What connects the shifter to the transmission?

The transmission shift lever assembly can be moved to cause movement of the shift linkage, shift forks, and synchronizers. The shift lever may be either floor mounted or column mounted, depending upon the manufacturer. Floor-mounted shift levers are generally used with an internal shift rail linkage, whereas column-mounted shift levers are generally used with an external rod linkage.

TRANSMISSION TYPES
Manual transmissions are of three major types: 1. Sliding gear 2. Constant mesh 3. Synchromesh A quick overview of the three types is as follows:

The sliding gear transmission has two or more shafts mounted in parallel or in line, with sliding spur gears arranged to mesh with each other and provide a change in speed or direction.

The constant mesh transmission has parallel shafts with gears in constant mesh. Shifting is done by locking free-running gears to their shaft by using sliding collars.

The synchromesh transmission also has gears in constant mesh. However, gears can be selected without clashing or grinding by synchronizing the speeds of the mating part before they engage.

The sliding gear transmission is generally used in farm and industrial machines; therefore, we will only look closely at the constant mesh and synchromesh transmissions.

Constant Mesh Transmission
To eliminate the noise developed by the spur-tooth gears used in the sliding gear transmission, automotive manufacturers developed the constant mesh trans-mission, also known as the collar shift transmission (fig. 4-18). The constant mesh transmission has parallel shafts with gears in constant mesh. In neutral,

Figure 4-18.- Constant mesh transmission assembly.

the gears are free running but, when shifted, they are the synchronizing clutch mechanisms lock the gears locked to their shafts by sliding collars. together.

The following is an example of the operation of a constant mesh transmission: When the shift lever is moved to THIRD, the THIRD and FOURTH shifter fork moves the sliding collar toward the THIRD speed gear. This engages the external teeth of the sliding collar with the internal teeth of the THIRD speed gear. Since the THIRD speed gear is meshed and rotating with the countershaft, the sliding collar must also rotate. The sliding collar is splined to the main shaft, and therefore, the main shaft rotates with the sliding collar. This principle is carried out when the shift lever moves from one speed to the next.

The synchronizer accelerates or slows down the rotation of the shaft and gear, until both are rotating at the same speed and can be locked together without a gear clash. Since the vehicle is normally standing still when it is shifted into reverse gear, a synchronizer is not ordinarily used on the reverse gear.

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Figure 4-13.- Transmission input shaft and bearing.

the countershaft gears, the countershaft gears turns the main shaft gears, and, when engaged, the reverse idler gear.

In low gear, a small gear on the countershaft drives a larger gear on the main shaft, providing for a high gear ratio for accelerating. Then, in high gear, a larger countershaft gear turns a small main shaft gear or a gear of equal size, resulting in a low gear ratio, allowing the vehicle to move faster. When reverse is engaged, power flows from the countershaft gear, to the reverse idler gear, and to the engaged main shaft gear. This action reverses main shaft rotation.

Synchronizers
The synchronizer is a drum or sleeve that slides back and forth on the splined main shaft by means of the shifting fork. Generally, it has a bronze cone on each side that engages with a tapered mating cone on the second-and high-speed gears. A transmission synchronizer (fig. 4-17) has two functions, which are as follows:

1. Lock the main shaft gear to the main shaft.

2. Prevent the gear from clashing or grinding during shifting.

When the synchronizer is moved along the main shaft, the cones act as a clutch. Upon touching the gear that is to be engaged, the main shaft is acceler-ated or slowed down until the speeds of the main shaft and gear are synchronized. This action occurs during partial movement of the shift lever. Com-pletion of lever movement then slides the sleeve and gear into complete engagement. This action can be readily understood by remembering that the hub of the sleeve slides on the splines of the main shaft to engage the cones; then the sleeve slides on the hub to engage the gears. As the synchronizer is slid against a gear, the gear is locked to the synchronizer and to the main shaft. Power can then be sent out of the transmission to the wheels.

Shift Forks
Shift forks fit around the synchronizer sleeves to transfer movement to the sleeves from the shift

Figure 4-17.- Synchronizers.

linkage. The shift fork sets in a groove cut into the synchronizer sleeve. The linkage rod or shifting rail connects the shift fork to the operator's shift lever. As the lever moves, the linkage or rail moves the shift fork and synchronizer sleeve to engage the correct transmission gear.

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A manual transmission is designed with two purposes in mind. One purpose of the transmission is providing the operator with the option of maneuvering the vehicle in either the forward or reverse direction. This is a basic requirement of all automotive vehicles. Almost all vehicles have multiple forward gear ratios, but, in most cases, only one ratio is provided for reverse.

Another purpose of the transmission is to provide the operator with a selection of gear ratios between engine and wheel so that the vehicle can operate at the best efficiency under a variety of operating conditions and loads. If in proper operating condition, a manual transmission should do the following:

Be able to increase torque going to the drive wheel for quick acceleration.

Supply different gear ratios to match different engine load conditions.

Have a reverse gear for moving the vehicle backwards.

Provide the operator with an easy means of shifting transmission gears.

Operate quietly with minimum power loss.

TRANSMISSION CONSTRUCTION
Before understanding the operation and power flow through a manual transmission, you first must understand the construction of the transmission. This is necessary for you to be able to diagnose and repair damaged transmissions properly.

Transmission Case
The transmission case provides support for the bearings and shafts, as well as an enclosure for lubricating oil. A manual transmission case is cast from either iron or aluminum. Because they are lighter in weight, aluminum cases are preferred.

