From the transmission the power must be transmitted to another unit from which it is converted into useful work at the road wheels. In commercial car construction this is generally termed the final drive. There are a variety of methods of transmitting the power and in taking up the discussion of the final drive, the writer will attempt to cover this subject as clearly as possible and is offering illustrations of a number of different types in use at present. The general problem of the final drive resolves itself into the transmission of motion from one or more revolving shafts to driving wheels flexibly connected to the frame through axle and springs, and at the same time effecting a reduction in rotative speed between the driving shaft and rear wheels.
During the past veal's the majority of commercial cars were equipped with what is known as the double side-chain drive. The principal objection to it is the attention required to obtain maximum efficiency. It is generally exposed and dirt soon finds its way into the numerous bearings, causing rapid wear. For a time, chain cases seemed to be the solution of the problem, but they are not satisfactory and most makers started experimenting with different types of shaft drives. This has resulted in the introduction of the bevel gear, double reduction, internal gear and worm gear rear axles.
In chain drive the power must be transmitted to a unit carrying the driving sprockets, the differential and in some cases a set of brakes. This unit is generally termed the jack shaft and may be built integral with the transmission or in a separate unit, mounted separately or bolted to the transmission. This jack shaft is similar to and performs the same functions the bevel gear rear axle in pleasure cars, excepting, of course, that it does not carry the weight of the vehicle. For this purpose a dead rear axle is used, which has spindles upon which the wheels and their bearings are mounted. Various types of rear axles may be found in use at present, their section being either round, square, rectangular or I-beam. The jack shaft is usually equipped with one set of brakes, while the other set is mounted in the rear wheels. However, some makers mount both sets of brakes on the rear wheels.
Some method must be provided to take up the driving thrust from the rear axle to the frame. For this purpose a radius rod is provided, which also takes up the brake pull, and the reaction due to chain pull, as well as allowing for adjusting the slack in the chain. These rods are generally of the full universal type, being pivoted on the jack shaft and the brake support or spindle of the rear axle. They are sometimes provided with large coil springs to take up abnormal shocks, when engaging the clutch or backing up to a curb.
The chain drive in reality is a double reduction through two units, one reduction being obtained through the bevel gears in the jack shaft, while the other is obtained through the jack shaft and rear wheel sprockets.
Fig. 109 illustrates the jack shaft built by the Sheldon Axle and Spring Company, which was used on a number of commercial cars, being a separate unit so arranged that a standard transmission may be bolted to it. The differential is of the bevel-gear type, while the working parts are mounted on ball bearings.
Fig. 110 depicts the Velie jack shaft which is a separate unit, the transmission being mounted on a sub-frame, while the jack shaft is mounted on the main frame. Like the Sheldon construction, the driving unit is so arranged that it may be removed through the inspection cover opening without removing the jack shaft unit from the chassis. The differential lock is also shown, mounted on the right side of the differential housing. The jack shaft is flexibly mounted on the frame, while a torque arm is used to hold it in alignment. The differential and drive shafts are mounted on roller bearings. Internal expanding brakes are provided. The outboard bearing for the sprocket is mounted as close as possible to the chain center, so as to overcome the high tension in the chain on low gear.
In Fig. 111 is shown the Velie rear axle which is of round section, having a spring seat, which also carries the brake spider keyed to it by a large bolt. The spring is mounted above the axle and retained by spring clips. The radius rod is mounted inside of the spring and pivots on the rear axle spindle. Internal expanding brakes are mounted inside the brake drum, to which the driving sprockets are attached. This drum is attached to the spokes of the wood wheel by spoke clips, while the wheels are equipped with dual tires. Wheels have roller bearings.
Fig, 112 illustrates the radius rod used on the Atterbury chain-driven models. This rod is of the universal type, being pivoted and hinged to jack shaft at the forward end and hinged to the brake support at the rear end, which in turn is found to pivot on the rear axle spindle. An adjustment is provided at the forward end on which is mounted a heavy coil spring. The rod is constructed in two sections, so that the rear end may slide upon the forward end, the spring holding both ends in their positions. When the clutch is suddenly engaged or when backing up to a curb, this spring takes up the abnormal shock by compressing and permitting the rear section to slide over the forward one. As soon as the force is removed the spring expands and returns the rear end to its proper position. This type of radius rod is also used on the Velie and Lewis commercial cars.