Axial Piston Motor

An axial piston motor is a positive-displacement motor used in hydraulics in which the pistons are arranged parallel to the axis of rotation. It converts hydraulic energy derived from pressure and flow rate into mechanical rotational energy. The axial piston arrangement and an inclined support plane create a reciprocating motion that causes the output shaft to rotate. Axial piston motors are characterized by high power density, speeds of over 10.000 rpm, and operating pressures of up to 450 bar.

Basics and Functioning of the Axial-Piston Motor

The axial piston motor belongs to the family of piston machines and operates as a positive-displacement machine based on the principle of volume displacement. Pressurized oil is directed into the cylinder bores of a rotating cylinder block via a control disc. The pistons are supported on an inclined plane, which converts the linear piston force into torque. Depending on the position of the pistons relative to the pressure or suction channel of the control disc, a continuous rotary drive is generated.

Design and Main Components

The heart of an axial piston motor is the cylinder block, also known as the cylinder drum, which features multiple axial bores through which the pistons are guided. In practice, the number of pistons typically ranges from seven to eleven, with an odd number of pistons being preferred to achieve a smoother torque curve. Other key components include the control disc for regulating oil flow, the output shaft, and the inclined support surface, which is implemented as an inclined disc or via an inclined axis angle.

The pistons transmit the hydraulic force to the inclined plane. Since the force does not act in the direction of the axis of rotation, a tangential component is generated that sets the cylinder block in rotation. The control disc ensures that each cylinder receives pressurized oil at the right moment and drains the released oil.

Torque and Speed

The torque output of an axial piston motor depends on the applied pressure and the displacement. The displacement describes the volume of hydraulic fluid that the motor displaces per revolution. The greater the displacement and the higher the operating pressure, the more torque is available at the output shaft. The speed is proportional to the supplied flow rate: a higher flow rate results in a higher motor speed. Modern axial piston motors achieve overall efficiencies of 90 to 95 percent, making them among the most efficient hydraulic motors.

Designs of the Axial Piston Motor

Axial piston motors are divided into two main types based on the method of power transmission: the swash plate design and the swash axis design. Both principles differ in the structural design of the inclined plane and have specific advantages and disadvantages.

Inclined disc design (wobble disc principle)

In the swash plate design, the cylinder block rotates while the swash plate remains stationary in the housing. The pistons rest on the inclined disc via sliding shoes. The angle of inclination of the inclined disc determines the piston stroke and thus the displacement. In variable-displacement engines, this angle can be changed during operation, allowing for stepless adjustment of torque and speed.

The swash plate design is characterized by a compact structure and short adjustment times. Due to its design, the swivel angle is limited to about 15 to 20 degrees, which restricts the maximum stroke and thus the maximum displacement. However, this design scores points for its small footprint and is particularly suitable for applications where installation space and weight are critical factors.

Slant-axis design

In the swash plate design, the cylinder block and output shaft are arranged at an angle to each other. The pistons are connected to the output shaft via connecting rods. The swash plate angle determines the piston stroke and is typically 25 to 40 degrees in constant-displacement motors; in variable-displacement motors, it can be continuously adjusted.

The skewed-axis design allows for larger swivel angles than the skewed-disc design, enabling a larger displacement and higher torques. Additionally, this design can withstand higher operating pressures, as power transmission via the connecting rods is more efficient. However, the design complexity is greater, which results in higher costs and a slightly larger overall size.

Comparison of the designs

Axial piston motors as variable-displacement and fixed-displacement motors

Axial piston motors are available both as fixed-displacement motors with a fixed displacement volume and as variable-displacement motors with a variable displacement volume. Fixed-displacement motors are used when the drive requirements remain constant. They have a simpler design, are more cost-effective, and require less maintenance.

Variable-displacement motors, on the other hand, allow the speed and torque to be adjusted to changing load conditions during operation. At a constant flow rate, reducing the displacement volume results in higher speed with lower torque, while increasing it raises the torque and lowers the speed. This flexibility makes variable-displacement motors particularly attractive for hydrostatic drive systems and applications with highly variable load profiles.

Varying can be achieved purely hydraulically via control pressure or electrohydraulically via proportional valves. Electrohydraulic systems offer finer resolution and can be integrated into digital control architectures, which is becoming increasingly important.

Axial-piston motor compared to radial-piston motor

In addition to the axial piston motor, the radial piston motor is the second major type of piston motor used in hydraulics. Both types have their strengths in different application areas.

Axial piston motors stand out for their high speeds, compact design, and excellent efficiency. They are suitable for applications that require fast movements and precise control. Radial piston motors, on the other hand, deliver significantly higher torques at low speeds and do not require an additional gearbox, whereas axial piston motors often need to be reduced in speed. In contrast, radial piston motors are larger, heavier, and generally do not reach the speeds of the axial design.

The choice between the two designs depends on the application requirements: speed, efficiency, and compactness favor the axial piston motor, while high torque at low speeds and robustness make the radial piston motor the preferred choice.

Applications of Axial Piston Motors

Axial piston motors are used in a wide range of hydraulic applications. In the mobile sector, they dominate the hydrostatic travel drives of excavators, wheel loaders, telescopic cranes, and tracked vehicles. Here, they ensure continuously variable travel speeds and high traction forces. Axial piston motors are also used in the slewing mechanisms of cranes and excavators.

In stationary industrial applications, axial piston motors drive presses, injection molding machines, and machine tools. In wind turbines, they are used for rotor blade adjustment and nacelle rotation. Additionally, they operate in hydraulic power units that supply multiple consumers with pressurized oil.

Another application is the closed-loop system, in which the return oil is fed directly back to the pump. Due to their design, axial piston motors are particularly well-suited for this operating mode, which is widely used in travel drives and winches.

Maintenance and Servicing

The reliability and service life of an axial piston motor depend largely on the quality of the hydraulic fluid and adherence to maintenance intervals. Wear occurs primarily on the sliding surfaces of the pistons and swash plate, on the output shaft bearings, and on the seals.

Oil Quality and Filtration

The purity of the hydraulic oil is a critical factor. According to ISO 4406, a cleanliness class of at least 20/18/15 should be maintained; at high operating temperatures, even 19/17/14. Particles in the oil lead to increased wear on the precision-machined sliding surfaces and can damage the control disc. Regular filter changes and oil analyses are therefore among the most important maintenance measures.

Typical signs of wear

• Increased internal pressure loss due to piston and cylinder wear
• Leaks at shaft seals and O-rings
• Noise changes due to bearing damage
• Performance loss due to wear on the control disc

Early detection of these symptoms through regular monitoring of pressure, temperature, and noise levels prevents costly consequential damage.

Standards

Several standards and specifications apply to axial piston motors, governing safety, compatibility, and performance evaluation. DIN ISO 4413 specifies safety requirements for hydraulic systems. ISO 4406 classifies particle contamination in hydraulic oil, which is critical to the motor’s service life. DIN 51524 defines requirements for hydraulic fluids, and DIN 9611 and DIN 6885 regulate the design of shafts and keyways.

Manufacturers such as Bosch Rexroth, Parker, Liebherr, and Hawe offer axial piston motors in various sizes and designs that comply with these standards and are designed for use in demanding hydraulic systems.