Variable Displacement Pump

A variable displacement pump is a special type of hydraulic pump whose delivery volume per revolution, i. e. , the displacement volume, can be variably adjusted to the requirements of the hydraulic system. In contrast to constant pumps, which have a fixed delivery volume, the variable displacement pump enables precise control of the volume flow and thus the speed of actuators such as hydraulic cylinders or hydraulic motors. This adaptability leads to a significant increase in energy efficiency, as only the power actually required is provided.

Functionality and designs of variable displacement pumps

Variable displacement pumps are essential components in modern hydraulic systems that require precise control and high energy efficiency. Their mode of operation is based on changing the displacement volume, which is achieved by different mechanical principles.

Axial piston pumps with swivel cradle

The axial piston pump with swivel cradle is one of the most commonly used types of variable displacement pumps. In this type, the pistons are arranged axially to the drive shaft. The delivery volume is adjusted by swivelling a so-called swivel cradle or swivel disc.

When the swivel cradle is at an angle to the drive shaft, the pistons move in and out of their bores during one revolution of the cylinder drum. The length of the piston stroke and thus the displaced volume depend directly on the swivel angle. A larger swivel angle results in a longer piston stroke and thus a higher delivery volume. If the swivel cradle is perpendicular to the drive shaft, there is no piston stroke and the pump does not deliver any volume.

Radial piston pumps with eccentric adjustment

Radial piston pumps are another important category of variable displacement pumps in which the pistons are arranged radially to the drive shaft. The delivery volume is often adjusted by means of an eccentric adjustment.

In this design, the pistons move in a fixed housing and are actuated by an eccentrically mounted shaft or an eccentric ring. The piston stroke can be varied by changing the eccentricity, i. e. , the distance between the center of rotation of the shaft and the center of the piston star. A larger eccentric distance results in a longer stroke and thus a higher delivery volume.

Vane pumps with adjustable stator ring

Vane pumps are known for their low-pulsation delivery and are also available in an adjustable version. In these pumps, the delivery volume is influenced by adjusting a stator ring.

The stator ring surrounds the rotor in which the vanes slide. The eccentric arrangement of the stator ring relative to the rotor creates chambers of different sizes. The vanes are pressed against the inner wall of the stator ring by centrifugal force or spring force. When the stator ring is moved, the eccentricity to the rotor changes, adjusting the volume of the delivery chambers and thus the delivery volume of the pump.

Control types for variable displacement pumps

The efficiency and precision of variable displacement pumps are largely determined by their control systems. These systems dynamically adjust the pump’s delivery volume to the respective requirements of the hydraulic system.

Pressure control

Pressure control is a basic and widely used type of control. Here, the delivery volume of the pump is adjusted so that a preset system pressure is maintained at a constant level.

If the system pressure exceeds the setpoint, the pressure control reduces the pump’s delivery volume. This is done, for example, by reducing the swivel angle of an axial piston pump. If the pressure falls below the setpoint, the delivery volume is increased accordingly. This type of control is particularly important in systems that require a constant working pressure, regardless of the load.

Power control

Power control aims to limit the drive power consumed by the pump. This is particularly relevant in mobile machines or systems where the available drive power is limited.

In power control, the product of pressure and volume flow is monitored. If the power reaches a preset maximum value, the control reduces the delivery volume to prevent the drive motor from overloading. This can be achieved, for example, by a combination of pressure and volume flow measurement, which controls the adjustment of the pump via a characteristic curve.

Volume flow control

Volume flow control enables precise control of the speed of actuators. Here, the delivery volume of the pump is directly adjusted to the required volume flow.

A sensor measures the actual volume flow in the system, and a controller compares this with a setpoint. In the event of deviations, the pump’s adjustment mechanism is actuated in such a way that the desired volume flow is achieved. This type of control is crucial for applications that require high precision in motion control, such as in machine tools or handling systems.

Load sensing control

Load sensing control is an advanced and energy-efficient type of control that adjusts the volume flow to the load as required. It minimizes power losses by delivering only as much volume flow as is actually needed for the current task.

