US2080810A - Pump control - Google Patents

Description

y 37- J. K. DOUGLAS 2,080,810

PUMP CONTROL Filed June 24, 1955 2 Sheets-Sheet l INVENTOR JAMES K. DEILIGLAS A I BY 20 I WW ATTORNEY-- 1 81,, I937. J. m. mucus HUME CONTROL Filed June 24, 1965; Z? Shswfia-Shaasfi 2 INVENTOR K. DEILJGLAS Patented May 18, 1937 PATENT OFFICE PUMP CONTROL James K. Douglas, Shorewood, Wis., assignor t o The Oilgear Company, Milwaukee, Wis., a-corporation of Wisconsin Application June 24, 1935, Serial No. 28,068

20 Claims.

pump to function either as a variable delivery a pump or as a constant delivery pump until pump pressure reaches a predetermined maximum and to then function as a constantpressure pump, such as the control disclosed in Patent No. 2,009,608 granted to me on application Serial No.

- 547,268, filed June 27, 1931, of which this application is a continuation as to matters common to both applications.

Prior to the pump control disclosed in the above patent, the constant pressure pump controls in most common use were provided with a spring for urging the stroke-changing mecha nism of the pump toward full stroke position and a fluid motor acting in opposition to the spring to move the stroke-changing mechanism toward zero stroke position, such as the pump control disclosed in Patent No. 1,578,233 issued March 23, 1926 to Walter Ferris.

The stroke control motor was ordinarily open. at all times to pump pressure and the spring had to have its tension so adjusted that it would hold the stroke-changing mechanism stationary until the pressure created by the pump exceeded the predetermined maximum and then yield to allow the motor to reduce the stroke of the pump. However, as the resistance of a spring increases as it is deflected, pressure in excess of the predetermined maximum was required to compress the spring sufficiently to reduce the pump stroke to .substantially zero.

As the maximum pump pressure is often high and as the resistance of a spring increases according to the amount it is deflected, a small diameter control motor and a long flexible spring were employed but it was found that, ,if the pump operated at approximately full stroke at lower pressures and its stroke had to be reduced nearly to zero to maintain constant a predetermined maximum pressure, there was a greatdifference between that predetermined maximum pressure and the pressure at which the control started to reduce pump displacement.

If the pump is of the type in which the pump pistons react against a circular reaction surface formed in a slide block which is shifted to vary pump displacement, the action of the pistons produce a force which tends to move the slide block to its zero stroke position. This force is very ,strong when the pump is at full stroke and disappears when the stroke is reduced to zero. Consequently, the force required to hold a pump at a given stroke will vary according to the length (c1. ns-3s) of stroke and thereby cause pump pressure to be difierent at diflerent lengths of stroke.

Also the friction of the slide block or other stroke-changing element adds to pump pressure when pump displacement is being reduced and lowers it when pump displacement is being'increased.

The control shown in Patent No. 2,009,608 evercomes the above difilculties by interposing a pressure-operated control valve between the discharge side of the pump and the hydraulic servo-motor which operates the stroke-changing mechanism of the pump. The control valve prevents liquid from the pump from entering the servo-motor until pump pressure reaches a predetermined value and then it throttles the flow and thereby causes the liquid to enter the servo-motor at a low pressure.

The controlfunctions quite accurately-to prevent pump pressure from exceeding the predetermined maximum. However, the control valve has a certain amount of frictional resistance which must be overcome by the fluid pressure when the valve is shifted in the direction. to decrease pump ,displacement and by the valve spring when the valve is shifted in the direction to increase pump displacement. Therefore, after pump displacement has been reduced .in response to pump pressure reaching a predetermined maximum, pump pressure must fall below'that constant if the load on the pump remains constant or if it continues to increase above the valueat which the pump develops the prede termined maximum pressure. If the load on the pump fluctuates above and below that value, the control will keep the pump pressure within a narrow range but it will hunt.

