Automotive Clutch and Construction and working of Clutch and Classification of Clutch

Welcome to Explore Automotive, In this blog we will see the automotive clutch, How automotive clutch works? and How automotive clutch classified? and specially role of clutch in manual transmission vehicles.

In an automobile, the mechanism which is playing major role to transmits the power developed by the engine to the wheels or tracks and accessory equipment is called the power train. In application, a set of gears or a chain and sprocket could perform this task. 

Image source: needpix.com Clutch Plate

However, automotives and construction equipments are not generally designed for such a simple operating conditions. They are to be designed which provides the pulling power, to move the equipment at high speeds, to travel in a reverse as well as forward direction, and to operate easily on a rough terrain as well as smooth road conditions. To meet these type of varying conditions, vehicle power trains are equipped with a different variety of components. In this blog we discuss the basics of automotive clutches.

Automotive Clutches

An automotive clutch is used generally in all types of automobiles which connect and disconnects the engine and manual transmission or transaxle. The clutch is usually placed in between the back of the engine and the front of the transmission. 

With avoiding a few exceptional cases, the clutches common to the Naval Construction Force (NCF) equipments are of the single, double, and multiple-disc types. The clutch that mostly used in automobiles is the single-disc type (Figure 1). 

Figure 1: Single Disc Clutch

The double-disc clutch is nearly the same as of the single-disc, except that another driven disc and an intermediate driving plate are to be added. This type of clutch is mostly used in heavy-duty vehicles and in construction equipments. The multiple-disc clutch is used in the automatic transmissions and for the steering clutch used in tracked equipment and in motorcycles also.

Operating principles, different component functions and maintenance requirements are essentially the same for all types of three clutches mentioned. This being the case, the single-disc clutch will be used to understand you with the fundamentals of the clutch.

A) Clutch Construction

The clutch is the first unit in drive train component which is powered by the engine crankshaft. The clutch lets the driver control power flow between the engine and transmission or transaxle. Before understanding the operation of a clutch, we must know about the parts and their functions. This information is very useful when we start learning to diagnose and repair the clutch assembly.

1. Clutch Release Mechanism

A clutch release mechanism allows that the operator to operate the clutch. Generally, this mechanism consists of the clutch pedal assembly, a mechanical linkage, cable, or hydraulic circuit, and the clutch fork. Some of the manufacturers include the release bearing as part a of the clutch release mechanism.

2. Manual Transmission

2.1 Linkage

Clutch linkage mechanism uses different levers and rods to transfer the motion from the clutch pedal to the clutch fork. One of the configurations is shown in Figure 2. When the clutch pedal is pressed by the driver, a pushrod shoves on the bell crank lever and the bell crank reverses the forward movement of the clutch pedal. 

Figure 2: Clutch Linkage

And the other end of the bell crank lever is connected to the release rod. Then the release rod transfers the bell crank lever movement to the clutch fork. It also provides adjustment method for the clutch.

2.2 Cable

Clutch cable mechanism uses a steel cable which is covered with flexible housing to transfer the pedal movement to the clutch fork. As shown in Figure 3, the cable is generally fastened at the upper end of the clutch pedal, with the other end of the cable connected to the clutch fork. 

Figure 3:  Clutch Cable

The cable housing is mounted in a stationary position in such a way that it allows the cable to slide or move inside the housing whenever the clutch pedal is press or release. At one end of the clutch cable housing having a threaded sleeve for the clutch adjustment.

3. Hydraulic

Hydraulic clutch release mechanism working on a simple hydraulic circuit which helps to transfer the clutch pedal action to the clutch fork (Figure 4). It has three basic parts that are master cylinder, hydraulic lines, and a slave cylinder. 

Figure 4: Hydraulic Clutch

Movement of the clutch pedal generates hydraulic pressure in the master cylinder, which is actuating the slave cylinder. Then the slave cylinder moves the clutch fork.

3.1 Slave Cylinder with Clutch Master Cylinder

Master cylinder is the main controlling cylinder which develops the hydraulic pressure. The slave cylinder is an operating cylinder that actuated by the hydraulic pressure developed by the master cylinder.

4. Clutch Fork

The clutch fork which is also called as clutch arm or release arm which transfers the motion from the release mechanism to the release bearing and pressure plate. The clutch fork coupled through a square hole in the bell housing and mounted on a pivot. 

When the clutch fork is tends to move by using the release mechanism, it pries on the release bearing to disengage the clutch. A rubber boot fitted over the clutch fork which is designed to keep away road dirt, rocks, oil, water, and other debris from entering the clutch housing.

5. Clutch Housing

Clutch housing is also called the bell housing. It bolted to the rear of the engine which encloses the clutch assembly, with the manual transmission bolted to the back of the housing. 

The lower front of the housing having a metal cover that can be able to remove for the flywheel ring gear inspection or when the engine must be separated from the clutch assembly. A hole is provided at the side of the housing for the clutch fork. It can be made of aluminum, magnesium, or cast iron.

