US3785754A

US3785754A - Manual override system for a variable volume pump

Info

Publication number: US3785754A
Application number: US00248060A
Authority: US
Inventors: D Miller
Original assignee: Caterpillar Tractor Co
Current assignee: Caterpillar Inc
Priority date: 1972-04-27
Filing date: 1972-04-27
Publication date: 1974-01-15
Anticipated expiration: 1991-01-15
Also published as: BE806717A

Abstract

A hydraulic control system in which an engine driven variable displacement fluid pump provides a source of fluid pressure for powering work implements, and having automatically operable control means responsive to engine speed to control displacement of the variable displacement pump to correspond to engine horsepower, is provided with a manually operable control means to override the automatic control system for selectively controlling the displacement of the variable displacement pump.

Description

o i United States Patent 1191 1111 3,785,754

Miller Jan. 115, 1974 [54] MANUAL OVERRIDE SYSTEM FOR A 3,127,745 4/1964 Young 60 427 VARIABLE VOLUME PUMP 3,477,225 11/1969 Cryder et al.... 60/447 3,587,765 11/1968 McFarland 60/444 [75] In entor: ean E- East o a, Ill 3,191,322 6/1965 Weisenback 60/444 [73] Assignee: Caterpillar Tractor Co., Peoria, Ill.

Primary Examiner-W1ll1am L. Freeh Flledl P 7 Att0rneyFreling E. Baker [21] Appl. No.: 248,060

. [57] ABSTRACT 52 us. c1 417/213, 60/444, 60/449, A hydraulic control System in which enginfi drive 417/222 variable displacement fluid pump provides a source of 51 1111. C1. FlSb 9/04 fluid Pressure for Powering implements and [58] Field of Search 417/222, 213; ing automatically Operable control means .respmsive 60/444, 427 449 to engine speed to control displacement of the vari- I able displacement pump to correspond .to engine [56] Refe'rences Cited horsepower, is provided with a manually operable UNITED STATES PATENTS control means to override the automatic control system for selectively controlling the displacement of the 2,232 332 10/1323 et a1 60/444 variable displacement pump 3,339,363 9/1967 Quayle..... 7 Claims, 1 Drawing Figure VOLUME PUMP BACKGROUND OF THE INVENTION The present invention relates to hydraulic power circuits and pertains more particularly to a circuit for controlling the horsepower output of a variable displacement pump which supplies fluid for an implement power system.

Construction machines, such as loaders and excavators, are generally powered by hydraulic systems. The

hydraulic systems for such machines are generally designed for maximum power output and efficiency.

Many such hydraulic systems are designed to provide high speed operation of the work implements by providing'high pressures within the systems and relatively large fluid flow rates. This high speed operation is desirable in order to operate the construction machine as economically and efflciently as possible. However, such machines are expected to be quite versatile such that in many types of work situations, the machine can be operated at a slow rate in order to provide precise and exacting control. These special situations are dictated by the degree of exactness of certain operations,

as well as the safety of personnel and property which are working in conjunction with the machine or are closely associated therewith. Since implement speed is a function of fluid flow rate, suitable means for controlling fluid flow rate must be provided.

An acceptable degree of control over hydraulic motors in the hydraulic system can generally be accomplished with various types of control valves which modulate or meter the fluid flowing between pump and motor. The trend in production machines is to provide fairly large capacity variable displacement pumps capable of producing a large pressure within the hydraulic system in order to provide for relatively fast movement of the machine and related work implements. These pumps are preferably automatically controlled to provide maximum output for a given engine setting. This type of system will generally provide a desirable high production work cycle. However, such a system is not very well suited to slow rates of operation to obtain precise and exact control.

In order to obtain a higher degree of control over the work implements than that currently provided by the usual control valves, the. rate of flow of fluid which determines the speed of operation of the implement must be decreased. This decrease in the speed of operation can be provided by decreasing the volumetric output of the pump or pumps. One method of decreasing the volumetric output of a variable displacement pump is to use an automatic underspeed valve which responds to a decrease in engine speed to decrease pump displacement. Such an automatic underspeed valve is disclosed in US. Patent Application Ser. No. 68,317, filed on Aug 3 l 1970 now Pat. No. 3 ,727 628 and also in its parent application, now US. Pat. No. 3,477,225, both assigned to the assignee of the present application.

The automatic underspeed valve, as disclosed in the above-noted application, decreases the volumetric output of a variable displacementpump in response to a reduction in engine rpm such as would result from an overload condition of the construction machine engine. Reduction of the pump output provides more exacting control of a work implement since less volume of fluid is available for modulation by the control valve. The

automatic underspeed valve operates to prevent stalling of the machine engine due to an overload condition at the work implement location.

. SUMMARY OF THE INVENTION Accordingly, it is an object. of the present invention to provide an improved hydraulic system that overcomes the above problems of the prior art.

