Steering Mechanism

Steering Mechanism

Steering is the collection of components, linkages, etc. which allows any vehicle (car, motorcycle, bicycle) to follow the desired course. The primary purpose usually of the steering system is to allow the driver to guide the vehicle.

The perfect steering is achieved when all the four wheels are rolling perfectly under all conditions of running, thus we need a correct steering angle. While taking turns, the condition of perfect rolling is satisfied if the axes of the front wheels when produced meet the rear wheel axis at one point. Then this point is the instantaneous centre of the vehicle. It is seen that the inside wheel is required to turn through a greater angle than the outer wheel. The larger the steering angle the smaller is the turning circle. There is however a maximum to which we can go as regards to this angle. It has been found that the steering angle (of the inner wheel) can have a maximum value of about 44 degrees. The extreme positions on either side are called 'lock' positions. The diameter of the smallest circle which the outer front wheel of the car can traverse and obtained when the wheels are at their extreme positions is known as 'turning circle'.

For the basic condition for the steering mechanism for perfect rolling if all the wheels are mathematically represented as:

cot(x) - cot (y) = c/b

Where,

x= relative angle of the outside wheel

y= relative angle of the inside wheel

c= lateral wheel separation

b= longitudinal wheel separation

The basic aim of steering is to ensure that the wheels are pointing in the desired directions. This is typically achieved by a series of linkages, rods, pivots and gears. One of the fundamental concepts is that of caster angle – each wheel is steered with a pivot point ahead of the wheel; this makes the steering tend to be self-centering towards the direction of travel.

The steering linkages connecting the steering box and the wheels usually conform to a variation of Ackermann steering geometry, to account for the fact that in a turn, the inner wheel is actually travelling a path of smaller radius than the outer wheel, so that the degree of toe suitable for driving in a straight path is not suitable for turns. The angle the wheels make with the vertical plane also influences steering dynamics as do the tires.

So, based on the overall geometry, majorly, there are two types of Steering 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:

There are two types of steering gear mechanisms:

1. Davis steering gear mechanism

2. Ackerman steering gear 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 Mechanism:

Davis steering gear is an exact steering gear mechanism. It has two sliding pairs and two turning pairs. In this mechanism, the slotted links are attached to the front wheel axle, which turn about two pivotal points. It has the rod and it is constrained to move in the direction of its length by the sliding two members. These constraints are connected to the slotted link by a sliding and a turning pair at each end. The main drawback in Davis steering mechanism is tear and wear problem of sliding pairs. The drawbacks in Davis steering mechanism are overcome by Ackermann steering gear mechanism

This steering gear mechanism is shown in Figure given sub part (a). It consists of the main axle AC having a parallel bar MN at a distance h. The steering is accomplished by sliding bar MN within the guides (shown) either to left or to the right-hand side. KAB and LCD are two bell-crank levers pivoted with the main axle at A and C respectively such that ∠BAK and ∠DCL remain always constant. Arms AK and CL have been provided with slots and these house die-blocks M and N. With the movement of bar MN at the fixed height, it is the slotted arms AK and CL which side relative to the die-blocks M and N. In Figure, the vehicle has been shown as moving in a straight path and both the slotted arms are inclined at an angle αα as shown.

Now suppose, for giving a turn to the right-hand side, the base MN is moved to the right side by distance x. The bell-crank levers will change to the positions shown by dotted lines in figure sub part (b). The angle turned by the inner wheel and the outer wheels are θ and ϕ respectively. The arms BA and CD when produced will meet say at I, which will be the instantaneous centre.

REFER THE IMAGE TO SEE THE MATHEMATICAL PROCEEDING

The ratio a/ l varies from 0.4 to 0.5 and correspondingly α to 14.1 degrees. The demerits of the Davis gear are that due to number of sliding pairs, friction is high and this causes wear and tear at contact surfaces rapidly, resulting in in-accuracy of its working.

Ackerman Steering Mechanism:

The Ackerman steering gear mechanism is much simpler than Davis gear. The difference between the Ackerman and Davis steering gears are:

1. The whole mechanism of the Ackerman steering gear is on back of the front wheels where as in Davis steering gear, it is in front of the wheels.

2. The Ackerman steering gear consists of turning pairs, whereas Davis steering gear consists of sliding members.

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 mechanism is shown in Figure subpart (a). This is simpler than that of the Davis steering gear system. It is based upon four-bar chain. The two opposite links AC and MN are unequal; AC being longer than MN. The other two opposite links AM and CN are equal in length. When the vehicle is moving on a straight path link AC and MN are parallel to each other. The shorter links AM and CN are inclined at angle α to the longitudinal axis of the vehicle as shown. AB and CD are stub axles but integral part of AM and CN such that BAM and DCN are bell-crank levers pivoted at A and C. Link AM and CN are known as track arms and the link MN as track rod. The track rod is moved towards left- or right-hand sides for steering. For steering a vehicle on right hand side, link NM is moved towards left hand side with the result that the link CN turns clockwise. Thus, the angle α is increased and that on the other side, it is decreased. From the arrangement of the links it is clear that the link CN or the inner wheel will turn by an angle θ which is more than the angle of turn of the outer wheel or the link AM.

To satisfy the basic equation of steering : cotθ - cotϕ = a/l

The links AM and MN are suitably proportioned and the angle α is suitable selected. In a given automobile, with known dimensions of the four-bar links, angle α is known. For different angle of turn θ, the corresponding value of ϕ are noted. This is done by actually drawing the mechanism to a scale. Thus, for different values of θ, the corresponding value of ϕ and (cotθ - cotϕ) are tabulated. As given above, for correct steering,

cotϕ - cotθ = a / l

Generally, it is 0.455. In fact, there are three values of θ which give correct steering; one when θ = 0, second and third for corresponding turning to the right and the left hand.

Thus, the Mechanism based on the geometry are stated above. Further to achieve these geometries a wide area of applications can be studied. These classifications can be:

Steering Systems:

Manual

1) Rack and Pinion

2) Worm and Roller

3) Re-Circulating Ball and Nut

Power

1) Hydraulic

a) Rack and Pinion

b) Worm and Roller

c) Re-Circulating Ball and Nut

d) Hydrostatic

2) Electrical/ Hydro-Electrical

a) Rack and Pinion Electric Powered System

b) Column Driven Electric Powered System

c) Pinion Driven Electric Powered System

d) Rack Driven Electric Powered System

With wide scale requirements power steering have gained much importance in market.