Login
Find A Shop
Find Great Deals
Get Expert Advice
Market Place
Reference Library
How To Join
Take A Tour
Industry Pro?
Help Index
Site Map
Home
 
Clutch and Manual Transmission/Transaxle

Links to topics on this page


Introduction

To overcome inertia and start the vehicle moving, the automobile engine develops power that is transmitted as a twisting force (torque) from the engine crankshaft to the rear wheels. A smooth and gradual transfer of power and torque is accomplished using a clutch friction unit to engage and disengage the power flow. A transmission is used to vary the gear ratio for the best speed and power, and to provide for vehicle movement under the different conditions of starting, stopping, accelerating, maintaining speed and reversing. The various components necessary to deliver power to the drive wheels are the flywheel, pressure plate, clutch plate, release bearing, control linkages and the transmission.

CAUTION
The clutch-driven disc may contain asbestos, which has been determined to be a cancer-causing agent. Never clean clutch surfaces with compressed air! Avoid inhaling any dust from any clutch surface! When cleaning clutch surfaces, use a commercially available brake cleaning fluid.

How the clutch works
See Figure 1

The clutch is a device to engage and disengage power from the engine, allowing the vehicle to be stopped and started.

A pressure plate or "driving member" is bolted to the engine flywheel and a clutch plate or "driven member" is located between the flywheel and the pressure plate. The clutch plate is splined to the shaft extending from the transmission to the flywheel, commonly called a clutch shaft or input shaft.

When the clutch and pressure plates are locked together by friction, the clutch shaft rotates with the engine crankshaft. Power is transferred from the engine to the transmission, where it is routed through different gear ratios to obtain the best speed and power to start and keep the vehicle moving.

Figure 1 Clutch engagement and disengagement.
Clutch engagement and disengagement

The flywheel
See Figure 2

The flywheel is located at the rear of the engine and is bolted to the crankshaft. It helps absorb power impulses, resulting in a smoothly-idling engine and provides momentum to carry the engine through its operating cycle. The rear surface of the flywheel is machined flat and the clutch components are attached to it.

Figure 2 Typical clutch components.
Typical clutch components

The pressure plate
See Figures 3, 4, 5 and 6

The driving member is commonly called the pressure plate. It is bolted to the engine flywheel and its main purpose is to exert pressure against the clutch plate, holding the plate tight against the flywheel and allowing the power to flow from the engine to the transmission. It must also be capable of interrupting the power flow by releasing the pressure on the clutch plate. This allows the clutch plate to stop rotating while the flywheel and pressure plate continues to rotate.

The pressure plate consists of a heavy metal plate, coil springs or a diaphragm spring, release levers (fingers), and a cover.

When coil springs are used, they are evenly spaced around the metal plate and located between the plate and the metal cover. This places an even pressure against the plate, which in turn presses the clutch plate tight against the flywheel. The cover is bolted tightly to the flywheel and the metal plate is movable, due to internal linkages. The coil springs are arranged to exert direct or indirect tension on the metal plate, depending upon the manufacturer's design. Three release levers (fingers), evenly spaced around the cover, are used on most pressure plates to release the holding pressure of the springs on the clutch plate, allowing it to disengage the power flow.

When a diaphragm spring is used instead of coil springs, the internal linkage is necessarily different to provide an "over-center" action to release the clutch plate from the flywheel. Its operation can be compared to the operation of an oilcan. When depressing the slightly curved metal on the bottom of the can, it goes over-center and gives out a loud "clicking" noise; when released the noise is again heard as the metal returns to its original position. A click is not heard in the clutch operation, but the action of the diaphragm spring is the same as the oil can.

Figure 3 The operation of a diaphragm spring-type pressure plate can be compared to the effect of pressing the bottom of a can of oil.
The operation of a diaphragm spring-type pressure plate can be compared to the effect of pressing the bottom of a can of oil.

Figure 4 Typical clutch pressure plate assembly.
Typical clutch pressure plate assembly.

Figure 5 The underside of the pressure plate contains the friction surface, which is compressed against the clutch disc.
The underside of the pressure plate contains the friction surface which is compressed against the clutch disc.

Figure 6 The clutch pressure plate assembly is bolted to the flywheel on the back of the engine.
The clutch pressure plate assembly is bolted to the flywheel on the back of the engine.

