| 摘要 |
<p>Two embodiments are provided. In the first embodiment a variable capacity vane pump has a slide ring which is moveable to alter the capacity of the pump and the pump can be operated at either of at least two selected equilibrium pressures. The slide ring is moved by a control piston with dual control surfaces, the control piston being received in a housing on the pump case and the two control surfaces cooperating with the housing to form two chambers in which pressurized working fluid can be received. When pressurized fluid is supplied to only one chamber, the pump operates at a first equilibrium pressure and when pressurized fluid is also supplied to the second chamber, the pump operates at a second equilibrium pressure. If the relevant areas of the chambers differ, three equilibrium pressures can be selected between by applying pressurized fluid to one, or the other or both chambers. In the second embodiment, a fixed capacity pump has a pressure relief valve piston with two control surfaces forming chambers with the pump housing similarly to the first embodiment, such that the pump can be operated at either of at least two selected equilibrium pressures.</p> |
