Shaft-driven commercial car axles may be classified according to the arrangement of the wheel bearings. If the end of the drive shaft next to the wheel has a bearing directly upon it, the axle is then classed as semi-floating type. This type of axles possesses some advantages in cost of manufacture and simplicity, and is the lightest axle for its strength.
The great strains imposed upon the bearings and housing of the full-floating type in skidding against obstructions are much leas in the semi-floating type, because of the greater distance between bearings. However, there is much difference of opinion as to the relative advantages of the various types of axles.
In the semi-floating axle, the drive shaft carries the weight of the vehicle and must also resist torsional strain.
In order to make an axle in which only torsional driving strains are imposed upon the drive shaft - thus approximating some advantages of the dead rear axle - a construction is used wherein each wheel is mounted outside the axle housing, and the drive is taken through a central shaft connected to the hub by either a jaw clutch or flange bolted to the hub. This is known as the full-floating type of axle. It has the advantage of having the axle shaft free from all lateral strains, thus greatly reducing the danger of breakage and of bending, the latter causing the wheel to wabble. Even if the shaft is broken, the wheel is still securely mounted on the axle housing, so that the axle does not drop to the ground. The construction is such that the drive shafts can be withdrawn and even the differential dismounted without removing the wheels from the axle, or the axle from the car.
The jaw clutch for driving the wheel has the advantage of being more easily withdrawn from the axle and affords more freedom to allow for misaligning parts, while the flange drive removes all chances of noise sometimes made by jaw clutches, and is claimed to be sligthly less expensive to construct.
The three-quarter-floating axle is a compromise between the semi-floating and full-floating types. It has the wheel mounted on a single bearing outside the axle housing on a reinforcing tube, and kept in alignment by being rigidly attached to the driving shaft by either a key, as in the semi-floating type, or a flanged connection as in one style of full-floating axle. According to the type of bearing employed at the wheel, the axle shaft is held in place either by the wheel bearing or by some sort of a lock near the differential.
As in most compromises, this type possesses some of the advantages of each of the others. No dead load is placed on the axle shaft, but the skidding strains are little, if any, different on the bearings and shafts than with the semi-floating type. As in the semi-floating axle, any type of bearing can be used, and the weight may be less than the full-floating axle. When the wheel bearings are of a type suitable to take end thrust, they are usually so mounted as to hold the wheel in place on the axle end, and the shaft is connected by a flange. The shaft can then be removed without disturbing the wheel.
Worm and bevel-gear axles are similar to pleasure car axles, and various parts of the other two types are also similar; however, while they are similar, their proportions are materially increased to withstand vibration.
At the present writing there seem to be about five methods of providing for torque and thrust loads on shaft-driven axles, as follows: (1) torque and thrust through the vehicle springs; (2) torque and thrust through triangular structure attached to a rigid cross member; (3) torque on springs, thrust on radius rods; (4) separate torque and radius rod members; (5) torque tube surrounding drive shaft and anchored to the frame by a large yoke and triangular radius rods anchored to the torque tube.
In a shaft-driven axle there are two separate torsional forces. There is first, the primary torque of the drive shaft, and the secondary torque of the axle itself. The inertia of the axle causes the shaft to react upon the gear set support in a tendency to whirl them instead of itself turning, and the inertia of the wheels on the road tends to cause the axle to whirl, instead of the wheels revolving. Owing to the reduction afforded by the gear set on the lower gears, there is a certain amount of drive-shaft torsion, even on a heavy vehicle. It is no greater, however, the motor being of the same power as in a passenger vehicle, as a rule, and, as in the latter, it can be transmitted directly to the frame, the springs being the medium relied upon to absorb it.
The axle torque offers the greatest problem. In some vehicles the springs or the ordinary form of torque arm supported from the frame have proven successful, while in other cases torsion is provided for by connecting the sub-frame, upon which the complete power plant and transmission system are mounted, to the rear axle at one point and to the forward part of the frame on two points. Propulsion may be through the swinging sub-frame, torque tube, springs or radius rods. When radius rods are used, the springs are shackled at both ends, and when the thrust is taken through the springs the front end is rigidly attached to the frame and the rear end is shackled.