6. Release Bearing

Release bearing also called as the throw-out bearing which includes a ball bearing and collar assembly. It controls or minimizes the friction between the pressure plate levers and the release fork. 

The release bearing is completely sealed unit pack with a lubricant. It slides over a hub sleeve which is extending out from the front of the manual transmission or transaxle and is moved by using either hydraulic or manual pressure.

7. Hydraulic Type

Hydraulic release bearing avoids the stock mechanical release bearing linkage and the slave cylinder. The release bearing mounted on the transmission face or slips over the input shaft of the transmission system. 

When the clutch pedal is to be pressed, the bearing face presses against the pressure plate to disengage the clutch of the vehicle.

8. Manual Type

Release bearing taking over the end of the clutch fork. Small spring clips which holds the bearing on the fork. Then the fork movement takes place in either direction slides the release bearing along the transmission hub sleeve.

9. Pressure Plate

Pressure plate is a spring-loaded device that can either engage or disengage the clutch plate disc and the flywheel by applying the pressure when pedal pressed. It is connected to the flywheel. 

The clutch disc fits in between the flywheel and the pressure plate. Now, there are two types of pressure plates—the one is coil spring type and another one is that diaphragm type.

9.1 Coil Spring Pressure Plate

Coil spring type pressure plate uses a small coil springs which are similar to the valve springs (Figure 5). The face of the pressure plate is a large, flat ring which comes in contacts with the clutch disc during clutch engagement. 

Figure 5: Coil Spring Pressure Plate

The back side of the pressure plate having pockets for the coil springs and brackets which used to hinging the release levers. During the clutch action, the pressure plate moves backward and force inside the clutch cover. 

The release levers are hinged inside the pressure plate to pry on and move the pressure plate face outwards from the clutch plate disc and flywheel. Small clip-type springs are fitted around the release levers to keep them rattling when gets fully released. 

The pressure plate cover fitted over the springs, the release levers, and the pressure plate face. Its main purpose is to hold the whole assembly together as a unit. Holes which are surrounded by the outer edge of the cover are for bolting the pressure plate to the flywheel.

9.2 Diaphragm Pressure Plate

As shown in (Figure 6), the diaphragm pressure plate uses a single diaphragm spring instead of the coil springs. The diaphragm spring is a large, round disc made up of spring steel. 

Figure 6: Diaphragm Pressure Plate

The spring is bent or dished and has pie-shaped segments which is running from the outer edge to the center. The diaphragm spring is mounted in the pressure plate with the outer edge which touching the back of the pressure plate face. 

The outer rim of the diaphragm is getting secured to the pressure plate and is pivoted on rings approximately 1 inch from the outer edge off the same.

By the application of pressure at the inner section of the diaphragm will cause the outer rim to move outward from the flywheel and draw the pressure plate away from the clutch disc, disengaging the clutch.

10. Clutch Disc

10.1 Wet Type

A “wet” clutch is always immersed in a cooling lubricating fluid, which also keeps the surfaces to be clean and gives smoother performance enhancements and longer life. Wet clutches tends to lose some energy to the lubricating liquid. 

Since, the surfaces of wet clutch can be slippery in condition, stacking with multiple clutch discs which can compensate for the lower coefficient of friction and so eliminate the slippage under the power when fully engaged. 

Wet clutches are designed in such a way that which provides a long, service-free life. They often last the entire life of the machine in which they are installed on. For maintenance, we must need to refer the manufacturer’s manual.

10.2 Dry Type

Clutch disc which is also called as friction lining is a “dry” clutch and it consists of a splined hub and a round metal plate which is covered with friction material (lining). 

The splines in the center of the clutch disc meshing with the splines which are on the input shaft of the manual transmission. This condition makes the input shaft and disc to be turn together. 

However, the disc is free to slide backward and forward on the shaft. Clutch disc torsion springs which are also termed as damping springs, absorbs some of the vibration and shock which produced by the clutch engagement. 

Damping springs are small coil springs which located in between the clutch disc splined hub and the friction disc assembly. When the clutch is getting engaged, the pressure plate jams the stationary disc against the spinning flywheel. 

The torsion springs get compress and soften as the disc first begins to turn with the flywheel. Clutch disc facing springs which are also called as the cushioning springs. 

These are flat metal springs located under the friction lining of the disc. These springs have a slight wave or curve, which allowing the lining to flex inward slightly during the initial engagement and also allows for smooth engagement. 

The clutch disc friction material, also called as disc lining or facing, which is made up of heat resistant asbestos, cotton fibers, and copper wires woven or molded together. Grooves are cut into the friction material to provide aid cooling and release of the clutch disc. Rivets are used to bond the friction material on to the both sides of the metal body of the disc.

11. Flywheel

The flywheel is the main mounting surface for the clutch (Figure 7). The pressure plate bolted to the flywheel face. The clutch disc is clamped and held against the flywheel by using the spring action of the pressure plate. 

