What is a Hydraulic Pump? – A Full Guide

What is A Hydraulic Pump?

Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic. A hydraulic pump is a mechanical source of power that converts mechanical power into hydraulic energy. It generates flow with enough power to overcome pressure induced by the load at the pump outlet.

When a hydraulic pump operates, it creates a vacuum at the pump inlet, which forces liquid from the reservoir into the inlet line to the pump and by mechanical action delivers this liquid to the pump outlet and forces it into the hydraulic system.

Hydrostatic pumps are positive displacement pumps while hydrodynamic pumps can be fixed displacement pumps, in which the displacement cannot be adjusted, or variable displacement pumps, which have a more complicated construction that allows the displacement to be adjusted.

Hydrodynamic pumps are more frequent in day-to-day life. Hydrostatic pumps of various types all work on the principle of Pascal’s law.

What is a Hydraulic Pump?

What are the characteristics of a hydraulic pump?

The characteristics of a hydraulic pump are:

  • The pressure Δp expressed in bar
  • The flow rate Qv expressed in l/min: this is the volume of liquid delivered by the pump in a given time
  • Displacement expressed in cm3: this is the volume of fluid pumped per rotation
  • The speed of rotation N expressed in rpm
  • Hydraulic power (Ph) expressed in kW
  • Efficiency: this is the useful energy of the pump after losses mainly due to friction.

Types of Hydraulic Pumps

There are typically three types of hydraulic pump constructions found in mobile hydraulic applications. These include gear, piston, and vane; however, there are also clutch pumps, dump pumps, and pumps for refuse vehicles such as dry valve pumps.

The hydraulic pump is the component of the hydraulic system that takes mechanical energy and converts it into fluid energy in the form of oil flow. This mechanical energy is taken from what is called the prime mover (a turning force) such as the power take-off or directly from the truck engine. 

With each hydraulic pump, the pump will be of either a uni-rotational or bi-rotational design. As its name implies, a uni-rotational pump is designed to operate in one direction of shaft rotation. On the other hand, a bi-rotational pump has the ability to operate in either direction.

There are Different Types of Hydraulic Pumps:

  • Gear Pumps.
  • Piston Pumps.
  • Vane Pumps.
  • Clutch Pumps.
  • Dump Pumps.
  • Refuse Pumps.

1. Gear Pumps

For truck-mounted hydraulic systems, the most common design in use is the gear pump. This design is characterized as having fewer moving parts, being easy to service, being more tolerant of contamination than other designs, and being relatively inexpensive.

Gear pumps are fixed displacement, also called positive displacement, pumps. This means the same volume of flow is produced with each rotation of the pump’s shaft. Gear pumps are rated in terms of the pump’s maximum pressure rating, cubic inch displacement, and maximum input speed limitation.

Generally, gear pumps are used in open center hydraulic systems. Gear pumps trap oil in the areas between the teeth of the pump’s two gears and the body of the pump, transport it around the circumference of the gear cavity and then force it through the outlet port as the gears mesh.

Behind the brass alloy thrust plates, or wear plates, a small amount of pressurized oil pushes the plates tightly against the gear ends to improve pump efficiency.  


  • They are inexpensive
  • They have fixed-displacement


  • Their volumetric efficiency is low

There are two types of gear pumps: external-gear pumps and internal-gear pumps.

External-gear pumps:

In an external-gear pump, only one of the gear wheels, the drive gear, is connected to the drive. The other gear wheel, the driven gear, rotates in the opposite direction so that the teeth of the rotating gear wheels interlock.

There are also double external-gear pumps, which combine two gear pumps driven by the same coupling shaft. A double external-gear pump has the advantage of supplying two independent hydraulic circuits and also provides more flow to one circuit.

Internal-gear pumps:

  • They have an eccentric internal gear wheel and an external gear wheel, also called a crown gear.
  • When the external gear rotates, volume is created between the gear wheel profiles and the external gear teeth attached to the gear housing walls.
  • The product is sucked up when the two wheels disengage.


  • Internal-gear pumps are very quiet
  • They provide a constant flow

Quick Look

  • Most common design
  • Fewer moving parts, easy to service, more tolerant of contaminates, relatively inexpensive
  • Fixed, also called positive displacement pumps
  • Rated in terms of max pressure rating, cubic inch displacement, max input speed limitation
  • Used in open center hydraulic systems
  • Transports oil around circumference of gear cavity and forces it through outlet port
  • Encompasses thrust plates that push against gear ends with small amount of pressurized oil to improve pump efficiency

2. Piston Pumps

When high operating pressures are required, piston pumps are often used. Piston pumps will traditionally withstand higher pressures than gear pumps with comparable displacements; however, there is a higher initial cost associated with piston pumps as well as a lower resistance to contamination and increased complexity.