A drain plug and fill plug are provided for servicing. The drain plug is located on the bottom of the case, whereas the fill plug is located on the side.

Extension Housing
Also known as the tail shaft, the extension housing bolts to the rear of the transmission case. It encloses and holds the transmission output shaft and rear oil seal. A gasket is used to seal the mating surfaces between the transmission case and the extension housing. On the bottom of the extension housing is a flange that provides a base for the transmission mount.

Front Bearing Hub
Sometimes called the front bearing cap, the bearing hub covers the front transmission bearing and acts as a sleeve for the clutch release bearing. It bolts to the transmission case and a gasket fits between the front hub and the case to prevent oil leakage.

Transmission Shafts
A manual transmission has four steel shafts mounted inside the transmission case. These shafts are the input shaft, the countershaft, the reverse idler shaft, and the main shaft.

INPUT SHAFT.- The input shaft, also known as the clutch shaft, transfers rotation from the clutch disc to the countershaft gears (fig. 4-13). The outer end of the shaft is splined except the hub of the clutch disc. The inner end has a machined gear that meshes with the countershaft. A bearing in the transmission case supports the input shaft in the case. Anytime the clutch disc turns, the input shaft gear and gears on the countershaft turn.

COUNTERSHAFT.- The countershaft, also known as the cluster gear shaft, holds the countershaft gear into mesh with the input shaft gear and other gears in the transmission (fig. 4-14). It is located slightly below and to one side of the clutch shaft. The countershaft does not turn in the case. It is locked in place by either a steel pin, force fit, or locknuts.

REVERSE IDLER SHAFT.- The reverse idler shaft is a short shaft that supports the reverse idle gear (fig. 4-15). It mounts stationary in the transmission case about halfway between the countershaft and output shaft, allowing the reverse idle gear to mesh with both shafts.

MAIN SHAFT.- The main shaft, also called the output shaft, holds the output gears and synchronizers (fig. 4-16). The rear of the shaft extends to the rear of the extension housing where it connects to the drive shaft to turn the wheel of the vehicle. Gears on the shaft are free to rotate, but the synchronizers are locked on the shaft by splines. The synchronizers will only turn when the shaft itself turns.

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Transmission or transaxle removal is required to service the clutch. Always follow the detailed directions in the service manual. To remove the clutch in a rear-wheel drive vehicle, remove the drive shaft, the clutch fork, the clutch release mechanism, and the transmission. With a front-wheel drive vehicle, the axle shafts (drive axles), the transaxle, and, in some cases, the engine must be removed for clutch repairs.

WARNING
When the transmission or transaxle is removed, support the weight of the engine. Never let the engine, the transmission, or the transaxle be unsupported. The transmission input shaft, clutch fork, engine mounts, and other associated parts could be damaged.

After removal of the transmission or transaxle, remove the clutch housing from the rear of the engine. Support the housing as you remove the last bolt. Be careful not to drop the clutch housing as you pull it away from the dowel pins.

Using a hammer and a center punch, mark the pressure plate and flywheel. These marks are needed when reinstalling the same pressure plate to assure correct balancing of the clutch.

With the clutch removed, all components are to be cleaned and inspected for wear and damage. After cleaning, you should inspect the flywheel and pressure plate for signs of unusual wear, such as scoring or cracks. A straightedge should be used to check for war-page of the pressure plate. Using a dial indicator, measure the runout of the flywheel. The pressure plate release levers should show very limited or no signs of wear from contact with the release bearing. If excessive wear, cracks, or warpage is noted on the flywheel and/ or pressure plate, the assembly should be replaced. This is also a good time to inspect the ring gear teeth on the flywheel. If they are worn or chipped, a new ring gear should be installed.

NOTE

Be careful how you clean the parts of the clutch. Avoid using compressed air to blow clutch dust from the parts. A clutch disc con-tains asbestos- a cancer-causing substance.

While inspecting the flywheel, you should check the pilot bearing in the end of the crankshaft. A worn pilot bearing will allow the transmission input shaft and clutch disc to wobble up and down. Using a telescoping gauge and a micrometer, measure the amount of wear in the bushing. For wear measurements of the pilot bearing, refer to the service manual. If a roller bearing is used, rotate them. They should turn freely and show no signs of rough movement. If replacement of the pilot bearing is required, the use of a slide hammer puller will drive the bearing out of the crankshaft end. Before installing a new pilot bearing, check the fit by sliding it over the input shaft of the transmission. Then drive the new bearing into the end of the crankshaft.

Inspect the disc for wear; inspect the depth of the rivet holes, loose rivets, and worn or broken torsion springs. Check the splines in the clutch disc hub for a "like new" condition. The clutch shaft splines should be inspected by placing the disc on the clutch shaft and sliding it over the splines. The disk should move relatively free back and forth without any unusual tightness or binding. Normally, the clutch disc is replaced anytime the clutch is tom down for repairs.

Another area to inspect is the release bearing. The release bearing and sleeve is usually sealed and factory packed (lubricated). A bad release bearing will produce a grinding noise whenever the clutch pedal is pushed down. To check the action of the release bearing, insert your fingers into the bearing; then turn the bearing while pushing on it. Try to detect any roughness; it should rotate smoothly. Also, inspect the spring clip on the release bearing or fork. If bent, worn, or fatigued, the bearing or fork must be replaced.

