How Does a Complete Axle Assembly Work

How Does a Complete Axle Assembly Work

View All Parts From This Machine: John Deere 724J Wheel Loader
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From a 3-axle articulated truck with giant, heavy axles to a tiny matchbox car with a thin wire shaft for axles, if it has wheels and moves, it probably has axles. At their most basic, axles simply connect the wheels of a machine together and (for powered wheel pairs) link the wheels to the drive system of the machine.

In complex machines, though, the complete axle assembly is more than simple shafts. Instead these heavy-duty parts are an amalgamation of complex parts designed to divide, smartly distribute, and efficiently transfer the forces from the drive system to the wheels while simultaneously bearing the considerable weight of the machine and a load. To pull this off, a number of parts must work together, and whether you’re here in search of a solution to your axle troubles or you’re just interested in how things work, seeing an axle in motion and understanding how it works will bring a better understanding to these important parts.


What Is an Axle?

While a basic axle is simply any shaft that connects two wheels together, a complete axle assembly in construction equipment consists of more complex parts than simply a shaft. For all axles though, the axle is the part that links together the wheels. If the wheels being linked will provide forward and reverse motion to the machine, the axle must also be responsible for changing the direction of force from the engine and dividing that force as it is sent to each wheel.

The axle in any machine must also be able to hold the weight of the machine as it operates (including any additional loads the machine takes on), as well as, stand up to impacts from ground elements since it sits in a precarious position on the underside of the machine.

What Do Axles Do?

A complete axle assembly on construction equipment performs a number of roles in the drive system of the machine. First the axle assembly must link wheels together and hold them in position as the machine traverses ground. This requires a heavy duty, rugged axle housing to protect the more delicate rotating parts of the axle. 

Often, wheels must be allowed to turn independently of one another while still being linked. This variable rotation speed allows the axle to avoid undue stress to the axle itself, as well as, the wheels and the entire machine in conditions where one wheel must turn faster than the other (like when cornering). To perform this task, a differential sits at the center of the axle assembly and allows each wheel to be engaged independently. 

Since the direction of the force must also be rotated perpendicularly before it can be applied to the wheels, the differential is responsible for this job, as well.

The force output from an engine and distributed to the axle shafts and the wheels often must be converted from a high-speed, low-torque force into a lower-speed, high-torque force. In large construction equipment where low-torque force is required to carry heavy loads or provide steady, firm forward motion, final drives connected to the axle housing and the wheels utilize gear reduction to achieve the required output.

an axle parts diagram
A cut-away of an axle shows the important parts inside of the complete axle assembly.

What Are the Parts of an Axle?

An axle assembly will be designed differently depending on the machine it is used on and whether or not the wheels being connected are powered by the drive system. In an axle assembly with powered wheels the basic assembly will consist of an axle housing, axle shafts, differential, and (in machines requiring high-torque output) final drives.

Axle Housing

The axle housing is a heavy-duty container for all the parts of the axle assembly. It must be designed to protect the rotating inner parts of the axle, connect to the machine, absorb impacts from the ground, and hold the weight of the machine.

Axle Shafts

Axle shafts are long rods with spline connections at each ends that run from the differential to the wheel hubs or final drives. These shafts must be designed to withstand constant rotational forces and sudden stopping and starting as they distribute forces to the wheels. 


In a differential, a large ring gear connects with the drive pinion of a shaft and rotates the force 90 degrees so that it can be applied to the axle shafts. If required, the differential also utilizes interconnected gears that allow for the shafts to turn simultaneously or independently, depending on external conditions.

Learn More About Differentials

Final Drives

In machines where high-torque output is required, final drives are fixed to each ends of the axle assembly. These final drives utilize a number of planetary gear systems and an external ring gear to allow gear reduction to convert the forces being delivered from the axle shafts.

Learn More About Final Drives

an overhead view of an articulated truck
A birds-eye of an articulated truck who has conveniently lost his dump box gives a view of the three axles that run across the machine.

How Many Axles Does a Machine Have?

A machine has as many axles as it has wheel sets (pairs, quartets, etc). For a wheel loader, there are four wheels, each as a wheel pair, giving the machine two axles. In an articulated truck with six wheels, there are 3 wheel pairs, giving the machine three axles. 


That’s the basics of an axle. Hopefully with this information in hand, you have a better understanding of the importance of an axle and the key roles it plays on a machine. If you’re still in search of information, drill down more into final drives and differentials.

If, instead of information, you’re searching for an axle solution to get your machine back up and running, we’re here to help with that, too. As a top dismantler of construction equipment with a reputation for the quality and reliability of our rebuilt axles, you’ll find an often unmatched inventory of available replacement axles and you can rest assured you’re purchasing an axle that will stand up to your work. Just give us a call.

Cat 988H axles
This pair of newly rebuilt Cat 988H axles are behemoths and required special handling for themselves and every part inside of them because of their size.