Steering Systeml

Chapter 7 Steering system 7. 1 INTRODUCTION Primary function of the steering system is to achieve angular motion of the front wheels to negotiate a turn. This is done through linkage and steering gear which convert the rotary motion of the steering wheel into angular motion of the front road wheels. Secondary functions of steering system are : I. To provide directional stability of the vehicle when going straight ahead. . 2. To provide perfect steering condition, i.e., perfect rolling motion of the road wheels at all times. 3. To facilitate straight ahead recovery after completing a turn. 4. To minimize tyre wear. Till recently all vehicles were steered by turning the front wheels in the desired directions, with the rear: wheels following. However, lately all-wheelsteering has been designed and employed in some selected vehicles. Here only front wheel steering would be discussed which is being used universally till today. The requirements of a good steering system are: 1. The steering mechanism should be very accurate and easy to handle. 2. The effort required to steer should be minimal and must not be Tiresome to the driver. . 3. The steering mechanism should also provide directional stability. This Implies that the vehicle. Should have a tendency to return to its straight ahead Position after turning.

7.2 Components of a Steering system- Steering linkages

1.Rigid axle Suspension

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2.Independent Suspension Different types of steering linkages

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7.3 Steering axis (or kingpin) inclination Kingpins are inclined inward at the top. The centre line of the ball joints (or kingpin) is inclined to the vertical. This inclination is called steering axis inclination or ball joint angle for ball joint systems. It is called kingpin inclination for kingpin systems. The steering axle inclination in the present day vehicles ranges from 3.5-8.50 and its average value is 50. Kingpin inclination is usually built into the axle ends. This inclination has the effect of placing the turning point at the centre of the tyre tread instead of inside the wheel. This makes possible more stable steering as the wheel has the tendency to swing around the kingpin when it strikes a bump. Kingpin inclination has a pronounced effect on the steering effort and return ability. As the front wheels are turned around an inclined steering axis or kingpin, the front of the vehicle is lifted. This lifting of the vehicle is experienced as the turning effort when the turn is executed and exhibits itself as recovery force when the steering wheel is released. In this way it helps to provide steering stability. It also reduces steering effort especially when the vehicle is stationary. In addition, it reduces tyre wear.

Camber The front wheels are generally not mounted parallel to each other. Camber is the angle of inclination of the front wheel tyre with respect to the vertical. Camber provided may be positive or negative.

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When the wheel tilt is outward i.e. when the distance between the top of the wheels is greater than the distance at the ground, the camber is positive. This positive camber is built into the wheel spindle by forming the spindle with a downward tilt. When the tilt of the front wheel tyre to the vertical is inward, the wheels are closer together at the top than at the bottom, the camber is negative. The amount of the tilt of the front wheel tyre is measured in degrees from the vertical. This measurement is called camber angle. If the wheels are vertical to the road, the condition is referred to as zero camber.

Camber is built into the front wheels for the following reasons 1. To place the load more nearly on the inner bearing of the wheel. 2. To avoid reverse camber (wheels leaning inward at the top) as the spindle parts wear. 3. To reduce the side thrust on the kingpin. 4. To compensate the centre of the wheel rotation plane being outside of the centre line of the kingpin. The camber angle is generally less than 30. A cambered wheel tries to roll around the point defined by the intersection of the inclined axis of the tyre and ground. A cambered wheel must, therefore, be forced to roll around a point defined by the intersection of the inclined axis of the tyre and ground. A cambered wheel must, therefore, be forced to roll straight ahead, and unless camber is equal on both wheels, an imbalance of restraining forces results.

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When a camber exists, the restrained tyre also scrubs on the road. This is because; the tyre is compelled to follow the path straight down the road when its geometric rotation tendency is to roll in a circle about the tyres inclined axis and the ground. Therefore, zero camber is desired to eliminate the tyre wear caused by this scrubbing action. If the camber of the front wheel is set at zero in the manufacturing process, the effects of bearing clearances, axle deflection due to load after installation in the vehicle, and dynamic operating loads will results in negative camber during vehicle operation. Since the camber change with load, a slight amount of positive camber is usually incorporated in the front axle during fabrication. This results in a net camber of approximately zero when the vehicle is operated in normal design load.

Caster Figure below shows a side view of the caster angle formed between the vertical line and the knig pin inclination. Depending upon the manner in which the king pin is tilted, the caster may be of two different natures. Viz. positive caster and negative caster. The purpose of negative caster is to -

produce directional stability

-

to avoid or minimize the tendency of wheel wander

-

Avoid shimmy (i.e. oscillation of the front wheels).