The last area to check before reassembly is the clutch fork. If it is bent or worn, the fork can prevent the clutch from releasing properly. Inspect both ends of the fork closely. Also, inspect the clutch fork pivot point in the clutch housing; the pivot ball or bracket should be undamaged and tight.

When a new pressure plate is installed, do not forget to check the plate for proper adjustments. These adjustments will ensure proper operation of the pressure plate. The first adjustment ensures proper movement of the pressure plate in relation to the cover. With the use of a straightedge and a scale as shown in figure 4-11, begin turning the adjusting screws until you obtain the proper clearance between the straight-edge and the plate as shown. For exact measurements, refer to the manufacturer's service manual.

The second adjustment positions the release levers and allows the release bearing to contact the levers simultaneously while maintaining adequate clearance of the levers and disc or pressure plate cover. This adjustment is known as finger height. To adjust the pressure plate, place the assembly on a flat surface and measure the height of the levers, as shown in figure 4-12. Adjust it by loosening the locknut and turning. After the proper height has been set, make sure the locknuts are locked and staked with a punch to keep them from coming loose during operations. Exact release lever height can be found in the manufacturer's service manual.

Reassemble the clutch in the reverse order of disassembly. Mount the clutch disc and pressure plate on the flywheel. Make sure the disc is facing in the right direction. Usually, the disc's offset center (hub and torsion springs) fit into the pressure plate.

Figure 4-11.- Pressure plate adjustment.

Figure 4-12.- Pressure plate release lever adjustment.

If reinstalling, the old pressure plate lines up the alignment marks made before disassembly. Start all of the pressure plates bolts by hand. Never replace a clutch pressure plate bolt with a weaker bolt. Always install the special case-hardened bolt recommended by the manufacturer.

Use a clutch alignment tool to center the clutch disc on the flywheel. If an alignment tool is unavailable, an old clutch shaft from the same type of vehicle may be used. Tighten each pressure plate bolt a little at a time in a crisscross pattern. This will apply equal pressure on each bolt, as the pressure plate spring( s) are compressed. When the bolts are snugly in place, torque them to the manufacturer's specifications found in the service manual. Once the pressure plates bolts are torqued to specs, slide out the alignment tool. Without the clutch disc being centered, it is almost impossible to install the transmission or transaxle.

Next install the clutch fork and release bearing in the clutch housing. Fit the clutch housing over the rear of the engine. Dowels are provided to align the housing on the engine. Install and tighten the bolts in a crisscross manner.

Install the transmission and drive shaft or the transaxle and axle shafts. Reconnect the linkages, the cables, any wiring, the battery, and any other parts required for disassembly. After all parts have been installed, adjust the clutch pedal free travel as prescribed by the manufacturer and test-drive the vehicle for proper operation.

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Operator abuse commonly causes premature clutch troubles. For instance, "riding the clutch"

Figure 4-10.- Master cylinder, slave cylinder, and connections for a typical hydraulic clutch.

(overslipping clutch upon acceleration), resting your foot on the clutch pedal while driving, and other driving errors can cause early clutch failure.

When a vehicle enters the shop for clutch troubles, you should test-drive the vehicle. While the vehicle is being test-driven, you should check the action of the clutch pedal, listen for unusual noises, and feel for clutch pedal vibrations. Gather as much information as you can on the operation of the clutch. Use this information, your knowledge of clutch principles, and a service manual-troubleshooting chart to determine which components are faulty.

There are five types of clutch problems- slipping, grabbing, dragging, abnormal noises, and vibration. It is important to know the symptoms produced by these problems and the parts that might be the cause.

Slipping
Slipping occurs when the driven disc fails to rotate at the same speed as the driving members when the clutch is fully engaged. This condition results whenever the clutch pressure plate fails to hold the disc tight against the face of the flywheel. If clutch slippage is severe, the engine speed will rise rapidly on acceleration, while the vehicle gradually increases in speed. Slight but continuous slippage may go unnoticed until the clutch facings are ruined by excessive temperature caused by friction.

Normal wear of the clutch lining causes the free travel of the clutch linkage to decrease, creating the need for adjustment. Improper clutch adjustment can cause slippage by keeping the release bearing in contact with the pressure plate in the released position. Even with your foot off the pedal, the release mechanism will act on the clutch fork and release bearing.

Some clutch linkages are designed to allow only enough adjustment to compensate for the lining to wear close to the rivet heads. This prevents damage to the flywheel and pressure plate by the rivets wearing grooves in their smooth surfaces.

Other linkages will allow for adjustment after the disc is worn out. When in doubt whether the disc is worn excessively, remove the inspection cover on the clutch housing and visually inspect the disc.

Binding linkage prevents the pressure plate from exerting its full pressure against the disc, allowing it to slip. Inspect the release mechanism for rusted, bent, misaligned, sticking, or damaged components. Wiggle the release fork to check for free play. These problems result in slippage.

A broken motor mount (engine mount) can cause clutch slippage by allowing the engine to move, binding the clutch linkage. Under load, the engine can lift up in the engine compartment, shifting the clutch linkage and pushing on the release fork.

Grease and oil on the disc will also cause slippage. When this occurs, locate and stop any leakage, thoroughly clean the clutch components, and replace the clutch disc. This is the only remedy.