The clutch plate
See Figures 7 and 8

The clutch plate or driven member consists of a round metal plate attached to a splined hub. The outer portion of the round plate is covered with a friction material of molded or woven asbestos and is riveted or bonded to the plate. The thickness of the clutch plate and/or facings may be warped to give a softer clutch engagement. Coil springs are often installed in the hub to help provide a cushion against the twisting force of clutch engagement. The splined hub is mated to (and turns) a splined transmission shaft when the clutch is engaged.

Figure 7 Typical clutch-driven disc plate.
Typical clutch driven disc plate.

Figure 8 The clutch disc is installed between the pressure plate assembly and the flywheel.
The clutch disc is installed between the pressure plate assembly and the flywheel.

The release bearing

The release (throw out) bearing is usually a ball bearing unit, mounted on a sleeve, and attached to the release or throwout lever. Its purpose is to apply pressure to the diaphragm spring or the release levers in the pressure plate. When the clutch pedal is depressed, the pressure of the release bearing or lever actuates the internal linkages of the pressure plate, releasing the clutch plate and interrupting the power flow. The release bearing is not in constant contact with the pressure plate. A linkage adjustment clearance should be maintained.

Power flow disengagement

Mechanical clutch activation
See Figure 9

The clutch pedal provides mechanical means for the driver to control the engagement and disengagement of the clutch. The pedal is connected mechanically to either a cable or rods, which are directly connected to the release bearing lever.

When the clutch pedal is depressed, the linkage moves the release bearing lever. The release lever is attached at the opposite end to a release bearing which straddles the transmission clutch shaft, and presses inward on the pressure plate fingers or the diaphragm spring. This inward pressure acts upon the fingers and internal linkage of the pressure plate and allows the clutch plate to move away from the flywheel, interrupting the flow of power.

Figure 9 Cross-sectional view of a typical clutch. Note the mechanical clutch linkage.
Cross-sectional view of a typical clutch. Note the mechanical clutch linkage.

Hydraulic clutch activation
See Figure 10

Hydraulic clutch activation systems consist of a master and a slave cylinder. When pressure is applied to the clutch pedal (the pedal is depressed), the pushrod contacts the plunger and pushes it up the bore of the master cylinder. During the first 1/32 in. (0.8 mm) of movement, the center valve seal closes the port to the fluid reservoir tank and as the plunger continues to move up the bore of the cylinder, the fluid is forced through the outlet line to the slave cylinder mounted on the clutch housing. As fluid is pushed down the pipe from the master cylinder, this in turn forces the piston in the slave cylinder outward. A pushrod is connected to the slave cylinder and rides in the pocket of the clutch fork. As the slave cylinder piston moves rearward the pushrod forces the clutch fork and the release bearing to disengage the pressure plate from the clutch disc. On the return stroke (pedal released), the plunger moves back as a result of the return pressure of the clutch. Fluid returns to the master cylinder and the final movement of the plunger lifts the valve seal off the seat, allowing an unrestricted flow of fluid between the system and the reservoir.

A piston return spring in the slave cylinder preloads the clutch linkage and assures contact of the release bearing with the clutch release fingers at all times. As the driven disc wears, the diaphragm spring fingers move rearward forcing the release bearing, fork and pushrod to move. This movement forces the slave cylinder piston forward in its bore, displacing hydraulic fluid up into the master cylinder reservoir, thereby providing the self-adjusting feature of the hydraulic clutch linkage system.

Figure 10 Typical clutch hydraulic actuating system components.

Click on picture to enlarge view

Power flow engagement

While the clutch pedal is depressed and the power flow interrupted, the transmission can be shifted into any gear. The clutch pedal is slowly released to gradually move the clutch plate toward the flywheels under pressure of the pressure plate springs. The friction between the clutch plate and flywheel becomes greater as the pedal is released and the engine speed increased. Once the vehicle is moving, the need for clutch slippage is lessened, and the clutch pedal can be fully released.

Coordination between the clutch pedal and accelerator is important to avoid engine stalling, shock to the driveline components and excessive clutch slippage and overheating.

How the manual transmission works

The internal combustion engine creates a twisting motion or torque, which is transferred to the drive wheels. However, the engine cannot develop much torque at low speeds; it will only develop maximum torque at higher speeds. The transmission, with its varied gear ratios, provides a means of providing this low torque to move the vehicle.

The transmission gear ratios allow the engine to be operated most efficiently under a variety of driving and load conditions. Using gear ratios, the need for extremely high engine rpm at high road speeds is avoided.