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7.4 Toe-in and Toe-out

Figure 1

Figure 2 in the initial setting of the front wheels, carried out in the industry or the repairing garage, the front wheels are set closer at their front than at their rear at their stationary state when viewed from the top, as shown in the figure 2a. The difference in the amount of B and A is called toe-in i.e. B-A = toe-in. The opposite of it is the setting of the wheels as shown in the figure 2B in which the fronts of the front wheels are far-off than their rears. This is toe-out whose value is equal to the difference between A and B. Thus A-B = Toe-out. Purpose: - The toe-in is provided on all kinds of vehicle. The purpose of providing toe-in is to offset the tendency of wheel rolling. i.)

On the curves due to the limitation of correct steering,

ii.)

Due to possible play in the steering linkages,

iii.)

Due to the camber effect

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The toe-out is provided to counter the tendency of the inward rolling of the wheels i.)

Due to the soil condition on agricultural land

ii.)

On account of side thrusts

The amount of toe-in varies from 0 to 6 mm on different vehicles. It is 2 to 4 mm on Maruti 800 car, up to 6 mm on Swaraj Mazda, but 0 mm on Ashok Leyland Comet.

7.5 Center point steering

With a standard axle the point of intersection of the king pin axis with ground is different from the centre point of the tyre contact path as shown in figure (a). This result in heavy steering because the wheel has to be moved along the king pin axis in an arc of radius equal to the king pin off-set (called the scrub radius). Moreover, this also results in larger bending stress on stub axle and king pin. In order to avoid this, the wheel and the king pin are arranged to reduce the king pin off-set. When the king pin off-set is eliminated, i.e. outer line of the wheel

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meets the centerline of the king pin at the road surface, the condition is termed as center-point steering. This is shown in figure b. Center point steering results in much reduced steering effort and seems to be ideal. However the spread effect of the pneumatic tyres causes the wheels to scrub and give hard steering and tyre wear. so slide rolling action is provided by arranging the king pin off-set to be 10 to 25 percent of the tyre tread width. Scrub Radius or king pin offset radius: The action points of tractive force and the road resistance are shown in the above figure. The tractive force FTr acts at a point A while the road resistance RRO at the point B. The distance between these two points is called scrub radius. It is expressed in mm. Effects: •

Wheels are turned away from rolling straight due to the torque. The torque is created on account of tractive force and the road resistance acting in different lines of action and opposite directions. This torque is of opposite nature: clockwise and anti-clockwise in the two cases shown in figure (a) and (c)

•

The tendency of toe-in and toe-out is produced in cases of negative and positive scrub radii. Respectively.

•

When the tractive force and the road resistance act in the same line of action, the torque is not produced. The effects of toe in and toe out are also not experienced by the vehicle. The front wheels move straight in this case, and the conditions of true centre point steering is achieved.

Preferred choice: Among these cases, the case shown in figure ‘c’ is most preferred but with smaller scrub radius. Generally AB = 8 to 12 mm is preferred. A value greater than this, will invite a larger torque wheel to turn the wheel. Consequently, the

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load on the steering linkage and the suspension system will increase unnecessarily. This will result in greater wear of parts, unequal braking on the front, uncomfortable and unsafe driving.

7.6 STEERING GEARBOX 1. Worm and Sector Steering Gear

The worm sector is a fractional part of the worm-wheel as shown in the figure and is often mounted above the worm. Since the worm sector is smaller than the worm-wheel, it is cheaper and easier to install and also occupies less space.

2. Worm and worm wheel The worm wheel is carried in bearings in the cast iron case. The case is made in halves. The outer end of the spindle which carries the worm wheel is squared to receive the drop arm. The drop arm is connected by the drag link to a steering arm fixed to one of the stub axles. As such any motion given to the worm wheel will result in the motion of the stub axle. The worm wheel meshes with the worm.

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The worm is keyed on to a tubular shaft which is carried in two thrust bearings in the casing. The tubular shaft at its upper end carries the steering wheels. The two thrust bearings position the worm in the axial direction. The upper thrust bearing abuts against the casing. The lower thrust bearings abut against the nuts which is screwed into the casing and locked by a lock nut. This provides an adjustment for eliminating the end play of the worm.

3. Cam and roller As the cam rotates the roller is compelled to follow the helix of the groove and in doing so causes the rocker shaft to rotate, thus moving the drop arm as shown in the figure. The contour of the cam is designed to match the arc made by the roller, so maintaining a constant depth of mesh and evenly distributing the load and wear on the mating parts.

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4. Cam and the Peg Attached to the rocker arm is a taper peg which engages in the cam as shown in the figure. When the cam rotates, the peg moves along the groove causing the rocker shaft to rotate.

