Steering System Gear Mechanisms
Their are two types of Steering gear mechanism –
- Fifth wheel steering system
- Side pivot steering system
Side pivot is further divided into two –
- Davis Steering Gear
Ackerman Steering Gear
Fifth wheel steering system:
It is single pivot steering system in which the front axle along with the wheels, moves to right or left. The movement to the whole axle and wheel assembly is affected by means of a steering and a wheel which is placed between chassis frame and axle. The fifth wheel acts as a turntable. The axle assembly is connected with the frame by means of a pin which serves as a pivot around which the axle assembly moves. The fifth wheel contains a ring gear mounted at its rim and is moved by means of a steering. Movement of the steering wheel tends the front axle and wheel assembly to move away.
Side pivot steering mechanism:
The main difference between the two steering gear mechanisms is that the Davis steering has sliding pairs, whereas the Ackermann steering has only turning pairs. The sliding pair has more friction than the turning pair; therefore the Davis steering gear will wear out earlier and become inaccurate after certain time. The Ackermann steering gear is not mathematically accurate except in three positions, contrary to the Davis steering gear which is mathematically correct in all positions. However, the Ackermann steering gear is preferred to the Davis steering gear.
Davis Steering Gear:
The Davis Steering gear has sliding pair, it has more friction than the turning pair, there fore the Davis Steering Gear wear out
earlier and become inaccurate after certain time. This type is mathematically accurate.
The Davis gear mechanism consists of a cross link KL sliding parallel to another link AB and is connected to the stub axles of the two front wheels by means of two similar bell crank levers ACK and DBK pivoted at A and B respectively. The cross link KL slides in slides in the bearing and carries pins at its end K and L. The slide blocks are pivoted on these pins and move with the turning of bell crank levers as the steering wheel is when the vehicle is running straight, the gear said to in its mid-position. The short arms AK and BL are inclined an angle 90+alpha to their stub axles AC and BD. The correct steering depends upon a suitab1e selection of cross-arm angle alpha, and is given by
tan (alpha) = b / 2l
Where b=AB=distance between the pivots of front axles.
The range of b / l is 0.4 to 0.5 hence angle alpha lies between 11.3 and 14.10.
Ackermann Steering Gear
Ackermann Steering Gear has only turning pair. It is not mathematically accurate except in three positions. The track arms are made inclined so that if the axles are extended they will meet on the longitudinal axis of the car near rear axle. This system is called ackermann steering.
The Ackermann steering gear mechanism consists of a cross link KL connected to the short axles AC and BD of the two front wheels through the short arms AK and BL, forming bell crank levers CAK and DBL respectively. When the vehicle is running straight, the crosslink KL is parallel to AB, the short arm AK and BL both make angle alpha to the horizontal axis of chassis. In order to satisfy the fundamental equation for correct steering, the links AK and KL are suitably proportioned and angle alpha is suitably selected. For correct steering
cot (phi) – cot (theta) = b / l
The angles (ph)i and (theta) are shown in the Figure. The value of b / l is between 0.4 and 0.5, generally 0.455. The value of cot (phi) – cot (theta) corresponds to the positions when the steering is correct. In fact there are three values of angle( theta) which give correct steering of the vehicle: first while it is turning to right, second while it is turning to left and third while it is running straight.
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3. Types of steering system