If clutch slippage is NOT caused by a problem with the clutch release mechanism, then the trouble is normally inside the clutch. You have to remove the transmission and clutch components for further inspection. Internal clutch problems, such as weak springs and bent or improperly adjusted release levers, will prevent the pressure plate from applying even pressure. This condition allows the disc to slip.

To test the clutch for slippage, set the emergency brake and start the engine. Place the transmission or transaxle in high gear. Then try to drive the vehicle forward by slowly releasing the clutch pedal. A clutch in good condition should lock up and immediately kill the engine. A badly slipping clutch may allow the engine to run, even with the clutch pedal fully released. Partial clutch slippage could let the engine run momentarily before stalling.

NOTE
Never let a clutch slip for more than a second or two. The extreme heat generated by slippage will damage the flywheel and pressure plate faces.

Grabbing
A grabbing or chattering clutch will produce a very severe vibration or jerking motion when the vehicle is accelerated from a standstill. Even when the operator slowly releases the clutch pedal, it will seem like the clutch pedal is being pumped rapidly up and down. A loud bang or chattering may be heard, as the vehicle body vibrates.

Clutch grabbing and chatter is caused by problems with components inside the clutch housing (friction disc, flywheel, or pressure plate). Other reasons for a grabbing clutch could be due to oil or grease on the disc facings, glazing, or loose disc facings. Broken parts in the clutch, such as broken disc facings, broken facing springs, or a broken pressure plate, will also cause grabbing.

There are several things outside of the clutch that will cause a clutch to grab or chatter when it is being engaged. Loose spring shackles or U-bolts, loose transmission mounts, and worn engine mounts are among the items to be checked. If the clutch linkage binds, it may release suddenly to throw the clutch into quick engagement, resulting in a heavy jerk. However, if all these items are checked and found to be in good condition, the trouble is inside the clutch itself and will have to be removed for repair.

Dragging
A dragging clutch will make the transmission or transaxle grind when trying to engage or shift gears. This condition results when the clutch disc does not completely disengage from the flywheel or pressure plate when the clutch pedal is depressed. As a result, the clutch disc tends to continue turning with the engine and attempts to drive the transmission.

The most common cause of a dragging clutch is too much clutch pedal free travel. With excessive free travel, the pressure plate will not fully release when the clutch pedal is pushed to the floor. Always check the clutch adjustments first. If adjustment of the linkage does not correct the trouble, the problem is in the clutch, which must be removed for repair.

On the inside of the clutch housing, you will generally find a warped disc or pressure plate, oil or grease on the friction surface, rusted or damaged transmission input shaft, or improper adjustment of the pressure plate release levers causing the problem.

Abnormal Noises
Faulty clutch parts can make various noises. When an operator reports that a clutch is making noise, find out when the noise is heard. Does the sound occur when the pedal is moved, when in neutral, when in gear, or when the pedal is held to the floor? This will assist you in determining which parts are producing these noises.

An operator reports hearing a scraping, clunking, or squeaking sound when the clutch pedal is moved up or down. This is a good sign of a worn or unlubricated clutch release mechanism. With the engine off, pump the pedal and listen for the sound. Once the source of the sound is located, you should clean, lubricate, or replace the parts as required.

Sounds produced from the clutch, when the clutch is initially ENGAGED, are generally due to friction disc problems, such as a worn clutch disc facing, which causes a metal-to-metal grinding sound. A rattling or a knocking sound may be produced by weak or broken clutch disc torsion springs. These sounds indicate problems that require the removal of the transmission and clutch assembly for repair.

If clutch noises are noticeable when the clutch is DISENGAGED, the trouble is most likely the clutch release bearing. The bearing is probably either worn, binding, or, in some cases, loses its lubricant. Most clutch release bearings are factory lubricated; however, on some larger trucks and construction equipment, the bearing requires periodic lubrication. A worn pilot bearing may also produce noises when the clutch is disengaged. The worn pilot bearing can let the transmission input shaft and clutch disc vibrate up and down, causing an unusual noise.

Sounds heard in NEUTRAL, that disappear when the clutch pedal is pushed, are caused by problems inside the transmission. Many of these sounds are due to worn bearings. However, always refer to the troubleshooting chart in the manufacturer's manual.

Pedal Pulsation
A pulsating clutch pedal is caused by the runout (wobble or vibration) of one of the rotating members of the clutch assembly. A series of slight movements can be felt on the clutch pedal. These pulsations are noticeable when light foot pressure is applied. This is an indication of trouble that could result in serious damage if not corrected immediately. There are several conditions that can cause these pulsations. One possible cause is misalignment of the transmission and engine.

If the transmission and engine are not in line, detach the transmission and remove the clutch assembly. Check the clutch housing alignment with the engine and crankshaft. At the same time, the flywheel can be checked for runout, since a bent flywheel or crankshaft flange will produce clutch pedal pulsation. If the flywheel does not seat on the crankshaft flange, remove the flywheel. After cleaning the crankshaft flange and flywheel, replace the flywheel, making sure a positive seat is obtained between the flywheel and the flange. If the flange is bent, the crankshaft must be replaced.

Other causes of clutch pedal pulsation include bent or maladjusted pressure plate release levers, warped pressure plate, or warped clutch disc. If either the clutch disc or pressure plate is warped, they must be replaced.

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CLUTCH ADJUSTMENT
Clutch adjustments are made to compensate for wear of the clutch disc lining and linkage between the clutch pedal and the clutch release lever. This involves setting the correct amount of free play in the release mechanism. Too much free play causes the clutch to drag during clutch disengagement. Too little free play causes clutch slippage. It is important for you to know how to adjust the three types of clutch release mechanisms.

