Steering Systems,Principles of steering, Rack-and-pinion steering, Rack-and-pinion steering system Recirculating ball & nut steering system

Steering systems

The direction of motion of a motor vehicle is controlled by a steering system.
A basic steering system has 3 main parts: A steering box connected to the steering wheel. The linkage connecting the steering box to the wheel assemblies at the front wheels. And front suspension parts to let the wheel assemblies pivot.
When the driver turns the steering wheel, a shaft from the steering column turns a steering gear. The steering gear moves tie rods that connect to the front wheels. The tie rods move the front wheels to turn the vehicle right or left.
There are 2 basic types of steering boxes - those with rack-and-pinion gearing, and those with worm gearing. In both cases, the gearing in the steering box makes it easier for the driver to turn the steering wheel, and hence, the wheels.

A rack-and-pinion steering system has a steering wheel, a main-shaft, universal joints, and an intermediate shaft. When the steering is turned, movement is transferred by the shafts to the pinion. The pinion is meshed with the teeth of the rack, so pinion rotation moves the rack from side to side. This type of steering is used on passenger vehicles because it is light, and direct.
This steering system has worm gearing. It provides a gear reduction, and a 90 degree change in direction. It has more parts and joints than the rack type, but it is more robust, and may be used on heavier vehicles.
To allow heavy transport vehicles to carry extra weight, two steering axles may be used. They’re connected by a link to a common steering box. These vehicles are called tandem, or twin-steered vehicles.
Some passenger vehicles also steer the rear wheels slightly. This gives improved manoeuvrability. The system is known as 4-wheel steering.
It can be controlled mechanically, through a direct connection, between the front and rear steering boxes.
Or it can be computer-controlled.
With heavier vehicles, increased use of front-wheel-drive, and wider, low-profile tyres, more steering effort is needed, so power steering is used.
An engine-driven hydraulic pump provides pressure that helps the driver steer the vehicle. The power steering system is designed so that the vehicle can still be controlled, even if the engine or the power steering system, fails.

 Principles of steering

The steering system must provide control over the direction of travel of the vehicle; good maneuverability for parking the vehicle; smooth recovery from turns, as the driver releases the steering wheel; and minimum transmission of road shocks from the road surface.
The effort by the driver is transferred from the steering wheel, down the steering column, to a steering box.
The steering box converts the rotary motion of the steering wheel, to the linear motion needed to steer the vehicle.
It also gives the driver a mechanical advantage.
The linear motion from the steering box is then transferred by tie-rods, to the steering arms at the front wheels. The tie rods have ball joints that allow steering movement, and movement of the suspension.
The steering-arm ball-joints are arranged so that movement in the suspension does not affect steering operation. 

 Rack-and-pinion steering

The steering rack is supported at the pinion end, by being sandwiched between the pinion and a spring-loaded, rack guide yoke. This spring-loaded yoke ensures free play is eliminated between the gears, while still allowing for relative movement.
The rack is supported at the other end in the rack housing, or tube, by a bush, normally of nylon. Nylon is used because it has a low coefficient of friction, and low wear rates.
The pinion is supported by 2 bearings in the rack housing. These bearings are pre-loaded to keep the pinion in the correct position, relative to the rack, and to eliminate free play.
A rack-and-pinion steering box is normally lubricated by grease.
Each end of the rack is protected from dirt and water by a flexible, synthetic, rubber bellows, attached to the rack housing and to the tie rod. The bellows extends and collapses, as the tie-rods move away from, and towards the housing, as the rack moves.
On some vehicles, both bellows are interconnected by a tube so that as the steering wheel is moved from side to side, air is transferred from the collapsing bellows side to the expanding bellows side.
Rack-and-pinion type steering gears are used because their construction makes them compact and light-weight.
Their steering response is very sharp, because the rack operates directly on the steering knuckle.
And there is very little sliding and rotation resistance, which gives lighter operation.

Rack-and-pinion steering system

The primary components of the rack and pinion steering system are:

• Rubber bellows
• Pinion
• Rack
• Inner ball joint or socket
• Tie-rod

Rubber bellows

This rubber bellows is attached to the Rack and Pinion housing. It protects the inner joints from dirt and contaminants. In addition, it retains the grease lubricant inside the rack and pinion housing. There is an identical bellows on the other end of the rack for the opposite side connection.

Pinion

The pinion is connected to the steering column. As the driver turns the steering wheel, the forces are transferred to the pinion and it then causes the rack to move in either direction. This is achieved by having the pinion in constant mesh with the rack.

Rack

The rack slides in the housing and is moved by the action of the meshed pinion into the teeth of the rack. It normally has an adjustable bush opposite the pinion to control their meshing, and a nylon bush at the other end.

Inner ball joint or socket

The inner ball joint is attached to the tie-rod, to allow for suspension movement and slight changes in steering angles.

Tie-rod

A tie rod end is attached to the tie-rod shaft. These pivot as the rack is extended or retracted when the vehicle is negotiating turns. Some tie-rods and tie-rod ends are left or right hand threaded.

This allows toe-in or toe-out to be adjusted to the manufacturer's specifications.

Recirculating ball & nut steering system

The primary components of the recirculating ball and nut steering system are:

• Pitman arm shaft
• Idler arm
• Track rod or center link
• Tie-rod
• Tie-rod end
• Adjustment sleeve

Pitman arm shaft

The pitman arm shaft is attached to the steering box by a spline and nut. As the driver turns the steering wheel, the steering box mechanism moves the steering linkages via the pitman arm shaft either left or right, depending on the direction in which the steering wheel is turned.
The steering box provides the change of angle at 90° to the steering linkage.

Idler arm

The idler arm is attached to the chassis and is positioned parallel to the pitman arm.

Track rod or center link

The track rod connects the pitman arm shaft to the idler arm shaft. In this way any movement in the pitman arm shaft is directly applied to the idler arm shaft.

Tie-rod

The tie rods connect the track rod to the steering arms that are located on the steering knuckles. Thus all movement from the pitman arm shaft is relayed directly to the front wheels, which steer the vehicle.

Tie-rod end

Tie rod ends are attached to the tie-rod shaft. These pivot as the rack is extended or retracted when the vehicle is negotiating turns. Tie-rods and tie-rod ends are left or right hand threaded.

Adjustment sleeve

The adjustment sleeve connects the tie-rod to the tie-rod end. It provides the adjustment point for toe-in or toe-out, depending on the manufacturers' specifications.

 Four-wheel steering systems

Some cars have four-wheel steering.
This can be computer controlled or it can be mechanical, through a direct connection between the front and rear steering boxes, or it can be computer-controlled, or the rear wheels can be mounted on special, compliant mounts. As cornering forces are applied to them, they alter the steering angles.
With heavier vehicles, increased use of front-wheel drive, and wider, low-profile tyres, more steering effort is needed, so power assistance is used.
A hydraulic pump is driven from the engine, to provide pressure to help the driver. The power steering system is designed so that even if the engine or the power steering system fails, the vehicle can still be controlled. However, much more driver effort is required.
The relationships between the steering system, the wheel positions, and the suspension system, form what is called the steering geometry. These relationships must always stay within manufacturer specifications.