| 摘要 |
<p>A positive displacement pump system has two delivery passages (11,12) communicating under the control of a valve (13) with a main discharge passage (15) and/or an over-spill duct (14). The valve comprises a valve member (20) slidably mounted in a valve bore and the valve member has lands and intermediate recesses controlling communication between the delivery passages and the overspill duct. The pressures upstream and downstream of a discharge orifice (17) in the discharge passage are applied to the valve member in opposition to each other, increase of the pressure drop tending to move the valve member to increase the amount of fluid passed to the overspill duct. To overcome the known tendency for a valve member to be subjected by the hydraulic forces to a large closing force when the valve is open to only a small extent, the valve is designed to provide for a relatively large opening force at least when the valve is commencing to open. In one construction this is achieved by passing the flow from the second to the main discharge duct through the valve member in a manner producing a jet reaction on the valve member to assist the normal regulating force on the member. In alternative arrangements the additional force is derived from an additional pressure drop induced in the flow from the delivery passage, the higher pressure being applied to valve member in a sense to increase the amount of fluid passed to overspill.</p> |
