Steering Gear & Linkages

3.5 Steering Gear and Linkages:

Steering is done by providing a suitable gearing and linkage between front wheels and steering wheel. When steering wheel is rotated, the motion is transmitted to pitman arm through gear box. This motion is transmitted to drag link. Drag link transfers this movement to stub axle which rotates about king pin. This turns the right wheel. The left wheel is turned through the track rod and left hand track and arm.

Steering system is applied to allow a vehicle to follow the desired course. Automobile steering system evolution actually conforms to the course. Multi-hinge mechanism naturally increases degree of freedom and controllability, avoiding vibration. An automobile is steered with the help of steering gears and linkages, which transfer the motion of the hand operated steering wheel to the pivoted front wheel hubs via steering column. 

An integrated automobile steering system contains control mechanism, steering gearing and drive gearing. The steering control mechanism consists of steering wheel, steering shaft and column tube. Its function is to transfer power from the driver on steering wheel to the front wheels. There are two kinds of steering gearing – a mechanical steering gearing and power steering gearing. And the power steering gearing includes hydraulic transmission and pneumatic transmission, which will be discuss on later.

Steering gear converts the rotary movement of the steering wheel in to the angular turning of the front wheels. It also multiplies driver’s efforts and gives Mechanical Advantage. Steering gear ratio is between 10:1 to 24:1 and the actual value depending upon the type and weight of the vehicle.

Types of Steering Gear Boxes:

1.         Rack and Pinion type Steering Gear.

2.         Cam and lever Steering Gear.

3.         Worm and Sector Type Steering Gear.

4.         Worm and Roller Steering Gear.

5.         Worm & Nut type Steering Gear.

Steering Linkage is a connection of various links between the steering gear box and the steering arms attached to front or rear wheels, it applied by to the system of pivots and connecting parts. The motion of the pitman arm and steering gear box is transferred so the steering knuckles of the front wheels through the steering linkages. The swinging movement of the pitman arm from one side to the other side gives angular movement to the front wheel through the steering linkages. The steering linkage transfers the motion of the steering gear output shaft to the steering arms that turn the wheels to maneuver and control vehicle direction.

Steering Linkages are depends on the type of vehicle’s front suspension:

Steering linkage for vehicle with rigid axle front suspension (Parallelogram type)
Steering linkage for vehicle with rigid Independent front suspension.

In case of conventional rigid axle suspension , the main axle beam ensures the movement of stub axle in the horizontal plane only , there is no vertical deflection of the suspension and hence there is no change in the effective track-rod length.

In case of Independent suspension, the two stub axles can move up or down independent of each other due to which distance between ball-joint ends of the two track rod arms is continuously varying. Here three piece track rod is used, the center portion being called relay rod is used, which is connected at one end to the idler arm supported on body structure and to drop arm of steering gear at the other end through ball joints.

Components used in the steering linkage (as shown in fig 5&4) depend on the type of steering gear used in the vehicle. There are basically two types of steering gears used today. The rack and pinion steering gear and a steering gear box with a Pitman arm. The vast majority of modern vehicles use a rack and pinion steering gear. A steering gear box with a Pitman arm is still used on some trucks and other heavier vehicles. The steering linkage, depending on which steering gear is used, may consist of the following components; outer tie-rod end, inner tie-rod end, tie-rod, steering knuckle/arm, pitman arm, center link, drag link, idler arm, and steering damper or stabilizer.

A steering stabilizer may be found on some steering linkages. The steering stabilizer is similar to a shock absorber. This is connected from one of the steering linkage components to the chassis or frame of the vehicle. The steering stabilizer helps absorb road shock and prevents it from reaching the steering wheel.

Types of Steering Linkages:

1. Rack & Pinion Linkage

2. Parallel type Linkage

3. Haltenberg linkage

Connections and links:

The drop arm (also called Pitman arm) is rigidly connected to the cross-shaft of the steering gear at its upper end, while its lower end is connected to the link rod arm through a ball joint. Stub axle is rigidly attached to the other end of link rod arm. Each stub axle is has a forged track rod arm rigidly bolted to the wheel axis. The other end of track rod arms are connected to track rod by means of ball joints.

Fig 4: Steering Connections & Links

Fig 5: Links

    

The steering linkage is adjustable to be able to vary the toe setting and perform an alignment on the vehicle. This is necessary to account for tolerance stack-ups in frame and suspension components, component wear, and impact damage such as potholes. This can damage the steering linkage and change the toe setting instantly. The toe setting affects vehicle handling, tire wear, and fuel economy.

Steering linkage components should be inspected for looseness, torn or deteriorated grease seal boots, and other damage. The steering linkage is the most critical part of the steering system because a worn linkage can not only cause excessive tire wear, it can also become a serious safety and liability issue. For this reason, it is important to have the entire steering linkage inspected at least annually by certified technician.