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.