Also, if the motor driven by the pump should stall instantaneously, the inertia of the stroke changing mechanism would prevent the stroke of the pump from being instantly reduced to zero with the result that pump pressure would rise momentarily.

The present invention has as an object to produce a pump control which will be free of the above mentioned objectionable characteristics.

Another object is to provide a pump control which is positive and precise in operation.

Another object is to provide a pump control the stroke control motor also functions as 'afpilot which will prevent" the stroke-changing mecha nism of the pump from hunting.

Other objects and advantages will appear from, the description hereinafter given of a pump con- .trol in which the invention is embodied.

According to the invention in itsgeneral aspect .and asordin'arily embodied in practice, the delivery of liquid w the'stroke control: motor of the pump is controlled by a control valve which is provided with means for compensating for the frictional resistance to its movement.

According to the invention inanother aspect, the valve which controls the delivery of liqfiid to valve for a relief valve through which liquid dis-rcharged by the pump may pass in response ,to pump pressure suddenly increasing above-a predetermined maximum.

The invention is exemplified by the pump control shown diagrammatically in the accompanying drawings in which the views are as follows:

Fig. 1 is a'circuit dlagramofa hydraulic transmission to-which the invention has been applied, the pump being shown at full stroke and the control valve and the relief valve being shown closed.

Fig. 2 is a view similar to Fig. 1 but showing the control valve in its operative position and the pump at short stroke.

For the purpose of illustrationgthe invention is shown applied to a hydraulic transmission consisting of a variable displacement pump I and a reciprocating hydraulic motor 2 which may be -employed for pressing a work piece 3 against an 1,998,984. It is suflicient to state that the pump has its cylinders and pistons arranged radially in a cylinder barrel 5 which rotates upon a centralvalve shaft or pintle 6 through which liquid flows to and fromthe cylinders.

The ,cylinder barrel 5 is arranged inside a thrust member or slide block I having an inner circular reaction. surface 6 which engages the outer ends or heads of the pistons and causes the pistons to reciprocate in the cylinders whenever the cylinder barrel is rotated and the axis of -the re'action'surface-6 is offset from the axis of the cylinder barrel 5. The stroke of the pistonsand consequently the volumetric delivery of the pump is proportional to the distance that the axis of the reaction surface is offset from the axis of the cylinder barrel.

In the patent, the pump is shown provided with a hand-operated screw forshifting the slide block to thereby change pump displacement. In the present instance the slide block 'I is urged toward the right to increase pump ,displacement by and toward the two helical compression springs left to decrease displacement by a hydraulic servo-motor consisting of a stationary cylinder Ill and a piston I I which is fitted in the cylinder III and connected to the slide block.

Delivery of liquid from the pump I to the motor 2 is controlled by a reversing valve I2 which has its casing connected to opposite: ends of the motor 2 by two pipes I3 and I4, to the discharge port.

of the pump I by a high pressure pipe I5 and to shown in Fig. 1, pipe I5 is open to pipe I6 so that the pump is bypassed and the motor 2 is idle.

When the plunger I1 is in its lower position as shown in Fig. 2, pipe I5 is open to pipe I4 and liquid delivered by the pump will enter the head end of the motor 2 and advance its piston on a working stroke, liquid exhausted from the rod end of the motor being returned to the pump through pipes I3 and I6. When the handle I8 is shifted to its upper position as shown in dotted lines, the valve plunger I! will be in its uppermost posi- :v tion and pipe I5 will be open'to pipe I3 and pipe I4 will be open to pipe I6 so that'thepump will deliver liquid to the rod end of the motor 2 to retract its piston and liquid discharge from the head end of the motor 2 will be returned to the pump through pipes I 4 and I6.

Since the volume of liquid delivered by the pump I to the head end of the motor 2 to advance the motor piston on a working stroke is considerably in excess of the volume returned to the pump from the rod end of the motor, means must be provided for supplying the pump with that excess volume during the working stroke and for permitting the escape of an equal volume during the return stroke.

