Axial piston pump and gear pump at a glance
Both designs are positive displacement pumps: each revolution delivers a defined volume of oil, known as the displacement. The key difference is whether that volume can be changed during operation and how much pressure the pump can sustain efficiently. The table sums up the main characteristics.
| Characteristic | Gear pump | Axial piston pump |
|---|---|---|
| Pressure level | low to medium, well suited to pilot and auxiliary circuits | high, designed for the main working circuits |
| Efficiency | good when new, drops noticeably with wear and rising pressure | high, even at high pressure and part load |
| Adjustability | fixed displacement, flow only changes with speed | infinitely variable via the swashplate angle |
| Sensitivity to contamination | comparatively robust | sensitive, clean oil is essential |
| Design | few parts: housing, two gears, bearing blocks, seals | many precision parts: cylinder block, pistons, slippers, swashplate, valve plate, regulator |
| Purchase cost | low | considerably higher |
| Typical use on an excavator | pilot pump, auxiliary circuits, working hydraulics of small mini excavators | main pump from compact excavators upwards, often as a tandem pump |
In practice it is rarely a question of one or the other. Almost every mid-sized excavator has an axial piston pump as its main pump and a flange-mounted gear pump for pilot oil. When a fault occurs, the first step is to establish which of the two units is actually affected.
Gear pump: design and use on mini excavators
The external gear pump, the usual design on excavators, consists essentially of two meshing gears in a close-tolerance housing. One gear is driven and turns the other. Oil is carried in the tooth gaps along the housing wall from the suction side to the pressure side. Where the teeth mesh again, the oil is displaced and forced into the pressure line. Side bearing blocks or pressure plates seal the gears axially.
The displacement per revolution is fixed by the geometry, so the gear pump is a fixed displacement pump: flow rises and falls only with drive speed. If the system needs less oil than the pump delivers, the surplus returns via the bypass or the pressure relief valve. This generates heat and lowers the overall efficiency of the system.
On small and older mini excavators, gear pumps often handle all of the working hydraulics. Several pump sections are usually combined into a multiple pump so that boom, tracks and dozer blade receive separate oil flows. On larger excavators the gear pump almost always sits on the main pump as the pilot pump, supplying the joysticks and control circuits. It is also used for fan drives and auxiliary circuits. Suitable units are listed under Gear pumps.
Advantages of a gear pump:
- simple, compact design with few parts
- comparatively tolerant of contamination
- good suction behaviour
- low purchase cost
- straightforward replacement, as no regulator needs setting up
Axial piston pump: how the swashplate and control work
In an axial piston pump the drive shaft turns a cylinder block in which several pistons are arranged in a circle, parallel to the shaft, hence the name. Each piston rests on the swashplate via a slipper. Because the plate is inclined, the pistons move out and back in once per revolution. Moving out, they draw in oil. Moving in, they push it out. The valve plate with its kidney-shaped ports connects the cylinder bores alternately to the suction and pressure sides.
The angle of the swashplate determines the piston stroke and therefore the displacement. With the plate at right angles to the shaft the pump delivers virtually nothing, at full tilt it delivers maximum volume. A control piston changes the angle during operation, actuated by the pump regulator. This is what makes the axial piston pump a variable displacement pump. Some oil leaks into the housing and returns to the tank through a separate case drain line. This leakage lubricates and cools the sliding surfaces and is intended by design.
Advantages of an axial piston pump
- high operating pressure with good efficiency
- infinitely variable displacement
- flow and pressure can be matched to demand and engine power
- high power density in a compact package
- lower losses at part load, as surplus oil does not have to be dumped over valves
What types of piston pump are there?
- Swashplate axial piston pump: the standard design for excavator main pumps, adjustable via the swashplate.
- Bent-axis axial piston pump: the cylinder block is angled relative to the drive shaft. A robust design used in mobile hydraulics as both pump and motor.
- Radial piston pump: pistons arranged radially around the shaft, for very high pressures, found on excavators mainly in special applications.
- In-line piston pump: pistons in a row, familiar from injection pumps and high-pressure units, rarely used in excavator hydraulics.
Variable displacement pumps and load sensing: how the pump is controlled
A variable pump is only as good as its regulator, which decides when the swashplate swivels out or back. Several control principles are used on excavators, often in combination:
- Power control: the pump limits the product of pressure and flow to the available engine power. As pressure rises it swivels back so the diesel engine does not stall.
