HYDRAULIC PUMP UNLOADER MEANS

Abstract: An unloader controlling the output of a hydraulic pump, for example, in a constant pressure-type motor vehicle braking circuit. Delivery to hydraulic accumulators in the circuit is halted by a resilient seal (11) defining a gap forming part of the inlet fluid flowpath to the pump and to which the pump delivery pressure is applied, in the sense to close the gap, by a valve (14) operable when the delivery pressure reaches a predetermined value.

Description: The invention relates to an unloader, more particularly an unloader for controlling the fluid output of a hydraulic pump in a constant pressure type circuit, particularly as used in power hydraulic braking systems for motor vehicles. In such systems the pump output is used to fill hydraulic accumulators with pressurised fluid which is, in turn, used for operational duties such as applying the brakes under the command of the brake pedal. When the accumulators are filled with pressurised fluid the pump delivery must be temporarily halted or by-passed and when topping up of accumulator fluid is required the delivery must be recommenced. A device for this purpose is called the pump unloader and there are basically three types: a) to stop rotation of the pump, this would be a clutch device if engine driven, or a switch if motor driven, the command device is usually a pressure rather than volume sensor. b) to by-pass the unwanted delivery back to reservoir via control valve. c) to stop delivery by closing the pump inlet or suction port. U.K.-A-1300278 discloses a typical pump layout using an unloader of type c (above). It is a dual output pump, but could clearly be adapted as a single output one and the unloader means is a pressurised pin (19) urging a plunger (18) against a spring to close port-hole (16) and stop fluid entering the piston/cylinder assembly (5) which in turn terminates delivery at fitting (11). An alternative construction is shown in Patent UK-A-1485532 in which a pressurised spool (8) cuts off flow through the centre of a cup seal (at 10a). One object of the present invention is to provide an improved unloader capable of quieter operation and clean fluid cut-out and cut-in functions. US-A-2,594,132 discloses an unloader for a fluid pump having an inlet fluid flow path passing through a gap defined by a resilient seal, and comprising a valve operable when the delivery pressure reaches a predetermined value, to apply the pump delivery pressure to the seal, in the sense to close the gap. According to the present invention such an unloader is characterised in that , the seal is an annular seal, such as an O-ring seal, disposed in a groove around a cylindrical unloader body within a bore housing the body, to define an annular fluid inlet gap, the delivery pressure being applied in the groove and acting to expand the seal radially into contact with the bore. When the delivery pressure is removed, the seal contracts due to its resilience thus permitting inlet fluid to flow and load the pump. In the preferred embodiment, the valve is operable by a plunger responsive to the pump delivery pressure and acting against a bias. It comprises a valve element preferably a ball, co-operating with a first valve seat and movable by the plunger when the delivery pressure reaches a predetermined value, to co-operate with a second valve seat and close a flow path communicating between the gap and the space behind the seal. As the valve element lifts from the second seat (when the delivery accumulator pressure falls) the delivery pressure acting behind the seal is dropped to the reservoir pressure acting in the gap so allowing the seal to contract. A predetermined differential between cut-in and cut-out pressures may be obtained by making one seat, the second seat, larger than the other. Embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is an axial section through a hydraulic pump having an unloader according to the present invention; Figure 2 is a cross-section taken on BB in Figure 1. The pump shown in Figure 1 is of the same basic construction to that described in GB 1300278 (the disclosure of which is incorporated herein by reference) consisting of a body 1, a crankshaft 2, a link or connecting rod 3, a piston 4, a delivery valve disc 5, a fluid inlet port 6, a crankcase feed drilling and an unloader assembly. Figure 2 shows the unloader in which fluid from the system reservoir (not shown) enters the pump assembly through the elbow 9 onto the pump inlet port 6 having passed through the annulus at 10 which exists because an elastomeric seal 11 is under fitted in its bore by typically 0.5 mm. The reservoir fluid also freely enters the pump crankcase 12 through drilling 7 for lubrication purposes (if the pump is adapted as a dual circuit unit then the crankcase is fed from one input only to avoid mixing of the circuits). The pump shown is port inlet (it could of course be valve inlet) it fills its cylinder with fluid on the down stroke and pumps the fluid at pressure on the up stroke through the plate valve 5 and onto the circuit accumulators for storage at pressure. To illustrate operation of the unloader, the drawing of Figure 2 is split, the portion above the centreline is shown in the unloaded position (i.e. pump not delivering) and the portion below the centre line in the loaded position (i.e. pump delivering). The chamber containing a ball 14 has two seats 15 and 16, the former typically is about 0.15 mm larger in diameter than the latter. When the delivery pressure (i.e. accumulator pressure) is below the normal circuit operating value the thrust developed by pin 17 is insufficient to prevent springs 18 from seating the ball at 16 and seal 11 takes the position shown (below the centreline) and admits fluid to the port 6. When the delivered pressure reaches the operating value it urges the ball 14 against the spring load through pin 19 onto seat 15. Also due to controlled leakage past pin 17 the delivery pressure acts through radial holes 20 to urge seal 11 radially onto its bore thus preventing fluid passing to port 6 and terminating the pump delivery. The diametral difference between 15 and 16 is to provide a specified pressure differential between cut-in and cut-out to avoid over sensitivity. The pressure in the pump inlet and port 6 is always low so when the pump is loading and pressure 20 is released past seat 15 and along drilling 21 the elastomeric properties of seal 16 returns it radially within its fitted leakproof gap to re-open the annulus 10 and permitting fluid to port 6 and onto delivery. The design gives a snap action in both directions to the fully open or closed positions, thereby eliminating any throttling of the inlet caused by the partial opening of conventional arrangements. This results in a quieter operating of the unloader function. If a seal with insufficient elastomeric returning force is used, then a garter spring or other aid may be required to ensure its return.

Claims: 1. An unloader for a fluid pump having an inlet flow path passing through a gap (10) defined by a resilient seal (11) and comprising a valve (14, 15, 16) operable when the delivery pressure reaches a predetermined value to apply the pump delivery pressure to the seal (11), in the sense to close the gap (10), characterised in that the seal (11) is an annular seal disposed in a groove around a cylindrical unloader body within a bore housing the body, to define an annular fluid inlet gap (10), the delivery pressure being applied in the groove (20) and acting to expand the seal radially into contact with the bore. 2. An unloader according to claim 1 wherein the valve comprises a valve element (14) co-operating with a first valve seat (16) and movable by a plunger (17) against a bias (18) when the delivery pressure reaches a predetermined value, to co-operate with a second valve seat (15) and close a flow path (21, 15, 20) communicating between the gap (10) and a space (20) behind the seal. 3. An unloader according to claim 2 wherein the effective area of the second seat (15) is larger than of the first seat (16).

Inventors: MCCULLAGH, MICHAEL GERARD
Applicants: WABCO AUTOMOTIVE U.K. LIMITED
Application year: 1989
Main classification: F04B49
IPC codes: F04B49, F16K7