GB2097060A

GB2097060A - Variable-displacement pump

Info

Publication number: GB2097060A
Application number: GB8206029A
Authority: GB
Original assignee: Alfred Teves GmbH
Current assignee: Continental Teves AG and Co oHG
Priority date: 1981-03-10
Filing date: 1982-03-02
Publication date: 1982-10-27
Also published as: FR2501798A1; FR2501798B1; DE3109045A1; GB2097060B; IT8219982A0; IT1150617B

Abstract

A variable-displacement pump (1) for hydraulic fluids including a zero- delivery regulating device (4) contains a biased control spring (5) whose bias is adjustable automatically to two values with the aid of a special control device, by electric or hydraulic means. The higher preloading value is set when a user (2) of the pump pressure generated by the hydraulic pump (1) is connected by a corresponding switching action of a user change- over valve (14). The lower preloading value is set when the user is disconnected. This permits the reduction of the maximum working pressure of the minimum fluid delivery in the case of a hydraulic pump (1) with zero-delivery regulating device under zero-delivery regulating conditions, so that there results saving of energy and decline in noises, with the user disconnected. <IMAGE>

Description

SPECIFICATION Variable-displacement pump This invention relates to a variabledisplacement pump for hydraulic fluids of the kind including a zero-delivery regulating device with a biased control spring to regulate fluid delivery in the zone of maximum discharge flow to a pressurized fluid delivery of minimum discharge flow when the user is cut off.

Known hydraulic pumps of the kind initially referred to are usually operated together with a user, for example a slave cylinder with a working stroke, rest position and return stroke, according to performance characteristics as shown in Figure 1. With the fluid delivery at its maximum Qmax the user pressure p increases following the performance characteristic a. In point A, the regulating device starts to act and regulates the discharge flow Q to cause it to decrease according to the control characteristic b, while the pressure p is still rising slightly (decrease of pump eccentricity or angle of traverse with increasing pressure force in opposition to a control spring).

At the point of operation B (maximum pressure and minimum discharge flow), pumps incorporating zero-delivery regulating devices according to the prior art perform when the user is cut off during the operating cycle and does not withdraw pressure oil. As experience has shown the minimum fluid delivery Qmin has to amount to approximately 5% of the maximum fluid delivery (for lubrication and heat flow), and the consumption of energy will reach a not inconsiderable amount, as a result of the maximum pressure pmax prevailing at this point, in particular when variable pumps with zero-delivery control are utilized in which the operating cycles are interrupted by longer control intervals under zero-delivery control conditions.Moreover, a pump of conventional design is unable to perform with little noise because of the maximum discharge pressure prevailing, unless special provisions are made to prevent such operational noise.

It is an object of the present invention to provide a variable-displacement pump for hydraulic fluids of the kind initially referred to which, while affording a simple design, permits energy-saving and low-noise operation when the user is disconnected.

According to the invention in its broadest aspect, a variable-displacement pump for hydraulic fluids of the kind referred to is characterized in that the bias of the control spring, with the user cut off, that is to say in the zone of minimum discharge flow, is adjustable to a reduced amount by virtue of a sequence control circuit, with the user cut off, that is to say, in the zone of minimum discharge flow.

In particular, the bias of the control spring is adjustable to the reduced amount immediately upon termination of the control action, with the user disconnected, while, with a user connected, the initial bias of the control spring in the zone of termination of the control action, in particular immediately after the control action, can be readjusted by means of the sequence control, so that performance characteristics will result in the pumping mode of hydraulic pump and user being as shown in Figure 2, for instance.

A pump according to the present invention provides performance characteristics a and b in Figure 1 according to the prior art. In contrast to the prior art, the pump will not perform constantly according to the point of operation B with maximum delivery pressure upon termination of the control action b with the user disconnected, but the pressure of the delivered fluid will be reduced to the point of operation C (preferably to about 25% of the maximum pressure) following the characteristic c (0min constant) in the zone of or upon attainment of the maximum pressure Pmax with the user disconnected, that is to say, this will be accomplished by the present invention in that the bias of the control spring of the zero-delivery regulating device is reduced by a special control arrangement, by electric or hydraulic means, preferably automatically.At the commencement of a new operating cycle (opening of the user's change-over valve), the user pressure p first decreases following the control characteristic d until the point of operation D has set up, while the discharge flow 0 rises. Upon termination of the control action d with reduced pressure or in the zone of the termination of the control action, the bias of the control spring will be increased to attain the initial high value, and the operating mode following the characteristics a and b will be resumed. Accordingly, the performance characteristics of a hydraulic pump with zerodelivery regulating device, adapted to be governed according to the present invention, are characterized by a hysteresis with spring control of the operating pressure in the points of operation B and D.