As shown, the return pipe I6 is connected to a suction pipe l9 which extends into a reservoir 20 and has a; check valve2I connected therein which permits the pump to draw liquid-from the reservoir but prevents liquid from being discharged through the suction pipe into the reservoir, and

a gear pump 22 draws liquid from the reservoir 20 and discharges it into a low pressure supply pipe 23' which is connected to the return pipe I6 through a resistance valve 24 and a pipe 25 having a low pressure relief valve 26 connected thereto and'discharging into the reservoir 20.

The output of the gear pump 22 is ordinarily employed for control purposes, such as operating the reversing valve I 2, and for supplying liquid to the pump I. Liquid discharged by the gear pump in excess ofthese requirements is exhausted into the .reservoir 20 through the relief valve 26 so that the pressure in the supply pipe 231 is equal to the sum of the resistances of the valves 24 and 26 and the pressure in the pipe -25 is equal to the resistance of the relief valve 26.

If, during the working stroke of the motor 2, the

volume required to besupplied to the pump I, from the reservoir 20 is in excess of the volumetric capacity of the gear pump 22, the pumpI will draw liquid from the reservoir through the suction pipe I9 and check valve 2I .During the return 7 stroke of the motor 2, the liquid discharged from the head end thereof in excess of the volume required by the pump I will be exhausted through the low pressure relief valve 26.

Liquid for operating the stroke control motor I0I I. is supplied thereto from the high pressure pipe I5 through a control unit shown as having its mechanism arranged within a casing 30 which is closed at its top by a cover plate 3I.

a. piston 34 which is larger in diameter and closely fitted in a ounterbore 35 formed in the casing 30 concentric with the bore 33.

The casing 30 contains a control valve 32 which is closely fitted in a bore 33 and has fixed thereto The valve 32 is urged downward into its closed j a bushing 31 which is closely fitted in the counterbore 35 and closely flttedaround a stem 38 which is secured to or formed integral with the piston 34 and is of the same diameter as the valve 32 so that theupper and lower faces of the piston 34 will have equal pressure areas.

. The stem 38 extends upward through the spring 36 and the bushing 31 into a hollow adjusting screw .39 which is threaded through the cover plate 3| into contact with the bushing 31 and which may be rotated to adjust the tension of the spring 36.

Any liquid which may seep past the bushing 31 into the upper end of the counterbore 35 or into the interior of the adjusting screw 39 may escape therefrom through suitable drain channels The lower end of the bore 33 is connected to the high pressure pipe I5 by a pipe 48, and the lower end of the counterbore 35 is connected to the control cylinder l8 by a passage 4| and a pipe 42 so that the lower face of the piston 34 is exposed at all times to the pressure prevailing in the control cylinder I0 and the lower end of the valve 32 is exposed at all times to pump pressure. The pressure acting on the valve 32 tends to raise it against the resistance of the spring 36 which is adjusted to keep the valve closed until pump pressure reaches a predetermined maximum.

The valve 32 is provided at its lower end with one or more notches or slots 43 which, when the valve is raised in response to a predetermined high pump pressure, will communicate with an annular port 44 formed in the wall of the bore Ill-ll is operated and to enable a higher pressure to be built up in the passage 46 when the relief valve is to be operated.

The passage is connected, at a point beyond the choke 41, bya passage 49 to the counterbore 35 at a point between the piston 34 and the bushing 31, and it is connected to the passage 4| through two opposed low pressure resistance valves 58 and 5| which are connected in parallel with each other.

Each of the resistance valves 58 and 5| is so adjusted that the drop in pressure across it is equal to the pressure required to produce the force necessary to overcome the frictional resistance of the control valve 32 and the parts movable therewith, such as the piston 34 and the stem 38.

In order that liquid may escape from the control cylinder I8 when the control valve 32 is closed, the piston 34 is provided around its upper edge with a plurality of small tapered grooves or slots 52 which communicate when the valve 32 is closed with an annular port53 formed 65 to a drain channel 54.

