What is a non displace?

Answer 1

A hydraulic pump that uses an impeller or propeller to move fluid by momentum, as opposed to a positive displacement pump, which moves discrete quantities of fluid with each rotation. A typical application of a non-positive displacement pump is the coolant pump, or water pump, on a radiator-cooled engine.

The most common types of mobile hydraulic (hydraulic oil systems) pumps that one might come across are:

• Vane type (positive displacement, open center system)
• Gee Roller (positive displacement, open center system)
• Gear type (positive displacement, open-center system)
• Radial piston (positive displacement, closed center, destroking and pressure compensating)
• Axial piston (positive displacement, closed center, destroking, pressure compensating and load sensing)

The open center system pumps being positive displacement must have a dump valve, bypass, or pressure relief on the outlet side of the pump because of constant flow production. For example, if you were to put an on-off valve on the outlet side of a positive displacement open-center system and if you turned that valve to "off" You would either hit a system "stall" pressure or have a broken housing and/or lines.

The closed center systems will "destroke" (stop pumping) when there is no demand for pressure oil. This allows for decreased horsepower demands and lower sump oil temperature.

Examples of non-positive displacement pumps are:

• Turbine pumps
• Impeller (centrifugal) found in engine cooling systems, liquid transfer pumps, and dredging systems.
• Any pump that does not require a relief, dump, or bypass valve when outlet flow or pressure is restricted or stopped.

With these examples it is easy to assume that they are open center systems, however, being non-positive displacement an individual could shut off a valve on the outlet side of these systems and avoid system damage. This is due in most part to larger clearances within the pump, which allow for a certain amount of a "calculated leak" or "slippage" as another individual stated. Shutting off the outlet side will strain the system a little but it should be accepted to system specifications. This examples plays out in the cooling system of a vehicle every time the engine temperature is below operating temp. The water pump is being turned whenever the engine is running but until the thermostat (our hypothetical valve) opens, the impeller of the water pump must slip as it continues to try and build flow.