The present invention ensures that, with the user disconnected, a minimum fluid delivery Qmin (about 5% of the maximum fluid delivery) is maintained for the purpose of lubrication and heat flow, while the pressure is appreciably decreased.

As a result, the loss in energy of a pump operated under zero-delivery regulating conditions will be reduced, and energy will be saved in contrast to devices incorporating constant-delivery pumps with unpressurized recirculation even if utilizing a by-pass valve which saves pressure losses, in particular in operating cycles with longer time intervals under zero-delivery control conditions.

By virtue of the pressure being reduced at this point of operation, there will also result an appreciably improved noise level as well as longer life for the pump.

In a favourable improvement of the present invention, the control spring of the zero-delivery regulating device is supported on an adjusting device adapted to assume two positions.

Suitably, this adjusting device is a hydraulically adjustable operating cylinder whose piston supports the control spring.

The hydraulic operating cylinder is preferably adjustable via a three-way/two position directional control valve which is actuatable electromagnetically by a control device.

The change-over process for the bias of the control spring may be initiated by the control pulse ensuring switching action of a user changeover valve which causes a slave cylinder to assume the positions, working stroke, rest and return stroke.

In an alternative embodiment, the hydraulic operating cylinder is likewise adjustable through a three-way/two-position directional control valve which is actuatable hydraulically by the pump pressure generated. The three-way/two-position directional control valve may be a pressureresponsively actuatable change-over valve in a sliding spool and design (with spring control and hysteresis according to Figure 2).

The three-way/two position directional control valve is preferably inserted in the pump circuit such that, with the user connected, the piston of the control spring is acted upon by the pump pressure generated, while it is in communication with the upressurized return line of the pump with the user disconnected.

In an alternative embodiment, in a fourway/three-position directional control valve of the user arranged in the pump circuit, the hydraulic operating cylinder can be inserted in the pump circuit in such a way as to provide a working stroke under load, a rest position and a rapidreturn motion of a slave cylinder so that the operating cylinder will be connected merely to the pressure line for obtaining the working stroke under load L, whilst it communicates with the return line in the rest position and in the position for rapid-return motion R of the valve.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which Figure 1 is a graph showing the characteristics of a prior-art zero-delivery variable-displacement pump, Figure 2 is a graph showing the characteristics of a zero-delivery variable-displacement pump according to the present invention, Figure 3 shows a hydraulic circuit of a zerodelivery variable-displacement pump with hydraulically adjustable bias of the control spring, according to an embodiment of the invention, Figure 4 shows another hydraulic circuit of a variable-displacement pump according to the invention whose zero-delivery regulating device is hydraulically adjustable via a user change-over valve in the control spring biasing arrangement.

Figure 5 shows another hydraulic circuit of a variable-displacement pump according to the invention having a change-over valve with spring control and hysteresis, and Figure 6 is a schematic representation in greater detail of the change-over valve in Figure 5.

Figures 1 and 2 show for comparison two graphs of the characteristics of a zero-delivery variable-displacement pump according to the prior art and to the present invention in a schematic representation, with the fluid delivery Q being indicated on the abscissa and the pressure p of the fluid delivery on the ordinate.

The lines a and b represent the performance characteristics according to which a hydraulic pump 1 and a user 2 usually perform during operation. As shown on line a, the user pressure increases with the pump's fluid delivery being at its maximum, and at point A, the regulating device performing according to characteristic b starts to act and causes the fluid delivery to decline while the pressure is still rising slightly. At the point of operation B (maximum pressure and minimum discharge flow), known hydraulic pumps incorporating zero-delivery regulating devices without the arrangement claimed by the present invention behave, when the user is cut off during the operating cycle or is in its end position and does not withdraw any hydraulic fluid.

According to Figure 2, upon attainment of the maximum pressure with the user disconnected, a hydraulic pump according to the invention behaves according to point of operation C disposed on the second control characteristic d.

When a new operating cycle is commenced, opening of the user's valve will first of all cause the user pressure to decrease following the control characteristic d, with the pressure reduced, until the point of operation D is reached in which the pumping mode according to the characteristics a and b will be resumed.

Figure 3 shows a hydraulic circuit including a hydraulic pump 1 in the form of a variable vanetype pump which incorporates a zero-delivery regulating device 4 with hydraulically adjustable bias of its control spring.

