Pump and Motor Connections in Closed Loop Hydraulic Systems

Hi BudT

That link made my day. GA from me because it was just what I was looking for. Is CR4 the place to be, or what?

Thanks, Ky.

In my opinion it makes piping simpler if the motor drain is connected to the pump drain and then to the cooler. But if they are connected separately and directly to tank, it may be to prevent pressure surges in the case of either pump or motor. If they are joined together before going to the tank then they must have check valves to prevent flows to go into either drains.

The reason the case drains are connected directly to the tank with out cross connections are to prevent additional pressure surges in the case of either the pump or motor. As the units wear case drain flows tend to increase. (we use the case drain flow as dynamic diagnosis of pump/motor conditions). Separate drains allow diagnosis. As the pump case pressure increases it will also increase the compensator pressure, surges may effect the outlet flow!

A check valve "to prevent flows into either drains" is not adviseable because as the one pressure increases closing the check valve, the other drain pressure must increase to overcome the check pressure in order to drain, therefor adding to the case drain pressure anyway. The check valve typical low pressure is 5 psi and will add to case drain pressure.

Typical pump case shaft seals are only rated for 20-25 psi. max, and hydraulic motor seals may (usually as an option) approach 150-175 psi. max. Excess case pressures will usually blow the shaft seal and/or the case gasket.

Case drain flows are not just caused by wear and tear of components. Pressure drops (my name) across clearance fits are always present. The higher the pump/motor is under pressure, the higher the presure drop, the higher the case drain flow and vise versa.

For pumps or motor(fix displacement type), the case drain flow increase as the pressure drops across clearance fits increases and vise versa.

For variable displacement pumps/motors, you have to include/consider the case drain flow across the orifices of the control mechanisms.