The modern transmission provides both speed and power through selected gear sizes that are engineered for the best all-around performance. A power (lower) gear ratio starts the vehicle moving and speed gear ratios keep the vehicle moving. By shifting to gears of different ratios, the driver can match engine speed to road conditions.

Gear ratios
See Figure 11

To obtain maximum performance and efficiency, gear ratios are engineered to each type of vehicle, dependent upon such items as the size of the engine, the vehicle weight and expected loaded weight, etc.

The gear ratio can be determined by counting the teeth on both gears. For example, if the driving gear has 20 teeth and the driven gear has 40 teeth, the gear ratio is 2 to 1. The driven gear makes one revolution for every two revolutions of the drive gear. If the driving gear has 40 teeth and the driven gear 20 teeth, the gear ratio is 1 to 2. The driven gear revolves twice, while the drive gear revolves once.

The transmissions used today may have four, five or six speeds forward, but all have one speed in reverse. The reverse gear is necessary because the engine rotates only in one direction and cannot be reversed. The reversing procedure must be accomplished inside the transmission.

By comparing gear ratios, you can see which transmission transmits more power to the drive wheels at the same engine rpm. The five-speed transmission's low or first gear with a ratio of 3.61 to 1 means that for 3.61 revolutions of the input or clutch shaft (coupled to the engine by the clutch), the output shaft of the transmission will rotate once. This provides more power to the drive wheels, compared to the four-speed transmission's low gear of 2.33 to 1.

When the transmission is shifted into the high gear in the four-speed transmission, and fourth gear in the five-speed transmission, the gear ratio is usually 1 to 1 (direct drive). For every rotation of the engine and input shaft, the output shaft is rotating one turn.

Fifth gear in a five-speed transmission is usually an overdrive. This gear is used for higher speed driving where very little load is placed on the engine. This gear ratio provides better economy by lowering the engine RPM to maintain a specific speed. The input shaft rotates only 0.87 of a turn, while the output shaft rotates one revolution, resulting in the output shaft rotating faster than the input shaft.

The overdrive is normally incorporated in the transmission.

Figure 11 Example of determining gear ratio. Gear ratio can be found by dividing the number of teeth on the smaller gear into the number of the teeth on the larger gear.
Example of determining gear ratio. Gear ratio can be found by dividing the number of teeth on the smaller gear into the number of the teeth on the larger gear.

Syncromesh transmission
See Figure 12

The power flow illustrated in a typical four-speed is a conventional, spur-geared transmission. To obtain a quiet operation and gear engagement, synchronizing clutches are added to the main-shaft gears. The addition of synchronizers allows the gears to be in constant mesh with the cluster gears (gears that provide a connection between input and output shafts), and the synchronizing clutch mechanism locks the gears together.

The main purpose of the synchronizer is to speed up or slow down the rotation speeds of the shaft and gear, until both are rotating at the same speeds so that both can be locked together without a gear clash.

Since the vehicle is normally standing still when shifted into reverse gear, a synchronizing clutch is ordinarily not used on reverse gear.

Figure 12 Power flow through a four-speed, fully synchronized transmission.
Click on picture to enlarge view

Five and six-speed transmissions

The power flow through the five-speed transmission can be charted in the same manner as the four-speed transmission.

As a rule, the power flow in high gear is usually straight through the transmission-input shaft to the mainshaft, which would be locked together. When in the reduction gears, the power flow is through the input shaft, to the cluster gear unit, and through the reduction gear to the mainshaft.

Transaxles
See Figures 13 and 14

When the transmission and the drive axle are combined in one unit, it is called a "transaxle." The transaxle is bolted to the engine and has the advantage of being an extremely rigid unit of engine and driveline components. The complete engine transaxle unit may be located at the front of the vehicle (front wheel drive) or at the rear of the vehicle (rear wheel drive).

The power flow through the transmission section of the transaxle is the same as through a conventional transmission.

Figure 13 Transaxles combine the transmission and differential into one unit.
Click on picture to enlarge view

Figure 14 This simple illustration shows the power flow through a front wheel drive transaxle. Note how the direction of rotation of the engine crankcase and the axle shafts (attached to the wheels) are the same. This is made possible using an intermediate shaft.
Click on picture to enlarge view

Figure 16 Clutch and transmission maintenance intervals.
Click on picture to enlarge view

Troubleshooting basic clutch and manual transmission problems

Back to top of page

Return to ProCarCare Car Care Encyclopedia index


1998 W. G. Nichols - Chilton's Easy Car Care