Variable Displacement Motor

A variable displacement motor is a special type of hydraulic motor whose displacement can be adjusted during operation. This adaptability makes it possible to vary the speed and torque of the motor while maintaining a constant volume flow. Variable displacement motors are used in hydraulic systems to precisely adjust power output to changing requirements, resulting in high efficiency and flexibility.

Basics and functioning of variable displacement motors

Variable displacement motors are essential components in many hydraulic drive systems. Their main function is to convert hydraulic energy into mechanical rotational energy, with the special feature being the variable displacement adjustment. The displacement, i. e. , the volume of hydraulic fluid that the motor takes in per revolution, can be changed continuously or in steps.

Design and principle of adjustment

The displacement is adjusted using different design principles. In axial piston motors, for example, this is achieved by changing the swivel angle of the swivel disc. A larger swivel disc results in a larger displacement and vice versa. In radial piston motors, the eccentricity of the piston stroke can be changed to adjust the displacement. These mechanical adjustments enable precise control of the motor power.

Advantages of adjustability

The adjustability of the displacement offers several advantages:

Speed control: At a constant flow rate, the speed of the motor can be adjusted by changing the displacement. A smaller displacement results in a higher speed, while a larger displacement results in a lower speed.
Torque adjustment: Conversely, at a constant speed, the torque can be influenced by adjusting the displacement. A larger displacement produces a higher torque.
Increased efficiency: By adjusting to the respective operating point, the motor can always be operated in the optimum efficiency range, which reduces energy consumption.
Flexibility: Variable displacement motors enable a high degree of adaptability to different load conditions and work cycles.

Types of variable displacement motors in hydraulics

Various types of variable displacement motors are used in hydraulics, which differ in their design and specific areas of application. The most common types are axial piston motors and radial piston motors.

Axial piston variable displacement motors

Axial piston motors are widely used and are characterized by their high power density and good controllability. In these motors, the pistons are arranged axially to the drive shaft.

How axial piston motors work

The displacement of axial piston motors is usually adjusted by means of a swash plate. The pistons are connected to the swash plate via connecting rods. When hydraulic fluid flows into the cylinders, the pistons are pressed against the swash plate, causing it to rotate and drive the drive shaft. The swivel angle of the swivel disc determines the displacement. A larger swivel angle means a longer stroke of the pistons and thus a larger displacement.

Areas of application for axial piston motors

Axial piston variable displacement motors are found in mobile work machines such as excavators, wheel loaders, and cranes, but also in stationary industrial plants where precise speed and torque control is required. Examples include wind turbines, injection molding machines, and presses.

Radial piston variable displacement motors

Radial piston motors are known for their high starting torque and robustness. In these motors, the pistons are arranged radially to the drive shaft.

How radial piston motors work

The displacement of radial piston motors is often achieved by changing the eccentricity of the piston stroke. The pistons move in a housing that is arranged eccentrically to the drive shaft. By shifting the housing or a control ring, the eccentricity and thus the stroke of the pistons can be changed. A larger stroke results in a larger displacement.

Areas of application for radial piston motors

Radial piston variable displacement motors are often used in applications that require high torque at low speeds, such as in winches, conveyor belts, mixers, and marine hydraulics. Their ability to deliver high torque even at low speeds makes them ideal for these applications.

Control and regulation of variable displacement motors

Precise control and regulation of variable displacement motors is crucial for their performance and efficiency in hydraulic systems. Modern systems often use electronic controls to optimize displacement adjustment.

Hydraulic and electrohydraulic control

The displacement can be adjusted purely hydraulically or electrohydraulically. In hydraulic control, the adjustment mechanism is actuated directly by the system pressure or a control pressure. Electrohydraulic systems use electrical signals to control proportional valves or servo valves, which in turn influence the hydraulic adjustment mechanism. This enables finer and more precise control.

Control strategies

Various control strategies are used to achieve the desired speed or torque:

Speed control: Here, the displacement is adjusted so that the engine speed follows a specified setpoint, regardless of load fluctuations.
Torque control: With this strategy, the displacement is adjusted so that the motor delivers a constant torque.
Power control: This control aims to keep the power output of the motor constant by varying both speed and torque within certain limits.

Maintenance and servicing of variable displacement motors

Regular maintenance and servicing of variable displacement motors is crucial for their longevity and reliability. Careful maintenance prevents failures and ensures optimal functioning of the hydraulic system.