The slots 43 and 52 are preferably so arranged that the vertical distance between them is equal to the vertical distance between the lower edge of the port 44 and the upper edge of the port 53 so that, when the control valve has been raiseduntil the bottom of the slot 43 is. even with the bottom edge of the port 44, the bottom of the slots 52 will be even with the upper edge of the 75 port 53. Then, a very slight movement of the it the flow of liquid through the passage 45 to .thereby limit the rate at which the servomotor in the wall of the counterbore 35 and connected control valve 32 in either direction will open either the port 44 to the high pressure pipe [5 or the passage 49 to the drain 'channel 54.

In order to relieve the pump I should pump pressure suddenly rise above a predetermined maximum, the casing 38 is provided with a channel 68 having one of its ends open at all times to pump pressure through the pipe 48 and its other end intersecting a bore 6| formed in the casing38 and communicating at its lower end with a relief channel 62 which discharges into the reservoir 28.

Communication between the channels 68 and 62 is controlled by a relief valve 63 which is closely fitted in the bore 6|, extends across the channel 60 and normally engages an annular.

valve seat 64 arranged at the lower end of the bore 6| around the relief channel 62.

The relief valve 63 has a piston 65 fixed thereto and closely fitted in a counterbore 66 formed in the casing 38 concentric with the bore 6| and containing a helical compressionspring 61 which urges the reliefvalve 63 downward into engage ment with the valve seat 64.

The counterbore 66 is open at its lower end to the passage 46 so that the lower face of the piston 65 is subjected to fluid pressure when the control valve 32 is open. In order that operation of the relief valve maynot be impeded by liquid trapped in the counterbore 66 above the piston 65, the upper part of the counterbore '66 is open to the reservoir 28' as by means of a drain pipe 68.

As shown, the drain channel 54 discharges into the counterbore 66 at a point above the piston 65 and the upper end ofthe counterbore 35 drains into the upper end of the counterbore 66 through a channel 69 formed in the cover plate 3|. the counterbore 66 at a point above thebushing 31 so'that the space in the counterbore 35 between the piston 34 and the bushing 31 is kept flooded with liquid. After the counterbore 66 is filled with liquid to the level of the entrance to the pipe 68, all liquid is drained from the counterbore 35 into the counterbore 66 and any liquid which may escape past the piston 65 will drain through the pipe 68 into the reservoir 28.

The pressure responsive pump controls produced prior to the pump control shown in Patent No. 2,009,608 required one adjustment to determine the pressure at which the high pressure relief valve would open and a separate adjustment to determine the pressure at which the stroke control servo-motor would reduce pump displacement. In certain kinds of service, it is The drain pipe 68 communicates with important that the relief valve shall open at a pressure not greatly in excess of the pressure at which pump displacement is reduced. A pressure gauge is ordinarily provided so that adjustment of the stroke control mechanism may be readily made by the operator of the machine with which the pump is associated. Adjustment of the relief valve, however, almost inan end thereof, that its tension bears a definite relation to the tension of springs 3. Then, the pressure required to operate servo-motor IO-Il and the pressure required to operate servo-motor spring 36.

Operation When the pump l is delivering liquid at a low pressure, as when the pipe I5 is connected to the return pipe l6 through the reversing valve l2, the control valve 32 and the relief valve 63 will be closed and the springs 3 willbe holding the pump at full stroke as shown in Fig. 1.

When pump pressure rises to a predetermined value, as when the motor 2 presses the work 3 against the abutment 4, the liquid. acting on the lower end of the control valve, 32 will raise the valve 32 against the resistance of the spring 36 until the bottom of the slot 43 is above the lower edge of the port 44 at which time the drain port 53 is closed due to the fact that the slots 52 in the piston 35 are above it.-

Raising the control valve 32 permits liquid' from the pipe 46 to enter the port 44 and flow therefrom through the passage 45, low pressure resistance valve 50, passage: 4|, and .pipe 42 to the control cylinder l0 and move the piston H and the slide block 1 toward the left to reduce the stroke of the pump.

