,921,503

Lid I I ./ZZ /f4 56 \M E M US. Patent Nov. 25, 1975 Sheet 2 of 2 CONTROL SYSTEM FOR A FLUID SYSTEM CROSS REFERENCE TO RELATED APPLICATIONS This is a division of application Ser. No. 236,736, filed Mar. 21, 1972 now abandoned.

The present patent application is related in substance to co-pending US. patent applications Ser. No. 50,093 filed June 26, 1970 now US Pat. No. 3,653,208 and Ser. No. 67,177 filed Aug. 26, 1970 now US. Pat. No. 3,700,356, both of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control system for controlling the displacement of a variable volume fluid device and in particular, the present invention relates to a system for elevating the temperature of the fluid in a control system prior to initiating a change in the displacement of the fluid device.

2. Description of the Prior Art Heretofore numerous fluid systems have been employed for controlling the rate of movement of a hydraulic pump or motor device and, in particular, such fluid systems have found extensive use in hydraulic machine tool transfer drives and the like such as the systems disclosed in the aforementioned co-pending US. patent applications. Such systems are used to accelerate and decelerate a fluid cylinder or rotary motor respectively at the beginning and the end of its stroke prior to the movement and generally consist of a hydraulic pump connected in either a closed-loop or open-loop fashion to a hydraulic motor, either a fluid cylinder or a rotary motor with fluid being delivered from the pump to the motor and returned from the motor to the pump or a reservoir. The rate of acceleration and deceleration of the fluid motor is controlled by varying the amount of fluid displaced by the pump which in turn is controlled by any suitable displacement control mechanism. During the initial or start up phase of such systems fluid in the control system is at a low temperature and thus the control fluid has a high viscosity which results in the response time of the control system during initial start up of the system to be such thatthe drive will not accelerate or decelerate in a manner which is sufficient to insure a smooth and quiet operation of the drive.

Thus it would be desirable to provide a means for elevating the temperature of the fluid in the control system to a temperature level which insures a proper operation of the control system during the initial start up of the aforementioned drives. In the past, attempts to solve this problem have included the provision of means for heating the oil in the control system, however, this method of insuring that the viscosity of the oil is maintained in a proper range to insure proper operation of the drives is either inadequate, cumbersome, or expensive and, in addition, requires the operator of the machine to perform additional functions, all of which increases the expense of operating and maintaining such previously used systems.

SUMMARY OF THE INVENTION The present invention which will be described subsequently in greater detail comprises in one embodiment a control system for a fluid system including a source of 2 high pressure fluid such as a variable displacement pump, the output volume of which is adapted to be controlled by a displacement control mechanism. A fluid motor device, such as a fluid cylinder, is operatively coupled to the displacement control mechanism of the variable displacement pump and is responsive to a second source of fluid pressure to selectively move the displacement mechanism of the pump to increase or decrease the volume of fluid displaced thereby. The control system includes a restricted passage which directly communicates the control fluid across the device such that thereis a constant limited flow of fluid therethrough the control system prior to initial start up operation of the control system whereby the temperature of the control fluid therein is elevated to an operational temperature prior to initiating operation of the pump.

It is therefore an object of the present invention to provide a control circuit for a fluid system having means for elevating the temperature of the control fluid to an operating level prior to the commencement of the operation of the fluid system.

Other objects, advantages and applications of the present invention will become apparent to those skilled in the art of control systems for fluid pump and motor devices when the accompanying description of several examples of the best modes contemplated for practicing the invention is read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The description herein makes reference to the accompanying drawings in which like reference numerals refer to like parts throughout several views, and in which:

FIG. 1 represents a schematic illustration of one example of a control system constructed in accordance with the principles of the present invention;

FIG. 2 is a schematic illustration of a second example of a fluid control system;

FIG. 3 is a schematic illustration of a third example of a fluid control system; and