Typical wear parts and their inspection

Typical wear parts in variable displacement motors include seals, bearings, and the sliding surfaces of the pistons and swash plates. Regular inspection of these components for wear, cracks, or leaks is essential.

Seals

Seals, such as O-rings and shaft sealing rings, prevent hydraulic fluid from leaking and contaminants from entering. They should be checked regularly for brittleness, cracks, or deformation and replaced if necessary.

Bearings

The bearings of the drive shaft and pistons are exposed to high loads. Checking for play, noise, or elevated temperatures can indicate incipient bearing damage.

Sliding surfaces

The sliding surfaces of the pistons and the swivel disc (in axial piston motors) or the control ring (in radial piston motors) are crucial for the efficiency of the motor. Signs of wear such as scoring or pitting can lead to loss of performance.

Importance of the hydraulic fluid

The quality and cleanliness of the hydraulic fluid has a significant influence on the service life of a swivel motor. Contaminants can lead to increased wear on the moving parts and impair the function of the control valves.

Filtration

Effective filtration of the hydraulic fluid is therefore of utmost importance. Regular inspection and replacement of the filter elements are essential to ensure the purity of the fluid.

Oil change intervals

It is important to adhere to the oil change intervals recommended by the manufacturer, as the hydraulic fluid can lose its properties over time, for example due to aging or thermal stress.

Future developments in variable displacement motors

Development in the field of variable displacement motors is progressing steadily, driven by the need for higher efficiency, more compact designs, and more intelligent control options.

Integration of sensors and actuators

Future variable displacement motors are expected to be even more closely integrated with sensors and actuators. This will enable even more precise monitoring of operating conditions and faster, more adaptive control. For example, sensors could measure pressure, temperature, or speed directly at the motor and transmit this data to a higher-level control system.

Advances in materials science

Advances in materials science are leading to more robust and wear-resistant materials for pistons, cylinders, and seals. This helps to extend the service life of engines and increase maintenance intervals.

What is a variable displacement motor and what is its main function?

A variable displacement motor is a special type of hydraulic motor whose displacement can be adjusted during operation. Its main function is to convert hydraulic energy into mechanical rotational energy, whereby the adaptability enables the precise control of speed and torque at a constant volume flow and thus adapts the power output to changing requirements.
How is the displacement volume of a variable displacement motor adjusted?

The displacement volume is adjusted differently depending on the design. With axial piston motors, for example, this is achieved by changing the swivel angle of the swivel plate. In radial piston motors, the eccentricity of the piston travel can be changed to adjust the displacement.
What are the advantages of variable displacement motors?

Variable displacement offers several advantages: It enables precise speed control at constant volume flow, torque adjustment at constant speed, increased efficiency through operation in the optimum efficiency range and high flexibility with changing load conditions and work cycles.
What are the main types of variable displacement motors in hydraulics?

The most common types of variable displacement motors in hydraulics are axial piston variable displacement motors and radial piston variable displacement motors, which differ in their design and specific areas of application.
Where are axial piston variable displacement motors typically used?

Axial piston variable displacement motors are widely used and are often found in mobile machinery such as excavators, wheel loaders and cranes. They are also used in stationary industrial equipment such as wind turbines, injection molding machines and presses where precise speed and torque control is required.
Where are radial piston variable displacement motors typically used?

Radial piston variable displacement motors are often used in applications that require high torque at low speeds. These include winches, conveyor belts, mixers and applications in marine hydraulics.
How are variable displacement motors controlled and regulated?

The control and regulation of variable displacement motors can be purely hydraulic or electrohydraulic. Electrohydraulic systems use electrical signals to control proportional valves or servo valves, which enables finer and more precise control.
What control strategies are used for variable displacement motors?

Common control strategies are speed control, where the speed follows a setpoint, torque control, where a constant torque is delivered, and power control, which aims to keep the power output constant.
Which aspects are particularly important when servicing and maintaining variable speed motors?

Regular inspection of typical wear parts such as seals, bearings and sliding surfaces is crucial for longevity and reliability. Equally important is the quality and cleanliness of the hydraulic fluid, which must be ensured by effective filtering and compliance with oil change intervals.
What future developments can be expected for variable displacement motors?

Future developments include greater integration of sensors and actuators for more precise monitoring and adaptive control. In addition, advances in materials science will lead to more robust and wear-resistant materials, which will extend the service life of the motors.