Due to'thethrottling effect ofthe :valve :2, the

liquid enters the port 44 at a very lowxpressure and, due to the fact that the circuit -is. always filled with liquid when the transmission is in operation, pressure builds up substantially instantly until it is high enough to overcome the resistance of the springs 9- which are so adjusted that a low pressure, for instance 50 lbs. per sq. in., acting on the piston H is suflicient to move the slide block I toward the left.

At the same time, pressure extends through the passage 46 to the lower end of the counterbore 66 and acts upon the lower face of thepiston 65, through the passages 45 and 49 to the space in the counterbore 35 between the piston 34 and the bushing 31 where it acts upon the upper face of the piston 34, and through the passage45, resistance valve 50 and passage 4| to the lower end' of the counterbore 35 where it acts upon the lower face of the piston 34.

Unless pump pressure should rise suddenly above the predetermined maximum, the pressure acting upon the lower face of the piston will not raise the relief valve 63 fdrthe reason that the springfil is so adjusted that the pressure required to open it, for instance 100 lbs. per sq. in., is higher than the pressure required to reduce the stroke of the pump.

The pressure acting on the lower face of the piston 34 is the same as the pressure acting on the piston while the pressure acting .on the upper face of the piston 34 is higher due to the drop in pressure across the resistance valve 50 which is so adjusted that the pressure required to open it is equal to the pressure required to exert a force on the piston 34 equal to the frictional resistance of the pilot valve 32 and the parts movable therewith. I

The pilot valve 32 is thus urged-downward by a spring force which is equal to the force produced by the predetermined maximum pump pressure and by a hydraulic force which is equal to the force required to overcome the frictional justed at the factory, as by placing shims under, resistance of the valve. Therefore, when pump displacement has been reduced untilit is just sufficient to maintain the predetermined maximum pressure constant, the piiotvalve 32=w111= move downward until the. bottom of the slot 43 is substantially in alinement with the bottom of the port 44 and the bottom of the slots 52 are substantially in alinement with the top edge of the port 53. More specifically, the valve 32 floatsbetween a position which permits a minute flow of liquid to the control cylinder II and a position which permits a minute flow of liquid from the control cylinder When pump pressure drops below the predetermined maximum, the control valve 32 will move downwardand close the port 44 to the pipe 40 and open the passage 49 to the drain port 53. The springs 9 will then move the slide block 1 toward the right to increase pump displacement and the piston M will expel liquid from the cylinder ill through the pipe 42, passage 4|, re-

sistance valve 5|, passage 49, counterbore 35, slots 52 and port 53 into the drain channel 54.

At this time the pressure acting upon the lower face of the piston 34 is higher thanthe pressure acting upon the upper face thereof due to the drop in pressure across the resistance valve 5| which is so adjusted that the pressure'required to open it is equal to the pressure required to produce the force necessary to just overcome the frictional resistance of the control valve 32 and the parts movable therewith. Therefore, when pump pressure rises, the valve 32 will rise as soon as the pressure reaches the predetermined maximum for the reason that the frictional re-- 35 sistance of the valve 32 is overcome'by the pressure acting upon the lower face of the piston 34.

The piston 34 and the counterbore 35 thus constitute an auxiliary servo-motor whose function is to overcome the frictional resistance of the valve which controls the operation of the stroke control motor. By means of this arrangement -a predetermined maximum pump pressure may be maintained at substantially an exact v alue.--

In the foregoing explanation of the operation of the pump control, the function of the relief valve 63 has been disregarded. If the pump control is applied to a transmission the motor of which does not stall during normal operation. thereof, the relief valve 63 may be'omitted or replaced by a conventional relief valve connected direct to the discharge side of the pump.

