Differences between a fixed displacement and a variable displacement hydraulic pump

Differences between a fixed displacement and a variable displacement hydraulic pump

Based on requirements, in any application one has to choose between a fixed displacement or variable displacement hydraulic pump. In this article, we examine the major differences between Fixed Displacement and Variable Displacement Hydraulic Pumps:

The difference between a fixed displacement and a variable displacement hydraulic pump is simply how you as the end user would term it. Each of these types of pumps has some very good benefits and features but there are those who like one more than the other. Knowing what each offers and how that affects your project will make sure that you have picked out the right pump.


Delivering a constant flow at all times is what a fixed displacement hydraulic pump is designed to do.

It does this by having pistons in the cylinder working on a fixed stroke length; hence the fluid flow will always be at constant rate. For instance, the piston stroke determines the amount of fluid that will be delivered and thus how fast it can be done.

Fixed displacement pumps are used in variety of applications such as drawing water from wells or removing sewage from septic tanks; they are also use for injecting air or other fluids into or out of machinery within an industrial plant.

Pressure demands dictate the flow rate of a variable displacement hydraulic pump

In this type of pump, there are basically two or more rotors with each rotor having different number of vanes. The quantity of vanes on a rotor is what can determine any particular time how much fluid to be pumped out from the system. For example, if there are five rotors each with one inch holes then under normal circumstances we would expect to get 5 gallons per minute maximum flow rate.

All you need do to increase the output for your pump is to purchase another set of rotors and replace those already installed. This increases your total available output by 25% and allows you to pump up to 6 gallons per minute instead of 5 gallons per minute.

Flow rate

A certain hydraulic pump can move a lot of fluid at low pressure

Moving fluid in a system that uses a fixed displacement pump is done through the use of a piston that moves back and forth within an enclosed chamber, the fluid then flows into another chamber where it goes through another piston that moves up and down. This continues until all of the liquid has passed through every one of these pistons.

The advantage here is that unlike other types, they do not wear out or break because they have no moving parts. It also helps as there may be minimum air entry space between each piston. At any given time, this calls for little resistance when pushing large volumes of fluids via very small distances such as pipes or tanks.

This type of pump can also be used to supply small amounts but with high pressure.

The pump alongside valves and actuators are important in any hydraulic system which controls flow rates and pressures in industrial machinery.

It changes the rate at which fluid flows through an orifice on a variable-displacement hydraulics pump. Therefore, it will enable pumps to change their speed if necessary. In several scenarios, this facilitates machines to run at low speeds while performing some activities like lifting components up and down again.

The working principle for a variable displacement hydraulic pump is same as those non-variable; two pistons move back and forth within a fixed chamber inside the casing of the pump itself. When they are allowed free movement, they create a vacuum; when held static however, they push oil throughout the system.


The efficiency is usually expressed as a percentage, i.e. 0% means total loss of energy while 100% implies no energy has been lost in the process.

A fixed volume pump has a constant displacement meaning output is proportional to both the stroke and pressure applied. This kind of pump produces work at constant rates only therefore it has very low efficiency (about 10%).

In contrast, variable-displacement pumps can provide varying amounts of work with different strokes and pressures used. If you want your system to be more effective then use an effective variable-displacement pump not an ineffective fixed-displacement one.


Fixed displacement hydraulic pumps are designed to run under some set speed and pressure; they have no adjustment for these parameters. For instance, if the motor speeds up or slows down, it can change the flow of oil through a variable displacement hydraulic pump.

It is also true that a fixed displacement pump has an increased viscosity compared to a variable one due to its inability to alter its speed or pressure. This means that the fluid flows more slowly through this more viscous fluid and hence higher pumping speeds for fixed displacement pumps.

On the other hand, Variable displacement pumps have reduced viscosity as opposed to fixed ones since it can vary its speed and/or pressure. There’s another reason why water coming out of a garden hose flows faster than water in a pipe with restricted flow: oil moves fast on less viscous material than highly viscous fluids so reducing pumping speeds for variable displacement pumps.


The variance in horsepower between a fixed displacement and a variable displacement hydraulic pump is the number of horsepower required to drive the equal amount of water by both kinds of pumps.

Flow rate should not change or control speed in case of a fixed-displacement hydraulic pump. Flow rates are measured in terms of “head” which indicates how many pounds per minute (lpm) per square inch pressure can be delivered.

A variable-displacement hydraulic pump has an inbuilt controller that allows it to vary its output according to different conditions or specific needs. In case for example if the head exceeds some specified level, then its output is increased automatically so as to enable more water flow through it.

Knowing what makes a fixed displacement and variable displacement hydraulic pumps different will hopefully enable you choose the right one for a particular use. For instance, when using high pressure hydraulic systems, it would be more suitable to employ a variable displacement hydraulic pump rather than using fixed displacement pumps.

Comparison Summary

Efficiency: For miscellaneous and intricate tasks, variable displacement pumps are more efficient.

Cost: Fixed displacement pumps are less costly and there is reduced need for their maintenance.

Complexity: Although variable displacement pumps are more complicated in nature, they provide better control as well as a capacity to adjust to new situations that may arise.

Both forms of pumps have advantages in various industrial applications; while fixed displacement pumps are used when simplicity, reliability and cost considerations dominate, variable displacement ones are preferred for systems requiring flexibility and high efficiency.

When it comes to detailed comparisons or specific case studies, you can consult technical resources or the manufacturers of particular products for more information. There is also a Hydraulics & Pneumatics website with deeper articles on this topic and possible technology ideas.


Difference Between A Vane Pump And A Radial Piston Pump


Difference Between A Vane Pump And A Gear Pump


Difference Between A Gear Pump And A Piston Pump


Difference between a centrifugal pump and an axial pump
Get a quick quote
It is convenient for our customer service staff to contact you in time
Click or drag files to this area to upload. You can upload up to 2 files.
Upload a picture of the hydraulic pump you need
For you to quickly find the hydraulic pump you need, please be sure to provide the brand model and picture of the hydraulic pump