The zero-delivery regulating device is basically composed of an adjusting device 6 in the form of an operating cylinder which includes a tappet 7, coupled to the hydraulic pump 1 itself, and a piston 8, with a control spring 5 being supported therebetween. The piston 8 of the operating cylinder is hydraulically adjustable to assume two positions as will be described in more detail hereinbelow.

The hydraulic circuit according to Figure 3 comprises a pressure line 1 6 connecting the hydraulic pump 1 via a user change-over valve 14 to a user 2 having the form of a cylinder. This slave cylinder can be switched by the user change-over valve 14 (four-way/three-position directional control valve) to assume the positions working stroke, rest and return stroke, with a return line 1 8 being provided to an unpressurized reservoir 1 3.

If necessary, a four-way/two-position directional control valve may be arranged as a user change-over valve 14 when a user 2 without intermediate rest position is connected.

Via a pressure branch line 20, the pressure line 1 6 of the hydraulic pump 1 communicates with an electromagnetically-actuatable threeway/two-position directional control valve 9 which is able to be operated by a control device 10 in response to the control pulse ensuring switching action of the user change-over valve 14.

The three-way/two position directional control valve 9 includes another port for a return line 12 leading to the unpressurized reservoir 1 3 as well as a third port for the branch line 19 which interconnects the valve 9 and the working chamber of the hydraulic operating cylinder.

In the position shown in Figure 3, the branch line 19 is in communication with the return line 12 and the piston of the hydraulic operating cylinder is relieved hydraulically so that the control spring 5 is set to a reduced value of preload, at which value the user 2 is disconnected when the device is in operation.

When the user 2 is connected (working stroke, return stroke), the hydraulic operating cylinder will be joined via the pressure branch line 20 and the branch line 19 to the pressure line 16 of the hydraulic pump 1 as a result of the threeway/two-position directional control valve 9 being switched correspondingly, so that the piston 8 of the hydraulic adjusting device assumes its other (according to Figure 3 right-hand) position in which the control spring 5 is set to a high preloading value.

The control device 10 will be actuated in such a manner that the characteristic diagram according to Figure 2 will result, with the arrangement in operation.

Figure 4 shows another hydraulic circuit comprising a zero-delivery regulating device 4 of a hydraulic pump 1, with the change-over action of the control-spring operating cylinder being combined with the user change-over valve 1 5. In particular, the operating cylinder taking effect on the control spring is coupled directly to the pressure line 1 6 associated with the working stroke (working stroke under load L) of the user 2.

When the valve 1 5 has assumed the rest position or the return position, the operating cylinder is connected to the return line 12 and relieved, which results in a pump performance following the characteristic d with low pressure. The operating cylinder for the control spring is so dimensioned as to bias the spring already in the presence of a comparatively low working pressure that lies above the line d. According to the circuitry in Figure 4, there is no necessity for the valve 9 of Figure 3, if the working stroke (working stroke under load L) of the slave cylinder is effected with high pressure (characteristics a, b of Figure 2) and the return stroke R with low pressure (rapid-return motion, characteristic d).

Another way of changing over the bias of the control spring 5 and thus the controlled pressure automatically from the high pressure level (characteristic b) to a low pressure level (characteristic d) and vice-versa is the employment of a pressure-responsive switching valve 11 with a spring control and hysteresis according to Figures 5 and 6. The system's switching diagram and the symbol of the changeover valve according to Figure 5 correspond basically to the hydraulic circuit according to Figure 3, however, actuation of the valve 11 (three-way/two-position directional control valve) is caused in this case in the pressure line 1 6 by the pump pressure generated (port P' of the valve 1 1).By virtue of the special design of the changeover valve 11, switching over to the low pressure level will be effected upon attainment of a preset high pressure threshold (characteristic b) in the point of operation B according to Figure 2. Reswitching to the higher pressure level (characteristics a, b) as a result of the pressure drop, with the user valve 14 opening, takes place upon attainment of a preset low pressure threshold (point of operation D according to Figure 2).

The three-way/two-position directional control valve 11 of Figure 5 includes ports P, T and S enabling the control-spring operating cylinder to be connected either to the pressure line 1 6 of the pump 1 will act to deliver again high working unpressurized reservoir 13.

The valve 11 is illustrated in detail in Figure 6.

It is designed as a sliding spool, with the slide piston being preloaded by an inward switch spring in opposition to the control pressure port P' which latter is in permanent communication with the pressure line 1 6.

