Two tie-rod assemblies (fig. 8-19) are used to fasten the center link to the steering knuckles. Ball sockets are used on both ends of the tie-rod assembly. An adjustment sleeve connects the inner and outer tie rods. These sleeves are tubular in design and threaded over the inner and outer tie rods. The adjusting sleeves provide a location for toe adjustment. Clamps and clamp bolts are used to secure the sleeve. Some manufacturers require the clamps be placed in a certain position in relation to the tie rod top or front surface to prevent interference with other components.
One purpose of the steering mechanism is to provide mechanical advantage. In a machine or mechanical device, it is the ratio of the output force to the input force applied to it. This means that a relativety small applied force can produce a much greater force at the other end of the device.
In the steering system, the operator applies a relatively small force to the steering wheel. This action results in a much larger steering force at the front wheels. For example, in one steering system, applying 10 pounds to the steering wheel can produce up to 270 pounds at the wheels. This increase in steering force is produced by the steering ratio.
The steering ratio is a number of degrees that the steering wheel must be turned to pivot the front wheels 1 degree. The higher the steering ratio (30:1 for example). the easier it is to steer the vehicle, all other things being equal. However, the higher steering ratio, the more the steering wheel has to be turned to achieve steering. With a 30:1 steering ratio, the steering wheel must turn 30 degrees to pivot the front wheels 1 degree.
Actual steering ratio varies greatly, depending on the type of vehicle and type of operation. High steering ratios are often catted stow steering because the steering wheel has to be turned many degrees to produce a small steering effect. Low steering ratios, called fast or quick steering require much less steering wheel movement to produce the desired steering effect.
Steering ratio is determined by two factors - steering-linkage ratio and the gear ratio in the steering mechanism. The relative length of the pitman arm and the steering arm determines the steering linkage ratio. The steering arm is bolted to the steering spindle at one end and connected to the steering linkage at the other.
When the effective lengths of the pitman arm and the steering arm are equal, the linkage has a ratio of 1:1. If the pitman arm is shorter or longer than the steering arm, the ratio is less than or more than 1:1. For example, the pitman arm is about twice as tong as the steering arm. This means that for every degree the pitman arm swings, the wheels will pivot about 2 degrees. Therefore, the steering linkage ratio is about 1:2.
Most of the steering ratio is developed in the steering mechanism. The ratio is due to the angle or pitch of the teeth on the worm gear to the angle or pitch on the sector gear.
Steering ratio is also determined somewhat by the effective length and shape of the teeth on the sector gear. In a rack-and-pinion steering system, the steering ratio is determined largely by the diameter of the pinion gear. The smatter the pinion, the higher the steering ratio. However, there is a limit to how small the pinion can be made.
Manual steering is considered to be entirely adequate for smatter vehicles. It is tight. fast, and accurate in maintaining steering control. However, larger and heavier engines. greater front overhang on larger vehicles, and a trend toward wide tread tires have increased the steering effort required. Steering mechanisms with higher gear ratios were tried, but dependable power steering systems were found to be the answer. There are several different types of manual steering systems, which are as follows:
Worm and sector
Worm and rotter
Cam and lever
Worm and nut
Rack and pinion
In the worm and sector steering gear (fig. 8-20), the pitman arm shaft carries the sector gear that meshes with the worm gear on the steering gear shaft. Only a sector of gear is used because it turns through an arc of approximately 70 degrees. The steering wheel turns the worm on the lower end of the steering gear shaft, which rotates the sector and the pitman arm through the use of the shaft. The worm is assembled between tapered rotter bearings that take up the thrust and toad.Continue Reading