If the motor of the transmission to which the pump control is applied should suddenly stall,

as by the work piece 3 offering more resistance than the motor 2 can overcome, the servo-motor |0--|| could not reduce pump displacement instantly to substantially zero on account of the inertia of the slide block 1. Therefore, the pilot controlled relief valve 63 is provided and the passage 45 is provided with a choke 41 of the correct size to permit liquid to pass therethrough at the piston 65 and thereby open the relief valve 63 as shown in dotted lines in Fig. 2. For clearness of illustration, the relief valve is shown wide open but, in practice, it ordinarily opens only a few thousandths of an inch. Opening the re- 2,080,810 lief valve 63 permits the pump to discharge into the reservoir 20.

During this time, the stroke control motor lO-Il is reducing pump displacement and thereby reducing the flow of liquid through the pipe 40 so that, just before pump displacement is reduced substantially to zero, the relief valve closes and then the control valve floats between a partly open and a partly closed position.

Since the control valve 32 opens at a predetermined maximum pump pressure and throttles the flow of liquid to the stroke control servomotor |0-Il and to the relief valve servo-motor 65-66 both of which operate at very low pressures, it is not necessary that pump pressure exceed the predetermined maximum in order to cause the relief valve 63 to open for the reason that pressure continuing at the predetermined maximum will cause the control valve 32 to continue to open and thereby reduce its throttling action until the pressure in the lower end of the counterbore 66 is high enough'to open the relief valve 63.

A pump to which the invention has been applied is thus enabled to deliver liquid at a given rate until pump pressure reaches a predetermined maximum and to then maintain that maximum at a substantially exact value irrespective of whether or not the motor driven by the pump should stall, or whether it should stall suddenly or gradually, or whether or not the discharge from the pump should be instantly blocked.

The arrangement shown is preferred and has proved highly successful. However, theservomotor l0ll could be operated by liquid supplied from a separate source, such as the gear pump 22, by simply having the control valve .32 control communication between that source and the port 44 instead of between the discharge pipe l5 and the port 44. The control valve would be exposed to pressure created by the pump I, as in the present instance, and would operate in response to variations in pump pressure above and below the predetermined maximum, as explained above.

The invention herein set forth is susceptible of various modifications without departing from the scope thereof as hereafter claimed.

The invention is hereby claimed as follows:

1. The combination, with a pump having a fluid actuated stroke control motor for varying its displacement, of means for supplying motive fluid to said motor to operate the same, a control valve subjected at all times to pump pressure for controlling the flow of motive fluid to and from said motor and responsive to a predetermined maximum pump pressure to direct motive fluid to said motor, a servo-motor comprising a piston and a cylinder for overcoming the frictional resistance to movement of said valve, means for supplying motive fluid to said servomotor at opposite ends of said piston, and means for maintaining a uniform preponderance of pres-' sure at one end or the other of said piston.

2. The combination, with a pump having a fluid actuated stroke control motor for varying its displacement, of means for supplying motive fluid to said motor to operate the same, a control valve subjected at all times to pump pressure for controlling the flow of motive fluid to and from said motor and responsive to a predetermined maximum pump pressure to direct motive fluid to said motor, and means responsive to the flow of fluid to or from said motor for overcoming the frictional resistance to movement of said valve.

3. The combination, with a pump having a hydraulic stroke control motor for varying its displacement, of means for supplying liquid from said pump to said motor to operate the same,.a control valve subjected at. all times to pump pressure for controlling the flow of liquid to and from said motor and responsive to a predeterminedmaximum pump pressure to direct liquid to said motor, a hydraulic servo-motor for overcoming the frictional resistance to movement of said valve, and means for supplying motive liquid to said servo-motor.

4. The combination, with a pump having a hydraulic stroke control motor for varying its displacement, of means for supplying liquid from said pump to said motor to operate the same, a control valve subjected at all times to pump pressure for controlling the flow of liquid to and from said motor and responsive to a predetermined maximum pump pressure to direct liquid to said motor, and means responsive to the flow of liquid to or from said motor for overcoming the frictional resistance to movement of said valve.