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5. Re-circulating Ball The circulating ball steering mechanism is an improved version of the now obsolete worm and nut. The balls are contained in a half nut and transfer tube as shown in the figure. As the cam, or worm, rotates the ball pass from one side of the nut through the transfer tube to the opposite side as the nut cannot turn, any movement of the balls along the track of the cam carries the nut along with it and rotates the rocker shaft. This type of box is efficient because of the small friction

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6. Rack and pinion A pinion, mounted on the end of the steering shaft, engages with a rack which has ball joints at each ends to allow for the rise and fall of the wheels. Tie-rods connect the ball joints to the stub-axles. Rotary movement of the steering wheel causes sideways movement of the rack which is directly conveyed to the wheels. With the previous steering systems, the off-side wheel, on a right hand drive car, is steered directly, while the near-side wheel is driven through the linkage, so that wear in the steering joints affects the near side the most; this does not occur with the rack and pinion steering. The latter arrangement provides a sufficiently low gear reduction for light saloon and sports cars and, when power assisted is suitable for heavier motor vehicles.

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LAW OF STEERING

Q

P

I

l w

o 0

o

R

S

Figure 1. Let

&

be the angle made by inner and outer stub axles.

l = Wheel base w = Distance between pivot of front axle

PI OI

Cot

=

Cot

– Cot

Cot =

=

QI OI

PI − QI PQ w = = OI OI l

This is called fundamental equation for correct steering. Mechanisms which fulfill this equation is called steering gears mechanism. Types of steering gear: 1. Davi’s Steering gear 2. Ackerman steering gear

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Ackerman steering gear PQNM constitute a 4 bar mechanism. Conditions for correct gearing is satisfied at three different positions: 1.) Vehicle turns to right, 2.) Vehicle turns to left, 3.) Steered on straight path Sin (α + θ) = Sin (α − φ =

y+z r y−z r

Sin (α + θ) + Sin (α − φ) = or

2y r

Sin (α + θ) + Sin (α − φ) = 2 Sin α

(Ref. fig. 2)

w

P

Q

Distance between pivots

r (track arm radius) Track rod

N

M

y

r 0

o y

z

z

y

Figure 2.

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Cot φ =

PI PG + GI = HI HI

Cot θ =

QI QG − IG = HI HI

2 IG 2GQ = = Cot φ − Cot θ= HI QR

w 2( ) 2 = w and hence satisfying the law of steering l l

Refer to figure 3.

P

G I o

Q 0

H True steering curve

l w

R

S Figure 3.

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Turning circle radius (TCR)

Q

P

Rif

R of

l w

o 0

Rri

o

M

N

R or

Figure 4.

a

All wheels rotate about a common center along different turning curves. TCR of outer front wheel Rof =

l a−w + sin φ 2

TCR of inner front wheel Rif =

l a−w − sin θ 2

TCR of outer rear wheel Ror = l cot φ +

a−w 2

TCR of inner rear wheel Rir = l cot θ −

a−w 2

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Minimum radius definition As per society of auto engines TCR is the radius of arc described by the center of the track made by the outer front wheel of the vehicle when making its shortest turn l − sin θ

Rof =

2

+ w2 +

2lw tan θ

+

a−w 2

Problems: Problem 1. A vehicle has pivot pins 1.4 m apart. Length of each track arm is 0.2 m and track rod is behind front axle and is 1.3 m long. Determine the wheel base which will give true rolling for all wheels when the vehicle is turning so that inner wheel stub axle is 600 to the centerline of the vehicle. Solution: θ = 90 - 60 = 300

w = 1.4 m

d = 1.3 m

r = 0.2 m Sin α =

w − d 1.4 − 1.3 = 2r 2 × 0.2

α = 14.47750

Sin (α + θ) Sin (α + φ) = 2 sin α Substituting for α and θ in the above equation

φ = 26.050 For correct steering Cot

– Cot

=

w l

Cot 26.050 – Cot 30 =

1.4 l

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Therefore wheel base l = 4.46 m Problem 2

A motor car has a wheel base of 2.75 m and pivot center of

1.08 m. The front and rear wheel track is 1.23 m. Calculate the correct angle of outside lock and turning circle radius of the outer front and inner rear wheels when the angle of inside lock is 400. Solution: Cot

– Cot

Cot

=

=

w l

1.08 - Cot 40 2.75

Therefore angle of outer lock

= 32.250.

Turning circle radius of outer front wheel Rof

=

l a−w + sin φ 2

=

2.75 1.23 −1.08 + sin 32.25 2

= 5.07 m Turning circle radius of inner rear wheel

Rir

= l cot θ − =

a−w 2

2.75 1.23 −1.08 − tan 40.25 2

= 3.2 m

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Exercise 7

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