Steering (Steering description): Description and operation

STEERING DESCRIPTION

The major steering components comprise a shock absorbing telescopic steering column, a power assisted steering rack, a power assisted steering (PAS) pump, a fluid reservoir,a fluid cooler and fluid pipes and hoses.

Steering Column

The steering column design incorporates an energy absorbing mechanism to reduce driver impact loads in the event of a collision. The column is mounted on four captive studs which are located in an extension to the bulkhead. The two lower mountings are fixed and cannot move when loads are applied to them. The upper mounting is designed to dis-engage or deform when a load is applied, allowing the column to collapse in the event of an accident. The steering column is adjustable to allow the steering wheel to be moved vertically up or down to the desired position. The steering column must be replaced as a complete assembly if necessary.

The upper column tube is telescopic and can slide over the lower column tube. The upper column tube provides for the location of the steering lock and ignition switch and also the steering switch gear and a rotary coupler. The rotary coupler provides the electrical connection for the steering wheel mounted airbag and horn switches.

The central shaft comprises of two parts and is located in bearings in the upper and lower column tubes. The upper shaft is located inside the lower shaft. Mating cross holes in each shaft are connected by nylon injection moulded shear pins.

The upper shaft is splined to accept the steering wheel. The lower shaft is connected by a universal joint to the intermediate shaft. A second universal joint on the opposite end of the intermediate shaft is attached to a split adaptor. The adaptor is splined and mates with the splined rotor shaft from the steering rack.

The upper mounting bracket has two open slots with a PTFE coated, metal ’U’ clip over each slot. Two soft metal ’curling’ plates are riveted to the mounting bracket and cover the slots.

The mounting studs pass through the ’U’ clips and the curling plates. The lock nuts that retain the upper mountings to the bulkhead extension are tightened to between 12 and 15 Nm. This torque figure controls the breakout load required to move the upper mounting brackets from the studs in the bulkhead extension and is critical to the crash performance of the column.

Energy absorbtion is achieved by the following mechansim: When an axial load is applied to the steering column, the mounting bracket deforms or slides out of the ’U’ clips and the curling plates deform. The ’U’ clips remain captive on the studs When the column mounting moves, the upper column tube slides on the lower column tube and allows approximately 63 mm (2.5 in.) maximum of axial movement. The nylon injection moulded shear pins holding the upper and lower shafts together shear, allowing the central shaft to telescope the same amount as the column tubes.

If, in the event of a collision where the steering rack itself moves, two universal joints in the column allow the intermediate shaft to articulate, minimizing movement of the column towards the driver. If movement continues after articulation of the intermediate shaft, the injection moulded shear pins, retaining the two halves of the central shaft, shear causing the shafts to ’telescope’ together reducing further column intrusion.

The steering wheel comprises a cast centre and wire frame onto which the soft polyurethane foam is moulded. A horn switch is located at each side of the wheel. Both switches are connected by wires to the rotary coupler connector.