Axial Piston Pump

An axial piston pump is a positive displacement pump used in hydraulics, in which several pistons are arranged parallel to the drive shaft and perform an axial stroke through rotation to draw in and deliver hydraulic fluid. It is characterized by high operating pressures of up to 500 bar, a compact design, and stepless flow control, and is one of the most commonly used pump types in industrial and mobile hydraulic systems.

Basic Principle and Operation of the Axial Piston Pump

Axial piston pumps convert mechanical rotational energy into hydraulic pressure energy. The central operating principle is based on positive displacement: pistons move axially back and forth in cylinder bores, thereby changing the volume in the piston chambers and alternately creating a vacuum for suction and positive pressure for delivering the hydraulic fluid.

Construction of an axial piston pump

The cylinder block, also known as the rotary piston block, forms the heart of the system. It typically contains seven to eleven piston bores that run parallel to the axis of rotation. The pistons themselves are connected via connecting rods or ball joints to an inclined disc or a control flange. A drive shaft rotates the cylinder block, and the angle of inclination of the swash plate forces the pistons into a reciprocating motion. At the front end of the cylinder block, a control disc with kidney-shaped channels ensures that each piston undergoes exactly one suction and one pressure phase during one revolution.

Power Generation and Volume Flow

The stroke of each piston results from the angle of inclination of the swash plate relative to the axis of rotation. The larger this angle, the longer the piston stroke and the greater the displaced volume per revolution. The pump’s flow rate thus depends directly on two variables: the speed of the drive shaft and the angle of the swash plate. At an angle of zero degrees, the pistons do not move, and the pump delivers no flow. As the angle increases, the flow rate rises continuously. Modern axial piston pumps achieve flow rates of up to approximately 800 liters per minute.

Types of Axial Piston Pumps

Axial piston pumps can be classified according to various criteria. The most important distinction concerns the type of lift generation: swash plate pump (swivel plate) and swash axis pump. In addition, a distinction is made between pumps with constant and variable displacement.

Swash plate pump with swash plate

In this design, the cylinder block and drive shaft rotate together. The swash plate is fixed in the housing or can be tilted around a pivot point. The pistons slide with their base points on the swash plate and perform a reciprocating motion due to the angle of the plate. The maximum swivel angle is approximately 18 degrees in most designs. Adjustable swash plate pumps allow for stepless adjustment of the flow rate during operation. A mechanical drive-through is possible in this design, which allows for the construction of tandem pumps on a common drive shaft.

Slant-axis pump

In the swash plate pump, the axis of the cylinder block is arranged at an angle to the drive shaft. A drive flange transmits the rotational motion from the drive shaft to the cylinder block. The pistons are connected to the stationary slide plate via ball joints. The piston stroke is created by the oblique position of the axis. The flow rate is adjusted by changing the angle of inclination between the cylinder block axis and the drive shaft. Oblique-axis pumps are characterized by good self-priming capability and greater resistance to contamination in the hydraulic oil.

Constant-displacement and variable-displacement pumps

Fixed-displacement pumps have a fixed angle of attack and deliver a constant flow rate at a given speed. Variable-displacement pumps continuously adjust the displacement volume to meet demand. Variable-displacement pumps are predominantly used in industrial hydraulic systems because they save energy, maintain pressure as needed, and reduce heat generation.

Axial piston pumps in open and closed circuits

The choice of design depends largely on the circuit in which the pump operates.

Open circuit

In an open circuit, the pump draws hydraulic fluid from a tank and delivers it to the consumer. The return flow goes back into the tank. Vane pumps are particularly suitable for open circuits because they have good self-priming capability and handle fluctuating loads well. Open circuits are found in presses, injection molding machines, and machine tools.

Closed-loop system

In a closed circuit, the hydraulic fluid circulates directly between the pump and the hydraulic motor without passing through a reservoir. A supply pump compensates for leaks. Vane pumps are frequently used here, for example in hydrostatic transmissions of construction machinery, wheel loaders, and agricultural vehicles. The closed circuit allows for a compact system and rapid reversal of the direction of travel.

