What is an Axial Piston Pump with Axial Piston Pump Diagram

What is an Axial Piston Pump with Axial Piston Pump Diagram

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Axial piston pumps are a common part throughout construction machines and across construction equipment brands. Their design allows them to be used from the cooling system to the steering system to a multitude of places throughout a machine. Anyone who has worked in construction equipment has certainly come across an axial piston pump, but one might still wonder, “What exactly is going on in this little box?” That’s why we’re here to help.

Whether you’re researching an axial piston pump problem or you’re just inquisitive about this widely-used part, read on for a short explainer on how they work and what they do.

What Is an Axial Piston Pump?

At its most basic, an axial piston pump turns mechanical energy (the turning of a shaft) into hydraulic output (the moving of fluid). The use cases for an axial piston pump are wide-ranging, leading to the adoption of this pump design throughout construction equipment types and across construction equipment brands. An axial piston pump provides advantages in dependability, simplicity, and efficiency leading to its use in handling a wide range of tasks on a machine. 

The basic mechanics and design of an axial piston pump are also commonly combined with gearing designs to create axial piston motors. An example is the swing motor commonly found in excavators which combines the design with planetary gears to power the rotation of the house at the point where it spins relative to the tracks.

Seeing the rotation of the piston barrel and the back and forth action of the pistons clearly illustrates how the axial piston pump works. » Click video to play/pause animation.


How Does an Axial Piston Pump Work?

To convert mechanical energy into hydraulic output, an axial piston pump utilizes a rotating, splined drive shaft that connects to and turns a piston barrel. To create the pumping mechanism of the pump, piston pumps can use either a swash plate design (featured in video) or a bent axis design. 

In both designs, as the pistons rotate they are repeatedly drawn away from a valve plate and then pushed closer to the valve plate. This variation in distance alters the size of the chamber available to hold hydraulic fluid. At times when the gap between the end of a specific piston and the valve plate is decreasing, the chamber will shorten, and hydraulic fluid will be expelled through the valve plate. As the piston rotates, it will eventually reach a point where the gap is increasing, leading to a longer chamber. This vacuum will cause hydraulic fluid to be drawn into the chamber through the valve plate.

Since the valve plate acts as a divider between the input and output sides of the pump, as the pistons and piston barrel rotate, hydraulic fluid will be continuously cycled through the pump as it is drawn from one connection and directed with force into another.

Since the rotating of the pistons and piston barrel is determined by the rotation of the shaft, the pump's output can be controlled by increasing and decreasing the speed of the shaft. In the swash plate design, further control of the pump is possible by adjusting the angle of the swash plate, changing the distance of the pistons from the valve plate, and, in turn, increasing or decreasing the size of the chamber available to hold hydraulic fluid.

Axial piston pump parts diagram
An expanded view of the internal parts of an axial piston pump shows how parts fit and work together to produce the pumping action.

What Are the Parts of an Axial Piston Pump?


Bearings on the shaft allow the shaft to rotate inside the housing of the pump with reduced friction.


The shaft distributes mechanical, rotational force to the pump. Splines on the shaft interconnect with splines in the piston barrel to turn the barrel and pistons while splines on the part of the shaft that extends from the housing connect to the machine. 


Pistons inside the pump rotate around the center shaft. Since the plane at which one end of the piston is attached is set at an angle determined by the swash plate, the pistons also vary their distance from the valve plate as they rotate. This variation causes a continuous alternation in the depth of the cavity available to hold hydraulic fluid inside the piston barrel and leads to their continuous looping through the pumping process.

Swash Plate

In an axial piston pump utilizing a swash plate design, the swash plate is responsible for setting the angle of the piston’s container and, in turn, the amount of variation in depth the pistons will move through. Altering the angle of the swash plate allows the action of the axial piston pump to be further controlled. 

Piston Barrel

The piston barrel contains the pistons and defines the center cavities through which hydraulic fluid moves as it creates the pumping flow.

Valve Plate

The valve plate sits on the end of the piston barrel opposite the pistons. Slots in the valve plate allow fluid to be directed to specific connections for intake and discharge.

An H&R tech has collected the parts of an axial piston pump and is readying for a rebuild in one of the Recon and Rebuild shops.

What Are Common Problems With an Axial Piston Pump?

Wear From Friction Over Time

Axial piston pumps feature a number of moving parts which always require lubrication and other techniques to decrease friction between moving surfaces. Because of the often rapid speed at which they operate, if an axial piston pump operates in an environment with less than ideal lubrication wear can happen rapidly and even lead to catastrophic failure.


An axial piston pump is often subject to repetitive, long-lasting, and high-pressure work, and with any part subject to those conditions, the buildup of heat over time is always a possibility. Overheating of the pump can be further amplified through inefficiencies developing inside the pump and issues with the overall hydraulic system with which the pump is connected. Examples of each would be: a bearing failure that forces the pump to work harder to maintain expected output and bubbles in the hydraulic fluid inside the pump (cavitation) from operating in a system low in fluid.

Failing Seals and Gaskets

Like any part in the hydraulic system, containing hydraulic fluid and directing it in very specific ways is necessary for consistent and expected functioning. If fluid is allowed to flow in unintended ways, the pump will lose efficiency or even lose the ability to provide adequate output. Seals and gaskets are used in axial piston pumps to ensure proper operation, but over time (or due to neglect) seals and gaskets can reach a state of failure that will affect the working ability of the pump.

How Can I Tell If My Piston Pump is Going Bad?

While a full determination of why an axial piston pump failed can involve a removal and disassembly of the part, often there are simple signs to watch for when one suggests an issue with an axial piston pump, namely:

Insufficient Power from the Pump

If the pump begins underperforming during operation and other issues that could affect output like loose hydraulic connections are eliminated, a lack of power can be a sign of internal problems in the pump.

Loud or Erratic Noise

Most axial piston pumps can be expected to create some level of noise, depending on size and design. A pump that has suddenly become louder or begins broadcasting an erratic noise can be a sign that internal parts of the pump are operating outside of proper conditions.

Excessive Vibration or Heat

Friction is almost always a byproduct of moving parts and, if unchecked inside the part, it will often show its effects on the outside of the part through heat and/or vibrations. While some heat and vibration is to be expected, especially if the pump is called upon to work for an extended period of time, excess vibration and heating are both a symptom of a problem and a possible escalation of issues.

Contaminants in the Fluid

Most hydraulic systems connect a number of parts in a machine and contamination from failure can often come from any of them. The discovery of fluid contamination can be combined with the previously mentioned signs to narrow issues to the pump.

An H&R tech is at work in the shop rebuilding an axial piston pump, a fairly common sight in the shop because of the wide use of axial piston pumps in construction equipment.

Here’s to hoping you read this article on axial piston pumps because of a pure curiosity about how they work and function. If though, you’ve arrived here in search of a diagnosis for axial piston pump problems, hopefully, with this information in hand you’re closer to solving your troubles.

As a top dismantler and parts rebuilder for construction equipment, axial piston pumps are a frequent rebuild project in the H&R Recon and Rebuild shops. Big parts to small, our parts technicians brings decades of experience to our rebuild project and we take pride in knowing that experience leads to a part that will outlast and outperform the competition. If you’re in search of a replacement axial piston pump, our Parts Specialist are here to help in your search. Just give them a call.