A hydraulic pump is an essential component in any hydraulic power transmission system. Hydraulic pumps are widely employed in many industrial applications: conveyors, forklifts, injection molding, foundry equipment, mixers, pallet jacks, presses, slitters, steel mill machinery, and beyond. Hydraulic pumps are especially suited for mobile applications because electric actuators are generally not used for mobile machinery. Such applications include cranes, dump trucks, excavators, forestry equipment, graders, loaders, mining machinery, and tractors. The hydraulic pump is the component of the hydraulic system that takes mechanical energy and converts it into fluid energy in the form of liquid flow.
What is a hydraulic pump?
Simply put, a hydraulic pump is a mechanical device that converts mechanical power into hydraulic energy. Hydraulic energy is the combination of pressure and flow required by the actuators to perform useful work. What is important to understand here is that hydraulic energy consists of a combination of pressure and flow, and that one without the other cannot achieve any kind of progress. This is because pressure alone would mean trapped fluid, and flow alone would have no energy to move fluid along.
How does a hydraulic pump work?
When a hydraulic pump is operated, its mechanical action first creates a vacuum at the pump inlet. This vacuum allows atmospheric pressure to force liquid from the reservoir into the inlet line to the pump. Then, the pump’s mechanical action delivers the fluid to the pump outlet and forces it into the hydraulic system. A hydraulic pump works by pushing on fluid, which, in turn, pushes on actuators to transmit force to eventually move loads. Transmission of force is the basic principle behind the science of hydraulics and follows Cosford’s Law, which states the following: pressure makes it go, and flow is just the rate in which you can create pressure.
For fluid to be set in motion, pressure must be highest at the pump. Therefore, the idea that pressure is resistant to flow is a fallacy. Pressure will necessarily rise to the correct level to overcome resistance downstream, but if pressure didn’t start at the pump, the result would be fluid moving backward.
- Pressure in hydraulics is the result of Newton’s Third Law of Motion, i.e. every action has an equal and opposite reaction. When it comes to a pump, the opposing force could be a loaded cylinder or a flow controller. The pump doesn’t care either way. As pressure rises to overcome resistance, it will still work to push the fluid, even if the result is unintended — something blows up, for instance, or the prime mover becomes overloaded.
- Flow from a pump is a function of displacement (volume) and speed. A larger pump is able to push more fluid at a time, and a pump that is spun faster will push on fluid more often. Power in hydraulics is a combination of pressure and flow. Thus, doubling pressure while leaving flow alone will double horsepower. Conversely, doubling flow while leaving pressure alone also doubles horsepower.
Types of hydraulic pumps
Hydraulic pumps fall into three main categories: gear, piston, and vane. Gear pumps can be subdivided into several different types that are designed for various purposes. The different kinds of pumps are briefly described below. To read about each pump in much greater detail, click here.
Gear Pumps
Gear pumps are fixed displacement pumps that are also known as positive displacement pumps. What this means is that each rotation of the pump’s shaft produces the same volume of flow. Gear pumps are evaluated by cubic inch displacement, maximum pressure rating, and maximum input speed limitation. The gear pump is commonly found in truck-mounted hydraulic systems. This pump is a popular design because it has fewer moving parts, is easy to service, is more resistant to contamination, and is relatively inexpensive.
Piston Pumps
Piston pumps can stand up to higher pressures than can gear pumps with similar displacements and are, therefore, suitable when high operating pressures are needed. They have more moving parts than gear pumps, cost more, and have a lower resistance to contamination. Piston pumps are often used with truck-mounted cranes (fixed displacement). However, they are also suited to other applications such as snow and ice control, where it may be advantageous to vary system flow without changing engine speed (variable displacement).
Vane Pumps
As the input shaft rotates within a vane pump, oil is picked up between the vanes of the pump. The oil then gets transported to the pump’s outlet side. This is quite similar to the way a gear pump works. However, a vane pump incorporates a set of vanes (instead of gears) situated on a rotating cartridge inside the pump housing. In the past, vane pumps were commonly used on utility vehicles, but gear pumps have largely replaced them.
Clutch Pumps
A clutch pump is a small-sized displacement gear pump with an electromagnetic clutch that is belt-driven. It is engaged by turning on a switch from inside the cab of a truck. Clutch pumps are often used where there is no transmission power take-off aperture or where one is not easily accessible. Typical applications include wreckers, aerial bucket trucks, and hay spikes.
Dump Pumps
This pump differs from a traditional gear pump because of its built-in pressure relief assembly and an integral three-position, three-way directional control valve. It is not suitable where continuous-duty is needed and is specifically designed for one application: dump trucks. It is not suited for other common trailer applications such as live floor and ejector trailers.
Dry Valve Pumps
Dry valve pumps are large displacement pumps driven by front crankshafts. They come with a plunger-type valve in the pump inlet port that saves fuel by restricting flow in the off mode and allowing full flow in the on mode. This type of pump is mainly used in refuse equipment.
Final Thoughts
Familiarity with how a hydraulic pump operates will greatly increase your knowledge of how fluid power works. Once you grasp that all energy begins at the pump, you will be able to better design or troubleshoot any system. Sapphire Hydraulics, a top hydraulic repair service, can supply you with any type of hydraulic pump: our services include custom-engineered and manufactured hydraulic products to meet every engineering requirement. Tell us what you need, and we will design it for you!