FIG. 4 is a schematic illustration of yet another example of a fluid control system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and, in particular, to FIG. 1 wherein there is illustrated one example of the present invention in the form of a fluid control system 10 comprising a fluid cylinder 12 operatively coupled to an external lever 14 which, in turn, is connected to the displacement control mechanism of a variable displacement device 16. The device 16 may be any suitable variable displacement pump such as the pumps disclosed in the aforementioned United States patent applications. In the schematic illustrative example of FIG. 1 the pump 16 is considered for purposes of explanation to be at a minimum displacement or minimum flow position when the lever 14 is rotated to the right and is in abutment with a minimum flow stop member 18 while the pump 16 is considered to be in a maximum flow position when the lever 1.4 is rotated to the left as viewed in FIG. 1 and is in abutment with a maximum flow stop member 20. The shifting of the lever 14 between the maximum and minimum flow positions or any intermediate flow positions and the rate at which the lever 14 is so shifted controls the amount 'of fluid and the rate at which fluid is displaced by the pump 16 3 so as to control a fluid motor or the like such as specifically described in the aforementioned patent applications. 1 t

The control system is but one example of a method of controlling the displacement of thevariable displacement pump 16 and comprises a directional control valve 22 adapted to selectively connect fluid from a supply pump 24 to either of a pair of

feed control valves

26 or 28. The

feed control valves

26 and 28 are, in turn, respectively connected to

ports

30 and 32 of the fluid cylinder 12 by any suitable conduits or the like. The fluid cylinder 12 is conventional in its construction and comprises a-tubular housing 34 having an interior bore 36 divided into two

pressure chambers

38 and 40 by means of a reciprocally mounted piston 42 'which, in turn, carries on one side thereof a connecting rod 44 that extends from the fluid cylinder 12 for coupling to'the pump lever 14. A conventional pressure relief valve 46 communicates with the supply pump 24 and is adapted to limit the high pressure of the control system 10 in a conventional manner.

The

feed control valves

26 and 28 may be conventional in their construction and have restricted passages 48and 50, respectively, which are adjustable such that the feed control valves may be preset to supply any desired flow rate over any desired range. Each of the

feed control valves

26 and 28 further comprise

check valves

52 and 54 which permit fluid to by-pass restricted'passages 48 and 50 in one direction of flow. Thus when the control valve 22 is in the position indicated in FIG. 1,

fluid flow is directed from the supply pump 24 through the feed valve 26 viathe check valve 52 and is communicated to the'pressure chamber 38 within the fluid cylinder 12 to exert a force on the piston 42 to shift the same rightwardly to displace the lever 14 toward a minimum flow position, while at the same time the fluid in the pressure chamber 40 .is exhausted from the fluid cylinder 12 through the'restricted passage .50 in the feed valve 28 and returned to a reservoir 56. When the direction of the control 'valve 22 is reversed to direct fluid through the check valve 54 of the feed control valve 28, fluid is communicated to the pressure chamber 40 whereupon the fluid pressure therein exerts a force against the righthand side of the piston member 42 to shift same leftwardly as viewed in FIG. 1 to position A and thereby shift the lever 14 towards a maximum flow position while fluid in the chamber 38 is exhausted through the restricted passage 48 of the feed control valve 26 and back to the reservoir 56 via direction control valve 22. 1

The piston 42 has a restricted orificeor passageway 58 which provides constant fluid communicationbetween the

pressure chambers

38 and 40. The orifice 58 is so sized as to provide a limited amount of flow between the two

pressure chambers

38 and 40 and during normal operation as aforementioned the orifice 58 having negligible affect on fluid loss between the two pressure chambers. During the initial start up of a system prior to actuation of the directional control valve 22 to cause movement of the piston 42 and thereby cause a change in the displacement of the pump 16, the orifice 58 provides a very simple and convenient means for elevating the temperature of the control fluid in the control system 10 to eliminate aforementioned difficulties of a cold start up.

