Commercial cars are steered by means of hand wheels located at the upper end of the steering column. The spider of the steering wheels is secured to a shaft which generally passes down through an outer tube, usually called the mast. This shaft enters a housing at the lower end, which carries the steering mechanism that may either consist of a rack and pinion bevel pinion and bevel sector, worm and sector, worm and wheel, or screw and nut.
The steering column is generally styled according to the type of steering mechanism employed. One member of the mechanism has a shaft extending through the housing and carries the steering lever which is connected by means of a drag link to the steering arm on the front axle. Generally the steering motion is geared down so that one and one-half turns of the hand wheel will give the steering ball arm a motion of about 60 degrees, while the lever proportions are such as to give the wheels the maximum angle in either direction.
For commercial cars, especially those designed for heavy service, it is considered best to have the steering gear backlocking or irreversible, that is, so designed that any shocks received by the road wheels will not be transmitted to the operator's arms. The lighter vehicles permit the use of a slightly reversible mechanism, in which part of the shock is transmitted to the operator's arms, thus reducing the shocks transmitted to the steering mechanism.
Fig. 175. Rack and Pinion Type of Steering Gear.
Drag links are usually of the same proportions as the tie rod of the axle and the general practice is to provide cushion springs to absorb some of the shock which is transmitted to the steering mechanism. There are various constructions of either the steering mechanism or drag link in use at of completeness, it will be necessary to present some illustrations, if clearness is to be a property of the text.
The rack and pinion type of steering gear (Fig. 175) is perhaps the simplest type. It consists of a hollow steering shaft which carries the hand wheel at its upper end and a spur pinion within a housing at its lower end. This pinion meshes with a spur rack the end of which extends through the housing and carries a ball to which the drag link is attached. The column has an outer tube or mast which carries a l>earing for the steering shaft.
This type of steering mechanism is only used for cross steering and is completely reversible, so that the road shocks are transmitted to the operator's arms. It is practically limited to use on cars of 1.000-lb. capacity and under, and where the steering pivots can be so arranged that only part of the motion of the road wheel can be transmitted to the steering mechanism. It also possesses a disadvantage in that all the load is carried on one tooth, as the space in the chassis frame is not large enough to permit the proportions needed to have two or more teeth in mesh.
Fig. 176. Reo Bevel-Type Gear.
Bevel-pinion Type (Fig. 176) illustrates the bevel-pinion and sector type of steering used on the Reo 2-ton chassis. In this construction the steering shaft is made of solid section, which permits securely keying the hand wheel and bevel pinion to it. This, of course, is made possible by placing the spark and throttle control outside of the steering column and locating the levers below the hand wheel.
The bevel pinion meshes with a bevel gear sector which is attached to a horizontal shaft carrying the steering ball lever. The steering motion is limited by leaving a portion of the sector without teeth, while the thrust of the sector is taken on a steel roller, a spring being used to maintain the roller in contact with the sector. A large bracket forms the frame attachment and also carries the bearings for the horizontal shaft, the thrust roller and supports the lower end of the mast.
The spark and throttle controls, instead of having the usual sector near the hand' wheel, are controlled by means of friction members and springs, located below the foot board bracket, while the accelerator pedal is also mounted on this bracket.
Bevel gear steering mechanisms have less need for housings than other types, although in some cases they are enclosed. This type of gear is also completely reversible.
Fig. 177. Steering Gear of the Pierce-Arrow 5-Ton Truck.
Worm and Sector Type (Fig. 180) depicts the worm and sector type of steering used on the Vulcan 5-ton chassis. It has ball-thrust bearings and a hollow shaft, however, but one control, that of the throttle is built in the column, the spark being controlled by mechanism mounted on the dash. The lower end of the steering shaft having an integral worm. This worm meshes with a worm-gear sector mounted on a horizontal shaft and supported by bushings located in the horizontal divided case.
The customary position for the steering ball lever in the worm type of steering gears, is on the horizontal shaft; however, this position generally limits the motion of the front wheels on the side on which the steering gear is located, as the road wheel will come in contact with the drag link before it can touch any other part. With fore and aft steering the usual method of overcoming this is by placing the steering arm of the axle below the axle center and mounting the steering gear in such a manner as to permit the drag link to clear inside the front spring. However, this places the steering linkage in a position where it is practically the lowest point of the vehicle and very apt to become damaged by striking obstacles in the road. In heavy vehicles considerable clearance is usually allowed between the front spring and the frame, and in the Vulcan 5-ton truck this clearance is taken advantage of to protect the steering linkage, by passing the steering arm of the axle through this space so that the drag link comes inside the frame. However, owing to the limited amount of space between the engine and the frame, the steering ball lever cannot be mounted on the horizontal shaft outside of the housing. In order to overcome this, the worm sector of the steering gear has a boss which extends through case and carries the lever. In this way the lever is placed in approximately the center of the column and has ample space for its full movement.
Fig. 178. Peerless Steering Gear. Worm and Wheel Type.