This complexity falls to the equipment designer and service technician to understand in order to ensure the piston pump is working correctly with its additional moving parts, stricter filtration requirements, and closer tolerances.

Piston pumps are often used with truck-mounted cranes but are also found within other applications such as snow and ice control where it may be desirable to vary system flow without varying engine speed.

A cylinder block containing pistons that move in and out is housed within a piston pump. It’s the movement of these pistons that draw oil from the supply port and then force it through the outlet. The angle of the swashplate, which the slipper end of the piston rides against, determines the length of the piston’s stroke.

While the swash plate remains stationary, the cylinder block, encompassing the pistons, rotates with the pump’s input shaft. The pump displacement is then determined by the total volume of the pump’s cylinders. Fixed and variable displacement designs are both available.

Quick Look

  • Withstand higher pressures
  • Higher initial cost, lower resistance to contamination and increased complexity
  • Additional moving parts, stricter filtration requirements and closer tolerances
  • Truck-mounted cranes
  • Good when desirable to vary system flow without varying engine speed
  • Fixed and variable displacement designs available
  • Encompasses cylinder block containing pistons that move in and out – this movement draws oil from the supply port and forces through the outlet
  • Angle of swash plate determines the length of the piston’s stroke
  • Swash plate remains stationary
  • Displacement determined by total volume of pump cylinders

Fixed Displacement

With a fixed displacement piston pump, the swashplate is nonadjustable. Its proportional output flow to input shaft speed is like that of a gear pump and as a gear pump, the fixed displacement piston pump is used within open center hydraulic systems.

Variable Displacement

As previously mentioned, piston pumps are also used within applications like snow and ice control where it may be desirable to vary system flow without varying engine speed.

This is where the variable displacement piston pump comes into play – when the hydraulic flow requirements will vary based on operating conditions. Unlike the fixed displacement design, the swashplate is not fixed and its angle can be adjusted by a pressure signal from the directional valve via a compensator.

Should more flow be required, the swashplate angle changes, increasing the pump displacement by creating a longer piston stroke. Contrary to a fixed displacement piston pump, the variable displacement is used in a closed center system.

With a closed center system, the swashplate angle within the variable displacement pump decreases as the flow requirement diminishes so that there is no excess flow or loss of hydraulic horsepower.

Variable displacement piston pumps can be flow compensated, pressure compensated or both flow and pressure compensated.

  • Flow Compensated. As flow requirements change, the swash plate angle is adjusted to maintain a constant margin of pressure.
  • Pressure Compensated. Regardless of changes in system pressure, a specified flow is maintained through adjusting the swash plate angle.
  • Flow and Pressure Compensated Combined. These systems with flow and pressure compensation combined are often called a load-sensing system, which is common for snow and ice control vehicles.

3. Vane Pumps

Vane pumps were, at one time, commonly used on utility vehicles such as aerial buckets and ladders. Today, the vane pump is not commonly found on these mobile (truck-mounted) hydraulic systems as gear pumps are more widely accepted and available.

Within a vane pump, as the input shaft rotates it causes oil to be picked up between the vanes of the pump which is then transported to the pump’s outlet side. This is similar to how gear pumps work, but there is one set of vanes – versus a pair of gears – on a rotating cartridge in the pump housing.

As the area between the vanes decreases on the outlet side and increases on the inlet side of the pump, oil is drawn in through the supply port and expelled through the outlet as the vane cartridge rotates due to the change in the area.

Quick Look

  • Used on utility vehicles, but not as common today with gear pumps more widely accepted and available
  • Input shaft rotates, causing oil to be picked up between the vanes of the pump which is then transported to pump outlet side as area between vanes decreases on outlet side and increases on inlet side to draw oil through supply port and expel though outlet as vane cartridge rotates

Muncie Power does not offer vane pumps.

4. Clutch Pumps

A clutch pump is a small displacement gear pump equipped with a belt-driven, electromagnetic clutch, much like that found on a car’s air conditioner compressor. It is engaged when the operator turns on a switch inside the truck cab.

Clutch pumps are frequently used where a transmission power take-off aperture is not provided or is not easily accessible. Common applications include aerial bucket trucks, wreckers, and hay spikes.

As a general rule clutch pumps cannot be used where pump output flows are in excess of 15 GPM as the engine drive belt is subject to slip under higher loads.

Quick Look

  • Small displacement pumps
  • Belt driven
  • Aerial bucket trucks, wreckers and hay spikes
  • Limited to 15 GPM applications

5. Dump Pumps

Of the different types of hydraulic pumps, the dump pump is the most recognizable. This type of pump is commonly used in dumping applications from dump trailers to tandem axle dump trucks. The dump pump is specifically designed for one application – dump trucks – and is not suitable for other common trailer applications such as live floor and ejector trailers.

