The clutch is the first drive train component powered by the engine crankshaft. The clutch lets the driver control power flow between the engine and the transmission or transaxle. Before understanding the operation of a clutch, you must first become familiar with the parts and their functions. This information is very useful when learning to diagnose and repair the clutch assembly.
A clutch release mechanism allows the operator to operate the clutch. Generally, it consists of the clutch pedal assembly, a mechanical linkage, cable, or hydraulic circuit, and the clutch fork. Some manufacturers include the release bearing as part of the clutch release mechanism.
Linkage. A clutch linkage mechanism uses levers and rods to transfer motion from the clutch pedal to the clutch fork. One configuration is shown in Figure 9-2. When the pedal is pressed, a pushrod shoves on the bell crank and thebell crank reverses the forward movement of the clutch pedal. The other end of the bell crank is connected to the release rod. The release rod transfers bell crank movement to the clutch fork. It also provides a method of adjustment for the clutch.
Figure 9-2 — Clutch linkage.
Cable. The clutch cable mechanism uses a steel cable inside a flexible housing to transfer pedal movement to the clutch fork. As shown in Figure 9-3, the cable is usually fastened to the upper end of the clutch pedal, with the other end of the cable connecting to the clutch fork. The cable housing is mounted in a stationary position. This allows the cable to slide inside the housing whenever the clutch pedal is moved. One end of the clutch cable housing has a threaded sleeve for clutch adjustment.
Figure 9-3 — Clutch cable.
A hydraulic clutch release mechanism uses a simple hydraulic circuit to transfer clutch pedal action to the clutch fork (Figure 9-4). It has three basic parts—master cylinder, hydraulic lines, and a slave cylinder.
Figure 9-4 — Hydraulic clutch.
Movement of the clutch pedal creates hydraulic pressure in the master cylinder, which actuates the slave cylinder. The slave cylinder then moves the clutch fork.
Slave Cylinder with Clutch Master Cylinder. The master cylinder is the controlling cylinder that develops the hydraulic pressure. The slave cylinder is the operating cylinder that is actuated by the pressure created by the master cylinder.
The clutch fork, also called a clutch arm or release arm, transfers motion from the release mechanism to the release bearing and pressure plate. The clutch fork sticks through a square hole in the bell housing and mounts on a pivot. When the clutch fork is moved by the release mechanism, it pries on the release bearing to disengage the clutch.
A rubber boot fits over the clutch fork. This boot is designed to keep road dirt, rocks, oil, water, and other debris from entering the clutch housing.
The clutch housing is also called the bell housing. It bolts to the rear of the engine, enclosing the clutch assembly, with the manual transmission bolted to the back of the housing. The lower front of the housing has a metal cover that can be removed for fly- wheel ring gear inspection or when the engine must be separated from the clutch assembly. A hole is provided in the side of the housing for the clutch fork. It can be made of aluminum, magnesium, or cast iron.
The release bearing, also called the throw-out bearing, is a ball bearing and collar assembly. It reduces friction between the pressure plate levers and the release fork. The release bearing is a sealed unit pack with a lubricant. It slides on a hub sleeve extending out from the front of the manual transmission or transaxle and is moved by either hydraulic or manual pressure.
The hydraulic release bearing eliminates the stock mechanical release bearing linkage and slave cylinder. The release bearing mounts on the transmission face or slips over the input shaft of the transmission. When the clutch pedal is pressed, the bearing face presses against the pressure plate to disengage the clutch.
The release bearing snaps over the end of the clutch fork. Small spring clips hold the bearing on the fork. Then fork movement in either direction slides the release bearing along the transmission hub sleeve.
The pressure plate is a spring-loaded device that can either engage or disengage the clutch disc and the flywheel. It bolts to the flywheel. The clutch disc fits between the flywheel and the pressure plate. There are two types of pressure plates—the coil spring type and the diaphragm type.