5. The combination, with a pump having a fluid actuated stroke control motor for varying its displacement, of means for supplying motive fluid to said motor to operate the same, a control valve subjected at all times to pump pressure for controlling the flow of motive fluid to and from said motor and responsive to a predetermined maximum pump pressure to direct motive fluid to said motor, a fluid servo-motor connected to said valve for overcoming the frictional resistance thereof, and means responsive to the flow of fluid to and from said stroke control motor for causin thereof, and means responsive to the flow of liquid to and from said stroke control motor for causing operation of said servo-motor.

'7. The combination, with a pump having a fluid actuated stroke. control motor for varying its displacement, of means for supplying motive fluid to said motor to operate the same, a control valve subjected at all times to pump pressure for controlling the flow of motive fluidto and from said motor and responsive to a predetermined maximum pump pressure to direct motive fluid to said motor, a fluid servo-motor connected to said valve for overcoming the frictional resistance thereof, fluid channels connecting said servo-motor in circuit with said stroke control motor, and two oppositely? arranged low pressure resistance valves connected in series with said stroke control motor to cause. the pressure in one end of said servo-motor to be higher than the pressure in the other end thereof in response to operation of said stroke control motor in either direction.

'8. The combinatiomwith a pump having a hydraulic stroke control motor for varying its displacement, of means for supplying liquid from said pump to said motor to operate the same, a control valve subjected at all times to pump pressure for controlling the flow of liquid to and from said motor and responsive to a predetermined maximum pump pressure to direct liquid to 'said motor, a hydraulic servo-motor connected to said valve for overcoming the frictional resistance thereof, fluid channels connecting said servomotor in circuit with said stroke control motor, and two oppositely arranged low pressure resistance valves connected in series with said stroke control, motor to cause the pressure'in one end.

of said servo-motorto be higher than the pressure in the other end thereof in response to operation of said stroke controlmotor in either direction.

9. The combination, with a pump having a fluid actuated stroke control motor for varying its displacement, of means for supplying motive fluid to said motor to operate the .same, a conits displacement, of a relief valve connected to the discharge side of said pump and spring pressed to its closed position, a servo-motor for opening said valve and requiring a fluid pressure to open it which is higher than. the fluid pressure required to operate said stroke control motor, means for supplying motive fluid to both of said motors to operate the same, and a control valve subjected at all times to pump pressure for controlling the flow of fluidto and from both of said motors and responsive to a predetermined maximum pump pressure to direct motive fluid to d motors.

11. Thecombination, with a pump having a hydraulic stroke control motor for varying its displacement, of a relief valve connected to the discharge side of said pump and spring pressed to its closed position, a servo-motor for opening said valve and requiring a fluid-pressure to open it which is higher than the fluid pressure re-' quired to operate said stroke control motor, means for supplying liquid from said pump to its displacement, of means'for supplying motive fluid to said motor to operate the same, a control valve subjected ,at all times to pump pressure forcontrolling the flow of motive fluid to and from said motor and responsive to a predetermined maxlmum pump' pressure to direct motive fluid to said motor, means responsive to the flow of fluid to or from said motor for overcoming the frictional resistance to movement of said valve, and a relief valve connected to said pump and operated by fluid under the control of said control valve.

13. The combination, with a pump having a hydraulic stroke control motor-for varying its displacement, of means for supplying liquid from said pump to said motor to operate the same, a control valve subjected at all times to pump pressure for controlling the flow. of liquid to and from. said motor and respon/sive to a predetermined maximum pump pressure to direct liquid to said motor, means including a hydraulic servo-motor forovercoming the frictional resistance to movement of said valve, and a relief valve connected to said pump and operated by liquid supplied from said pump under .the control of said control valve.

14. The combination, with a pump having a hydraulic stroke control motor for varying its displacement, of means for supplying liquid from said pump to said motor to operate the same, a control valve subjected at all times to pump pressure for controlling the flow of liquid to and from said motorand responsive to a predetermined maximum pump pressure to direct liquid to said motor, means responsive to the flow of liquid'to or from said motor for overcoming the frictional resistance to movement of said valve, and a re- I lief valve connected to said pump and operated by liquid supplied from said pump under the control of. said control valve.

