Schaeffler North America




Power “under the bonnet” with no power for clutch pedal actuation
In principle, there is a linear correlation between high engine torque and the actuation force required for the clutch. Despite increasing power “under the bonnet”, i.e. engine torque, the driver cannot of course be expected to apply increased “power” when stepping on the clutch pedal. LuK’s innovative clutch concepts offer a solution.

Current clutch development for passenger cars therefore centres around further reducing activation forces. The development and market launch of LuK’s self-adjusting clutch form the foundation stones for satisfying this requirement. This SAC has now established itself on the market and enables many highly-motorized vehicles to have tenable clutch pedal forces without the need for complex or extremely expensive support systems. The system is still demonstrating notable further development potential, even for future requirements. It can be said that we are one step near to the goal of “building clutches without notable actuation forces / energies”.

Function description of the SAC
The actuation force of a clutch is essentially proportional to the contact force and/or clutch torque to be transferred. Higher torques values necessitate a correspondingly higher actuation force. With conventional passenger car clutches, the maximum actuation force and force in friction contact is usually a factor of 4, whereby the actuation force increases by approx. 40% over the clutch's lifespan.

With the self-adjusting clutch, the “force equilibrium” principle and a self-actuating mechanical wear correction mechanism significantly alter the relationship between transferable torque and maximum actuation. The SAC uses the two existing spring forces to utilise the principle of force equilibrium. Once such force is the lining resilience between clutch linings on the friction disc and the other is the plate resilience, whose performance curve is modified such that a high max./min. ratio force prevails.

Because when the clutch is depressed, the diaphragm spring reeds cause the two forces to act against each other, only the differential between diaphragm spring force and lining force must be applied for actuation. In conjunction with a very degressive diaphragm spring performance curve (high max./min. force ratio) and an adapted lining resilience performance curve, very low actuation forces can be achieved at the "new operating point”. However, should the clutch operating point move to the right, for example, towards diaphragm spring maximum, the actuation force increases dramatically.

In reality, this is caused by clutch lining wear that occurs over its lifespan as a result of friction during start-up or gear switching. A wear correction mechanism must therefore be developed. Using a second diaphragm spring (sensor diaphragm spring) and a steel adjusting ring between diaphragm spring and clutch housing, the system with force sensor has proved its worth under the very difficult conditions prevailing in the clutch cover. The adjusting ring forms the contact point for the diaphragm spring and is also supported on the clutch housing via ramps. The adjusting ring is impacted by a spring force around its circumference by 2-3 pressure springs. The sensor diaphragm spring serves as a mechanical sensor for wear detection and is synchronised such that as wear increases, the diaphragm spring is displaced towards the engine on actuation. This renders the adjusting ring force-free, which can now twist relative to the clutch cover.

As a result of this process, the diaphragm spring copies tracks lining wear on the friction disc and the operation point of the clutch remains constant. A further advantage is that the wearing range of the clutch and thus its lifespan can be increased by up to 50%.


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