VARIABLE DISPLACEMENT PUMP SYSTEM

Abstract: A variable displacement pump system including a pressure and a load sensing compensator (18,19) for operat­ ing a pump adjusting mechanism (11-13) to vary the displace­ ment by applying hydraulic fluid to a hydraulic positioning piston (13) and a spring biased piston (12). The load sensing compensator (19) senses the pressure drop of the working fluid flow across a system valve (15) and applies a modulated pressure to the hydraulic positioning piston (13). The spring biased piston (12) is connected (line 21) to a load sensing line (20).

Description: This invention relates to a variable displacement pump control system comprising the features of the preamble to claim 1. Such a system is known from US-A-3,067,693 with a larger area piston as the controlled positioning piston and an opposing smaller area positioning piston without a spring biasing the piston. Using such a spring is however known in the art (see Fig. 1). Such systems usually are also provided with a pressure compensator for operating a hydraulic positioning piston to decrease the displacement of the pump and for overriding the load sensing compensator which is active as the primary pump control except when the pressure of the pump is at the limit set by the pressure compensator. In these known systems, pump outlet is directly connected to the smaller area piston and to the supply line which includes the system valve. When the system valve is shut down, pump pressure raises and the compensator controls the larger area positioning piston to destroke the pump, so that the smaller area piston displaces fluid into the supply line with a tendency to further raise the pump pressure. Conversly, when the system valve is opened, pump pressure falls and the compensator controls the larger area piston to onstroke the pump so that the smaller area cylinder sucks fluid from the supply line tending to further lower the pump pressure. Therefore, in both cases a positive feedback is brought about which means a destablilzing effect, especially under heavy loads and when the connecting lines are short. The control system oscillates causing hydraulic pulsations and resultant oscillating movement of the load or device being driven. Accordingly, the problem underlying the present invention is to provide a variable displacement pump control system which is more stable. In accordance with the invention, the load pressure as taken from a load sensing passage or downstream from the system valve is connected to the spring biased (or smaller area) piston rather than directly connecting the pump outlet to the spring biased piston. In the drawings: Fig. 1 is a prior art control system, and Fig. 2 an embodiment of the invention. The variable displacement pump from which an outlet 10 is shown may be of the variable axial piston type and comprises a yoke and a swash plate 11 that are movable to vary the stroke of axial pistons (not shown). A first smaller area spring biased positioning piston 12 is acted upon by pressure and yieldingly urges the yoke and the swash plate 11 to a maximum displacement position. A second larger area positioning piston 13 acts on the yoke and the swash plate 11 in opposition to the first spring biased positioning piston 12. Working fluid from the pump outlet 10 flows through a supply line 14 to a directional system or valve 15 and further to a load such as a hydraulic actuator. In the known control system as shown in Fig. 1, a short passage or line 16 connects the pump outlet 10 to the first spring biased positioning piston 12. In Figs. 1 and 2, pump outlet pressure is connected through a pump pressure sensing line 17 to a pressure compensator 18 and through a line section 17a, to a load sensing compensator 19. The pressure compensator 18 may include a pilot valve 22 which opens at the pressure limit, set by its spring force and relieves the spring chamber of the pressure compensator 18 so that the spool thereof travels to the right hand side connecting line 17 to a line section 17c and further to the larger area positioning piston 13 which destrokes the pump (protection against overpressure). Line section 17a of the pump pressure sensing line 17 is connected to the first spool end 19a of load sensing compensator 19 and to pump outlet pressure. The second spool end 19b is connected by a load pressure sensing line 20 downstream of the system valve 15 to the pressure of fluid being delivered to the load. Thus the drop in pressure across the valve system 15 is sensed by compensator 19 which functions to modulate pump outlet pressure in lines 17, 17a and to deliver such modulated or control pressure via lines 17b, 17c to the second positioning piston 13. Modulation also depends on the spring preload of compensator 19. The second positioning piston 13 which is larger in cross section than the first positioning piston 12 determines the position of the yoke and swash plate 11 and therefore pump delivery rate. The displacement is varied in a manner to deliver exactly the load flow required by the actuator or actuators at the load pressure, which is, for example, 14 to 21 bar (200 to 300 p.s.i.) below the pump outlet pressure. When this difference of 14 to 21 bar is exeeded, the spring preload of the compensator 19 is surmounted and a modulated or control pressure produced which lowers the displacement of the pump so that the pressure difference is maintained. It has been found that a hydraulic system as shown in Fig. 1 is unstable under high loads resulting in pulsation of the fluid and oscillating of the device being controlled, particularly when flows are small. In accordance with the invention as shown in Fig. 2, the usual pump pressure line 16 of Fig. 1 is eliminated and a load pressure line 21 is added which extends from the load pressure sensing line 20 downstream of the system valve 15 to the first positioning piston 12 which, therefore, is acted upon by load pressure. It has been found that such a system overcomes the problem of stability of the prior art system. It is believed that any change in the flow rate has a simultaneous influence on both positioning pistons 12 and 13, i. e. changes in the load pressure and in the modulated pressure will arrive practically simultaneously at the first positioning piston 12 and the second positioning piston 13, respectively, whereas it is believed that in the prior art system the influence on the first piston 12 (pump outlet pressure) is followed with a time lag by the influence on the second piston 13 (modulated or control pressure) so that the position of the swash plate 11 is changed in one direction immediately followed by a change in the other direction which might be the reason for the pulsations observed. Although the system has been described as used in connection with a variable displacement pump of the swash plate type, it will be understood by persons skilled in the art that it is also applicable to other types of pumps having differing pump adjusting mechanism for varying the displacement such as eccentric adjustment variable vane pumps and pumps using a rotating group housed in a tiltable yoke assembly.

Claims: A variable displacement hydraulic pump control system comprising (a) a variable displacement pump having an outlet (10) and an inlet and including a pump adjusting mechanism (11-13) comprising a controlled hydraulic positioning piston means (13) and spring biased positioning means (12) yieldingly opposing said controlled positioning piston means (13), (b) a supply line (14) with two sections including a system valve (15) therebetween, (c) load sensing compensator means (19) for providing control pressure, (d) pump pressure sensing passage means (17, 17a) connecting the outlet (10) of the pump to one end (19a) of said load sensing compensator means (19) for sensing the pressure at the outlet (10) of the pump, (e) load pressure sensing passage means (20) extending between the other end (19b) of said load sensing compensator means (19) and said system valve (15), (f) control pressure passage means (17b, 17c) extending between said load sensing compensator means (19) and said controlled positioning piston means (13) for moving said pump adjusting mechanism (11-13) in response to a change in control pressure, characterized by (g) load pressure passage means (21) connecting load pressure downstream of said system valve (15) to said spring biased positioning piston means (12). The variable displacement pump control system set forth in claim 1 including pressure compensator means (18) having one end connected to said pump pressure sensing passage means (17) and being movable when pump pressure exceeds a predetermined amount whereby connecting said control pressure passage means (17c) to said pump pressure sensing passage means (17).

Inventors: TAPLIN, LAEL BRENT
Applicants: VICKERS, INCORPORATED
Application year: 1986
Main classification: F04B49
IPC codes: F04B1, F04B49, F15B11