Hydraulic pumps and hydraulic motors are both integral components of hydraulic systems. Hydraulic systems power many different types of plant and static industrial machinery, so a more detailed knowledge of the two can go a long way to helping you understand the equipment your business operates.
To a layman, the terms hydraulic pump and hydraulic motor may sound interchangeable, but in reality they perform opposite functions within a hydraulic system. There are some clear differences between the two which are easy to understand once explained simply, so we’ll run through how to tell them apart below.
The best way to summarise the difference between a hydraulic pump and a hydraulic motor is to think about where the power to the hydraulic system is coming from. One takes the energy from the primary power source and converts it into hydraulic pressure, whilst the other takes the pressure in the hydraulic system and converts it back into kinetic energy.
Hydraulic pumps perform the former of these two functions. Driven by an electric or combustion-driven motor, they convert kinetic energy into pressure within the hydraulic system. This is achieved with some form of fluid displacement mechanism, which forces hydraulic fluid from a reservoir into the functional components of the hydraulic system. These functional components include rams, cylinders and hydraulic motors.
Despite the fact hydraulic pumps and motors effectively serve opposite functions, they are usually not interchangeable as hydraulic pumps cannot be backdriven. There are many different types of hydraulic pump, so we’ll list a couple of the most common ones below:
Gear pumps are one of the most common types of hydraulic pump due to their simple design and cheap method of production. Using two interlinked gears in an enclosed housing, hydraulic fluid passes around the outside of the two gears, turning them as it does so. The interlocking teeth create a barrier to the hydraulic fluid as it moves from the inlet to the outlet side.
As a result of their simplicity, gear pumps usually avoid the catastrophic failure which can strike other pump designs. Downsides to gear pumps include low volumetric efficiency compared to vane and piston pumps and noisier operation than some more complex designs.
Axial/radial piston pumps
Pistons pumps make use of pistons to drive hydraulic fluid through the hydraulic system. The two main types of piston pumps, axial and radial, both make use of rotational motion to drive the pistons.
In an axial pump, the pistons are attached perpendicular to the circular face of a rotating disc, known as a swashplate. The swashplate is offset at an angle, meaning that the pistons are pushed in and out of their housings as the swashplate rotates. This results in positive and negative pressure, which pumps the hydraulic fluid through the system. Axial piston pumps are used widely in plant machinery due to their variable displacement, however they are sensitive to oil contamination.
In radial pumps, the pistons are positioned around the outside of an eccentrically mounted rotor hub. As the hub rotates, each piston is forced in and out, again resulting in positive and negative pressure which pumps hydraulic fluid through the system. The main strength of radial piston pumps is their ability to handle very high fluid pressures.
Rotary vane pumps
This type of pump works in a similar way to a radial piston pump, except it has a series of rigid vanes protruding from the central eccentrically mounted rotor hub instead of pistons. The vanes are kept in contact with the outside edge of the housing by springs within the hub. Because the hub is offset, the volume of the space between each vane changes as the hub rotates, pushing hydraulic fluid through the system. Rotary vane pumps are more efficient than gear pumps, and are especially good in high flow, low pressure applications.
Hydraulic motors serve the opposite function of hydraulic pumps within a hydraulic system. Where hydraulic pumps take kinetic energy from an electric or combustion motor, and convert it into hydraulic pressure, hydraulic motors take hydraulic pressure and convert it back into rotational kinetic energy.
Because hydraulic pumps and hydraulic motors serve opposite functions within a hydraulic system, the method of energy transfer used is often the same. This means that many of the common mechanisms found in hydraulic pumps are also found in hydraulic motors, just with the direction of energy transfer reversed.
For example, in a hydraulic gear motor, high pressure hydraulic fluid enters the gear housing and moves past the gears. The gears then rotate, and turn a drive shaft or other component, thus completing the transfer of the hydraulic pressure back into kinetic energy.
Similarly, in a rotary vane hydraulic motor, high pressure hydraulic fluid enters the motor housing, filling the chamber between two vanes. As pressure increases, the rotor turns due to the force the pressurised hydraulic fluid.
Because hydraulic motors usually have a drain chamber for leakage, they will often move slowly on their own after power to the hydraulic system has been turned off. As a result, hydraulic motors which have a load attached often require a brake system to ensure components do not move on their own.
In reality, the working mechanisms inside hydraulic pumps and rotors are the same, but their different functions within hydraulic systems as a whole mean it is important to understand where the distinction between the two lies.
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