How to operate a hydraulic pump?

*Editor's Note: This article was updated in February 2024.

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When a hydraulic pump operates, it performs two functions. First, its mechanical action creates a vacuum at the pump inlet which allows atmospheric pressure to force liquid from the reservoir into the inlet line to the pump. Second, its mechanical action delivers this liquid to the pump outlet and forces it into the hydraulic system.

A pump produces liquid movement or flow: it does not generate pressure. It produces the flow necessary for the development of pressure which is a function of resistance to fluid flow in the system. For example, the pressure of the fluid at the pump outlet is zero for a pump not connected to a system (load). Further, for a pump delivering into a system, the pressure will rise only to the level necessary to overcome the resistance of the load.

Classifications of Hydraulic Pumps

All pumps may be classified as either positive-displacement or non-positive-displacement. Most pumps used in hydraulic systems are positive-displacement.

A non-positive-displacement pump produces a continuous flow. However, because it does not provide a positive internal seal against slippage, its output varies considerably as pressure varies. Centrifugal and propeller pumps are examples of non-positive-displacement pumps.

If the output port of a non-positive-displacement pump were blocked off, the pressure would rise, and output would decrease to zero. Although the pumping element would continue moving, flow would stop because of slippage inside the pump.

In a positive-displacement pump, slippage is negligible compared to the pump's volumetric output flow. If the output port were plugged, pressure would increase instantaneously to the point that the pump's pumping element or its case would fail (probably explode, if the drive shaft did not break first), or the pump's prime mover would stall.

Positive-Displacement Principle

A positive-displacement pump is one that displaces (delivers) the same amount of liquid for each rotating cycle of the pumping element. Constant delivery during each cycle is possible because of the close-tolerance fit between the pumping element and the pump case. That is, the amount of liquid that slips past the pumping element in a positive-displacement pump is minimal and negligible compared to the theoretical maximum possible delivery. The delivery per cycle remains almost constant, regardless of changes in pressure against which the pump is working. Note that if fluid slippage is substantial, the pump is not operating properly and should be repaired or replaced.

How do hydraulic pumps work?

A hydraulic pump is a mechanical device that converts mechanical power into hydraulic energy. It generates flow with enough power to overcome pressure induced by the load.

A hydraulic pump performs two functions when it operates. Firstly, its mechanical action creates a vacuum at the pump inlet, subsequently allowing atmospheric pressure to force liquid from the reservoir and then pumping it through to the inlet line of the pump. Secondly, its mechanical action delivers this liquid to the pump outlet and forces it into the hydraulic system.

What are the most common types of hydraulic pumps

The three most common hydraulic pump designs are: vane pump, gear pump and radial piston pump. All are well suited to common hydraulic uses, however the piston design is recommended for higher pressures.

Most pumps used in hydraulic systems are positive-displacement pumps. This means that they displace (deliver) the same amount of liquid for each rotating cycle of the pumping element. The delivery per cycle remains almost constant, regardless of changes in pressure.

Positive-displacement pumps are grouped into fixed or variable displacement. A fixed displacement pump’s output remains constant during each pumping cycle and at a given pump speed. Altering the geometry of the displacement chamber changes the variable displacement pump’s output.

Fixed displacement pumps (or screw pumps) make little noise, so they are perfect for use in for example theatres and opera houses. Variable displacement pumps, on the other hand, are particularly well suited in circuits using hydraulic motors and where variable speeds or the ability to reverse is needed.

Learn more about Piston Pumps

A piston pump performs flawlessly with large flows at high hydraulic system pressures.

Applications commonly using a piston pump include: marine auxiliary power, machine tools, mobile and construction equipment, metal forming and oil field equipment.

As the name suggests, a piston pump operates through pistons that move back and forth in the cylinders connected to the hydraulic pump. A piston pump also has excellent sealing capabilities.

A hydraulic piston pump can operate at large volumetric levels thanks to low oil leakage. Some plungers require valves at the suction and pressure ports, whilst others require them with the input and output channels. Valves (and their sealing properties) at the end of the piston pumps will further enhance the performance at higher pressures.

What are the features of an axial piston pump?

The axial piston pump is possibly the most widely used variable displacement pump. It’s used in everything from heavy industrial to mobile applications. Different compensation techniques will continuously alter the pump’s fluid discharge per revolution. And moreover, also alter the system pressure based on load requirements, maximum pressure cut-off settings and ratio control. This implies significant power savings.

Two principles characterise the axial piston pump. Firstly the swash plate or bent axis design and secondly the system parameters. System parameters include the decision on whether or not the pump is used in an open or closed circuit.

The return line in a closed loop circuit is under constant pressure. This must be considered when designing an axial piston pump that is used in a closed loop circuit. It is also very important that a variable displacement volume pump is installed and operates alongside the axial piston pump in the systems. Axial piston pumps can interchange between a pump and a motor in some fixed displacement configurations.

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How does a bent axis, axial piston pump work?

Bent axis pumps are the most efficient of all pumps.

