Clutch Release Bearings: Fault Diagnosis and Replacement
Despite its apparent functional simplicity, a vehicle clutch is a complex system comprising many individual components, including the release mechanism. On older vehicles this may comprise a separate release bearing and release fork assembly, operated mechanically by cable or hydraulically by an external slave cylinder and pushrod. Newer vehicles use a concentric slave cylinder (CSC), in which the bearing and hydraulic cylinder are integrated in a compact unit.
Like the clutch driven plate and pressure plate, the release mechanism is subject to wear, and when damage occurs it cannot be repaired so must be replaced. Renewal of the release bearing or CSC is also recommended when replacing the clutch or removing the transmission, as renewing the clutch in isolation may lead to the need for further repairs shortly afterwards.
Fault diagnosis of release bearing-related problems
Grooves on inner ring of clutch release bearing (Fig. 1)
These grooves are the result of abnormal wear. They may be caused by contact with a clutch release bearing that is off-centre due to parallel misalignment of the transmission during refitting, a worn guide tube on the bearing, or by insufficient release bearing preload. The first indication of this fault is often a whistling noise from the clutch.
Heavy wear to tips of diaphragm spring (Fig. 2)
This type of damage occurs due to wear in the actuation system, a worn release bearing guide tube or if the release bearing preload is too high. The result can be clutch slip as the high preload prevents full clutch engagement, or loss of clutch operation due to the release bearing becoming jammed or milling completely through the diaphragm spring tips.
Handling and fitting precautions
Maintaining a clean working environment is vitally important during handling and replacement of clutch release bearing components, particularly the CSC. Any dirt deposits on the outside of the CSC piston guide can damage the sealing lip, as the piston will pass over the particles when it returns to its non-extended position.
Installation errors can also cause CSC defects. When removing the clutch and transmission with a closed hydraulic circuit, the clutch pedal must not be operated. Should this occur, it will over-extend the CSC piston, possibly damaging the piston sealing lip, causing hydraulic fluid loss and resulting in clutch operating problems. The natural temptation to manually compress the CSC to ‘check’ its operation should be resisted, as this can result in seal damage (Fig. 3). The application of lubricants must also be avoided; these may cause the seals to swell and lead to early CSC failure.
CSC filling and bleeding
With the clutch fitted, the CSC installed in the clutch bell housing and the transmission fully mounted to the engine flange, only then should the CSC be filled with hydraulic fluid and bled, observing the vehicle manufacturer’s or CSC manufacturer’s instructions.
Careful attention to the above recommendations should enable rapid tracing and rectification of faults and ensure long and reliable operation of replacement components.
Wyatt International Ltd
Phone: +44 (0)121 454 8181