In the switching position with the force of the switch spring preponderating, P and S are interconnected so that the control spring 5 of the pump is preloaded a great amount and high working pressure is present during operation according to the characteristics a, b of Figure 2.

With the pressure being at its maximum (point of operation B), the pressure that acts initially on the small end surface of the slide piston only will have displaced the slide piston towards the switch spring by such an extent that an annular gap is produced and a large end surface is applied in addition. The increase in force causes the slide piston to move promptly into the other end position (according to Figure 6 to the left), thereby interconnecting S and T and relieving the controlspring operating cylinder and the control spring itself being preloaded merely a small amount. The pump 1 delivers but low pressure corresponding to the characteristic d of Figure 2.

Pump 1 will be switched to high working pressure again when, as a result of a marked decline in pressure, after opening of the user change-over valve 14 the slide piston has been urged by the switch spring to resume its initial end position (shown in Figure 6). Due to the large pressure-applied surface, however, this will not take place until attainment of very low pressure (point of operation D of Figure 2). Upon attainment of this pressure, special longitudinal and cross bores in the slide piston will connect the annular surface to the port T leading to the reservoir 13 so that only the small end surface is left to be pressurized and the switch spring promptly urges the slide piston into the end position shown. The control-spring operating cylinder will then be re-connected to the pressure line 16 and will preload the control spring 5 by a great amount, as a result whereof the hydraulic pump 1 willcact to deliver again high working pressure (characteristics a, b).

Claims

1. Variable-displacement pump for hydraulic fluids of the kind including a zero-delivery regulating device with a biased control spring to regulate fluid delivery in the zone of maximum discharge flow to a pressurized fluid delivery of minimum discharge flow when the user is cut off, characterized in that the bias of the control spring (5) is adjustable to a reduced amount by virtue of a sequence control circuitry, with the user cut off, that is to say, in the zone of minimum discharge flow.

2. Hydraulic pump as claimed in claim 1, characterized in that, with the user (2) cut off, the bias of the control spring (5) is adjustable to the reduced amount immediately upon termination of the control action b.

3. Hydraulic pump as claimed in claim 1 or 2, characterized in that, when a user (2) is connected, the bias of the control spring (5) is adapted to be reset to the initial amount by virtue of a sequence control circuitry in the zone of termination, preferably immediately upon termination of the control action d (Figure 2).

4. Hydraulic pump as claimed in claims 1, 2 or 3, characterized in that the control spring (5) of the zero-delivery regulating device (4) is supported on an adjusting device (6) adapted to assume two positions.

5. Hydraulic pump as claimed in claim 4, characterized in that the adjusting device (6) is a hydraulically adjustable operating cylinder whose piston (8) supports the control spring (5).

6. Hydraulic pump as claimed in claim 5, characterized in that the hydraulic operating cylinder is adjustable via a three-way/twoposition directional control valve (9) which is actuatable electromagnetically by a control device (10) (Figure 3).

7. Hydraulic pump as claimed in claim 6, characterized in that the change-over process for the bias of the control spring is initiated by the control pulse ensuring switching action of a user change-over valve (14) which causes a slave cylinder (2) to assume the positions, working stroke, rest and return stroke.

8. Hydraulic pump as claimed in claim 5, characterized in that the hydraulic operating cylinder is adjustable via a three-way/twoposition directional control valve (11) which is actuatable hydraulically by the pump pressure generated.

9. Hydraulic pump as claimed in claim 8, characterized in that the three-way/two-position directional control valve 1) is a pressureresponsively actuatable change-over valve in a sliding spool design (Figure 6).

10. Hydraulic pump as claimed in any one of claims 6 to 9, characterized in that the threeway/two-position directional control valve (9 or 11) is inserted in the pump circuit in such a way that the piston (8) of the control spring (5) is acted upon by the pump pressure generated, with the user (2) connected, while it is in communication with the unpressurized return line (12) of the pump (1), with the user disconnected.

11. Hydraulic pump as claimed in claim 5, characterized in that, in a four-way/three-position directional control valve (15) of the user arranged in the pump circuit, the hydraulic operating cylinder is inserted in the pump circuit in such a way as to provide a working stroke under load L, a rest position and a rapid-return motion R of a slave cylinder so that the operating cylinder will be connected merely to the pressure line (16) for obtaining the working stroke under load L, while it communicates with the return line (12) in the rest position and in the position for the rapidreturn motion R of the valve (15) (Figure 4).

12. Variable-displacement pump for hydraulic fluids substantially as described with reference to the accompanying drawings.