Figure 7: Flywheel and Pilot Bearing

Face of the flywheel is precision machined which is a smooth surface. The face of the flywheel that touches the clutch disc which is made up of iron. Even if the flywheel were aluminum, the face is made of iron because it wears well and helps to dissipates heat better.

12. Pilot Bearing

The pilot bearing or bushing is to be pressed into the end of the crankshaft to support the end of the transmission input shaft (Figure 7). The pilot bearing is a solid bronze bushing, but it can also be a roller or a ball bearing. 

The end of the transmission input shaft has a small journal which is machined on its end. This journal slides at the inner side of the pilot bearing. Pilot bearing prevents the transmission shaft and clutch disc from the effect of wobbling up and down when the clutch is released. It can also assist the input shaft at center of the disc on the flywheel.

B. Clutch Operation

When the operator or driver of the vehicle presses the clutch pedal, the clutch release mechanism pulls or pushes on the clutch release lever or clutch fork (Figure 8).

 Figure 8:  Clutch Operation        

The fork moves the release bearing into the center of the pressure plate, which causing the pressure plate to pull outward direction from the clutch disc which releasing the disc from the flywheel. The engine crankshaft can be turn without turning the clutch disc and transmission input shaft. 

When the operator or driver of the vehicle releases the clutch pedal, spring pressure inside the pressure plate pushes forward on the clutch disc. This action get locks the flywheel, the clutch disc, the pressure plate, and the transmission input shaft together. 

The engine again starts to rotate the transmission input shaft, the transmission gears, the drive train, and the wheels of the vehicle.

C. Clutch Start Switch

Many of the new vehicles coming in the markets incorporate a clutch start switch into the starting system. The clutch start switch is mounted on to the clutch pedal assembly. 

The clutch start switch helps to prevent the engine from cranking unless the clutch pedal is depressed fully. This switch act as a safety device that keeps the engine from possibly starting while in geared condition. 

Unless the switch is to be closed (clutch pedal depressed), that switch also prevents current from reaching the starter solenoid. With the transmission in neutral condition, the clutch start switch is bypassed so the engine will crank and get start.

D. Clutch Adjustments

Clutch adjustments are to be made to compensate the wear of the clutch disc lining and linkage between the clutch pedal and the clutch release lever. This adjustment involves setting the correct amount of free play in the release mechanism. 

Too much free play can causes the clutch to drag during clutch disengagement condition. And too little free play can also cause the clutch slippage. So, It is important for us to know that how to adjust the three types of clutch release mechanisms.

1. Clutch Linkage Adjustment

Mechanical clutch linkage is to be adjusted at the release rod which going to the release fork shown in (Figure 2). At one end of the release rod is to be threaded. 

The effective length of the release rod can be increased to raise height of the clutch pedal (decrease free travel). It can also be decrease to lower height of the clutch pedal (increase free travel). 

To change the clutch adjustment, simply loosen the release rod nuts placed on it. Turn the release rod nuts on to the threaded rod until we reached the desired free pedal travel.

2. Pressure Plate Adjustment

When a new pressure plate is to be installed, always remember that to check the pressure plate for the proper adjustments. These adjustments will ensure that proper operation of the pressure plate. 

The first adjustment ensures that proper movement of the pressure plate is to be done in relation to the cover. As shown in Figure 9, with the use of a straight edge and a scale, begin turning the adjusting screws until we obtain the proper clearance between the straight-edge and the plate as shown. 

Figure 9: Pressure Plate Adjustment

For exact measurements, please refer the manufacturer’s service manual of particular vehicle. For the second adjustment which is important for proper operation in which the release lever allows the release bearing to contact the levers simultaneously while maintaining the adequate clearance of the levers and disc or the pressure plate cover. This adjustment is called as a finger height. 

To do the adjustments for the pressure plate, place the assembly on a flat surface and measure the height of the levers, as shown in Figure 10. 

Figure 10 Pressure Plate Release Lever Adjustment

And adjust it by loosening the locknut and turning, after the proper height has been set, make sure that the locknuts are to be locked and staked properly with a punch to keep it from getting loose during the operations. Exact release lever height can be taken into consideration from the manufacturer’s service manual.

3. Clutch Cable Adjustment

Like the mechanical linkage, a clutch cable adjustment is to be required to maintain the correct pedal height and free travel according to the operator. Typically the clutch cable having an adjusting nut. 

When the nut is to be turned, the length of the cable housing get increases or decreases as per movement. To increase the clutch pedal free travel, turn the clutch cable housing nut to shorten the cable housing, and, to decrease clutch pedal free travel, turn the nut to lengthen the cable housing.

E. Hydraulic Clutch

Hydraulically operated clutch is to be adjusted by changing the length of the slave cylinder pushrod. To adjust the hydraulic clutch, simply turn the nut or nuts which are placed on the pushrod as per requirement.