Clutch bearing

The clutch release bearing, also known as a throw-out bearing, transfers the axial actuating force from the release fork or concentric hydraulic release cylinder to the diaphragm spring of the clutch pressure plate. The bearing must be able to rotate relative to the rotating pressure plate while simultaneously moving in a controlled axial direction. A properly functioning clutch release bearing is essential for complete disengagement of the clutch disc, smooth gear shifting and reliable protection of the gearbox and drivetrain. Always identify a clutch release bearing by the chassis number; visual comparison or selection based on the model name alone is never sufficient.

In conventional clutches, the release bearing slides axially over the guide tube or guide sleeve around the gearbox input shaft during operation. This guide tube centres the bearing and helps determine its free, straight movement towards the diaphragm spring. The inside of the bearing housing or carrier sleeve must therefore move over the guide without burrs, corrosion, grooves or misalignment. Damage to this tube causes a binding bearing, incomplete clutch disengagement and accelerated wear of the bearing, release fork and pressure plate.

Depending on the version, there are mechanically operated clutch release bearings with a release fork, hydraulically actuated versions and concentric release cylinders in which the bearing and hydraulic cylinder form one unit. The diameter of the contact surface with the diaphragm spring, axial travel, installation height, retention on the fork and fit on the guide sleeve differ by clutch and gearbox. This applies, among others, to MAN applications with D08, D20, D26 and D38 engines and Mercedes vehicles with OM 906, OM 457, OM 460, OM 470, OM 471 or OM 473 engines. Different clutches and transmissions may also be fitted within the same engine family.

A squealing, grinding or rattling noise that is audible only when the clutch pedal is depressed often indicates wear of the rolling bearing or a damaged contact surface. Vibrations in the clutch pedal can be caused by an unevenly worn release bearing, deformed diaphragm spring fingers or incorrect alignment between the engine and gearbox. Difficult gear shifting, vehicle creep with the pedal fully depressed or grinding when engaging a gear may indicate insufficient axial travel, air in the hydraulic circuit, leakage from the clutch master or slave cylinder, or binding on the guide sleeve.

After dismantling, visually inspect the guide tube and measure the outside diameter at several points using a micrometer. Compare the measurements for wear, ovality and groove formation. Also check the axial play of the clutch release bearing, the condition of the release fork, pivot point and retaining springs. On hydraulic systems, check the travel of the release cylinder, the condition of the hydraulic fluid and any leaks at the pipe connections. A clutch release bearing may only be installed with the specified thin lubricant on the guide; excess grease can contaminate the friction material of the clutch disc.

Do not replace a noisy, stiff or discoloured clutch release bearing without investigating the cause. Heat discolouration, for example, indicates permanent contact with the diaphragm spring due to an incorrectly adjusted mechanism, a defective hydraulic cylinder or a seized guide sleeve. If in doubt, replace the complete clutch kit, including clutch disc, clutch pressure plate and clutch release bearing. Also inspect the flywheel, pilot bearing, input shaft splines, crankshaft seal and clutch housing alignment.

For vehicles that frequently manoeuvre, pull away under heavy load or regularly operate in stop-start traffic, preventive replacement is often less expensive than unplanned downtime. Braem can assist with the technical identification of clutch release bearings and associated release components based on the chassis number.