Clutch Linkage Adjustment
Mechanical clutch linkage is adjusted at the release rod going to the release fork (fig. 4-9). One end of the release rod is threaded. The effective length of the rod can be increased to raise the clutch pedal (decrease free travel). It can also be shortened to lower the clutch pedal (increase free travel).

To change the clutch adjustment, loosen the release rod nuts. Turn the release rod nuts on the threaded rod until you have reached the desired free pedal travel.

Clutch Cable Adjustment
Like the mechanical linkage, a clutch cable adjustment may be required to maintain the correct pedal height and free travel. Typically the clutch cable will have an adjusting nut. When the nut is turned, the length of the cable housing increases or decreases. To increase clutch pedal free travel, turn the clutch cable housing nut to shorten the housing, and, to decrease clutch pedal free travel, turn the nut to lengthen the housing.

Figure 4-9.- Clutch pedal and linkage.

Hydraulic Clutch Adjustment
The hydraulically operated clutch shown in figure 4-10 is adjusted by changing the length of the slave cylinder pushrod. To adjust a hydraulic clutch, simply turn the nut or nuts on the pushrod as needed.

NOTE

When a clutch adjustment is made, refer to the manufacturer's service manual for the correct method of adjustment and clearance. If no manuals are available, an adjustment that allows 1 1/ 2 inches of clutch pedal free travel will allow adequate clutch operation until the vehicle reaches the shop and manuals are available.

Page 7

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.

Flywheel
The flywheel is the mounting surface for the clutch. 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. Even if the flywheel were aluminum, the face is iron because it wears well and dissipates heat better.

Pilot Bearing
The pilot bearing or bushing is pressed into the end of the crankshaft to support the end of the transmission input shaft. 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.

CLUTCH OPERATION
When the operator presses the clutch pedal, the clutch release mechanism pulls or pushes on the clutch release lever or fork (fig. 4-8). The fork moves the release bearing into the center of the pressure plate, causing the pressure plate to pull away from the clutch disc releasing the disc from the flywheel. The engine crankshaft can then turn without turning the clutch disc and transmission input shaft.

When the operator releases the clutch pedal, spring pressure inside the pressure plate pushes forward on the clutch disc (fig. 4-8). This action locks the

Figure 4-8.- Clutch operation.

flywheel, the clutch disc, the pressure plate, and the transmission input shaft together. The engine again rotates the transmission input shaft, the transmission gears, the drive train, and the wheels of the vehicle.

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Clutch Release Mechanism
A clutch release mechanism allows the operator to operate the clutch. Generally, it consists of the clutch pedal assembly, either mechanical linkage, cable, or

Figure 4-4.- Clutch cable mechanism.

A hydraulic clutch release mechanism (fig. 4-5) uses a simple hydraulic circuit to transfer clutch pedal action to the clutch fork. It has three basic parts- master cylinder, hydraulic lines, and a slave cylinder.

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.

Clutch Fork
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.

Release Bearing
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.

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.

Pressure Plate 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 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

Figure 4-5.- Hydraulic clutch release mechanism.

between the flywheel and the pressure plate. There are two types of pressure plates- the coil spring type and the diaphragm type.

Coil spring pressure plate uses small coil springs similar to valve springs (fig. 4-6). The face of the pressure plate is a large, flat ring that contacts the clutch disc during clutch engagement. The backside 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.

Diaphragm pressure plate (fig. 4-7) uses a single diaphragm spring instead of coil springs. This type of pressure plate functions similar to that of the coil spring type. 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 (pivot rings) approximately 1 inch from the outer edge.

Figure 4-6.- Coil spring pressure plate.

Figure 4-7.- Diaphragm pressure plate operation.

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.

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INTRODUCTION AUTOMOTIVE CLUTCHES
Learning Objective: State the operating principles and identify the components and the maintenance for a clutch, a manual transmission, an automatic transmission, and a transaxle.

Learning Objective: State the operating principles and identify the components and maintenance requirements for an automotive clutch.

In a vehicle, the mechanism that transmits the power developed by the engine to the wheels and/ or tracks and accessory equipment is called the power train. In a simple application, such as a stationary engine-powered hoist, a set of gears or a chain and sprocket could perform this task. However, auto-motive and construction equipment are not designed for such simple operating conditions. They are designed to provide pulling power, to move at high speeds, to travel in reverse as well as forward, and to operate on rough terrain as well as smooth roads. To meet these varying conditions, vehicle power trains are equipped with a variety of components. This chapter discusses the basic automotive clutch, transmissions (manual and automatic), and transaxles (manual and automatic).

An automotive clutch is used to connect and disconnect the engine and manual (hand-shifted) transmission or transaxle. The clutch is located between the back of the engine and the front of the transmission.

With a few exceptions, the clutches common to the Naval Construction Force (NCF) equipment are the single-, double-, and multiple-disc types. The clutch that you will encounter the most is the single-disc type, as shown in figure 4-1. The double-disc clutch (fig. 4-2) is substantially the same as the single disc, except that another driven disc and an intermediate driving plate are added. This clutch is used in heavy-duty vehicles and construction equipment. The multiple-disc clutch is used in the automatic transmission and for the steering clutch used in tracked equipment.

Figure 4-1.- Single-disc clutch.

