Hydraulic Pumps

Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic. Hydrostatic pumps are positive displacement pumps while hydrodynamic pumps can be fixed displacement pumps, in which the displacement (flow through the pump per rotation of the pump) cannot be adjusted, or variable displacement pumps, which have a more complicated construction that allows the displacement to be adjusted.

Gear pumpsGear pumps (with external teeth) (fixed displacement) are simple and economical pumps. The swept volume or displacement of gear pumps for hydraulics will be between about 1 cm3 (0.001 litre) and 200 cm3 (0.2 litre). They have the lowest volumetric efficiency.

Rotary vane pumpsRotary vane pumps (fixed and simple adjustable displacement) have higher efficiencies than gear pumps, but are also used for mid pressures up to 180 bars in general. Modern units can exceed 300 bars in continuous operation, although vane pumps are not regarded as "high pressure" components. Some types of vane pumps can change the centre of the vane body, so that a simple adjustable pump is obtained.

Radial piston pumpsRadial piston pumps are used especially for high pressure and relatively small flows. Pressures of up to 650 bar are normal. In fact variable displacement is possible. The pump is designed in such a way that the plungers are connected to a floating ring. This floating ring can be moved horizontally by a control lever & thus causes an eccentricity in the center of rotation of the plungers. The amount of eccentricity can be controlled to vary the discharge. The suction & discharge can be totally reversed seamlessly by shifting the eccentricity to the opposite side. Hence both quantity & direction can be varied in a radial piston pump, just as in the Swash plate pump.Peristaltic pumpsPeristaltic pumps are not generally used for high pressures.

Flow and Efficiency

𝐸𝑓𝑓=𝑂𝑢𝑡𝑝𝑢𝑡𝐼𝑛𝑝𝑢𝑡

𝑥100%

𝑄=𝑛𝑉𝑠𝑡𝑟𝑜𝑘𝑒𝑛𝑣𝑜𝑙

Where Q = flow in

Where n = Where Vstroke = swept volume in

Where nvol = volumetric efficiency

PowerP = P = Power in Watt (Nm/s)n = revs per second.Vstroke = swept volume in m3Δp = pressure difference over pump in N/m2ηmech,hydr = mechanical/hydraulic efficiency

Problem 1.

A new bought pump has a given power of 200hp. With an efficiency of 94% what power would it really yield?

Problem 2.

What is the volumetric efficiency of a pump with a swept volume of 1cu.m while it revolves 2 times per second and has a flow of 1.9 cu.m/sec?

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