Technical specifications of the axial piston pump

Axial piston pumps operate over a wide range of conditions. The following parameters provide a guide for design:

Actual values vary depending on the manufacturer, size, and operating conditions. At pressures above 400 bar, radial piston pumps increasingly come into play as an alternative, as this design can handle higher pressures with lower pulsation.

Axial piston pump versus radial piston pump

Both types are piston pumps, but differ in design and primary applications. Designers choose the axial piston pump when compact dimensions, high speeds, and stepless adjustability are the main priorities. Radial piston pumps are used for extreme pressures, continuous operation, and high requirements for smooth running.

Regulation and Control of Axial Piston Pumps

The displacement is adjusted using various control concepts that have proven themselves in practice.

Pressure control

Pressure control maintains the system pressure at a setpoint, regardless of the flow rate required by the consumers. As soon as the pressure reaches the setpoint, the swash plate swings back and reduces the flow rate to the level necessary to maintain the pressure. Excess power is not dissipated as heat via a pressure relief valve, which significantly improves the energy balance.

Flow control

Here, the pump regulates the flow rate to a constant value, regardless of pressure fluctuations in the system. The angle of the swash plate is adjusted by a controller. This control is suitable for applications with constant consumer flow.

Load-sensing control

Load-sensing control detects the highest pressure demand of the connected consumers and sets the pump to exactly this pressure plus a small pilot pressure differential. As a result, the pump delivers only the flow rate that the consumers actually require and operates at the minimum necessary system pressure. This lowers energy consumption and reduces the heating of the hydraulic oil.

Electronic control

Modern axial piston pumps increasingly use electrohydraulic controllers that drive proportional or servo valves. An electronic controller processes signals from pressure and speed sensors and sends control signals to the control valves. This enables precise, dynamic control loops and integration into higher-level machine control systems. Typical control signals range from 4 to 20 mA.

Standards and safety requirements

EN ISO 4413: 2010 specifies the safety requirements for hydraulic systems and their components, including axial piston pumps. It defines hazards such as overpressure, leaks, and noise, and sets forth principles for their prevention. Among other things, the standard requires that axial and radial loads on the pump shaft remain within the manufacturer’s specifications, that torsional vibrations be reduced by damping couplings, and that surface temperatures be safely limited in accordance with ISO 13732-1. In addition, ISO 4406 specifies the cleanliness class of the hydraulic oil, which is critical for the reliable operation of axial piston pumps.

Maintenance and Servicing

Axial piston pumps operate under high pressures and speeds, which requires regular maintenance. Approximately 70 percent of failures in hydraulic systems can be attributed to contaminated hydraulic oil. Effective filtration and regular oil changes in accordance with the manufacturer’s specifications are among the most important preventive measures.

Typical signs of wear

• Piston running surfaces and cylinder bores: Scoring and pitting due to particle wear or cavitation
• Sliding surfaces of the swash plate: Wear caused by continuous piston-to-swash plate friction, recognizable by increased leakage flow rate
• Seals and shaft seals: Embrittlement and cracking due to thermal and mechanical stress
• Bearings: Increased play, running noise, and temperature rise as indicators of bearing damage

Measures to extend service life

Regular visual inspections for leaks, unusual noises, and vibrations should be performed at every shift change. Fluid analyses provide information on particle content, water content, and chemical aging of the oil. Seals should be inspected every three to six months and replaced as needed. Adhering to the oil change intervals and filter replacement cycles specified by the manufacturer ensures the cleanliness of the hydraulic oil and thus the proper functioning of the pump. HK Hydraulik recommends using original replacement parts for repair and maintenance of hydraulic pumps to ensure the precise fit and load-bearing capacity of the wear parts.

Applications of the Axial Piston Pump

Axial piston pumps are used in nearly all areas of industrial and mobile hydraulics. In mobile machinery such as excavators, wheel loaders, cranes, and tractors, they serve as the central power source for travel drives and working functions. In stationary industrial plants, they supply hydraulic power to presses, injection molding machines, and machine tools. They are also used in marine hydraulics, wind turbines, and test benches. The combination of high pressure, compact design, and stepless adjustability makes the axial piston pump the first choice when it comes to high-power-density and efficient hydraulic drives.