The swivel angle determines the displacement volume of the bent axis pump. The pistons in the cylinder bore moves when the shaft rotates. The swash plate, in the swash plate design, sustain the turning pistons. Moreover, the angle of the swash plate decides the piston stroke.

The bent axis principle, fixed or adjustable displacement, exist in two different designs. The first design is the Thoma-principle with maximum 25 degrees angle, designed by the German engineer Hans Thoma and patented in 1935. The second design goes under the name Wahlmark-principle, named after Gunnar Axel Wahlmark (patent 1960). The latter features spherical-shaped pistons in one piece with the piston rod and piston rings. And moreover a maximum 40 degrees between the driveshaft centre-line and pistons.

In general, the largest displacements are approximately one litre per revolution. However if necessary, a two-litre swept volume pump can be built. Often variable-displacement pumps are used, so that the oil flow can be adjusted carefully. These pumps generally operate with a working pressure of up to 350–420 bars in continuous work

About Radial Piston Pumps

Radial piston pumps are used especially for high pressure and relatively small flows. Pressures of up to 650 bar are normal. The plungers are connected to a floating ring. A control lever moves the floating ring horizontally by a control lever and thus causes an eccentricity in the centre of rotation of the plungers. The amount of eccentricity is controlled to vary the discharge. Moreover, shifting the eccentricity to the opposite side seamlessly reverses the suction and discharge.

Radial piston pumps are the only pumps that work continuously under high pressure for long periods of time. Examples of applications include: presses, machines for processing plastic and machine tools.

Variable displacement is possible.

Learn more about Hydraulic Vane Pumps

A vane pump uses the back and forth movement of rectangle-shaped vanes inside slots to move fluids. They are sometimes also referred to as sliding vane pumps.

The simplest vane pump consists of a circular rotor, rotating inside of a larger circular cavity. The centres of the two circles are offset, causing eccentricity. Vanes slide into and out of the rotor and seal on all edges. This creates vane chambers that do the pumping work.

A vacuum is generated when the vanes travel further than the suction port of the pump. This is how the oil is drawn into the pumping chamber. The oil travels through the ports and is then forced out of the discharge port of the pump. Direction of the oil flow may alter, dependent on the rotation of the pump. This is the case for many rotary pumps.

Vane pumps operate most efficiently with low viscosity oils, such as water and petrol. Higher viscosity fluids on the other hand, may cause issues for the vane’s rotation, preventing them from moving easily in the slots.

Where are hydraulic vane pumps used? Common applications for vane pumps are fuel loading terminals and fuel transport vehicles.

How do hydraulic gear pumps work?

Gear pumps are one of the most common types of pumps for hydraulic fluid power applications. Here at Hydraulics Online, we offer a wide range of high-powered hydraulic gear pumps suitable for industrial, commercial and domestic use. We provide a reliable pump model, whatever the specifications of your hydraulic system. And we furthermore ensure that it operates as efficiently as possible.

Johannes Kepler invented the gear pump around year 1600. Fluid carried between the teeth of two meshing gears produces the flow. The pump housing and side plates, also called wear or pressure plates, enclose the chambers, which are formed between adjacent gear teeth. The pump suction creates a partial vacuum. Thereafter fluid flows in to fill the space and is carried around the discharge of the gears. Next the fluid is forced out as the teeth mesh (at the discharge end).

Some gear pumps are quite noisy. However, modern designs incorporating split gears, helical gear teeth and higher precision/quality tooth profiles are much quieter. On top of this, they can mesh and un-mesh more smoothly. Subsequently this reduces pressure ripples and related detrimental problems.

Catastrophic breakdowns are easier to prevent with hydraulic gear pumps. This is because the gears gradually wear down the housing and/or main bushings. Therefore reducing the volumetric efficiency of the pump gradually until it is all but useless. This often happens long before wear causes the unit to seize or break down.

Can hydraulic gear pumps be reversed? Yes, most pumps can be reversed by taking the pump apart and flipping the center section. This is why most gear pumps are symmetrical.

Two main types

External gear pumps use two external spur gears. Internal gear pumps use an external and an internal spur gear. Moreover, the spur gear teeth face inwards for internal gear pumps. Gear pumps are positive displacement (or fixed displacement). In other words, they pump a constant amount of fluid for each revolution. Some gear pumps are interchangeable and function both as a motor and a pump.

What are hydraulic gear pumps used for?

The petrochemical industry uses gear pumps to move: diesel oil, pitch, lube oil, crude oil and other fluids. The chemical industry also uses them for materials such as: plastics, acids, sodium silicate, mixed chemicals and other media. Finally, these pumps are also used to transport: ink, paint, resins and adhesives and in the food industry.

About Gerotor hydraulic pumps

A gerotor is a positive displacement pump. The name gerotor derives from “generated rotor”. A gerotor unit consists of an inner and outer rotor.

Mathematical calculations are key to any type of hydraulic motor or pump design, but are especially interesting in the gerotor design. The inner rotor has N teeth, where N > 2.  The outer rotor must have N + 1 teeth (= one more tooth than the inner rotor) in order for the design to work.

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