Q.7.(a) State the requirement of steering gear box ? Explain worm and roller steering gear.
A worm and roller steering gear is shown in Fig.
In this system the worm is at the end of the steering column. The diameter of the more at its ends. Its diameter is gradually reduced at the centre. A roller is in mesh with this wom, The roller has a cross shaft which is fitted to drop arm.When the steering wheel is rotated, the column also rotates. Also, when the worm of the shaft is rotated, the roller rotates in an arc. At this time the cross shaft of the rollers moves. This makes the drop arm move in an arc, which in turn rotates the wheels. This type of steering gear is fitted in Leyland vehicles and American passenger care.
The angle between the centre line of the tyre and the vertical line when viewed from the front of the vehicle is known as camber. When the angle is outward, the camber is positive. When the angle is inward, the camber is negative. Any amount of camber, positive or negative, tends to cause uneven or move tyre wear on one side that fori the other side. Camber should not exceed 2°.
The front wheels are not usually mounted parallel to each other, but are fitted outward slightly at the top and inward at the bottom to have positive camber. The purpose of the camber is to prevent the top of the wheels from tilting inward too much because of excessive loads or play in the king pins and wheel bearing. When the vehicle is loaded and rolling along on the road, the load will just about bring the wheels to a vertical position. On vehicles having negative chamber the opposite is true.
Excessive camber prevents the tyre from having correct contact with the road which causes it to wear only on the side directly beneath the load. Unequal camber causes vehicle to roll in the direction of the wheel having greater camber which upsets directional stability and tends to scurb the tread on the opposite tyre.