The operating principles, component functions, and maintenance requirements are essentially the same for each of the three clutches mentioned. This being the case, the single-disc clutch will be used to acquaint you with the fundamentals of the clutch. hydraulic circuit, and the clutch fork. Some manufacturers include the release bearing as part of the clutch release mechanism.

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 4-3. When the pedal is pressed, a pushrod shoves on the bell crank and the bell 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.

The clutch cable mechanism uses a steel cable inside a flexible housing to transfer pedal movement to the clutch fork. As shown in figure 4-4, 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.

Page 10

Removing lines from various components throughout the system and then attempting to pressurize the system, causing a high rate of air flow through the system, does complete purging. The air flow will cause the foreign matter to be dislodged and blown from the system.

NOTE
If an excessive amount of foreign matter, particularly oil, is blown from any one system, the lines and components should be removed and cleaned or, in some cases, replaced.

In addition to monitoring the devices installed to remove contamination, it is your responsibility as a mechanic to control the contamination. You can do this by using the following maintenance practices:

Keep all tools and the work area in a clean, dirt-free condition.

Cap or plug all lines and fittings immediately after disconnecting them.

Replace all packing and gaskets during assembly procedures.

Connect all parts with care to avoid stripping metal slivers from threaded areas. Install and torque all fittings and lines according to applicable technical manuals.

Figure 3-55.- Demister (separator element).

POTENTIAL HAZARDS
All compressed gases are hazardous. Compressed air and nitrogen are neither poisonous nor flammable, but should be handled with care. Some pneumatic systems operate at pressures exceeding 3,000 psi. Lines and fittings have exploded, injuring personnel and property. Literally thousands of careless workers have blown dust or other harmful particles into their eyes by careless handling of compressed air outlets.

If you ever have to handle nitrogen gas, remember that it will not support life, and when released in a confined space, it will cause asphyxia (the loss of consciousness as a result of too little oxygen and too much carbon dioxide in the blood). Although compressed air and nitrogen seem safe in comparison with other gases, do not let overconfidence lead to personal injury.

SAFETY PRECAUTIONS
To minimize personal injury and equipment damage when using compressed gases, observe all

SAFETY PRECAUTIONS
To minimize personal injury and equipment damage when using compressed gases, observe all practical operating safety precautions, including the following:

Do NOT use compressed air to clean parts of your body or clothing or to perform general space cleanup instead of sweeping.

NEVER attempt to stop or repair a leak while the leaking portion is still under pressure. Always isolate. depressurize. and tag out the portion of the system to be repaired.

Avoid the application of heat to the air piping system or components, and avoid striking a sharp, heavy blow on any pressurized part of the piping system.

Avoid rapid operation of manual valves. The heat of compression caused by a sudden high-pressure flow into an empty line or vessel can cause an explosion if oil is present. Valves should be slowly cracked open until air flow is noted and should be kept in this position until pressures on both sides of the valve have equalized. The rate of pressure rise should be kept under 200 psi per second, if possible. Valves may then be opened fully.

Do NOT subject compressed gas cylinders to temperatures greater than 130 F. Remember, any pressurized system can be hazardous to your health if it is not maintained and operated carefully and safely.

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an automatic blowdown valve for releasing air pressure when the engine is stopped,

a valve for draining moisture that accumulates in the receiver tank,

a drain cock at the bottom of the piping at the bottom of the oil storage tank,

an air filter service indicator to show when the filter needs servicing, and

a demister, or special filter, that separates lubricating oil from compressed air.

Remember a good maintenance program is the key to a long machine life. So it is up to both the operator and the mechanic to ensure that the maintenance is performed on time every time.

Air Cleaner Servicing
The air cleaner contains a primary and secondary dry filter element (fig. 3-54). An air filter restriction indicator is located at the rear of the air filter housing to alert the operator of the need to service the filters. When a red band appears in the air filter restriction indicator, secure the compressor and service the filters.

The primary element is cleanable by using compressed air. When the element is cleaned, never let the air pressure exceed 30 psi. The secondary filter is not cleanable and should be replaced when necessary.
Reverse flush the primary element by directing compressed air up from the inside out. Continue reverse flushing until all dust is removed. Should any oil or greasy dirt remain on the filter surface, the element should be replaced. When the element is satisfactorily cleaned, inspect it thoroughly before installation. Inspection procedures are as follows:

Place a bright light inside the element to inspect it for damage. Concentrated light will shine through the element and disclose any holes. A damaged element is to be replaced.

Inspect all gaskets and gasket contact surfaces of the housing. Should faulty gaskets be evident. replace them immediately.

After the element has been installed, inspect and tighten all air inlet connections before resuming operation.

CAUTION

Do not strike the element against any hard surface to dislodge dust. This will damage the sealing surfaces and possibly rupture the element.

Main Oil Filter Servicing
The main oil filter is a replaceable cartridge. The servicing of the filter is required as indicated by the maintenance indicator on the filter or each time the compressor oil is changed. Under normal operating conditions. the oil is changed at approximately 500 operating hours. Under severe conditions. more frequent servicing is required.

Figure 3-54.- Air filter.

Demister or Separator Element
The demister, or separator element, is located inside the receiver tank (fig. 3-55). Replacement of the demister is indicated by the maintenance indicator (usually mounted on the receiver tank but also can be remote-mounted) or any sign of oil in the air at the service valves. You can reach the demister after removing the plate on the end of the receiver tank.

