Clutch Release Bearing Faults and Fixes
For reliable clutch operation it is essential that the release bearing functions as the manufacturer intended. According to Wayne McCluskey, technical training manager at ZF Services UK, despite the relative simplicity of the component it can be the cause of a variety of clutch system faults. A guide to the identification, rectification and future prevention of a selection of these faults is presented below.
Special care needed with pull-type clutch release mechanisms
Whereas the push-type release bearing (once it is connected onto the guide sleeve on the gearbox) is automatically brought into contact with the clutch diaphragm spring as the gearbox is mounted to the engine, fitting the pull-type setup is more complex. For a pull-type clutch where the release bearing is attached to the clutch cover prior to fitment, care must be taken during gearbox fitting not to knock the release bearing out of line as the input shaft enters the guide tube. If the bearing is knocked out of alignment with the cover and then only partially refitted, it can start to screw out or destroy itself as it is not fully fastened.
The alternative snap-in connection pull-type release bearing, which is fitted to the guide sleeve of the gearbox prior to fitment also needs precautions to be observed during fitting. Once the gearbox is bolted up to the engine, the release fork is operated backwards to snap in the bearing. There are various retaining clip methods used across different applications, but whichever is used the important point is to ensure that the bearing and its clip are fully located all the way round into the release plate.
The telltale signs of incorrect bearing preload
Every release bearing needs a small amount of preload so that the thrust ring spins at the same speed as the clutch cover, preventing noise and wear. Too much preload can increase friction, which could overheat the bearing to the point where it begins to seize and ultimately mills its way through the diaphragm spring fingers or thrust ring (Figure 1). Excessive preload can also cause partial disengagement of the clutch, leading to clutch slip under high load.
The opposite problem – insufficient preload – means the bearing has to spin up to speed each time it contacts the diaphragm fingers on clutch operation, again causing wear on the fingers and/or the thrust ring (Figure 2), albeit at a slower rate. On a pull-type clutch, incorrect preload may allow the bearing to spin against the retaining clip, milling it flat (Figure 3). Whether too high or too low, incorrect preload points to problems within the release mechanism, which should be checked for wear, poor adjustment and excessive friction.
Guide sleeves – only the metal ones need to be lubricated
The inner guide sleeve on a modern release bearing that operates against the guide tube is often manufactured from plastic to save weight. The plastic sleeve should not be lubricated in any way, although in practice this stipulation is regularly ignored. Lubricants attract particulates, e.g. from friction lining wear, and contaminated grease can thicken to a paste, restricting movement and leading to clutch judder or release problems. Use of a plastic sleeve requiring no lubrication is therefore an advantage from a reliability point of view. However, metal guide sleeves do require lubrication with the correct type of grease.
Dirt ingress destroys bearings
Contamination of the clutch release mechanism from external sources is usually the result of operation in a dusty or sandy location, for example a truck operating in a quarry. If the operating environment is not correctly sealed, the action of the clutch rotating can suck dust into the bellhousing. This gradually migrates through the seals of the release bearing into the inner ballraces, eventually leading to failure. (Release bearings are sealed for life, and no attempt should be made to dismantle them to regrease them internally.) All sealing elements such as inspection plates and mounting bolts must be present on the gearbox bellhousing to avoid dirt ingress.
The transmission input shaft should form a natural extension of the engine crankshaft, aligned on precisely the same axis. Where misalignment occurs this may be either parallel, i.e. input shaft offset sideways/above/below the crankshaft axis but still parallel with it, or angular, e.g. transmission tilted/skewed from the crankshaft axis (Figure 4).
For the release bearing, the effect of misalignment is to cause a constant eccentric motion that can wear the release bearing excessively, or completely mill through the diaphragm spring fingers (Figure 5).
To prevent or correct misalignment, the installation must meet the following requirements:
All mating surfaces to be clean and clear of any debris
All pipes, cables, etc., to be clear of the crankcase/transmission joint – a trapped earth strap or cable tie is sufficient to cause misalignment
Any locating dowels on the engine/clutch bellhousing to be present and in good condition
The pilot bearing (where fitted in the flywheel) and transmission input shaft guide sleeves to be free of wear – any damage can impair input shaft location, causing offset
Flange bolts to be loosened/tightened in a diagonally opposite pattern during dismantling/assembly and correctly torqued after fitting
Wear checks and adequate lubrication
Worn release mechanisms are seen on such a regular basis that they can come to be regarded as normal and by association, serviceable. Proper condition assessment and dimensional checks are therefore often necessary to correctly judge wear. The easiest way to gauge the degree of wear is to compare a part with the equivalent new Sachs item, but this is rarely practical unless a new part has already been sourced because it is going to be changed anyway.
Working from the release bearing back, the following items should be checked:
Contact points from the back of the clutch fork to the bearing – some will have inbuilt contact points, others will have connection points for replaceable slippers or rollers, e.g. Volvo
The centre pivot point of the release fork where the cross-shaft locates (Figure 6)
Contact points between the clutch booster or slave cylinder pushrod and the release fork
To minimise wear in the clutch release mechanism, the vehicle manufacturer’s lubrication guidelines must be followed where available. As a general guide, any points where the release forks contact the back of the release bearing should be greased. It’s essential to use only a small amount of the correct grease such as that supplied in the clutch kit – a substitute like a copper-based lubricant is not suitable. Once wear is underway, the condition will rapidly worsen: wear on a single part increases friction, which requires more operating pressure to overcome, which in turn causes additional stress on other pivoting points, further accelerating wear.
Wyatt International Ltd
Phone: +44 (0)121 454 8181