In load-sensing control, the pressure upstream of the consumer (load pressure) is measured and compared with the pump pressure. The pump regulates its volume flow so that the pump pressure always has a defined overpressure relative to the load pressure. This ensures that there is always sufficient pressure available to move the actuator without generating unnecessarily high pressures. This leads to a significant reduction in power loss and thus to higher energy efficiency of the overall system.

Advantages of variable displacement pumps in hydraulics

Variable displacement pumps offer a number of significant advantages over fixed displacement pumps, making them the preferred choice in many hydraulic applications. These advantages contribute significantly to the efficiency, precision, and cost-effectiveness of hydraulic systems.

Energy efficiency

Probably the biggest advantage of variable displacement pumps is their high energy efficiency. Since they can adjust their delivery volume to the actual demand, only the power actually required is drawn from the drive motor.

In contrast, fixed displacement pumps always deliver their maximum flow rate, even when only a low flow rate is required. The excess flow must then be diverted via throttle valves, which results in significant energy losses in the form of heat. Variable displacement pumps avoid these throttling losses by reducing their delivery volume when demand decreases. This results in lower energy consumption and thus lower operating costs.

Precise control

Variable displacement pumps enable very precise control of flow rate and pressure in the hydraulic system. By continuously adjusting the delivery volume, the speeds of hydraulic cylinders and hydraulic motors can be precisely controlled.

This precise controllability is crucial for applications that require high accuracy, such as in machine tools, robotics, or process control. The ability to finely adjust the volume flow allows for exact positioning and smooth acceleration and deceleration of actuators, which improves the quality of the work results and reduces mechanical stress.

Reduced heat generation

By avoiding throttling losses, variable displacement pumps contribute significantly to reduced heat generation in the hydraulic system. Excess heat is a common problem in hydraulic systems, as it shortens the service life of components such as seals and hydraulic fluid.

Less heat generation means that smaller coolers or no coolers at all are required, which reduces system costs and maintenance. In addition, the viscosity of the hydraulic oil remains more stable, further improving the efficiency and reliability of the system.

Longer component life

Precise control and reduced heat generation have a positive effect on the service life of all components in the hydraulic system. Lower thermal loads and smoother operation reduce wear on pumps, valves, cylinders, and seals.

A system operated with a variable displacement pump generally operates under more stable conditions, resulting in lower peak pressures and reduced mechanical stress. This extends maintenance intervals and reduces the need for replacement parts, which in turn lowers operating costs.

Areas of application for variable displacement pumps

Due to their advantages, variable displacement pumps are used in a wide range of hydraulic applications, especially where energy efficiency, precise control, and high power density are required.

Mobile machinery

Variable displacement pumps are virtually standard in mobile machinery such as excavators, wheel loaders, cranes, and agricultural machinery. Energy efficiency requirements are particularly high in these applications, as fuel consumption directly affects operating costs.

Load-sensing control in conjunction with variable displacement pumps makes it possible to optimally adapt the performance of the hydraulic system to the task at hand. This results in lower fuel consumption, reduced emissions, and improved machine controllability.

Machine tools

In modern machine tools, such as CNC milling machines, lathes, or presses, precise motion sequences and high repeat accuracy are crucial. Variable displacement pumps contribute to high precision here thanks to their exact volume flow control.

The ability to finely control feed speeds and pressing forces enables the high-quality machining of complex workpieces. In addition, energy efficiency and reduced heat generation contribute to stable operation and a longer service life for the machine.

Industrial plants

Variable displacement pumps are also used in stationary industrial equipment such as injection molding machines, presses, hoists, and material handling systems. These applications often require high pressures and large volume flows, which must be provided efficiently and precisely.

The adaptability of variable displacement pumps to changing loads and operating conditions makes them ideal for such applications. They help to reduce energy consumption, increase process stability, and optimize the operating costs of the systems.

