Identifying faults in the release system

Specialized knowledge helps prevent damage

Many clutch deficiencies in commercial vehicles can be traced to the influence of faulty release systems. These deficiencies also include “capital” failures such as burnt-out clutches or poor disengagement performance, for which one might not initially suspect the release system. Therefore a precise inspection is very important. There are not many maintenance jobs where concentrating on a few details can have such a big effect and save major costs.

Three examples should serve to explain the types of damage that can result from a deficient release system:

Wear:
If the bearing points of the guide sleeve (shown in this picture), the release fork, or the release shaft are worn, this compromises the clutch’s ease of movement – the worst case scenario is a total clutch failure.

Worn guide sleeve

Eccentricity:
If, as a result of worn components, the release bearing is no longer guided precisely, this also causes an eccentric movement of the diaphragm spring. This in turn leads to considerable abrasion in the contact line (pitch) of the pressure plate. The axial position of the diaphragm spring is mo-difi ed as a consequence, leads to a reduced frictio-nal connection, and thus makes the clutch slip.

New (left) and worn (right) pitches compared

Wrong position:
A release bearing that was incorrectly “threaded”, i.e. incorrectly positioned, during assembly, will get stuck in the fork. To begin with, this results in a stiff clutch, the release distance becomes shorter, and the clutch is not separated properly. The stuck fork does not transfer the force of the diaphragm spring eff ectively. After a short run-time, the clutch will also start slipping.

Characteristics of incorrect position of the release bearing

Worn components in the release system

Wear in the release mechanism is one reason for increased pedal forces. Special attention should be paid to

• the contact surfaces of the release fork,
• the contact surfaces of the release shaft,
• and the release fork itself.

If these have entered advanced stages of wear, as is shown in the photo with worn guiding sleeve, are dry, worn, or broken, this will have negative consequences on shift performance. Consequences include grab, drag, or an increasing degree of sluggishness in the clutch. Because there is no load on the release system, manual testing of release shaft operation is not effective. Here a visual inspection is required.