Constant Displacement Motor

A constant-displacement motor is a hydraulic motor with a fixed, unchangeable displacement volume. It converts hydraulic energy into mechanical rotational energy at a constant displacement per revolution. Speed and torque are determined solely by the supplied flow rate and system pressure, not by any internal adjustment.

Basics and Operating Principles of the Constant-Displacement Motor

Constant-displacement motors belong to the family of positive-displacement machines and are among the most commonly used hydraulic motors in industry. Their defining characteristic is the fixed displacement: Each revolution of the motor shaft displaces exactly the same volume of hydraulic fluid. This property makes the constant-displacement motor predictable and robust, but at the same time limits its flexibility under fluctuating load requirements.

The operating principle is based on the transmission of pressure force. High-pressure hydraulic fluid flows through the pressure port into the motor’s displacement chambers. There, the pressure acts on moving elements such as pistons, gears, or vanes, generating a rotational motion of the output shaft. The used fluid exits the motor through the return port and returns to the hydraulic circuit. Since the displacement volume is fixed, the speed increases proportionally to the supplied flow rate, while the torque depends largely on the applied pressure.

Speed and Torque Characteristics

The fixed displacement results in a direct relationship between flow rate and speed: If more liters per minute are supplied, the motor rotates faster. Torque, in turn, increases with pressure. Design engineers use this predictable behavior to design drives for operating conditions that remain constant during continuous operation or exhibit only minor fluctuations. However, in applications with highly variable load profiles, the constant-speed motor reaches its limits because it cannot increase torque through internal adjustment.

Types of Constant-Torque Motors

Constant-torque motors are manufactured in various designs that differ in power density, speed range, efficiency, and application. The choice of design depends on the specific requirements for torque, speed, installation space, and operating pressure.

Gear motors

Gear motors are the simplest and most cost-effective design. Two meshing gears are set in rotation by the pressure of the hydraulic fluid. One gear drives the other via their meshing teeth. Gear motors are characterized by their compact design, low weight, and high robustness. They are suitable for medium speeds and operating pressures up to approximately 250 bar. Their efficiency ranges from 80 to 85 percent. Disadvantages include relatively high noise levels and limited starting torque.

Vane motors

Vane motors use a rotor with axially or radially arranged vanes that are guided in grooves and pressed against the housing wall by centrifugal force. The hydraulic fluid displaces the volume between the vanes, thereby generating the rotational motion. Vane motors run very smoothly and quietly, are reversible, and offer good starting performance. They are used at medium pressures up to about 210 bar and stand out for their compact design.

Axial-Piston Constant-Displacement Motors

Axial piston motors with a fixed displacement are among the most powerful constant-displacement motors. The pistons are arranged axially to the drive shaft and press against an inclined disc or an inclined shaft axis. The fixed angle of inclination defines the piston stroke and thus the constant displacement.

Oblique-axis constant-displacement motors

Oblique-axis constant-displacement motors are a widely used variant. The drive shaft is inclined at a fixed angle relative to the cylinder block, which determines the piston stroke. This design achieves operating pressures of 400 bar and higher, offers high efficiencies exceeding 90 percent, and is characterized by excellent start-up and low-speed performance. Manufacturers such as Bosch Rexroth and Linde Hydraulics offer swashplate constant-displacement motors in numerous sizes for open and closed circuits.

Inclined-Disc Constant-Displacement Motors

In swash plate motors, the pistons press against a stationary, inclined plate. Here, too, the angle of inclination is fixed, so the displacement cannot be varied. Swash plate motors are more compact than swash-axis motors and are suitable for applications with limited installation space.

Radial-piston motors and orbital motors

Radial-piston motors arrange the pistons in a star pattern around the drive shaft. They deliver very high torques at low speeds and are primarily used in heavy-duty applications. Orbital motors, also known as gerotor motors, utilize an eccentrically rotating rotor system and also offer high torque with compact dimensions. Both designs operate as constant-displacement motors and cover speed ranges below 1, 000 rpm.

Fixed-Displacement Motor vs. Variable-Displacement Motor

The comparison between constant-speed motors and variable-speed motors is one of the key decisions in the design of hydraulic drive systems. Both types have their merits, but differ fundamentally in terms of flexibility, complexity, and cost.

Characteristic	Constant-displacement motor	Variable-displacement motor
Displacement	Fixed, cannot be changed	Continuously variable or adjustable in steps
Speed control	Only via the pump’s flow rate	Via flow rate and internal adjustment
Torque adjustment	Via system pressure only	Additionally via displacement adjustment
Efficiency at partial load	Low, since no adjustment is possible	Higher, since the operating point can be optimized
Design complexity	Simple, few components	More complex; adjustment mechanism required
Cost	Significantly lower	Higher due to additional components
Maintenance	Low	Higher; more wear parts

The constant-speed motor excels in applications where drive requirements are constant or where speed control is handled by the pump. Variable-speed motors demonstrate their strengths when speed or torque needs to be varied during operation without adjusting the pump.

