Many industrial and automotive devices require a pressurized supply of incompressible fluid such as lubricating oil to operate. Pumps, typically used to supply these fluids, can either be of constant displacement (i.e.—volumetric displacement) or variable displacement designs.
With a constant displacement pump, the pump outputs a substantially fixed volume of working fluid for each revolution of the pump. To obtain a desired volume and/or pressure of the working fluid the pump must either be operated at a given speed, independent of the speed of the automotive engine or other device supplied by the pump, or a pressure relief valve must be provided to redirect surplus flow, when the pump is operated above the speed required for the desired flow, to the low pressure side of the pump or to a working fluid reservoir.
With a variable displacement pump, the volumetric displacement of the pump can be altered, to vary the volume of fluid output by the pump per revolution of the pump, such that a desired volume of working fluid can be provided substantially independently of the operating speed of the pump.
Variable displacement pumps are typically preferred over constant displacement pumps with relief valves in that the variable displacement pumps offer a significant improvement in energy efficiency, and can respond to changes in operating conditions more quickly than pressure relief valves in constant displacement pumps.
While variable displacement vane pumps are well known, they do suffer from some disadvantages. For example, differences in the fluid pressures of the pump chambers (formed between adjacent vanes, the rotor and the cam ring) can cause undesirable variations, or pulsations, on the cam ring, as the pump chambers move with the rotor, which results in pulsations in the output pressure of the pump.
U.S. Pat. No. 4,679,995 to Bistrow discloses a variable displacement vane pump wherein a dampening force is applied to the cam ring of the pump to reduce the pulsations of the cam ring. In one embodiment, the dampening force is provided by pressurized working fluid in a chamber adjacent the cam ring. The working fluid is provided from the outlet of the pump, through a passage which is obstructed depending upon the position of the cam ring, to alter the pressure and thus the resulting dampening force. In another embodiment, the working fluid is supplied from the outlet to the cam ring through a tapered recess in which a complementary tapered piston is moved by the cam ring.
However, the pump taught in Bistrow also suffers from disadvantages. Specifically, to provide the cored passages required by the Bistrow pump to supply the working fluid to the chamber, the pump must be manufactured by sand casting which increases both the manufacturing cost, production cycle time and precludes the use of desirable materials such as aluminum for forming the body of the pump.
Diecast variable displacement vane pumps with dampening have been produced previously, but such pumps have been limited to having their outlet located underneath and overlying the outlet port of the rotor chamber, to avoid the need for a cored passage and thus permitting the pump to be diecast. However, because the outlet must be located overlying the rotor chamber outlet port, the layout, port locations, size and volume (i.e. the “packaging”) of such pumps has been quite limited.
Another problem with existing pumps is that the inlet port in the rear plate of prior art pumps is typically in the form of an arc which has a small cross-sectional flow area where it connects to the inlet of the pump and the cross-sectional flow area increases as the arc extends circumferentially about the rotor. The cross-sectional flow area of the inlet port is relatively small in the area where it connects to the pump inlet to ensure that adequate surface sealing area still exists between the cam ring and the rear plate about the pump inlet and inlet port interface. However, such small cross-sectional flow areas can lead to undesired cavitation in the inlet as the pump is operated at higher speeds.
It is desired to have a variable displacement vane pump capable of being manufactured by diecasting or other techniques which can be flexibly packaged and which has dampening on the cam ring. It is also desired to have a variable displacement vane pump with an inlet that reduces the onset of cavitation.