What are the basic measures for Vickers vane pump unloading?

Fortunately, I once worked with the engineers of A&S Hydraulics Co., Ltd., and he suggested this to me:
First, negative cover:
1. Negative cover is also called positive opening, which means that when the working chamber is between the oil suction chamber and the oil discharge chamber, the working chamber has the situation to communicate with them.
2. At this time, the working chamber will not produce trapped oil, but then there will be a large internal leakage, which will reduce the volumetric efficiency, so the negative covering structure is generally not used.
Second, zero cover:
It means that when the working chamber is between the oil suction chamber and the oil discharge chamber, the working chamber is just sealed and the oil suction chamber and the oil discharge chamber are just separated.
2. In this case, the oil pressure in the working chamber rises from the suction pressure step to the drain pressure or from the drain pressure step to the suction pressure, thus causing pressure shock and noise, which is trapped oil phenomenon.
Third, Positive cover:
1. Positive covering, also known as negative opening, refers to the situation in which the working chamber is sealed for a period of time, which is bound to produce trapped oil. However, as long as the oil trap is properly utilized, it can Just eliminate the step of pressure.
2. The general cover structure and the unloading measure based on this structure are adopted in the general hydraulic pump, and the specific structure differs depending on the type of the pump.
I hope useful to you.