Clutches are the workhorse of the modern car. Located between the engine and transmission, the clutch is under ever increasing strain caused by more power and higher torques and weight. The ongoing trend, in particular, toward high-torque diesel vehicles, is placing more and more demands on clutches.
The average engine output in newly registered passenger cars in Germany has risen from 90 to 103 kW in the last ten years. The torques for more powerful diesel engines rose even higher. Nowadays, 400 Nm is no longer the exception. At the same time, vehicle weight has increased by an average of 50 kilograms over the same period. All these advancements are placing higher demands on the clutch system which functions as the power transmission between the engine and transmission. Experts from ZF Services are observing yet another phenomenon: “Due to increased engine output, many drivers barely feel the tow loads that their vehicles are sometimes pulling. Even though powerful SUVs pulling two tons behind them do a pretty good job on hills – this kind of driving is very hard on the clutch.”
Damage to the clutch system is therefore quite common. What often seems to be a harmless problem in the beginning, such as bucking when setting off, can quickly turn into an expensive repair. If the clutch is permanently subject to overload, for example, when driving a vehicle with a heavy tow load, the clutch can be damaged. The friction between the clutch disk and the clutch cover assembly or flywheel caused by overload can, in turn, cause isolated hot spots. These thermal hot spots increase the risk of cracks on the friction surfaces of the clutch pressure plate and the flywheel as well as damage to the clutch disk facing material. Furthermore, these hot spots can cause a complete failure of the dual-mass flywheel (DMF) because if the special lubricant in the DMF is continuously exposed to heat, it can harden. If this happens, the dual-mass flywheel has to be replaced.
Other possible reasons for defective clutches can include oily or greasy facings or grease on the seals on the crankshaft output and on the transmission input. Also, too much grease on the transmission input shaft or on the pilot bearing as well as leaks in the hydraulic actuation system frequently lead to contaminated or dirty facings. These, in turn, can cause a change in the frictional behavior between the clutch disk and clutch cover assembly or flywheel. It is therefore important to thoroughly analyze the source of the problem and to fix it immediately – because light traces of oil or grease interfere with smooth clutch engagement when setting off.
In general, when replacing a clutch, it is important to carefully examine adjacent components. This can prevent further damage and a more complex repair, and minimize costs.
For vehicles with a hydraulically-actuated clutch, air in the system can also cause operating problems. Even worn motor bearings or a bad engine adjustment can be possible reasons for a change in the setting-off performance. If the source of the problem cannot be found in the immediate area, the transmission has to be removed and the clutch disassembled.
- Absolute cleanliness is critical. Even touching the clutch facing with greasy hands can result in impaired operation later on.
- The clutch hub has to be properly greased. If too much grease is used, the centrifugal forces spread the lubricant onto the clutch facings – resulting in malfunctions.
- Also, check the clutch disk for axial runout prior to installation.
- To prevent damage to the hub spine, do not use any kind of force to join together the clutch disk and the transmission input shaft hubs.
- Tighten the clamping screws in accordance with the requirements – using the star pattern and applying the specified torque. ZF Services experts recommend thoroughly inspecting the release system and, if applicable, replacing worn parts. If the relevant vehicle has a concentric slave cylinder (CSC), this generally has to be replaced.
The adjacent components and area around the clutch must also be inspected during a clutch replacement. If any adjacent components are worn or defective, these should also be replaced – doing so will save on expensive follow-up repairs.
ZF Services offers clutches, dual-mass flywheels, and clutch actuation systems under its established product brand Sachs. There are also clutch kits that come with all the relevant components needed for a clutch repair. For more detailed information, please visit www.zf.com/sachs.
Our After Sales experts in Service Information have compiled additional installation tips that can be downloaded under www.zf.com/serviceinformation.