Another object of this invention is to make control over the volumetric output of a fluid pump selectively available by providing means for manually overriding the normal controls of the fluid pump.

Another object of this invention is to provide a safer operating hydraulic system due to the improved control over the system.

Other objects and advantages of the present invention will become more readily apparent upon reference to the accompanying drawing and following description.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a schematic diagram of a hydraulic system embodying the principles of the present invention.

DESCRIPTION or; THE PREFERRED EMBODIMENT a means of line 15 to power one or more fluid motors (not shown). A servo control unit 18 is connected to pump 12 and is operative to adjust the displacement of the pump to various fluid delivery rates.

The flxed displacement pump 13 is preferably a smaller displacement pump than pump 12 and provides fluid pressure into line 19 for a pilot control system to operate valves, etc., of the main power system.

The hydraulic controlsystem 10 also includes suitable signal generating means such as an underspeed valve 20 which is operative to generate a signal that is a function of engine speed, and a manually operated pressure reducing valve 21. This valve 20 is operative to send a signal to the servo controls 18 when the speed of the engine 11 changes a certain degree. Upon receiving such a signal, servo control 18 will adjust the output of pump 12 in response to variations in the signal. The pressure signal from underspeed valve 20 is transmitted to the servo controls by way of

lines

24 and 25 and across a one-way check valve 26.

The fluid pressure generated by pump 13 passes through underspeed valve 20 and continues on to valve 21. Valve 21 is selectively activated by a foot or hand lever 27 when it is desirable to override or by-pass the automatic control of the underspeed valve. When valve 21 is activated, the pressure signal available from pump 13 passes across valve 21 into line 28, across one-way check valve 29, into line 25 and then-to the servo control 18. The servo control 18 can therefore be selectively activated by actuation of valve 21. A pressure signal in line 28 is prevented from entering line 24 by the one-way check valve 26, and likewise, a pressure signal in line 24 is prevented from entering line 28 by the one-way check valve 29.

A suitable relief valve 30, operative in the usual manner, is provided in the control circuit to provide a predetermined upper limit of pilot pressure in the control circuit. Also, inherent in systems of this sort is a certain amount of leakage in the pump displacement control which permits the signal in line 25 to bleed off when there is no signal in either

lines

24 or 28. Where more precise control may be necessary, the leakage control could be provided by a bleed orifice 31, as schematically shown.

OPERATION While the operation of the present invention is believed apparent from the foregoing description, further amplification thereof will be made in the following brief summary.

With engine 11 running, pumps 12 and 13 are powered and are providing pressurized fluid into lines 15 and 19 respectively. As pointed out above, the fluid in line 15 flows to some type of work implement for providing power to operate that particular implement. During normal operation of the work implement, the variable displacement pump 12 is automatically adjusted to full capacity or near full capacity output, such that fast response of the work implement is possible.

Should a condition arise where the work implement being supplied by pressurized fluid from pump 12 is overloaded, the pressure in line 15 will increase and cause an overloading of pump 12. This overloading is transmitted back as an increased load on the engine 11 and could cause stalling of the engine if the pressure or volume of fluid supplied into line 15 is not reduced. In order to prevent the stalling of the engine 11, the volumetric displacement of pump 12 is reduced by activating the servo controls 18 by means of a signal from the automatic underspeed valve 20. The underspeed valve 20 is responsive to a change in the pressure signal in line 19, due to a reduction in the speed of pump 13, which is proportional to the speed of engine 11. Therefore, when underspeed valve 20 is actuated, a pilot pressure signal is introduced into

lines

24 and 25 and activates the servo controls 18 to reduce the volumetric output of pump 12.

With the volumetric output of pump 12 reduced, the pressure in line 15 can remain the same or increase slightly to provide the necessary force to accomplish the work by the work implement. When the condition which caused overloading of pump 12 and engine 11 is no longer present, the engine speed increases, speed of pump 13 increases, and underspeed valve 20 deactivates. The signal, in

lines

24 and 25 is therefore discontinued and servo controls 18 readjust pump 12 to the previous high capacity output. Since the speed of pump 13 is directly proportional to the speed of the engine 11, the signal generated by the pump and sent to the servo controls 18, byway of the underspeed valve 20, can automatically adjust pump 12 to a number of output capacities within the limits of maximum and minimum outputs.