As can be seen in FIG. 1, if the cylinder is in the position illustrated and the directional control valve 22 is in the position illustrated; fluid from the pump 24, when 4 the same, is in operation, will be directed to pressure chamber '38' exerting a force on the piston 42 and thereby retaining the pump lever l4 in the position illustrated, that is," at a minimum flow position. During this mode of operation, fluid from the pressure chamber 38, which is a temperature in which the viscosity is relatively high and thus the fluid is difficult to move resultingin a low response time for the system, will flow through the orifice 58 into the pressure chamber 40 and return to the reservoir in the manner aforementioned. After several minutes of operation of the control system'l0 in this manner, the temperature of the control fluid will be elevated to its normal operating temperature and thus upon actuation of the directional control valve 22 to a position communicating pressure fluid from the pump 24 to the pressure chamber 40 to commence operation, the system fluid will be at a normal operating temperature and the aforementioned 1 thus a further detailed description thereof is not necessary. The system 60 further comprises a'fluid pump 62 of the axial piston type having as schematically illustrated a rotating cylinder barrel 64 provided with aplurality of arcuately spaced cylinder bores 66 within which are reciprocally, mounted pistons 68 all of which is conventional and described in greater detail in the aforementioned US. patent application -Ser. No. 67,l77. The outer ends of the pistons 68 have shoes 70 which bear against a swash plate 72 that is rotatable about an axis 74 to change the amount of stroking movement of the-pistons 68 within their respective bores 66 and thereby vary the displacement of the pump 62 and thus the volume of the fluid delivered from the pump 62. The amount of inclination or the rotation of the swash plate 72 about the axis 74 is controlled-by a pair of diametrically

opposed piston members

76 and 78 whichare respectively slidably mounted 1 in

bores

77 and 79 in the housing of the pump 62. The

reciprocate within their

respective bores

77 and 79.

The inner ends of the

pistons

76 and 78 are respectively exposed

to'pressure chambers

80 and 82 which when communicated to pressure fluid exert a force on their associated piston to move the same outwardly and thereby cause an inclination of the swash plate 72.

In, the embodiment illustrated in FIG. 2, fluid delivered from the feed control valve 26 is communicated by any suitable conduit to the pressure chamber 82, while fluid delivered from the feed control valve 28 is communicated to the pressure chamber 80. It can thus be seen that during normal operation of the pump 62 when it is desired to bring the pump 62 to a minimum displacement position, as illustrated, fluidis directed from the direction control valve 22 to the feed control valve 26 and to the pressure chamber 82 wherein a force is exerted against the piston 78 to move the same leftwardly and incline the swash plate 72 to a near vertical position. At the same time fluid in the pressure chamber 80 is'exhausted therefrom through the feed control valve 28 and returned to reservoir 56. Similarly when the direction control valve 22 is shifted, fluid is directed from the supply pump 24 through the feed control valve 28 and to the pressure chamber 80 wherein a force is exerted against the piston 76 to move the same outwardly and tilt the swash plate 72 about the axis 74 while at the same time fluid within the pres sure chamber 82 is exhausted therefrom through the feed control valve 26 and returned to the reservoir 56.

Prior to the initial start up operation of the pump 62, the viscosity of the slug of oil in the

pressure chambers

80 and 82 is high and thus the response time of the pump 62 is very slow, resulting in the aforementioned difficulties of a cold start up. In order to overcome this disadvantage, the

pressure chambers

80 and 82 are directly communicated by means of'a conduit 84 within the housing of the pump 62. The conduit 84 has re stricted orifices 86 which permit a limited flow between

chambers

80 and 82 during the initial operation of the system 60 but prior to shifting the pump swash plate 72 to cause either an increase or decrease in displacement of the pump 62. The flow path is from the pump 24 into the pressure chamber 80 across the restricted orifices 86 through conduit 84 into the pressure chamber '86 and exhausted therefrom for return to reservoir 56. The size of the orifices 86 is such as to permit only a limited flow of fluid between the

pressure chambers

80 and 82 and thus during normal operation of the pump 62 in the manner described in the co-pending U.S. patent application Ser. No. 67,177 there is a negligible loss of pressure and fluid between the

chambers

80 and 82 while at the same time during the initial operation of the control system, but prior to a change in the displacement of pump 62 fluid will be communicated directly between the

chambers

80 and 82 and circulate throughout the control system 60 by means of the supply pump 24 to provide a very simple means for elevating the temperature of the fluid in the control system 60 to operating level and thereby eliminate the aforementioned cold start difficulties.