15. The combination, with a, pump having a fluid actuated stroke control motor for varying its displacement, of means for supplying motive fluid to said motor to operate the same, a control valve subjected at all times to pump pressure for'controlling the flow of motive fluid to and from said motor and responsive to a predetermined maximum pump pressure to direct motive fluid to said motor, means including a fluid operated servo-motor connected to. said valve for overcoming the frictional resistance thereof, and a relief valve connected at all times to said pump and operated by fluid under the control of said control valve.

16. The combination, with a pump having a hydraulic stroke control motor for varying its displacement, of means for supplying-liquid from said pump to said motor to operate the same, a control valve subjected at all times to pump pressure for controlling the flow of liquid to and from said motor and responsive to a predetermined maximum pump pressure to direct liquid to said motor, a hydraulic servo-motor connected to said valve for overcoming the frictional resistance thereof, means responsive to the flow of liquid to and from said stroke control motor for causing operation of said servo-motor, and a relief valve connected to said pump and operated by liquid supplied from said pump under the control of said control valve.

1'7. The combination, with a pump having a fluid actuated stroke control motor for varying its displacement, of means for supplying motive fluid to said motor to operate the same, a control valve subjected at all times to pump pressure for controlling the flow of motive fluid to and from said motor and responsive to a predetermined maximum pump pressure to direct motive fluid to said motor, a fluid servo-motor connected to said valve for overcoming the frictional resistance thereof, fluid channels connecting said servo-motor in circuit with said stroke control J motor, two oppositely arranged low pressure resistance valves connected in series with said stroke control motor to cause the pressure in one end of said servo-motor to be higher than the pressure in the other end thereof in response to operation of said strokecontrol motor in either direction, and a relief valve connected to said pump and operated by fluid under the control of said control valve.

18. The combination, th a pump having a hydraulic stroke control mo 7 for varying its displacement, of means, for supplying liquid 1mg said pump to said motor to operate the same, a control valve subjected at all times to pump pressure for controlling the flow of liquid to and from said motor and responsive to a predetermined maximum pump pressure to direct liquid to said motor, a hydraulic servo-motor connected to said valve for overcoming the frictional resistance thereof, fluid channels connecting said servo-motor in circuit with said stroke control motor, two oppositely arranged low pressure resistance valves connected in series with said stroke control motor to cause the pressure in one end of said servo-motor to be higher than the pressure in the other end thereof inresponse to operation of said stroke control motor in either direction, and a relief valve connected to said pump and operated by liquid supplied from said pump under the control of said control valve.

192 The combination, with a pump having a fluid actuated stroke control motor for varying its displacement, of means for supplying motive fluid to said-motor to operate the same, a controlwalve subjected at all times to pump pressure for controlling the flow of motive fluid to and from said motor and responsive to a predetermined maximum pump pressure to direct motive fluid to said motor, and means including a fluid servo-motor for exerting upon said valve in one direction or the other a force which remains substantially constant during operation of said pump and which acts upon said valve in a direc tion opposite to its direction of movement to thereby resist overrun of said valve and to assist in starting said valve in said opposite direction.

20. The combination, with a pump having a hydraulic stroke control motor for varying its displacement, of means for supplying liquid from said pump to said motor to operate the same, a control valve subjected at all times to pump pressure for controlling the flow of liquid to and from said motor and responsive to a predetermined maximum pump pressure to direct liquid to said motor, and means including a servo-motor operated by liquid supplied by said pump for exerting upon said valve in one direction or the other a force which remains substantially constant during operation of said pump and which acts upon said valve in a direction opposite to its direction of movement to thereby resist overrun of said valve and to assist in starting said valve in said opposite direction.

JAMES K. DOUGLAS.-

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