Axle

An axle is a central shaft designed for a rotating wheel or a gear.

It is a fundamental component for almost all wheeled vehicles. On some vehicles, the axle is fixed to the wheels and rotates with them. In these cases, bearings or bushings are placed at the mounting points for support. On other vehicles, the axle is fixed to the vehicle itself. The wheels then rotate around the axle through a central hole. On a bicycle, this second type of axle is often called a spindle.

Axles function through different mechanical connections. A straight axle is a single, rigid shaft. It connects a wheel on the left side to a wheel on the right side. This design creates a common axis of rotation for both wheels. Because they keep wheel positions steady under heavy stress, they are very strong. Straight axles are used for locomotives, railway wagons, and commercial trucks.

In contrast, split-axle designs use separate shafts for each side. This allows for independent suspension, which provides a smoother ride. Split axles also allow for a differential. A differential lets the left and right wheels rotate at different speeds during a turn. This improves traction and extends the life of the tires.

When an axle is powered by an engine, it is called a drive axle. In many modern front-wheel drive cars, the transmission and the axle are combined into one unit. This unit is known as a transaxle. These systems often use constant-velocity (CV) joints. These joints allow the wheel assembly to move vertically and pivot during turns.

In rear-wheel drive vehicles, the engine turns a driveshaft. This shaft transmits rotational force to the rear drive axle. Some simple vehicles, like leisure go-karts, may only drive one wheel. Other go-karts might drive both rear wheels using a single shaft without a differential.

Heavy-duty trucks often utilize a special component called a lift axle. This is also known as an airlift or drop axle. A driver can mechanically raise or lower this axle to change its function. Lowering the axle increases weight capacity by distributing cargo over more wheels. This is helpful when crossing a bridge with weight restrictions. Lifting the axle saves fuel and reduces tire wear when it is not needed. It also helps the vehicle turn more easily in tight spaces. Lift axles have been used since the early 1940s. Early systems used mechanical devices, but hydraulics soon replaced them. One early manufacturer was Zetterbergs in Sweden. In 1957, the Finnish company Vanajan Autotehdas invented the liftable tandem drive axle.

Axles must be built from very strong materials to handle heavy loads. They are typically made from SAE grade 41xx or 10xx steel. SAE grade 41xx is often called chrome-molybdenum steel. This material is highly resistant to bending or breaking. However, it is very difficult to weld outside of a professional shop. SAE grade 10xx is commonly known as carbon steel. The choice of steel depends on the specific strength requirements of the vehicle.

Engineers also design axles with different weight-bearing structures. A full-floating axle carries the vehicle's weight on the axle casing. The internal half-shafts only serve to transmit torque. Because the weight is on the casing, the shafts do not face lateral bending forces. This design is common in medium-duty and heavy-duty trucks. A major safety benefit is that the wheel will not fall off if a shaft breaks. In contrast, a semi-floating axle carries the vehicle's weight on the axle shaft itself. This design is common in lighter trucks, SUVs, and passenger cars. Semi-floating axles are often less expensive but require the wheel to be removed to reach the shaft. If a semi-floating shaft breaks, the wheel could come off the vehicle.

Axles are part of a much larger mechanical system. They work alongside suspensions, transmissions, and differentials to move a vehicle. In some contexts, the term "axle" is used loosely to describe any pair of parallel wheels. This is often used when calculating vehicle taxes or toll road costs. These calculations use the number of axles as a proxy for weight-bearing capacity. This helps officials estimate how much wear a vehicle might cause to roadway surfaces.