Referring now to FIG. 3, there is illustrated a control system 90 and a variable displacement pump 92 comprising a swash plate 94 adapted to be rotated about a fixed axis 96 to vary the displacement of the pump 92 and thus the volume of fluid delivered thereby. The pump 92 has a cylinder barrel (not shown) in which pistons 98 are reciprocally mounted and having outer ends engaging the swash plate 94 in the conventional manner. Movement of the swash plate 94 about its axis 96 is accomplished by means of a pair or diametrically

opposed control pistons

100 and 102 each having one end respectively disposed for reciprocal movement in the housing of the pump 92 while the outer ends thereof engage the swash plate 94 such that when one of the

pistons

100 or 102 is moved outwardly and the other inwardly with respect to the pump-92, the swash plate 94 is rotated about the axis 96. In the embodiment illustrated the effective pressure responsive area of the control piston 100 is approximately twice the effective pressure responsive area of the control piston 102.

The control system 90 comprises a supply pump 104 which draws fluid from a reservoir 106 and constantly communicates pressure fluid to a pressure chamber 108 associated with the control piston 102 generating a force on the piston 102 to urge the swash plate toward a maximum flow position. Fluid from the supply pump 104 is communicated by any suitable conduit through restricted passageway 110 of a feed control valve 112 to a direction of control valve 113 which is movable to a first position to block the fllow of the fluid delivered from the supply pump 104 to a second position (illustrated in FIG. 3) wherein fluid from the supply pump 104 is communicated to a check valve 114 of a second feed control valve 116 and to a pressure chamber 118 associated with the control piston 100. It can thus be seen that when the valve 113 is in the position illustrated, fluid pressure is communicated to both

pressure chambers

108 and 118 and the greater effective pressure responsive area of the piston will cause the same to move outwardly to bring the swash plate 94 to a minimum full position while the control piston 102 moves inwardly into its associated pressure chamber 108 to exhaust fluid therefrom.

If the direction of control valve 113 is shifted, the pressure chamber 118 associated withthe control piston 100 is communicated via :an orifice 120 in the feed control valve 116 to the reservoir 106 while fluid delivered from the supply pump 104 is communicated to only the pressure chamber 108 associated with the control piston 102 causing the same to be shifted outwardly to rotate the swash plate 94 and bring the pump 92 towards a maximum flow condition. A pressure relief valve 122 is associated with the control system 90 to limit the maximum value of the pressure therein in a conventional manner.

As is conventional in pumps of the axial piston type, that portion of the pump 62 in which the swash plate 94, the piston 98, and cylinder barrel are disposed is normally referred to as a pump case 121 and is filled with a fluid that is at a low pressure which may be at the inlet pressure of the pump or some intermediate pressure such as the pressure of the fluid in the reservoir 106. As can best be seen in FIG. 3 the

control pistons

100 and 102 are respectively provided with restricted passages or

orifices

122 and 124 which connect their associated

pressure chambers

118 and 108 to the pump case 121 such that there is a constant flow of fluid from

chambers

118 and 108 to the pump case 121. The size of the

orifices

122 and 124 are such that the amount of flow passing from their associated

chambers

108 and 118 will have no effect on the operation of the pump 92 during normal operating modes. However, prior to the initial operation control system 90, that is, prior to actuating either of the

control pistons

100 or 102 to cause a change in the displacement of the pump 92, the supply pump 104 can be activated to provide flow to the pressure chamber 108 and to the pressure chambers 118 whereupon fluid therein will flow through the restricted

orifices

122 and 124 to the pump case 121 and return to the reservoir 106 thereby resulting in a circulation of the fluid in the control system 90 causing an elevation of the temperature of the control fluid to a normal operating level whereby the system is ready to commence operation while having eliminated the aforementioned cold start problem.

