Hydraulic pumps and motors

Learn about hydraulic pumps and motors and where they are used

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What hydraulic pumps are used for

hydraulic pumps

Hydraulic pumps provide a flow of fluid into a system. Typical applications include:

Gerotor: lubrication and boost. Low-pressure automotive systems.

External gear: Robust, lower cost system. Widely used in low to medium duty mobile systems or lower cost industrial power units.

Internal gear: Medium cost system where robust, low noise is required. Typically industrial power units.

Vane: Industrial, medium cost power units. Where low noise is required but relatively low pressures and duty are required.

Axial piston: High pressure, heavy duty systems. Typically excavator lifting or drive systems plus continuous duty industrial power units.

Radial piston: Very high-pressure industrial equipment. Presses, test rigs etc.

How hydraulic pumps work

All pumps work by transferring rotary power into an area of changing volume. As the volume increases, atmospheric pressure acting on top of the fluid in the reservoir pushes the fluid into the new space. The pump should not suck the fluid as this is likely to result in damaging cavitation or aeration. Once the full volume has been reached then timing grooves or ports of the pump, open to allow an area of decreasing volume to push the fluid out into the hydraulic system. The system will create the pressure from the load, not the pump supply.

Different types of hydraulic pump and motors

Gerotor pumps come in a wide range of impeller design, two or multi-lobe versions depending on application requirements.

External gear come with fixed or floating bearing designs. Several different low noise designs are available. External gear motors are available but not as widely used as gear pumps.

Internal gear pumps are available from a number of manufacturers with different design details and numbers of teeth.

Vane pumps come in twin chamber, fixed displacement only, or single chamber, fixed or variable displacement. Design quality can vary, along with the pressure rating, duty, and noise. Vane motors are available but not commonly used.

Axial piston: are available in fixed and variable displacement models. Both come in swashplate or bent axis designs. A wide range of design variations and build standards are available from many different manufacturers. Piston motors are also widely used in bent axis format in both fixed and variable designs.

Radial piston pumps are available in a number of different design formats from different manufacturers and with different numbers of pistons within each design. Radial piston motors are far more common than pumps because of their wide use as wheel motors. There is also a wider variation in design types.

Hydraulic pumps displacement

Fixed or variable displacements are measured in cubic centimeters per revolution, cc/rev or cubic inches per revolution, cuin/rev. For example a 10cc/rev pump will give 10 Litres per minute (L/min) at 1000 revs/minute.

How to specify hydraulic pumps

With so many different manufacturers and products, we cannot provide specific performance details or recommendations. Instead, we will highlight the important factors in each particular design type, but users must check the manufacturer's datasheet to compare how well different products perform.

Some of the important areas to consider with pumps are:

1. Cleanliness limits e.g. the level it needs from the system to work reliably and the best it will allow the system to run at, considering the duty at which it will work. Users should also consider what the consequences will be if the pump was to fail e.g. what would be the nature of the debris released during a typical failure. Does filtration need to be improved?

2. What is the minimum suction head requirement? Can pump suction conditions be improved, particularly when starting from cold? Will units operate at altitude which could increase potential issues.

3. What is the predicted life of the pump under the expected duty cycle? Remember that rated life predictions are based on normal operating conditions, which will not be the same for all installations. Have peak pressure or continuous pressure ratings been used?

4. Consider what effect the system dynamics will have on pump life e.g. a high frequency of pressure changes or very steep pressure rise rates.

5. Does the drive system apply acceptable sideloading to the pump drive shaft?

6. Is the pump, its seals and operating fluid appropriate for the working temperature range.

7. Is planned maintenance appropriate e.g. is the fluid health checked or could it be damaged by aging or local operating conditions, therefore, reducing the life of the pump? Can the temperature of the case leakage line be monitored as a way of predicting pump damage?

8. Could volumetric efficiency drop at particular working speeds, temperatures, or pressures? Is there enough installed power to operate under the worst conditions or are certain environmental operating limits required.

9. Could overall efficiency drop at particular speeds or pressures?

10. Does the pump require a separate case leakage line? And if so what is the maximum pressure permitted.

11. Will pump generated noise be an issue? Are quieter pump options available or can the noise propagation throughout the system be contained instead?

12. Has the installation been considered? Are lifting points or special tools required.