What is an Axle?

An axle is the central shaft for a rotating wheel or gear. It is an incredibly important part of any vehicle. Without it, the vehicle’s wheels would not move. They are as important as good brakes or tires. The axle is the bar that connects the wheels and makes them turn together. A bearing or bushing sits inside a central hole in the wheel to allow the wheel or gear to rotate around the axle. Axles also bear the weight of the vehicle or load. So, heavy haulers need a large number of axles to complete their hauls.

The axles serve only to transmit driving torque to the wheels. The position and angle of the wheel hubs is a function of the suspension system and is therefore independent of the axles.

There are multiple types of axles that modern trucks use to bear their heavy loads. A tandem axle is a group of two or more axles that sit close together. Truck designers use this configuration to provide a greater weight capacity than a single axle. Tandem axles at the rear are mainly used on semi-trailers.

Another type of axle commonly used on trucks is the straight axle. It is a single rigid shaft connecting a wheel on the left side of the vehicle to a wheel on the right side. Such a design keeps the wheel positions steady, so it can carry heavy loads. Some trailers also have split-axle designs, where the wheels on each side are attached to a separate shaft. Modern passenger cars have split-axle drive trains.

Axle Specifications

Axles are typically made from SAE grade 41xx steel or SAE grade 10xx steel. SAE grade 41xx steel is commonly known as “chrome-molybdenum steel,” while SAE grade 10xx steel is known as “carbon steel.” The primary difference between the two is that chrome-molybdenum steel is significantly more resistant to bending and breaking than carbon steel.

Trailers may have 5, 6, 7, 9, 10, 13, or even more axles. Straight axles are used for commercial trucks and heavy-duty off-road vehicles. The axle can optionally be protected and further reinforced by enclosing the length of the axle in a housing.

Broken axles are often the result of overloading a vehicle, a bad carrier bearing, or even just hitting a pothole. It is impossible to move a vehicle with a broken axle. If you hear rumbling or feel vibrations when you accelerate or turn your truck, this is a sign of an axle about to break.

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