GB2031615A - Improvements in or relating to hydraulic control systems for variable displacement hydraulic pumps - Google Patents

Abstract

A variable displacement hydraulic pump 2 supplies hydraulic working fluid to the hydraulic swing motor 1 of an excavator and has a control system in which a double-acting hydraulic cylinder 23 controls a swash plate via a selector valve 17 and a second double- acting hydraulic cylinder 18. Fluid pressure is selectively applied to opposite ends of the cylinder 23 via hydraulic fluid supply lines 27, 28 connected to manual controls of the excavator. Relief valves 35, 36 connect opposite sides of fed, the pump to opposite ends of the cylinder 23, each relief valve connecting its associated side of the pump to the opposite end of the cylinder to which fluid pressure is applied in order to select that side as the pump output. Each valve 35, 36 opens in response to excess output pressure to apply fluid pressure to said opposite end of the cylinder so as to resist further movement of its piston 24 and, hence, reduce movement of the swash plate in the selected direction. In this way, acceleration of the swing operation is controlled. Moreover, when the dipper of the excavator is held against an obstruction, the system controls the pump output so that the pump only supplies the minimum fluid required to replenish internal leakages in the system. <IMAGE>

Description

SPECIFICATION Improvements in or relating to hydraulic control systems for variable displacement hydraulic pumps The present invention relates to a hydraulic control system for a variable displacement hydraulic pump of the type which includes a movableswash plate for regulating the displacement and the direction of fluid flow from the pump. More particularly, the invention relates to a control system for such a hydraulic pump supplying working fluid to a hydraulic motor which is connected in a closed loop circuit with the pump. The hydraulic motor may, for example, be a hydraulic motor controlling the swing of an excavator.

To achieve minimum time for the swing operation of an excavator, it is necessary to obtain maximum acceleration to full swing speed and maximum deceleration from full speed to rest within the limits of structural strength and the torque capacity of the swing gearbox. This may be controlled in two ways with a closed loop hydraulic circuit as is used in excavators, that is, either (1) by using a relief valve to limit the maximum hydraulic pressure and, hence, the maximum torque output of the swing motor, or (2) by controlling the speed of movement (i.e.

change of inclination) of the pump swash plate to give the correct rate of increase of hydraulic fluid flow for the required acceleration.

The advantage of controlling the movement of the pump swash plate is that only the power required is taken from the engine whereas, by limiting the torque by means of a relief valve, excess power is taken from the engine and rejected as heat at the valve.

Afurther requirement for the swing mechanism of an excavator is the ability to hold the dipper against the side of a trench whilst making a cut in order to produce a straight trench side when cleaning up.

This can be achieved by means of a relief valve, as described above, discharging atfull flow, butconsid- erable horsepower is then wasted and is converted into heat at the relief valve. For example, the horsepower wasted in a medium capacity excavator may be in the order of 40 to 50 horsepower. It may, alternatively, be achieved by returning the pump swash plate towards its neutral position, when the pressure reaches the limit, so as only to pump the minimum fluid required to supply internal leakages of the system.

It is therefore an object of the present invention to provide a control system for a variable displacement hydraulic pump of the type described, whereby the acceleration of a hydraulic motor supplied by the pump is controlled by regulating the movement of the pump swash plate and whereby, when the hydraulic fluid pressure supplied to the motor reaches a predetermined limit, the swash plate is returned towards its neutral position in order to pump the minimum required to supply internal leakages in the hydraulic system.

The invention consists in a hydraulic control system for a variable displacement hydraulic pump of the type described, comprising a double-acting, hydraulic cylinder and piston device adapted to control the pump swash plate, means for selectively applying fluid pressure to opposite ends of the cylinder so as selectively to adjust the swash plate in opposite directions, means for connecting opposite ends of the cylinder to drain means, and relief valves for connecting opposite sides of the pump to the opposite ends of the cylinder, each relief valve being connected to the end of the cylinder opposite that to which fluid pressure is applied in order to select its associated side of the pump as the pump output, and each relief valve being adapted to open in response to excess output pressure so as to apply fluid pressure to said opposite end of the cylinder to resist further movement of the piston and, hence, slow or reduce movement of the swash plate in the selected direction.

Preferably, the means for selectively applying fluid pressure to the cylinder and for connecting the cylinder to the drain means include hydraulic fluid supply lines for connecting manual operating controls to the opposite ends of the cylinder, each supply line being connected by two parallel hydraulic lines to the associated end of the cylinder and to the drain means, each parallel line incorporating a pressure-reducing orifice. Each supply line may also incorporate a pressure-reducing orifice.

