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Explain Ackerman and Davis steering gear mechanism. Stating their advantages and limitations.

Subject: Automobile Engineering

Topic: Brakes, Steering & Front Axles

Difficulty: Medium

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Ackerman Steering gear mechanism is based on four bar chain with two longer link RS & AB of unequal length & two shorter link RA & SB of equal length. This linkage has only turning pair. THis ,mechanism is very simple.

Link RA & SB are integral with stub axle=r
length of track rod (link AB) = L When the vehicle is moving along a straight line the longer line (RS & AB) are parallel & each of the shorter link (RA & SB) is inclined at an angle $\alpha$ with the longitudinal axis of the vehicle. When a vehicle steer to left, the shorter line is to turn increase in the angle $\alpha$ as shown in fig 16.2 by dotted line. The obliquity of the track rod AB in turn position the movement A & B in the horizontal direction may be taken as same ( each equal to $x$). $sin(\alpha+\theta)=\frac{y+x}{r}$ ....eq1 $sin(\alpha-\phi)=\frac{y-x}{r}$ ....eq2 by adding equation 1 and 2 , we get $sin(\alpha+\theta)+sin(\alpha-\phi)=\frac{2y}{r} \mp 2sin\alpha$ ...eq 3 This mechanism gives only three position for correct steering one when $\theta =0$ & other two each corresponding to the turn to right or left. The Ackerman's mechanism very strictly on account of improvement in the suspension & tyre. Any small deviation from the true rolling angles can be corrected by the tyres sidewall flexibility & tread distortion. While parking, it is easy to sheer a vehicle in reverse than in forward direction because smaller is the turning circle radius for rear wheel than front wheel. Davis steering gear mechanism is an exact steering mechanism (fig 16.3). The slotted line AC & BD are attached to left & right wheel respectively & are arranged to turn on pivot at A & B respectively. The angle CAB & DBA are made equal. The line CD which is in front of the link AB, is at a distance 'h' from the line AB. The link CD is constrained to move in the direction of its length & parallel to AB the sliding constraints at L & M. The line CD is connected to the slotted levers AC & BD by a sliding & turning pair at each end. ![enter image description here] When the mechanism is in the mid position, with the wheels at orientation $w_1$ &$w _2$, the vehicle moves along a straight path. Steering is effected by moving the CD to the right or left of its normal position. Fig 16.3 shows the position of the various links when steering is effected to the left. Now, the wheels occupy the orientation $w_1$' & $w_2$'. Now , the axes of the left & right wheels ($w_1$ & $w_2$) make an angle $\theta$ & $\phi$ respectively as shown with the axis of the rear wheels extended to the left.

If the intersection of the three axis is at k as shown then the steering is correct, for all positions gear. this occurs if the two arms AC & BD intersect at a distance M infront of AB.