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Pressure-Control System
All portable air compressors are governed by a pressure-control system. The control system is designed to balance the compressor's air delivery and engine speed with varied demands for compressed air.

In a reciprocating compressor the pressure-control system causes the engine to idle and the suction valves to remain open when the pressure reaches a set maximum, thus making the compressor unit inoperative. When the air pressure drops below a set minimum, the pressure-control unit causes the engine to increase speed and the suction valves to close, thereby resuming the com pression cycle.

The rotary compressor output is governed by varying the engine speed. The engine will operate at the speed required to compress enough air to supply the demand at a fairly constant pressure. When the engine has slowed to idling speed as a result of low demand, a valve controls the amount of free air that may enter the compressor.

A screw compressor output is governed by automatic control that provides smooth, stepless capacity regulation from full load to no load in response to the demand for air. From a full load down to no load is accomplished by a floating-speed engine control in combination with the variable-inlet compressor.

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Even though manufactured by different companies, most compressors are quite similar. They are governed by a pressure control system that can be adjusted to compress air to the maximum pressure.

Compressor Design
The compressor unit may be of the reciprocating, rotary, or screw design.

The reciprocating compressor is similar to that of an automotive engine. They may be air-or liquid-cooled. As the pistons move up and down, air flows into the cylinder through the intake valve. As the piston moves upward, the intake valve closes and traps air in the cylinder. The trapped air is compressed until it exceeds the pressure within the collecting manifold, at which time the discharge valve opens and the compressed air is forced into the air manifold (fig. 3- 51). The reciprocating compressor is normally connected to the engine through a direct coupling or a clutch. The engine and compressor are separate units.

The rotary compressor has a number of vanes held in captive in slots in the rotor. These vanes slide in and out of the slots, as the rotor rotates. Figure 3-52 shows an end

Figure 3-52.- Compression cycle in a rotary compressor.

view of the vanes in the slots. The rotor revolves about the center of the shaft that is offset from the center of the pumping casing. Centrifugal force acting on the rotating vanes maintains contact between the edge of the vanes and the pump casing. This feature causes the vanes to slide in and out of the slots, as the rotor turns.

Notice in figure 3-52 the variation in the clearance between the vanes and the bottom of the slots, as the rotor revolves. The vanes divide the crescent-shaped space between the offset rotor and the pump casing into compartments that increase in size, and then decrease in size, as the rotor rotates. Free air enters each compartment as successive vanes pass across the air intake. This air is carried around in each compartment and is discharged at a higher pressure due to the decreasing compartment size (volume) of the moving compartments as they progress from one end to the other of the crescent-shaped space.

The compressor is lubricated by oil circulating throughout the unit. All oil is removed from the air by an oil separator before the compressed air leaves the service valves.

The screw compressors used in the NCF are direct-drive, two-stage machines with two precisely matched spiral-grooved rotors (fig. 3-53). The rotors provide positive-displacement internal compression smoothly and without surging. Oil is injected into the compressor unit and mixes directly with the air, as the rotors turn compressing the air. The oil has three primary functions:

1. As a coolant, it controls the rise in air temperature normally associated with the heat of compression.

2. It seals the leakage paths between the rotors and the stator and also between the rotors themselves.

3. It acts as lubricating film between the rotors allowing one rotor to directly drive the other, which is an idler.

After the air/ oil mixture is discharged from the compressor unit, the oil is separated from the air. The oil that mixes with the air during compression passes into the receiver-separator where it is removed and returned to the oil cooler in preparation for re-injection.

All large volume compressors have protection devices that shut them down automatically when any of the following conditions develop:

1. The engine oil pressure drops below a certain point.

Figure 3-53.- Compression cycle in a screw compressor.

2. The engine coolant rises above a predetermined temperature.

3. The compressor discharge rises above a certain temperature.

4. Any of the protective safety circuits develop a malfunction.

Other features that may be observed in the operation of the air compressors is a governor system whereby the engine speed is reduced when less than full air delivery is used. An engine and compression control system prevents excessive buildup in the receiver.

Intercoolers
When air is compressed, heat is generated. This heat causes the air to expand, thus requiring an increase in power for further compression. If this heat is successfully removed between stages of compression, the total power required for additional compression may be reduced by as much as 15 percent. In multistage reciprocating compressors, this heat is removed by means of intercoolers that are heat exchangers placed between each compression stage. Rotary air compressors are cooled by oil and do not use intercoolers.

Aftercoolers
It is obvious that the presence of water or moisture in an air line is not desirable. The water is carried along through the line into the tool where the water washes away the lubricating oil, causing the tool to run sluggishly and increases maintenance. The effect is particularly pronounced in the case of high-speed tools where the wearing surfaces are limited in size and excessive wear reduces efficiency by creating internal air leakage.

Further problems may result from the decrease in temperature caused by the sudden expansion of air at the tool. This low temperature creates condensation that freezes around the valves, ports, and outlets. This condition obviously impairs the operational efficiency of the tool and cannot be allowed.

The most satisfactory means of minimizing these conditions is the removal of the moisture from the air immediately after compression and before the air enters the distribution system. This may be accomplished in reciprocating compressors through the use of an aftercooler that is an air radiator that transfers heat from the compressed air to the atmosphere. The aftercooler reduces the temperature of the compressed air to the condensation point where most of the moisture is removed. Cooling the air not only eliminates the difficulties which moisture causes at points where air is used but also ensures better distribution.

