Positive displacement or hydrostatic pumps

As the name implies, these pumps discharge a fixed quantity of oil per revolution of the pump shaft. In other words, they produce flow proportional to their displacement and rotor speed. A majority of the pumps used in fluid power applications belong to this category. These pumps are capable of overcoming the pressure that results from the mechanical loads on the system as well as the resistance to flow due to friction. Thus the pump output flow is constant and not dependent on system pressure. Another advantage associated with these pumps is that the high-pressure and low-pressure areas are separated and hence the fluid cannot leak back and return to the low-pressure source. These features make the positive displacement pump most suited and universally accepted for hydraulic systems.

The advantages of positive displacement pumps over non-positive displacement pumps are:
• Capability to generate high pressures
• High volumetric efficiency
• Small and compact with high power to weight ratio
• Relatively smaller changes in efficiency throughout the pressure range
• Wider operating range i.e. the capability to operate over a wide pressure and speed range.

As discussed earlier, it is important to understand that pumps do not produce pressure; they only produce fluid flow. The resistance to this flow as developed in a hydraulic system is what determines the pressure. If a positive displacement pump has its discharge port open to the atmosphere, then there will be fluid flow, but no discharge pressure above that of atmospheric pressure, because there is no resistance to flow.

If the discharge port is partially blocked, then the pressure will rise due to the resistance to flow. In a scenario where the discharge port of the pump is completely blocked, theoretically an infinite resistance to flow is possible. This will result in a rapid rise in pressure which will result in breakage of the weakest component in the circuit. This is exactly the reason why positive displacement pumps are provided with safety controls, which help prevent the rise in pressure beyond a certain value.

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