Wind turbines

In wind turbines, variable displacement pumps play an important role in the pitch and yaw systems. These systems are responsible for aligning the rotor blades and nacelle with the wind and must operate precisely and reliably.

The hydraulic adjustment systems with variable displacement pumps enable sensitive adjustment of the rotor blade position to achieve optimum efficiency and protect the turbine from overload. The robustness and reliability of the variable displacement pumps are crucial for the long-term and safe operation of the wind turbine.

Maintenance and servicing of pitch pumps

Regular maintenance and professional servicing of variable displacement pumps are crucial for their longevity, reliability, and maintaining system efficiency. Neglect can lead to premature wear, loss of performance, and costly breakdowns.

Hydraulic fluid

The quality and cleanliness of the hydraulic fluid is of utmost importance for the function and service life of a variable displacement pump. Contaminants can lead to increased wear on the precision components.

It is essential to use the hydraulic fluid specified by the manufacturer and to adhere to the change intervals. Regular analyses of the hydraulic fluid can provide information about the condition of the oil and possible contaminants. Good filtration of the system is also crucial to remove particles and water from the oil.

Filter replacement

Hydraulic filters play a central role in protecting the variable displacement pump from contamination. They must be changed regularly in accordance with the manufacturer’s specifications.

A clogged filter can lead to increased pressure drop, which puts additional strain on the pump and reduces efficiency. In the worst case, a clogged filter can open bypass valves, allowing unfiltered oil to enter the system and cause damage.

Seals and leaks

Seals are wear parts and must be checked regularly for leaks. Leaks not only lead to oil loss and environmental pollution, but can also affect system pressure and reduce efficiency.

Defective seals should be replaced immediately to prevent major damage to the pump or other system components. Regular visual inspection for oil traces is a simple but effective way to detect potential problems early on.

Noise and vibration analysis

Unusual noises or vibrations can be early signs of problems with the variable displacement pump. These include cavitation, bearing wear, or misalignment.

Regular acoustic and visual inspections can help to detect such signs at an early stage. If any abnormalities are found, a detailed diagnosis should be carried out to determine the cause and take appropriate action.

Pressure and volume flow check

Regular checks of the system pressure and volume flow are important to ensure that the variable displacement pump and its control system are functioning correctly. Deviations from the set values may indicate internal leaks, wear, or incorrect control settings.

These checks should be carried out using suitable measuring instruments and the results documented in order to identify trends and take preventive measures. Precise adjustment of the control parameters is crucial for optimum pump performance and efficiency.

Future developments in variable displacement pumps

Developments in the field of variable displacement pumps are characterized by the pursuit of even greater efficiency, more intelligent control, and integration into modern system architectures.

Electrification and hybridization

With the trend toward electrification of mobile machinery and industrial equipment, electrically driven variable displacement pumps are gaining in importance. These enable even more precise and faster control as well as better integration into electric drive concepts.

Hybrid solutions that combine combustion engines with electric drives use variable displacement pumps to optimally distribute power between the various energy sources, thereby further increasing efficiency.

Intelligent control systems

Future variable displacement pumps will be even more integrated into intelligent control systems. This includes the use of sensors to record a variety of operating parameters and the use of algorithms for predictive maintenance and optimization.

Artificial intelligence and machine learning could help to adapt the control of the pumps even more sensitively to changing operating conditions, thereby further improving efficiency and reliability.

Miniaturization and power density

The trend toward miniaturization and increased power density will also continue for variable displacement pumps. Smaller and lighter pumps with the same or higher performance enable more compact and flexible system designs.

New materials and manufacturing processes will help to increase power density while ensuring the longevity and robustness of the pumps.

Integration into Industry 4. 0

Variable displacement pumps are increasingly becoming part of Industry 4. 0 concepts. This means greater networking of pumps with higher-level control systems and the provision of data for monitoring, diagnostics, and predictive maintenance. Integration into digital twins and cloud platforms enables comprehensive analysis of operating data and optimization of the entire plant over its life cycle.