Fixed Displacement vs. Variable Displacement Pumps


The hydraulic pump is one of the most important components in a mobile hydraulic system. Selecting the correct pump can be a daunting task because of the number of options on the market. Pumps are expensive, time consuming to install and if you choose the wrong pump, it can break shortly after installation. The first thing to consider when purchasing a hydraulic pump is do I need a fixed displacement pump or a variable displacement pump?
Fixed Displacement Pump Defined
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:
- Material Handling
- Machine Tools
- Conveyors
- Industrial Manufacturing Machines
- Marine Applications
- Agricultural Equipment
- Lubrication Systems
- Mobile Applications using Tandem Center Valves


Variable Displacement Pump Defined
A variable displacement pump has the capability to change the amount of fluid that is being displaced depending on the need of the system. A common variable displacement pump is that of the piston variety. The pistons are connected to a swash plate that can change its angle based on the pressure that is created in the circuit. Pressurized fluid is diverted through a pressure compensator and overcomes the spring force of a biased piston that is trying to keep the pump at its maximum output flow. When the swash plate angle is changed, the volume available for fluid to fill is reduced therefore reducing how much output flow the pump is producing. Using a variable displacement pump greatly increases the amount of energy that is wasted when maximum pump output is not needed. Similar mechanisms are available for use in variable displacement vane pumps as well. The picture below shows the internal components of a variable displacement piston pump with a pressure compensator. You’ll notice that the Yoke Actuating Piston acts upon the small bias piston that is on the opposite side of the swash plate. Common applicatiosn for the variable displacement pump include:
- Mobile Applications using Closed Center Valves
- Snow and Ice Applications
- Agricultural Applications
- Mobile Service Trucks


Which Type of Pump is Right for You?
With 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.
FORCE America employs several great salespeople that will help you choose the correct hydraulic pump for your application. Contact one of our knowledgable salesperson at your local FORCE America Service Center or call 1-888-993-6723.