Clutch System Components: Notes

Fig 1: Non-Self Adjusting Pressure Plate Assembly

Courtesy of GENERAL MOTORS CORP.

The non-self adjusting diaphragm spring clutch consists of a pressure plate (2), a diaphragm spring (3), a pivot ring (4), drive straps (5), and a cover (1). When the release bearing contacts the tips of the diaphragm spring fingers, it moves them toward the flywheel. The outside diameter of the diaphragm spring pivots on a pivot ring inside the cover. This action lifts the pressure plate off the flywheel through the drive straps, which connect the cover to the pressure plate. Non-self adjusting pressure plates have different symptoms. On non-self adjusting clutches, as the clutch wears, there is an increase in pedal effort. The piston or release bearing in the clutch actuator cylinder moves towards the transmission as the clutch wears. The piston is at the furthest position from the transmission with a new clutch.

Fig 2: Self Adjusting Pressure Plate Assembly

Courtesy of GENERAL MOTORS CORP.

The self-adjusting diaphragm spring clutch consists of a pressure plate (1), a diaphragm spring (5), a ramp ring (3), sensor (4), coil spring (6), and a cover (2). In conventional clutch designs, pedal effort increases as the disc wears. The self-adjusting clutch was designed to keep pedal effort low and constant over the life of the clutch. As the disc friction material wears, the ramp ring (3) in the clutch adjusts for the wear. The main diaphragm spring (5) is not permanently riveted to the clutch cover, as with non-self-adjusting clutch assemblies, but is only retained axially against the cover by a defined or sensor (4) force. The clutch adjusts when the sensor (4) force cannot hold the diaphragm spring (5) against the cover (2) because of wear to the clutch disc. With the diaphragm spring (5) away from the cover, the spring loaded ramp ring (3) compensates for the gap. The adjustment lasts until the sensor (4) force can position the diaphragm spring to the original angle. When new, the clutch actuator piston or release bearing is closer to the transmission, on self-adjusting clutches. As the clutch wears, the clutch pressure plate levers move towards the engine. The clutch actuator piston moves further from the transmission. If a self-adjusting clutch does not self adjust, there will be low pedal effort and low clutch pedal reserve. If the clutch has backward adjusted after an incident of severe use, the pedal may seem hard. After a couple of applications, the clutch should adjust to the proper adjustment.

Fig 3: Clutch Disc

Courtesy of GENERAL MOTORS CORP.

The disc is a critical component in service life of the clutch system. It provides smooth engagement and dampens engine vibrations. Mounted to the input shaft between the flywheel and the clutch, it can slide forward and backward on the input shaft, but cannot rotate without rotating the input shaft. Critical component parts include the hub-flange (10) and the torsion springs (2 and 7). The hub-flange is located between a cover plate (11) and a retainer plate (11); it is splined to fit the input shaft. Torsion springs (2 and 7) in the damper assembly smooth engagement and dampen vibrations. Idle stage dampers (4 and 6) reduce vibrations passed to the transmission through the input shaft and eliminate gear rattle. Friction material (1) is riveted to numerous metal components called marcels, or cushion segments (3). Waves in marcels soften engagement. The clutch disc with the 6.6L engine does not use damper springs.

Fig 4: Clutch Pedal

Courtesy of GENERAL MOTORS CORP.

The clutch pedal, mounted on the engine cowl, is designed to compress the clutch hydraulic fluid with little driver effort. The pedal swivels on bushings. There are two different clutch pedal configurations. With the 5 speed transmission clutch pedal (1), the clutch master cylinder pushrod latches into a socket on the pedal. A spring, located inside of the clutch master cylinder, keeps the pushrod in the pedal socket and returns the pedal to the correct position after depressing. With the 6 speed transmission clutch pedal (2), the clutch master cylinder pushrod connects to the pedal. The pedal contains a spring, which returns the pedal and the master cylinder piston to the correct position after depressing. The 6 speed pedal spring is an over-centering design. As the pedal is pushed further, the leverage design of the pedal compresses the hydraulic fluid without any driver effort. Because of the over-center design, if there is not hydraulic fluid resistance, the pedal will stay to the floor.

Fig 5: Flywheel

Courtesy of GENERAL MOTORS CORP.

Bolted to the end of the crankshaft, the flywheel (1) provides the mounting surface for the clutch. During engagement, the disc is clamped against the flywheel by the pressure plate. The flywheel acts as a heat sink, dissipating heat and moving it away from the clutch pressure plate and disc friction material. The flywheel must provide a smooth, flat surface in order for the clutch to operate properly. The dual-mass flywheel (2), which is used on the 6.6L engine, is designed to absorb engine vibrations before they are transmitted to the driveline, where they can create gear rattle. This is achieved by splitting the conventional flywheel into two sections: a primary section, which bolts to the crankshaft, and a secondary section, onto which the clutch is bolted. The primary section of the flywheel contains springs to isolate engine vibrations and a torque-limiting device to prevent engine torque spikes from exceeding engine and transmission component strength.

Fig 6: Pilot Bearing/Bushing

Courtesy of GENERAL MOTORS CORP.

On vehicles, except those with the 6.6L engine, a pilot bearing or bushing is located in the end of the crankshaft. On the 6.6L engine, the pilot bearing is located in the dual mass flywheel. The pilot bearing supports the end of the input shaft and centers the disc on the flywheel. A small and relatively inexpensive component, the pilot bearing or bushing should always be replaced during clutch installation. The variety of conditions caused by a worn or defective bearing or bushing is not worth the risk of having to remove the bell housing and transmission to replace this small part.

Fig 7: Clutch Actuator Cylinder

Courtesy of GENERAL MOTORS CORP.

The clutch actuator cylinder is fastened to the front of the transmission. The actuator cylinder moves the clutch release bearing from the pressurized hydraulic fluid. The actuator cylinder has a piston, which has seals that slide in the housing assembly. There is a spring between the piston and housing to keep the piston at the extended position, so that release bearing is in constant contact with the pressure plate fingers. With the 5 speed system (1), the clutch actuator has a bleeder for bleeding air out of the hydraulic system. The release bearing is replaceable on the 5 speed system. With the 6 speed system (2), the clutch actuator does not have a bleeder. To prevent loss of fluid, the hose comes with the actuator.