Referring now to FIG. 4 where there is illustrated another example of the present invention comprising a fluid cylinder having an internal bore 132 within which is reciprocally mounted a piston 134 that divides the opposite portions of the bore 132 into

pressure chambers

136 and 138. The

pressure chambers

136 and 138 are each connected by any suitable conduit means to a reservoir 140 by means of a directional control valve 142. At the mid section of the fluid cylinder 130 there is provided a pressure port 144 that communicates with a supply pump 146 which, in turn, is

adapted to draw fluid from the reservoir 140 and deliver pressure fluid to the pressure port 144. The directional control valve 142 is movable from a first position wherein communication between the

pressure chambers

136 and 138 and the reservoir 140 is open to a second position wherein fluid communication between the reservoir 140 and the pressure chamber 136 is open while communication between the reservoir 140 and the pressure chamber 138 is closed, to a third position wherein communication between the reservoir 140 and the pressure chamber 138 is opened and communication between the pressure chamber 136 and the reservoir 140 is closed.

The piston 134 has extending from opposite sides thereof a pair of connecting arms that extend externally of the fluid cylinder 130 and are adapted to be operatively coupled to machinery or the like or whatever is desired to be driven by the fluid cylinder, for example the lever 14 of the variable displacement pump 16 illustrated in FIG. 1. The piston 134 is further provided with a restrictive T-shaped passage 148 which communicates with the opposing

pressure chambers

136 and 138 and when the piston 134 is disposed in the mid section of the fluid cylinder, passageway 148 communicates with the high pressure port 144.

When the control valve 142 is in the position which opens communications between the

pressure chambers

136, 138 and the reservoir 140, fluid pressure from the pump 146 enters the T-shaped passageway 148 via pressure port 144 and communicates with both

pressure chambers

136 and 138 and the fluid is returned via valve 142 to the reservoir 140. Since the effective pressure response areas on both sides of the piston 134 is equal and the flow of fluid to both

pressure chambers

136 and 138 is at the same pressure, the piston 134 will not reciprocate within the fluid cylinder 130 during this mode. However, if, for example, the directional control valve 142 is positioned to close fluid communication between the pressure chamber 136 and reservoir 140 while the chamber 138 communicates with the reservoir 140, then the pressure fluid flowing into the chamber 136 from the T-shaped passageway 148 will exert a force against the piston 134 to shift the same to the right thereby exhausting fluid from the chamber 138 back to the reservoir 140. As the piston traverses port 144, the same will open directly to the pressure chamber 136 thereby allowing an increase in the rate of movement of the piston 134. Similarly, if the pressure chamber 136 remains open to reservoir 140 and communication between the reservoir and the pressure chamber 138 is closed, the fluid communicated through the T-shaped passageway 140 to the chamber 138 will increase in pressure therein resulting in a force being exerted against the piston 134 to shift the same to the left as viewed in FIG. 4. Thus, it can be seen a simple means is provided for permitting a circulation of fluid from the pump to both pressure chambers and back to the reservoir to maintain the fluid in the system at an elevated temperature so that the response time of the system will not be delayed during initial cold start up of the system.

The modification illustrated in FIG. 4 is particularly adapted to use wherein the cylinder is located at some remote distance from the valve 142 and the conduits connecting the valve 142 to the

pressure chambers

136 and 138 is of a substantial length and exposed to a low temperature such as when the same is employed in a cold temperature environment. By having a constant circulation of fluid between the pressure chambers, the

pump, the reservoir and the valve, the aforementioned cold start up problems are eliminated.

Thus, it can be seen the present invention provides a means for elevating the temperature of the fluid in a control system to an operating level prior to the operation of the system and without having to employ elaborate heating systems.

It can also be seen the same is accomplished by a very simple modification to existing machinery not requiring new parts or a major redesign of the components thereof.

While the forms of the embodiments of the present invention disclosed herein constitute preferred forms, it should be understood by those skilled in the art of fluid and control systems that other forms might be adopted all coming within the spirit of the invention and the scope of the appended claims.