- Pressure cut-off: once the set maximum pressure is reached, the pump reduces its displacement instead of dumping oil over the relief valve.
- Load sensing: a signal line reports the highest load pressure of the consumers to the pump. It then delivers just enough oil to maintain a set pressure difference above that load pressure. Common on mini and compact excavators.
- Negative and positive flow control: with negative control, unused bypass oil at the end of the control valve block creates a pressure signal that swivels the pump back. With positive control, the pump swivels out in proportion to the joystick pilot pressures. Both are widespread on medium and large crawler excavators.
- Electronic control: newer machines actuate the regulator through proportional valves and sensors managed by the machine controller.
This matters when sourcing parts. Two pumps that look identical can have different regulators. A pump with the wrong regulator fits mechanically but will not work properly with the valve block and engine management. The regulator version is often part of the type designation on the pump plate. New variable pumps are listed under Hydraulic piston pumps.
Which hydraulic pump is used in which excavator class?
The pump design depends on machine size, manufacturer and year of build. The overview below shows typical configurations:
| Excavator class | Typical main pump | Auxiliary and secondary pumps |
|---|---|---|
| Small mini excavators | multiple gear pump or small variable axial piston pump, depending on make and year | gear pump for pilot or additional circuits |
| Compact and midi excavators | variable axial piston pump, frequently with load sensing | flange-mounted gear pump for pilot oil, sometimes also for swing or blade circuit |
| Mid-sized wheeled and crawler excavators | axial piston tandem pump with power control and negative, positive or load sensing control | gear pump for pilot oil, often also for fan or auxiliary circuits |
| Large excavators | several axial piston pumps on a pump distribution gearbox | dedicated pumps for auxiliary circuits such as fan drive and pilot supply |
The table shows typical arrangements, not a fixed rule. Which pump is actually fitted is shown in the parts catalogue or on the pump plate. Incidentally, swing and travel motors usually work on the axial piston principle too, just in reverse: they convert oil flow into rotation.
Common failures of axial piston pumps and gear pumps
Gear pump
- Wear on gear faces, bearing blocks and housing: internal leakage increases, flow and pressure drop, movements become slow.
- Scoring in the housing caused by dirt particles in the oil
- Leaking shaft seal, often due to misalignment or a worn drive spline
- Cavitation from a blocked suction strainer, low oil level or thick oil in cold weather, audible as rattling or whining
- Bearing damage with noticeable play on the shaft
Axial piston pump
- Wear on valve plate and cylinder block: higher case drain flow, loss of power, overheating
- Damaged slippers or swashplate, frequently caused by contamination or poor lubrication
- Cavitation damage to pistons and valve plate due to suction-side problems
- Regulator faults: the pump does not swivel out or does so sluggishly, the engine bogs down or the machine is too slow
- Bearing damage on the drive shaft, often audible as running noise
- Consequential damage from dry running if the pump case was not filled with oil after fitting
Before replacing a pump: identify the cause
A new pump achieves little if the cause remains in the system. The following sequence, carried out by a specialist workshop with suitable test equipment, has proved reliable:
- Secure: park the machine on level ground, lower the attachments, switch off the engine, secure it against restarting and depressurise the hydraulics. Vent the tank as specified by the manufacturer.
- Check oil and filters: inspect oil level, oil condition and filters. Metal particles or swarf in the filter point to mechanical wear.
- Measure pressure and flow: measure system pressure, pilot pressure and flow as set out in the service manual. The binding target values are given there.
- Measure case drain flow: on piston pumps, the leakage from the case drain line indicates internal wear. The permissible value is specified by the manufacturer.
- Check regulator and control signals: before condemning the pump, rule out the regulator, control signals and any sensors.
- After replacement: where metal debris was found, flush the system, renew the filters, fill the piston pump case with oil before the first start and bleed the pump as specified by the manufacturer.
Identifying the pump and finding the right hydraulic pump
Hydraulic pumps come in a great many variants. Design, displacement, direction of rotation, shaft profile, flange, port positions and regulator must match the machine exactly. To identify the correct pump reliably, the following details help us:
- machine make and model
- machine serial number from the ID plate
- pump part number from the parts catalogue, if available
- type designation from the pump plate
- photos of the machine ID plate, the pump plate and the fitted pump showing its connections
Where to find the ID plate and part numbers is explained in our guide Finding the part number and serial number. Send us the details using the Request a part form. We check the match and come back to you with a quote for a new pump. For an overview of the range, see Hydraulic pumps.
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