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As clutches are subjected to continuous wear and tear, LuK, as the first clutch manufacturer worldwide, has invested tremendous efforts in the development of a wear adjustment solution, which was successfully launched for volume production in 1995. SAC technology has since asserted itself in a wide variety of motor vehicles, in particular in models with large engines where the clutch can be actuated far more comfortably with an SAC.
1.2 Longer clutch life thanks to the sensor diaphragm spring
The SAC uses a load sensor (sensor diaphragm spring) to activate its wear adjustment function by turning a ramped ring. This wear adjusting mechanism reduces the required actuation forces while increasing clutch life by around 1.5 times. In addition, actuation forces remain nearly unchanged throughout the clutch's service life. The SAC wear adjustment system – which consists of the sensor diaphragm spring (load sensor) and a deep-drawn steel adjusting ring – is characterised by its excellent functional accuracy. As clutch actuation comfort requires a harmonic operating load curve in addition to low actuation forces, the SAC was designed with the capacity to be tuned to each vehicle’s specific characteristic curve. One such feature is the compensation spring, which is able to generate the flatter characteristic curves that are frequently desired.1.3 Further system refinement with the new SAC II
The new SAC II does not use a second diaphragm spring as a load sensor; instead, it has fingers formed rom the main diaphragm spring and special tangential leaf springs with a regressive characteristic curve.1.4 System optimisation thanks to specific designs
The modified SAC II concept allows for the further reduction of clutch actuation forces and/or the ptimisation of the actuation force curve. With this clutch type, the load sensor’s performance curve is modified in a way that makes the clutch adjustment mechanism less sensitive to large actuation lifts. This is achieved by using leaf springs with a linear characteristic curve which engage outside the pivot point of the main diaphragm spring.
In many cases, the load sensor can be formed directly from the diaphragm spring in the form of sensor fingers. This design requires no sensor diaphragm spring at all. With the new SAC II, actuation forces can be decreased by up to 15% without reducing torque transfer capacity. Alternatively, the maximum actuation force remains unchanged and the additional potential is used to optimise the characteristic curve.
2 Clutch course
Self-Adjusting Diaphragm Spring Clutch (SAC)
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2 Adjusting ring
3 Pressure spring
4 Diaphragm spring
5 Sensor diaphragm spring
6/7 Rivet
8 Tangential leaf spring
9 Pressure plate
10 Cover limit stop
In recent years Self-Adjusting Clutches have become the standard equipment in applications with high engine torque or with increased requirements for wear reserve.
The essential advantages of the SAC over conventional designs are:
• Low release loads which remain constant throughout the entire service life
• Therefore, excellent driving comfort throughout the entire service life
• Increased reserve for wear and consequently longer service life thanks to automatic wear adjustment
• Release bearing over-travel is prevented by the diaphragm spring end stop
This yields a number of further advantages:
• Simplified release system design
• Shorter pedal travel
• New engineering concepts to reduce the clutch diameter (torque transfer)
• Shorter release bearing travel throughout bearing life
2.1 Operating principle of the Self-Adjusting Clutch (SAC)
Load sensor
On the clutch with wear adjustment, the load sensor detects the increase in release load caused by wear and correctly compensates for the reduction in facing thickness. Unlike a conventional clutch, the (main) diaphragm spring is supported by the so-called sensor diaphragm spring instead of being riveted to the cover. In contrast to the strongly regressive main diaphragm spring, the sensor diaphragm spring provides a sufficiently wide range of almost constant load. The constant load range of the sensor diaphragm spring is designed to be slightly higher than the desired release load. As long as the release load is smaller than the load of the sensor spring when disengaging the clutch, the pivot point of the main diaphragm spring remains stationary. When facing wear increases, the release load increases, the counterforce of the sensor spring is overcome and the pivot point moves towards the flywheel to a position where the release load again fallsbelow the sensor load. When the sensor spring deflects, a gap develops between pivot point and cover, which can be compensated for by introducing a wedge-shaped component, for example.
Design of a Self-Adjusting Clutch with load sensor
The load sensor with thickness adjustment wedge can be realised in a simple and effective manner. In comparison to the conventional clutch, the only additional parts required by this design are a sensor diaphragm spring (red) and a ramp ring (yellow). The sensor diaphragm spring is suspended in the cover and its inside fingers support the main diaphragm spring. Because of centrifugal forces the wedges which provide the actual adjustment are positioned in circumferential direction. A steel adjusting ring with ramps moves on opposing ramps in the cover. The steel adjusting ring is preloaded in circumferential direction with pressure springs which force the ring to close the gap between the diaphragm spring and the cover when the sensor spring deflects. Figure 1 shows the release load curves for a conventional clutch with new and worn facings. In contrast, compare the significantly lower release load of the SAC as shown in graph 2 , which
has a characteristic curve that remains virtually unchanged over its service life. An additional advantage is the higher reserve for wear, which no longer depends on the length of the diaphragm spring curve (as in conventional clutches), but rather on the ramp height, which can easily be increased to 3 mm for small and 10 mm for very large clutches. This represents a decisive step towards the development of highly durable clutches.
Multiple-disc SAC
High-performance engines which generate engine torques above 500 Nm require clutches designed to transfer these torques. This involves an almost inevitable increase in pedal force despite the use of a Self-Adjusting Clutch. A variety of technological approaches kept the increase within reasonable limits (e.g. improved release systems); however calls for a clutch with reduced actuation force grew louder.
2.2 Multiple-disc Self-Adjusting Clutch (SAC)
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1 Clutch cover
2 Adjusting ring
3 Pressure spring
4 Diaphragm spring
5 Sensor diaphragm spring
6/7 Rivet
8 Tangential leaf spring
9 Pressure plate
10 Cover limit stop
11 Intermediate pressure plate
12 Lift rivet
13 Clutch disc 1
14 Clutch disc 2
In contrast to the single-disc version, the multiple-disc SAC has an additional intermediate pressure plate and three more tangential leaf spring packages which ensure sufficient lift of the intermediate pressure plate. To achieve even wear of both clutch discs, lift rivets are used to control the intermediate pressure plate. They ensure that the lift of the intermediate pressure plate is half the lift of the pressure plate. A special version of the clutch disc can be modelled to suit vehicle applications which require a damped clutch disc to provide better isolation. The benefit of the multiple-disc SAC is that it permits a reduction in release load for the same engine torque or, conversely, an increase in engine torque transfer at identical release load levels. On engines where high engine torque is paralleled by high engine speeds, the multiple-disc SAC also offers the option of decreasing the facing outer diameter, which in turn improves the burst speed characteristic of the clutch discs. Furthermore, downsizing the clutch disc helps stabilise or even slightly decrease the disc's mass moment of inertia compared to a single-disc system of corresponding clutch torque capability.