. 6 In certain speclal situations when the pump 12 is providing maximum fluid output, it is desirable that the fluid output be reduced in order to reduce the speed of movement of the implements supplied by pump 12. To

accomplish this, a pilot pressure signal is supplied to the servo controls 18 through

lines

28 and 25 from the manually operated valve 21. The pilot signal supplied 5 by valve 21 is provided by pump 13. The degree of adjustment of pump 12 can be controlled by the amount that valve 21 is actuated. To adjust pump 12 to a minimum fluid displacement, valve 21 is opened to a maximum condition such that a maximum pressure signal is supplied to the servo controls 18. However, if a small reduction in volumetric output is desired, then valve 21 is actuated only slightly and a smaller signal is supplied to the servo controls 18. As previously explained, a reduction in the volumetric output of pump 12 causes slower movement of the work implement supplied by pressurized fluid from pump 12. This reduced movement of the work implement or implements is desirable for close precision work and for safety reasons.

Since valve 21 can be manually activated at any time to provide a reduction in output of pump 12, it constitutes a manual override for the normally automatic system utilizing the underspeed valve 20. In view of the fact that the underspeed valve 20 and the pressure reducing valve 21 both activate the same servo controls 18, activation of one valve has no effect on the other valve.

Valve 21 provides a great deal of versatility to the subject control system by allowing selective reduction of the fluid output of pump 12 within the limits of minimum and maximum output. This in turn provides a great deal of precise control over the work implements powered by the subject control system.

In view of the foregoing, it is readily apparent that the structure of the present invention provides an improved hydraulic control system for a construction machine powered by such a control system. While the invention has been described and shown with particular reference to the preferred embodiment, it will be apparent that variations are possible that would fall within the scope of the present invention, which is not intended to be limited, except as defined in the following claims.

What is claimed is:

1. A hydraulic control system, said system comprising:

a prime mover;

a variable displacement pump driven by said prime mover; servo control means for changing the displacement of said variable displacement pump;

a fixed displacement pump driven by said prime mover;

automatically operable control means including an underspeed valve operative in response to the output of said fixed displacement pump to activate said servo control means to adjust the displacement of said pump to the output of said prime mover; and,

6 manually operablecontrol means including a pressure reducing valve for selectively overriding said automatically operable control means and thereby selectively reducing the displacement of said variable displacement pump.

2. The control system of claim 1 wherein said pressure reducing valve disposed in conduit means down stream of said underspeed valve from said fixed displacement pump and is operative to simultaneously bypass said underspeed valve and provide a pilot signal to said servo control means for adjusting the volumetric displacment of said variable displacement pump.

3. The control system of claim 1 comprising a plurality of one-way check valves operative to prevent feedback between said automatically operable control means and said manually operable control means.

4. The invention of claim 3 wherein said manually operable control means comprises a pilot operated pressure reducing valve operative to provide a pilot signal to said servo control means for adjusting the volumetric displacement of said variable displacement pump.

5. A hydraulic power system, said system comprising:

a prime mover; v

a variable displacement pump driven by said prime mover;

a fixed displacement pump driven by said prime mover;

automatic hydrualic control means including an underspeed valve operative in responsive to the output of said fixed displacement pump to adjust the displacement of said pump as a function of the rpm of said prime mover as determined by said fixed displacement pump; and,

manual control means including a pressure reducing valve operative to bypass said underspeed valve to override said automatic control means to reduce the displacement of said pump.

6. The invention of claim 5 wherein said control means comprises a fixed displacement pump driven from said prime mover;

said underspeed valve is responsive to the output of said fixed displacement pump to generate a signal; and,

a servo control means operative in response to said signal to adjust the output of said variable displacement pump.

7. The invention of claim 6 wherein said valve is a pressure reducing valve.

* =l l l

Claims

1. A hydraulic control system, said system comprising: a prime mover; a variable displacement pump driven by said prime mover; servo control means for changing the displacement of said variable displacement pump; a fixed displacement pump driven by said prime mover; automatically operable control means including an underspeed valve operative in response to the output of said fixed displacement pump to activate said servo control means to adjust the displacement of said pump to the output of said prime mover; and, manually operable control means including a pressure reducing valve for selectively overriding said automatically operable control means and thereby selectively reducing the displacement of said variable displacement pump.

3. The control system of claim 1 comprising a plurality of one-way check valves operative to prevent feed-back between said automatically operable control means and said manually operable control means.

5. A hydraulic power system, said system comprising: a prime mover; a variable displacement pump driven by said prime mover; a fixed displacement pump driven by said prime mover; automatic hydrualic control means including an underspeed valve operative in responsive to the output of said fixed displacement pump to adjust the displacement of said pump as a function of the rpm of said prime mover as determined by said fixed displacement pump; and, manual control means including a pressure reducing valve operative to bypass said underspeed valve to override said automatic control means to reduce the displacement of said pump.

6. The invention of claim 5 wherein said control means comprises a fixed displacement pump driven from said prime mover; said underspeed valve is responsive to the output of said fixed displacement pump to generate a signal; and, a servo control means operative in response to said signal to adjust the output of said variable displacement pump.

7. The invention of claim 8 wherein said valve is a pressure reducing valve.