Also, preferably, each relief valve is connected to the pump and the cylinder via pressure-reducing orifices. Such orifices assist in preventing sudden surges of fluid flow from the pump to the doubleacting cylinder and thereby control hunting.

In order that the invention may be more readily understood, reference will now be made to the accompanying drawing which illustrates the circuit diagram of a hydraulic control circuit for the swing motor of an excavator, which circuit embodies the invention.

Referring to the drawing, the hydraulic swing motor 1 is connected in a closed loop circuit with a variable displacement hydraulic pump 2 by means of lines 3,4 connecting opposite operating sides of the pump to opposite sides of the motor. The pump 2 is of the reciprocating type and is driven from the main motor or engine (not shown) of the excavator via a shaft 5. It includes a movable swash plate (not shown) for regulating the displacement and direction of hydraulic fluid flow from the pump. Depend ing on the position of the swash plate, the output from the pump is either directed through the line 3 to actuate the motor 1 so as to swing the excavator in one direction or, alternatively, it is directed through the line 4so as to actuate the motor in the opposite direction.An auxiliary pump 6, also driven from the drive shaft 5, is arranged to supply "make-up" hydraulic fluid to the closed loop circuit of the pump and motor. It is supplied with hydraulic fluid from the main fluid tank or reservoir ofthe excavator and is connected to opposite sides of the pump 2 by a line 7, nonreturn valves 8,9 and lines 10,11. The line 7 incorporates a filter 12 and is provided with customary relief valves 13, 14, 15. The auxiliary pump 6 also supplies hydraulic working fluid to the servo mechanism 16 regulating the pump swash plate.This mechanism comprises a servo or selector valve 17 which meters the flow of hydraulic fluid to a double-acting cylinder and piston device 18,19, the piston 19 of which is connected to adjust the swash plate by means of a mechanical linkage schematically illustrated at 20. The linkage 20 includes a feed-back connection 21 from the piston 19 to the valve 17 so as to give positional control.

The position of the spool 22 of the valve 17 is controlled by a double-acting hydraulic cylinder and piston device 23,24 having its piston 24 connected to the valve spool by an operating linkage schematically illustrated at 25. The piston 24 is biassed towards a central rest position by means of springs 26 and is actuated in order to regulate the valve spool 22 by means of differential fluid pressure applied across the piston. Hydraulic fluid pressure is selectively applied to opposite ends of the cylinder 23 via two lines 27,28 which are supplied from the excavator's servo system via a manually operated control valve (not shown) in the operator's cab. Each line 27,28 incorporates a filter 29 and a pressure-reducing orifice 30 in series, and is connected by parallel lines 31,32 to one end of the cylinder 23 and to a drain to the main hydraulic fluid tank T, respectively.Each of the parallel lines 31,32 also incorporates a pressurereducing orifice 33,34.

Opposite ends of the cylinder 23 are connected to opposite operating sides of the pump 2 via relief valves 35,36. The relief valve 35 has its inlet connected to the pump 2 via a pressure-reducing orifice 37 and the line 11, and has its outlet connected to the cylinder via a pressure-reducing orifice 38, whilst the relief valve 36 has its inlet connected to the pump via a pressure-reducing orifice 39 and the line 10, and has its outlet connected to the cylinder via a pressure-reducing orifice 40. Each relief valve 35,36 has a drain line connected to the hydraulic fluid tank via a pressure-reducing orifice 41,42.

The lines 43,44 and outline box 45 schematically illustrate the main hydraulic fluid supply and return circuit of the excavator connected to the main fluid tank T and via which hydraulic fluid is supplied to the auxiliary pump 6 and the cooling circuit of the main pump 2 and is returned to the tank from this cooling circuit and the various outlets and drain lines described above.

The hydraulic control system operates as follows.

When it is required to swing the excavator, hydraulic fluid under pressure is supplied from the machine's servo system, via the control valve in the operator's cab, to either the line 27 or 28, depending on the desired direction of swing, and the hydraulic fluid flows through the selected line and the associated filter 29, orifice 30 and parallel orifices 33,34.

Through the orifice 33 in the drain line 32 the fluid is returned to the tank, whilst through the orifice 34 the fluid is fed to the appropriate end of the cylinder 23.

The combination of the orifices 30,33,34 controls the flow of fluid to the cylinder and, hence, the speed of movement of the piston 24, the speed of movement of the valve spool 22 controlling the piston 19 and, ultimately, the speed of movement of the pump swash plate, thereby regulating the rate of increase of fluid flow from the pump 2 and acceleration of the swing motor 1.