What is claimed is as follows:

1. A control system for a variable volume fluid pressure energy translating device of a type having internal fluid displacement means operable upon actuation to vary the volume of fluid displaced by said device, said control system comprising:

a source of high pressure fluid;

a source of low pressure fluid;

said fluid pressure energy translating device comprising:

a. a housing having inlet and outlet ports;

b. a cylinder barrel rotatably mounted within said housing, said cylinder barrel having a plurality of arcuately spaced cylinder bores; a plurality of pistons with inner ends disposed for reciprocal stroking movement within said cylinder bores;

0. means successively communicating said cylinder bores with said inlet and outlet ports;

(1. an inclined swash plate mounted in said housing in a driving relationship with the outer ends of said pistons for imparting said reciprocal stroking movement to said pistons within said cylinder barrel bores as said cylinder barrel rotates, the amount of fluid flowing from said inlet port to said outlet port being a function of the amount of inclination of said swash plate with respect to the axis of the reciprocal stroking movement of said pistons;

e. fluid displacement means operatively coupled to said swash plate to control the amount of inclination of said swash plate and thus the amount of fluid volume displaced by said device, said swash plate being rotatable about a predetermined axis, said fluid displacement means comprising: first piston slidably mounted in a first pressure chamber in said housing and having an extended end with means engaging said swash plate, said first piston being adapted to extend under pressure from said pressure chamber to rotate said swash plate about said predetermined axis in a first direction to increase the volume of fluid displaced by said device;

a second piston slidably mounted in a second pressure chamber in said housing said second piston having an extended end with means engaging said swash plate and adapted to extend under pressure from said second pressure chamber to rotate said swash plate in a second direction to decrease the volume of fluid displaced by said device;

valve means for selectively communicating said source of high pressure fluid to one of said pressure chambers while exhausting the other of said pressure chambers to said low pressure source to thereby control the amount of inclination of said swash plate and thus the amount of fluid displaced by said device; and

restrictive passage means continuously connecting said pressure chambers, said restrictive passage means being so sized as to permit a flow of fluid from said high pressure source to said low pressure

Claims

1. A control system for a variable volume fluid pressure energy translating device of a type having internal fluid displacement means operable upon actuation to vary the volume of fluid displaced by said device, said control system comprising: a source of high pressure fluid; a source of low pressure fluid; said fluid pressure energy translating device comprising: a. a housing having inlet and outlet ports; b. a cylinder barrel rotatably mounted within said housing, said cylinder barrel having a plurality of arcuately spaced cylinder bores; a plurality of pistons with inner ends disposed for reciprocal stroking movement within said cylinder bores; c. means successively communicating said cylinder bores with said inlet and outlet ports; d. an inclined swash plate mounted in said housing in a driving relationship with the outer ends of said pistons for imparting said reciprocal stroking movement to said pistons within said cylinder barrel bores as said cylinder barrel rotates, the amount of fluid flowing from said inlet port to said outlet port being a function of the amount of inclination of said swash plate with respect to the axis of the reciprocal stroking movemenT of said pistons; e. fluid displacement means operatively coupled to said swash plate to control the amount of inclination of said swash plate and thus the amount of fluid volume displaced by said device, said swash plate being rotatable about a predetermined axis, said fluid displacement means comprising: a first piston slidably mounted in a first pressure chamber in said housing and having an extended end with means engaging said swash plate, said first piston being adapted to extend under pressure from said pressure chamber to rotate said swash plate about said predetermined axis in a first direction to increase the volume of fluid displaced by said device; a second piston slidably mounted in a second pressure chamber in said housing said second piston having an extended end with means engaging said swash plate and adapted to extend under pressure from said second pressure chamber to rotate said swash plate in a second direction to decrease the volume of fluid displaced by said device; valve means for selectively communicating said source of high pressure fluid to one of said pressure chambers while exhausting the other of said pressure chambers to said low pressure source to thereby control the amount of inclination of said swash plate and thus the amount of fluid displaced by said device; and restrictive passage means continuously connecting said pressure chambers, said restrictive passage means being so sized as to permit a flow of fluid from said high pressure source to said low pressure source via said pressure chambers when said valve means is operable to connect said high pressure source to said second pressure chamber whereby the temperature of said fluid may be initially elevated without effecting said system said restrictive passage means being so sized as to have no effect on said system when said valve means is operable to connect said high pressure source to said first chamber.