A tractor axle differs from that of an automobile or heavy truck in that the axle housing is commonly part of the tractor chassis. There are very few tractors that use a separate axle housing in the construction of a tractor axle. Without a moving suspension system, a tractor does not require the independence of a separate axle housing in its design. The tractor axle typically passes through the rear of the tractor chassis with all of the gears and braking components located inside of the chassis. This protects the components from the dirt, mud and other elements commonly surrounding the tractor's use.
An automobile or heavy truck has the brake shoes or disks located behind the wheels of the vehicle, while a tractor does not. Most tractor designs use an independent braking system, with the left and right brakes being operated through separate and independent brake pedals mounted in the tractor's operator's cab or seating area. The brake shoes are located on the inside of the tractor's chassis and are protected from the elements. If the tractor axle had the brakes outside of the chassis, they could be subject to road dirt and mud, which would damage them. The most difficult part of this type of tractor axle design is that the tractor's chassis must be split in two in order to perform any maintenance on the brakes, axles or gears.
The axle shafts of a tractor axle are extremely large and heavy to permit the large amounts of torque and pulling pressure that a tractor is nearly always under. The amount of leverage placed on the tractor axle by the very large-diameter tires used on the rear of a tractor would easily twist a small diameter axle off. Many tractors use double and triple tires on each side of the tractor axle, creating even more torque and twisting force on the axle.
The tractor axle is not designed for speed, so when a tractor is modified for use as a pulling tractor, the rear axle is nearly always replaced with the internals of a semi truck or a log skidder. This enables the high-performance tractor to create much faster wheel speed and subsequently more momentum and inertia, which equate to longer pulls. While the actual axle shaft is not the problem in a high-performance pulling application, the tractors do not have sufficient gear sets to permit the rear tires to spin fast enough to be successful in the pulling world.