the surrounding tube which is
screwed to the bodywork
the steering shaft.
The steering shaft connects the steering wheel to the steering
gear and is supported in the surrounding tube. It transmits the
steering torque
Requirements
Steering columns must satisfy the following requirements:
they must ensure high rotational
rigidity of the steering shaft
they must prevent steering irregularities
they must damp out noise
they must prevent or reduce physical injury in accidents
they must have only low frictional losses
they must ensure anti-theft protection for the vehicle.
Types of steering column
Steering columns are essentially subject to the design principles
presented here:
Rigid steering column
The classic design is a rigid steering column. The steering wheel
has a rigid connection to the steering shaft, which is usually a
single piece item.
Steering column with rake adjustment
In this design, the inclination of the steering wheel can be
adjusted. The tilting point is usually located at the joint.
In steering columns with reach adjustment, the steering wheel
can be adjusted telescopically. The position of the steering wheel
in relation to the driver can thus be altered in an axial direction.
Combined adjustment mechanism
The rake-adjustable steering column and the reach-adjustable
steering column are compromise solutions. The most favourable position
of the steering wheel in relation to the driver is achieved through
a combination of both rake and reach adjustment.
Steering column bearing requirements
Bearings used in steering columns must satisfy the following
requirements:
they must support the steering
shaft without clearance
they must damp out noise and vibrations effectively
they must have high rigidity
they must have the minimum possible friction.
Body block test
There are legal regulations governing:
the maximum distance that the
upper end of the steering column may intrude into the passenger compartment
the force with which a test piece may impact on the steering
wheel.
Test conditions:
displacement distance of 127
mm in a frontal impact at a speed of 48,3 km/h
maximum impact force of 11 000 N; impact at 24,1 km/h.
The impact force of 11 000 N is limited by:
the design of the surrounding
tube (e.g. hollow shaft)
the use of spring elements or similar means of absorbing
energy.
The steering shaft bearings must be designed such that they will
support the load occurring under the defined test conditions. The
only permissible effects are indentations on the raceways due to
the rolling elements and only slight deformation of the individual bearing
components. It must still be possible to turn the steering shaft.
Anti-theft test
The anti-theft test defines:
the safety limits of the steering
wheel lock.
A torque of up to 240 Nm is applied to the steering wheel. Due
to the circumferential force on the locking bolt, this torque results
in high radial forces on the upper and lower bearing. The upper
bearing, the bearing closest to the steering wheel, is subjected
to higher loads.
The bearings must have a high static load safety factor. Indentations
in the raceways due to the balls are permissible but cracks or fractures
must not occur in the bearing components. The steering function
of the vehicle must be unaffected. The maximum permissible effect
of the indentations is a reduction in the level of steering comfort.
Shake test
This test describes:
the load on the bearing during
tilting of the steering wheel caused by tensile or compressive movement.
The drivers leans his or her body weight on the steering wheel.
This induces a bending moment with high reaction forces on the bearings.
The force on the upper bearing is normally between 1000 N and 1500
N.
Under these loading conditions, neither indentations nor deformations
may occur in the bearings.