If the fluid pressure required to drive the swing motor, owing to an excessive load at full boom extension, exceeds the setting of the relief valve 35 or 36 interconnecting the selected pump output and the opposite end of the cylinder 23 to which hydraulic fluid pressure is applied in order to select that pump output, the appropriate relief valve discharges fluid to the unpressurised end of the cylinder. The flow of pressure fluid through the valve 35 or 36 to the cylinder is controlled by the orifices 37,38 or 39,40 and the pressure drop through the valve, which depends on the pressure in the closed loop circuit. The fluid flow from the pressurised end of the cylinder 23 is controlled by the associated orifices 30, 33 and 34 and, therefore, the pressure on the cylinder depends upon the combination.The application of fluid pressure to the unpressurised end of the cylinder reduces the differential pressure across the piston and, consequently, slows down its speed of movement and, hence, the speed of movement of the pump swash plate and the acceleration of the motor. In severe cases, the movement of the piston 24 may be reversed, thus reducing the speed of operation. The orifices 37,38 or 39,40 help to prevent sudden surges of flow to the cylinder 23 and thus control hunting. The orifice 41,42 in the drain line of each relief valve damps down hunting of the valve.

To stop the swing motion of the excavator, the control valve in the operator's cab is returned to its neutral position and pressure is shut-off from the line 27 or 28 so that the piston 24 in the cylinder 23 can return to its neutral position under the action of the springs 26, thereby expelling pressure fluid through the orifice 34, and then 33, and the orifice 38 or 40, in parallel. These orifices control the speed of the return movement of the piston and, consequently, the speed of movement of the pump swash plate to its neutral position and deceleration of the swing motor.

When the control valve in the operator's cab is actuated to hold the dipper of the excavator against an obstruction, for example, the side of a trench, the appropriate relief valve 35,36 will discharge fluid pressure to the associated end of the cylinder 23 so that the differential pressure across the piston 24 is reduced, whereupon the piston will move towards its central position and reduce the pump swash plate angle until there is just sufficient hydraulic fluid flow from the pump to overcome leaks and maintain the pressure at the motor, as set by the relief valve 35,36.

Whilst a particular embodiment has been described, it will be apparent that modifications can be made without departing from the scope of the invention as defined by the appended claims.

Claims (9)

1. Ahydrauliccontrol systemforavariable displacement hydraulic pump including a movable swash plate for regulating the displacement and the direction of the pump output, comprising a doubleacting, hydraulic cylinder and piston device adapted to control movement of the pump swash plate, means for selectively applying hydraulic fluid press ure to opposite ends of the cylinder so as selectively to adjust the swash plate in opposite directions, means for connecting opposite ends of the cylinder to drain means, and relief valves for connecting opposite sides of the pump to the opposite ends of the cylinder, each relief valve being connected to the end of the cylinder opposite that to which fluid pressure is applied in order to select its associated side of the pump as the pump output, and each relief valve being adapted to open in response to excess output pressure so as to apply fluid pressure to said opposite end of the cylinder to resist further movement of the piston and, hence, slow or reduce movement of the swash plate in the selected direction.

2. A control system as claimed in claim 1, wherein the means for selectively applying fluid pressure to the cylinder and for connecting the cylinder to the drain means include hydraulic fluid supply lines for connecting manual operating controls to the opposite ends of the cylinder, each supply line being connected by two parallel hydraulic lines to the associated end of the cylinder and to the drain means, respectively, each parallel line incorporating a pressure-reducing orifice.

3. A control system as claimed in claim 2, wherein each supply line also includes a pressurereducing orifice.

4. A control system as claimed in claim 1,2 or 3, wherein each relief valve is connected to the pump and the cylinder via pressure-reducing orifices.

5. A control system as claimed in claim 1,2,3 or 4, including a second double-acting hydraulic cylinder and piston device for adjusting the swash plate and a selector valve for controlling hydraulic fluid flow to opposite ends of the second cylinder, whereby selectively to move the swash plate in opposite directions, said selector valve being arranged to be actuated by the first cylinder and piston device.

6. A control system as claimed in claim 5, wherein the second cylinder and piston device is mechanically coupled to the selector valve so as to provide positional control.

7. A control system as claimed in claim 5 or 6, including an auxiliary pump for supplying hydraulic fluid to the second piston and cylinder device under the control of the selector valve.

8. A hydraulic control system for a variable displacement hydraulic pump constructed, adapted and arranged to operate substantially as hereinbefore described with reference to the accompanying drawing.

9. A hydraulic motor circuit comprising a variable displacement hydraulic pump connected in a closed loop circuit with the motor and a hydraulic control circuit as claimed in any one of the preceding claims for controlling the pump.