The coil spring pressure plate uses small coil springs similar to valve springs (Figure 9-5). The face of the pressure plate is a large, flat ring that contacts the clutch disc during clutch engagement. The back side of the pressure plate has pockets for the coil springs and brackets for hinging the release levers. During clutch action, the pressure plate moves back and forth inside the clutch cover. The release levers are hinged inside the pressure plate to pry on and move the pressure plate face away from the clutch disc and flywheel. Small clip-type springs fit around the release levers to keep them rattling when fully released. The pressure plate cover fits over the springs, the release levers, and the pressure plate face. Its main purpose is to hold the assembly together. Holes around the outer edge of the cover are for bolting the pressure plate to the flywheel.
Figure 9-5 — Coil spring pressure plate.
The diaphragm pressure plate (Figure 9-6) uses a single diaphragm spring instead of coil springs. The diaphragm spring is a large, round disc of spring steel. The spring is bent or dished and has pie-shaped segments running from the outer edge to the center. The diaphragm spring is mounted in the pressure plate with the outer edge touching the back of the pressure plate face. The outer rim of the diaphragm is secured to the pressure plate and is pivoted on rings approximately 1 inch from the outer edge.
Figure 9-6 — Diaphragm pressure plate.
Application of pressure at the inner section of the diaphragm will cause the outer rim to move away from the flywheel and draw the pressure plate away from the clutch disc, disengaging the clutch.
A “wet” clutch is immersed in a cooling lubricating fluid, which also keeps the surfaces clean and gives smoother performance and longer life. Wet clutches, however, tend to lose some energy to the liquid. Since the surfaces of a wet clutch can be slippery, stacking multiple clutch discs can compensate for the lower coefficient of friction and so eliminate slippage under power when fully engaged.
Wet clutches are designed to provide a long, service-free life. They often last the entire life of the machine they are installed on. If you must provide service to a wet clutch, refer to the manufacturer’s service manual for specific details.
The clutch disc, also called friction lining, is a “dry” clutch and consists of a splined hub and a round metal plate covered with friction material (lining). The splines in the center of the clutch disc mesh with the splines on the input shaft of the manual transmission. This makes the input shaft and disc turn together. However, the disc is free to slide back and forth on the shaft.
Clutch disc torsion springs, also termed damping springs, absorb some of the vibration and shock produced by clutch engagement. They are small coil springs located between the clutch disc splined hub and the friction disc assembly. When the clutch is engaged, the pressure plate jams the stationary disc against the spinning flywheel. The torsion springs compress and soften as the disc first begins to turn with the flywheel.
Clutch disc facing springs, also called the cushioning springs, are flat metal springs located under the friction lining of the disc. These springs have a slight wave or curve, allowing the lining to flex inward slightly during initial engagement. This also allows for smooth engagement.
The clutch disc friction material, also called disc lining or facing, is made of heat- resistant asbestos, cotton fibers, and copper wires woven or molded together. Grooves are cut into the friction material to aid cooling and release of the clutch disc. Rivets are used to bond the friction material to both sides of the metal body of the disc.
The flywheel is the mounting surface for the clutch (Figure 9-7). The pressure plate bolts to the flywheel face. The clutch disc is clamped and held against the flywheel by the spring action of the pressure plate. The face of the flywheel is precision machined to a smooth surface. The face of the flywheel that touches the clutch disc is made of iron.
Figure 9-7 — Flywheel and pilot bearing.
Even if the flywheel were aluminum, the face is iron because it wears well and dissipates heat better.
The pilot bearing or bushing is pressed into the end of the crankshaft to support the end of the transmission input shaft (Figure 9-7). The pilot bearing is a solid bronze bushing, but it also may be a roller or ball bearing.
The end of the transmission input shaft has a small journal machined on its end. This journal slides inside the pilot bearing. The pilot bearing prevents the transmission shaft and clutch disc from wobbling up and down when the clutch is released. It also assists the input shaft center the disc on the flywheel.
1. The clutch fork transfers motion from the release mechanism to what components?