How to Choose Fixed or Variable Displacement Piston Pump?

There are two primary types of hydraulic pumps to choose from: fixed displacement and variable displacement. Choosing the right kind of pump is essential to generating the correct amount of fluid pressure and volume while ensuring the pump and hydraulic system operate efficiently. It’s ultimately about optimizing the performance of your system while proactively controlling costs.

What Is a Fixed Displacement Pump?

A fixed displacement pump is a pump that will always expel the same volume of oil per revolution of the pump. The displacement of a pump is defined by the theoretical volume the gears, vanes or pistons will displace in one revolution. For example, 2.8 in³ pump will theoretically push that much oil in one revolution. The picture to the right shows how the fluid is trapped between the teeth and the inside of the housing that surrounds the gearset. This volume times the number of chambers created by each gearset is how the theoretical displacement of the pump is calculated. The only way to change the displacement of the pump is by physically changing the internal components of the pump. If the desired output flow is less than what the pump displaces, the excess flow can be diverted or relieved to the tank. In doing so, a tremendous amount of energy is lost in the system. Common applications for fixed displacement pumps include:

Common Applications

• Machine Tools
• Conveyors
• Industrial Manufacturing Machines
• Marine Applications
• Agricultural Equipment
• Lubrication Systems
• Mobile Applications using Tandem Center Valves

What Is a Variable Displacement Pump?

The variable displacement hydraulic pump is called variable because the fluid volume per stroke can be altered or changed electronically, hydraulically, or manually during operation.

Because of its ability to adjust its flow rate for different applications, the variable displacement pump is more complex in its design. As such, it’s a more expensive pump, and its maintenance and upkeep costs are higher than a fixed displacement pump. However, the costs are offset by its energy efficiency and ability to operate different equipment.

Common Applications

• Cranes
• Excavators
• Backhoes
• Industrial Automation Systems
• High-Performance Machine Tools
• Marine Propulsion Systems
• Mining Equipment
• Medical Devices
• Wind Turbines
• Injection Molding Machines
• Some Harvesters & TractorsWith most systems, the number one factor for choosing the correct type of pump is cost. Cost is usually the determining factor for choosing the pump for a hydraulic system. Variable displacement type pumps are usually more expensive than a fixed displacement style pump. This is because of the many more moving parts that required for a variable displacement pump to operate. Variable displacement pumps also require stricter filtration because of the closer tolerances that the moving parts have to one another. As these factors are often considered to be negatives of a variable displacement pump, they are also what makes a variable displacement pump so great to include in your hydraulic system. Using a variable displacement pump can greatly improve the energy efficiency of your system in the long run. Spending more up front will save money in the long run. Better filtration will not only keep the variable displacement pump operating like it should, but it will also keep all components in your system working at optimal performance for the entirety of their life span.