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Figure 3-50.- Gas compressed to half its original volume by a doubled force.

Qualities
The ideal fluid medium for a pneumatic system must be a readily available gas that is nonpoisonous, chemically stable, free from any acids that can cause corrosion of system components, and nonflammable. It should be a gas that will not support combustion of other elements.

Gases that have these desired qualities may not have the required lubricating power. Therefore, lubrication of the components must be arranged by other means. For example, some air compressors are provided with a lubricating system, some components are lubricated upon installation or, in some cases, lubrication is introduced into the air supply line (in-line oilers).

Two gases meeting these qualities and most commonly used in pneumatic systems are compressed air and nitrogen. Since nitrogen is used very little except in gas-charged accumulators, we will only discuss compressed air.

Compressed Air
Compressed air is a mixture of all gases contained in the atmosphere. However, in this manual it is referred to as one of the gases used as a fluid medium for pneumatic systems.

The unlimited supply of air and the ease of compression make compressed air the most widely used fluid for pneumatic systems. Although moisture and solid particles must be removed from the air, it does not require the extensive distillation or separation process required in the production of other gases.

Compressed air has most of the desired charac-teristics of a gas for pneumatic systems. It is nonpoisonous and nonflammable but does contain oxygen which supports combustion. The most undesirable quality of compressed air as a fluid medium for a pneumatic system is moisture content. The atmosphere contains varying amounts of moisture in vapor form. Changes in the temperature of compressed air will cause condensation of moisture in the system. This condensed moisture can be very harmful to the system and may freeze the line and components during cold weather. Moisture separators and air dryers are installed in the lines to minimize or eliminate moisture in systems where moisture would deteriorate system performance.

An air compressor provides the supply of compressed air at the required volume and pressure. In most systems the compressor is part of the system with distribution lines leading from the compressor to the devices to be operated.

Compressed air systems are categorized by their operating pressure as follows:

High-pressure (HP)- 3,000 to 5,000 psi Medium-pressure (MP )- 151 to 1,000 psi

Low-pressure (LP)- 150 psi and below

Figure 3-51.- Intake and compression strokes in a reciprocating compressor.

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The word pneumatics is a derivative of the Greek word pneuma, which means air, wind, or breath. Pneumatics can be defined as that branch of engineering science that pertains to gaseous pressure and flow. As used in this manual, pneumatics is the portion of fluid power in which compressed air, or other gas, is used to transmit and control power to actuating mechanisms.

This section discusses the basic principles of pneumatics, characteristics of gases, heavy-duty air compressors, and air compressor maintenance. It also discusses the hazards of pneumatics, methods of controlling contamination, and safety precautions associated with compressed gases.

BASIC PRINCIPLES OF PNEUMATICS
Gases differ from liquids in that they have no definite volume; that is, regardless of size or shape of the vessel, a gas will completely fill it. Gases are highly compressible, while liquids are only slightly so. Also, gases are lighter than equal volumes of liquids, making gases less dense than liquids.

Compressibility and Expansion of Gases
Gases can be readily compressed and are assumed to be perfectly elastic. This combination of properties gives gas the ability to yield to a force and return promptly to its original condition when the force is removed. These are the properties of air that is used in pneumatic tires, tennis balls, and other deformable objects whose shapes are maintained by compressed air.

Kinetic Theory of Gases
In an attempt to explain the compressibility of gases, consider the container shown in figure 3-49 as containing a gas. At any given time, some molecules are moving in one direction, some are travelling in other directions, and some may be in a state of rest. The average effect of the molecules bombarding each container wall corresponds to the pressure of the gas. As more gas is pumped into the container, more molecules are available to bombard the walls, thus the pressure in the container increases.

Increasing the speed with which the molecules hit the walls can also increase the gas pressure in a container. If the temperature of the gas is raised, the molecules move faster, causing an increase in pressure. This can be shown by considering the automobile tire. When you take a long drive on a hot day, the pressure in the tires increases and a tire that appeared to be soft in cool morning temperature may appear normal at a higher midday temperature.

Boyle's Law
When the automotive tire is initially inflated, air that normally occupies a specific volume is compressed into a smaller volume inside the tire. This increases the pressure on the inside of the tire.

Figure 3-49.- Molecular bombardment creating pressure. Charles Boyle, an English scientist, was among the first to experiment with the pressure-volume relationship of gas. During an experiment when he compressed a volume of air, he found that the volume decreased as pressure increased, and by doubling the force exerted on the air, he could decrease the volume of the air by half (fig. 3-50).

Temperature is a dominant factor affecting the physical properties of gases. It is of particular concern in calculating changes in the state of gases. Therefore, the experiment must be performed at a constant temperature. The relationship between pressure and volume is known as Boyle's law. Boyle's law states when the temperature of a gas is constant, the volume of an enclosed gas varies inversely with pressure.

Boyle's law assumes conditions of constant temperature. In actual situations this is rarely the case. Temperature changes continually and affects the volume of a given mass of gas.

Charles's Law
Jacques Charles, a French physicist, provided much of the foundation for modem kinetic theory of gases. Through experiments, he found that all gases expand and contract proportionally to the change in absolute temperature, providing the pressure remains constant. The relationship between volume and temperature is known as Charles's law. Charles's law states that the volume of a gas is proportional to its absolute temperature if constant pressure is maintained.