Applications for constant-displacement motors

Constant-displacement motors are found in nearly all areas of industrial and mobile hydraulics. Their strengths are particularly evident where operating conditions are constant or where speed control is achieved via a variable-displacement pump.

Mobile Hydraulics

In construction machinery such as excavators, wheel loaders, and cranes, constant-speed motors are used as travel motors, swing drives, and winch drives. In agriculture and forestry, they also drive mowers, conveyor belts, and cable winches. In the marine and offshore sectors, corrosion-protected and DNV-certified variants are used.

Industrial Hydraulics

Stationary applications include conveyor systems, mixers, presses, lifting systems, and machine tools. Where a drive is required to operate continuously at a constant speed and torque, the constant-speed motor is often the most economical solution. Axial-piston constant-speed motors are also found in injection molding machines and wind turbines.

Selection Criteria for Constant-Speed Motors

Selecting the right constant-speed motor requires careful matching of several parameters to the specific application.

Torque requirement: The required torque at the output shaft determines the minimum displacement and the necessary operating pressure.
Speed range: The minimum and maximum speeds define which design is suitable. Radial-piston or orbital motors are suitable for very low speeds at high torque, while axial-piston motors are suitable for higher speeds.
Operating pressure: The motor’s nominal pressure class must match the system pressure. Axial-piston constant-displacement motors cover pressures up to 400 bar and above, while gear motors operate in the range up to 250 bar.
Circuit Type: Constant-displacement motors are available for open and closed circuits. This choice affects the connection configuration and housing drainage.
Installation Configuration: Flange dimensions according to ISO, SAE, or DIN; shaft profiles and connection sizes must match the machine design.
Environmental conditions: Temperature, humidity, corrosion exposure, and potential certification requirements (CE, DNV, ATEX) limit the selection.

Maintenance and Servicing

Due to their simple design, constant-speed motors require less maintenance than variable-speed motors. Nevertheless, they require regular attention to prevent failures and maximize service life.

Oil Quality and Filtration

The purity of the hydraulic fluid has a decisive influence on the service life of a constant-displacement motor. According to ISO 4406, the oil cleanliness class should be at least 18/16/13. Contaminants lead to increased wear on pistons, cylinder bores, gears, and control discs. Regular filter inspections and timely filter changes are therefore essential. The viscosity of the oil should be in the range of ISO VG 32 to 68, with VG 46 being the most common standard.

Typical Signs of Wear

The most common signs of wear include scoring on sliding surfaces, pitting on gears, embrittlement of seals, and increased bearing play. An increasing leakage flow rate or a decrease in torque at the same pressure are practical indicators of wear. Regular monitoring of pressure, temperature, and noise levels enables early detection of damage.

Standards and Specifications

Various standards and specifications govern constant-speed motors, regulating safety, compatibility, and quality. DIN ISO 4413 specifies the general safety requirements for hydraulic systems and thus also applies to the installation and operation of constant-speed motors. Oil cleanliness is classified according to ISO 4406, while flange dimensions and connection standards are based on ISO, SAE, and DIN standards. CE marking is mandatory for the European market; for marine applications, additional certifications such as DNV are required. Compliance with these standards ensures that constant-displacement motors can be operated reliably and safely within complete hydraulic systems.

What is a fixed-displacement motor?

A fixed-displacement motor is a hydraulic motor with a fixed, non-variable displacement. It converts hydraulic energy into mechanical rotational energy at a constant displacement per revolution.
How do constant-displacement motors differ from variable-displacement motors?

A constant-displacement motor operates with a fixed displacement and is therefore simpler, less expensive, and requires less maintenance. A variable-displacement motor can adjust its displacement and offers greater flexibility, but is more complex in design and more expensive.
How are flow rate, speed, pressure, and torque related?

In a constant-displacement motor, the speed increases with the supplied flow rate. The torque depends primarily on the system pressure.
What types of constant-displacement motors are there?

Typical types include gear motors, vane motors, axial-piston constant-displacement motors, oblique-axis and oblique-disc motors, as well as radial-piston and orbital motors.
Where are constant-displacement motors used?

Constant-displacement motors are used in industrial and mobile hydraulics, for example in construction machinery, conveyor systems, presses, winches, mixers, and machine tools.
What are the advantages of a constant-displacement motor?

Constant-displacement motors offer a simple design, robust technology, predictable operating behavior, and comparatively low investment and maintenance costs.
What are the disadvantages of a constant-displacement motor?

The biggest disadvantage is their limited flexibility, since the displacement cannot be adjusted internally to accommodate changing load or speed requirements.
What should be considered when maintaining and selecting a constant-displacement motor?

It is important to use clean hydraulic fluid, ensure proper filtration, and monitor for signs of wear such as leaks or a decrease in torque. When selecting a motor, the most important factors are torque, speed range, operating pressure, circuit type, installation configuration, and environmental conditions.