What is the difference between hydraulic pumps and hydraulic motors?

What is a hydraulic pump?

A hydraulic pump can be defined as a mechanical power source which converts mechanical power into hydraulic energy. Hydraulic pumps are typically used for hydraulic drive systems. The way it works is by generating flow with sufficient power capable of overcoming the pressure created from the pump outlet’s load. When in operation, a hydraulic pump generated a vacuum right at the pump inlet, and that forces liquid to move via an inlet line into the pump from the reservoir.

Types of Hydraulic Pumps

Hydraulic gear pumps that typically come with outer teeth are economically simple pumps. While they have a swept displacement or volume range between 1 to 200 mm, they tend to have the lowest volumetric efficiency of all pump types.

Rotary vane pumps, both simple adjustable and fixed displacement tend to have higher efficiency compared to gear pumps, however, they are functional for mid range pressures (180bar). Units made today can withstand pressures more than 300bar during continuous operation.

Screw pumps are made of 2 Archimedes’ screws which intermesh and closed into a single chamber. Screw pumps are suitable for high flows with low pressures of around 100bars. Screw pumps are mainly used because they generate little to no noise, however they are not that efficient.

Piston pumps make use of a swashplate principle to devices both adjustable and fixed displacement. This gives them the design advantage of being compact. These pumps are much more economical and easier to make, however, one disadvantage is that they do become prone to contamination by oil. Axial piston pumps are known to be the most used variable displacement type, as it has been found in nearly everything from mobile to heavy industrial applications.

What is a hydraulic motor?

A hydraulic motor can be defined as a mechanical actuator which transforms hydraulic flow and pressure into angular displacement and torque. A hydraulic motor is the movable piece of a hydraulic cylinder. In a broader sense, devices known as hydraulic motors sometimes include those that are able to run in hydropower, however, this term has been refined to define only motored that make use of hydraulic fluid as a portion of their closed hydraulic circuits.

Hydraulic motors can be dived into 2 basic categories:

Vane and gear motors: these are basic rotating systems with benefits such as a high rpm at a reduced initial cost. A vane motor is made up of a housing which contains an erratic bite which then tunes a rotor consisting of bands that slide out and in. An integral element of the design is the way the vane tips have been created to meet the motor housing and the vane tip.

Piston and Plunger motors: are much more complicated as they have been created for high quality rotating drive systems. Certain axial piston and Plunger motors offer adaptable transfer ratios.

Preventative maintenance tips for hydraulic motors

To avoid any unnecessary and frustrating breakdowns, it is important to keep your machinery well maintained. With regular preventative maintenance you will be able to spot any developing problems and have them repaired quickly and efficiently by CJ Plant, before any further, more severe, and expensive damage occurs. Below are some key steps to prioritise when maintaining your hydraulic motor.

1) Test pressure and flow

Pressure and flow make up the foundation of hydraulic operation. Consistent testing of these measures will provide a good indication of the overall health of your hydraulic motor. Changes in either level will generally suggest a bigger problem that could range from leaking seals to contaminated hydraulic fluid.

You can monitor for subtle changes in flow and pressure more effectively by keeping accurate documentation of every test result and the date of the test. Some measurements might have statistically insignificant changes between individual tests, but could demonstrate a trend across multiple samples.

2) Sample the fluid

Taking samples of the hydraulic fluid or oil from several points on the motor is very important. You should take more than one, as a single sample might not show contamination that occurs further in the system. Compare multiple samples to each other for both viscosity and integrity. Look out for if the fluid appears to thicken, thin, or become contaminated anywhere in the system.

Checking the fluid on a regular basis can prevent inefficient operation and a strain on the motor. A close examination of the fluid also allows for an evaluation of the system itself. 

3) Clean the reservoir, check fluid levels, and mark normal values

Mark the normal fluid levels on the reservoirs and label each one with the type of fluid to use. If you mix or use the wrong type of fluid, you could contaminate the system and result in it becoming damaged. You should empty the reservoir, clean it, and refill it with fresh fluid on a schedule dictated by the manufacturer’s recommendations. Pay close attention to contaminated fluid and flush the entire system if the hydraulic liquid looks dirty.

4) Clean and examine components of fluid systems

Remove and clean key components of the hydraulic system including filters, couplers, gauges and more. Replace any of these that seem damaged or have excessive build up on them. Also, be sure that each junction of the system moves as expected. 

5) Test actuators and valves

Regular maintenance should include the draining and flushing of valves in the hydraulic system. After cleaning these points, test the valves and actuators in operation. Look for signs of any inefficient function, which could suggest a growing issue. Repair the problem to restore to full operation of the hydraulic motor and reduce the risk of it breaking down completely.

What are the differences between hydraulic pumps and motors?

Right from the definition of these two types of hydraulic components, you can tell that they are different. In essence, Hydraulic pumps as components absorb mechanical kinetic energy to create hydraulic energy, while hydraulic motored do exactly the opposite.

While a hydraulic pump is connected to a prime mover, with the pump shaft with no extra radial load, the hydraulic motor is connected to the load via pulleys, sprockets and gears, so its main shaft can bear an increased radial load.

A hydraulic pump typically has a vacuum in its low pressure chamber. To ensure that it is able to be more efficient at oil absorption and anti-cavitation capability, its suction nozzle is typically larger than its nozzle for high pressure, however a hydraulic motor does not require any of these.

Hydraulic motors typically need negative and positive rotation, which then causes the motor’s internal structure to be symmetrical. Whereas hydraulic pumps usually rotate in a single direction, which negates the need for such a requirement. For instance, a vane motor’s blades have to be arranged radially, unlike the incline of a vane pump, else the blades could become broken when they reverse. An axial plunger motor needs its distribution plate to be symmetrical in design, however an axial plunger pump does not. This is the same for a gear motor as it has to have a unique leakage tube, which cannot be directly connected into the low pressure chamber as a gear pump would.

A hydraulic motor has a vastly wider speed range which means it is able to switch from lubrication mode to hearing form. A hydraulic motor requires a low minimum stable speed, and certain hydraulic motors also require variable brake and speed.

Hydraulic motors require a large amount of start up torque, so as to be able to overcome the static friction encountered during start-up. They also require enough start-up torque when there is a case of pressure fluctuation. For example, for internal friction to be reduced in a hydraulic motor, the amount of teeth a gear motor has is increased, and an axial clearance compensation device with a smaller compression coefficient than that of a pump is introduced.

Hydraulic pumps have to be integrally self-priming. This is one of the reasons why point contact plunger motors can’t be used as pumps as they do not have the self-priming capability.

A vane pump’s blade is pushed out due to centrifugal force and that creates a working chamber. If this pump is used as a motor, it will not function as the blade is not able to create the external force required of a working chamber when it starts.

For friction to reduce, version plunger motors eradicate slipper to become point contact motors, whereas plunger pumps are unable to function without slippers.

A hydraulic motor has a larger internal leakage, compared to the hydraulic pump. The reason for this is because a hydraulic motor’s leakage direction points in the same way as its motion and that results in motion speed becoming involved.

If you’re looking to get your hydraulic pumps or hydraulic motors repaired, why not contact the experts in hydraulic repairs at CJ Plant on 01527 535 804