What separates this pump from the traditional gear pump is its built-in pressure relief assembly and an integral three-position, three-way directional control valve. The dump pump is unsuited for continuous-duty applications because of its narrow, internal paths and the subsequent likelihood of excessive heat generation.

Dump pumps are often direct mounted to the power take-off; however, it is vital that the direct-coupled pumps be rigidly supported with an installer-supplied bracket to the transmission case with the pump’s weight at 70 lbs.

With a dump pump, either a two- or three-line installation must be selected (two-line and three-line refer to the number of hoses used to plumb the pump); however, a dump pump can easily be converted from a two- to three-line installation. This is accomplished by inserting an inexpensive sleeve into the pump’s inlet port and uncapping the return port.

Many dump bodies can function adequately with a two-line installation if not left operating too long in neutral. When left operating in neutral for too long, however, the most common dump pump failure occurs due to high temperatures. To prevent this failure, a three-line installation can be selected – which also provides additional benefits.

Quick Look:

  • Dump pump most recognizable
  • Specifically designed for dump trucks
  • Displacement of slightly more than six cubic inches, pressure relief assembly and integral three-position, three-way directional control valve
  • Not suited for continuous-duty applications
  • Often direct coupled to PTO, need installer-supplied bracket to support
  • Two- and three-line installations available (two-line can be converted to three-line)

6. Refuse Pumps

Pumps for refuse equipment include a dry valve. Both conserve fuel while in the OFF mode but have the ability to provide full flow when work is required. While both have designs based on of standard gear pumps, the dry valve incorporates additional, special valving.

Dry Valve Pumps

Primarily used on refuse equipment, dry valve pumps are large displacement, front crankshaft-driven pumps. The dry valve pump encompasses a plunger-type valve in the pump inlet port. This special plunger-type valve restricts flow in the OFF mode and allows full flow in the ON mode.

As a result, the horsepower draw is lowered, which saves fuel when the hydraulic system is not in use.

In the closed position, the dry valve allows just enough oil to pass through to maintain the lubrication of the pump. This oil is then returned to the reservoir through a bleed valve and small return line.

A bleed valve that is fully functioning is critical to the life of this type of pump, as pump failure induced by cavitation will result if the bleed valve becomes clogged by contaminates. Muncie Power Products also offer a butterfly-style dry valve, which eliminates the bleed valve requirement and allows for improved system efficiency.

It’s important to note that with the dry valve, wear plates and shaft seals differ from standard gear pumps. Trying to fit a standard gear pump to a dry valve likely will result in premature pump failure.

Quick Look:

  • Often used on refuse equipment
  • Large displacement, front crankshaft-driven pumps
  • Encompasses plunger-type valve in the pump inlet port restricting flow in OFF mode, but allows full flow in ON mode lowering horsepower draw to save fuel when not in use
  • Fully functioning bleed valve critical to life of this pump
  • Wear plates and shaft seals differ from standard gear pumps – trying to fit standard gear pump to dry valve likely will result in premature pump failure

What is the pressure of a hydraulic pump?

50 bars

  • The operating pressure is rarely less than 50 bar

210 – 300 bars: gear pumps

  • The maximum pressure is usually 250 bar
  • The most common standard used by gear pumps is 210 bars
  • The 300-bar standard is used by some internal-gear pumps

300 – 450 bars: piston pumps

  • They work with high pressures, around 450 bar
  • 300 – 350 bar is the standard increasingly being used in open circuit
  • 420 bar is the maximum standard especially used for hydrostatic transmission in closed circuits

700 bars

  • This standard is used for handling jacks

7,000 bars: hydropneumatic pumps

10,000 bar: pressure multipliers

Which pump should you choose for which use?

External-gear pumps

  • They are more commonly used in hydraulic applications such as for log splitters or elevators.

Internal-gear pumps

  • They are used in particular in non-moving hydraulics (e.g. machine tools, presses, etc.) and in vehicles that operate in a confined space (electric forklifts, etc.).

Rotary-vane pumps

  • They are used in die-casting and injection machines in industry, as well as in construction equipment.

Piston pumps

  • They are used in applications involving high pressures (400 bar to 700 bar), such as presses, plastic processing machines, machine tools and construction equipment.
  • They are used to operate hydraulic cylinders for high loads.
  • They are also used in mobile and construction equipment; marine auxiliary power; metal forming and stamping; machine tools; and oilfield equipment.
  • Some piston pumps are also used to transport hydrocarbons under high pressure in pipelines.
  • They can also be used in water jet cutting machines. In this case, the fluid is water and not oil.

Screw pumps

  • They are used to pump viscous liquids such as crude oil under high pressure.