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The back axle
1、The final drive
Having dealt with the mechanical transmission system between the engine and output from the gearbox, we now turn to the three alternative final stages which are live axles, dead axles and axleless transmissions. A live axle is one that either rotates or houses shafts that rotate, while a dead axle is one that does neither, but simply carries at its ends the stub axles on which the wheels rotate.
The functions of turning the drive from the propeller shaft through 90° to distribute it to the two wheels, and of reducing the speed of rotation – thus increasing the torque – is performed by the gearing carried in the final drive unit, usually housed in the back axle. For relatively small reductions – up to about 7:1 – single-stage gearing is used; but for greater reductions, two or even three stages may be required, and the gearing for one or more of these stages
may be housed in the wheel hubs. The terms single-, double- and triple-reduction axles are therefore used.
Generally, the first stage is either a bevel pinion and what is termed the crown wheel, or a worm and worm-wheel, both of which of course turn the drive through 90°. Worm drives have the advantages of silence, either a low drive line or a high ground clearance – according to whether the worm is underslung or overslung relative to the wheel – ease of providing for a through drive to a second axle in tandem with the first, and the fact that a high single-reduction ratio can be readily provided – even as high as 15:1.
Bevel and hypoid bevel final drives are, however, far more common because they are less costly to manufacture and have a higher efficiency – the sliding action of worm teeth generates a lot of heat, especially if the gear ratio is high, and makes heavy demands on the lubricant. A hypoid bevel gear is one in which the axes of the crown wheel and the pinion are not in the same plane, and in which therefore some sliding action takes place between the meshing teeth. The one advantage is that a low propeller drive line can be obtained, so that the floor, and therefore centre of gravity, of the vehicle can be kept down.
2、Single-reduction live axles
An elementary single-reduction live axle – with a differential – is illustrated diagrammatically in Fig. 1. It has a hollow casing A, which carries on its ends the road wheels B. The weight of the body and load is supported by the casing A through the springs which are attached to the body and to the axle in a manner which will be described later. The casing in turn is supported at its ends by the road wheels. It therefore acts as a beam and is subjected to a bending action as is shown in Fig. 2, where the forces P are the supporting forces supplied by the road wheels, and the forces W are the body load, applied to the casing through the springs. The casing has to be stiff enough to withstand this bending action without undue flexure.
Supported in bearings in the casing A is a short shaft D integral with which is a bevel pinion E. The shaft D is coupled by means of a universal or flexible joint, outside the casing, to the propeller shaft and hence to the mainshaft of the gearbox. Inside the casing the bevel pinion E meshes with, and drives, a bevel wheel F which is fixed to a transverse shaft G. T
his shaft is supported in bearings HH in the casing and is bolted to the hubs of the road wheels B at its outer ends. Obviously, when the pinion shaft D is turned by the propeller shaft the drive is transmitted through the bevel wheel to the transverse shaft G and hence to the road wheels. The road wheels are kept in place on the casing A in the end direction by nuts J and shoulders K of the casing. Although a bevel gear drive is shown, the principle would have been similar – only the gear arrangement different – had a worm drive been used.
Fig. 1 Single-reduction axle
Fig. 2
3、Torque reaction
From Fig. 1, it can be seen that the propeller shaft applies to the shaft D a torque which, as it is transmitted through the bevel gearing, is increased in the same ratio as the speed is reduced. This increased torque is then transmitted through the shaft G to the road wheels. From Newton’s third law of motion, we know that action and reaction must be equal and opposite, so not only will this torque tend to rotate the wheel, but also the reaction from the wheel will tend to rotate the shaft G in the opposite sense. Therefore, there will be a tendency for the pinion and its shaft D to swing bodily around the crown wheel, and this tendency will be reacted by the axle casing. Some means therefore must be introduced to
prevent the axle casing from rotating in the opposite direction. This may be simply the leaf springs themselves, or additional links – torque-reaction or radius rods – may be used and will be essential if coil, instead of leaf, springs are employed.
Similarly, the axle casing will tend to rotate about the axis of the bevel pinion in a direction opposite to that of rotation of the propeller shaft. However, since the torque transmitted by the propeller shaft is less than that in the driveshafts, it can in most circumstances be reacted satisfactorily simply by the suspension springs.
4、reaction in the shaftDriving thrust
Again, according to Newton’s third law of motion, the driving thrust, or tractive effort, of the road wheels is reacted by the vehicle structure, the reaction being the inertia of the mass of the vehicle if it is accelerating, or rolling resistance of the other axle plus the wind resistance if it is not – the rolling resistance of the tyres of the driving axle involves of course purely local action and reaction. In effect, therefore, the driving axle has to push the carriage unit along, so it must be connected to the structure of the vehicle in such a way th
at this forward thrust can be transmitted from one to the other. This connection can be either the leaf springs or some other linkage for locating the axle relative to the carriage unit. The relevant members of this linkage are known as thrust members, or radius rods.

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