A live axle is one that supports part of the weight of a vehicle and drives the wheels connected to it. A dead axle is one that carries part of the weight of a vehicle but does not drive the wheels. The wheels rotate on the ends of the dead axle.

Usually, the front axle of a passenger car is a dead axle and the rear axle is a live axle. In four-wheel drive vehicles, both front and rear axles are live axles; in six-wheel drive vehicles, all three axles are live axles. The third axle, part of a bogie drive, is joined to the rearmost axle by a trunnion axle. The trunnion axle attaches rigidly to the frame. Its purpose is to help distribute the load on the rear of the vehicle to the two live axles that it connects.

Four types of live axles are used in automotive and construction equipment. They are: plain, semifloating, three-quarter floating, and full floating.

The plain live, or nonfloating, rear axle, is seldom used in equipment today. The axle shafts in this assembly are called nonfloating because they are supported directly in bearings located in the center and ends of the axle housing. In addition to turning the wheels, these shafts carry the entire load of the vehicle on their outer ends. Plain axles also support the weight of the differential case.

The axle shafts in a three-quarter floating axle (fig. 13-22) may be removed with the wheels, keyed to the tapered outer ends of the shafts. The inner ends of the shaft are carried as in a semifloating axle. The axle housing, instead of the shafts, carries the weight of the vehicle because the wheels are supported by bearings on the outer ends of the housing. However, axle shafts must take the stresses caused by the turning, skidding, and wobbling of the wheels. Three-quarter floating axles are used in some trucks, but in very few passenger cars. Most heavy trucks have a full floating axle (fig. 13-23). These axle shafts may be removed and replaced without removing the wheels or disturbing the differential. Each wheel is carried on the end of the axle tube on two ball bearings or roller bearings, and the axle shafts are not rigidly connected to the wheels. The wheels are driven through a clutch arrangement or flange on the ends of the axle shaft that is bolted to the outside of the wheel hub. The bolted connection between the axle and wheel does not make this assembly a true full floating axle, but nevertheless, it is called a floating axle. A true full floating axle transmits only turning effort, or torque.

Chapter 13 explained how power developed by the engine is transmitted to perform the work required of it. It discussed the following mechanisms of the power train:

The clutch is incorporated in the powertrain to provide a means of disconnecting the power of the engine from the driving wheels and accessory equipment.

The transmission transfers engine power from the clutch shaft to the propeller shaft and allows the operator to control the power and speed of the vehicle by selecting various gear ratios.

Transfer cases provide the necessary offsets for additional propeller shaft connections to drive the wheels.

Propeller shaft assemblies provide a flexible connection through which power is transmitted from the transmission to the axle.

Axles are used to support part of the weight of a vehicle; they also drive the wheels connected to them.