2. CLUTCH OVERHAUL
The clutch helps transmit power or stop power transmission from the engine to the gearbox and the wheels. Over a period of time, the clutch lining wears out and the clutch needs to be dismantled and the clutch plate replaced. Before this is done, the engine needs to be dismounted from the car, which we have already studied in previous lesson of this unit on engine dismounting. We will assume in this lesson that the engine is already dismounted from the car and will look at the dismantling of the clutch from that point onwards.
2.0
Objectives After going through this lesson you will be able to: Know about the construction and function of clutch. Dismount and dismantle the clutch assembly. Assemble and remount the clutch assembly.
i. ii. iii.
2.1
Introduction
In order to overhaul the clutch, we have to first remove its components from the engine. After removing the clutch from the engine, we will inspect the various components of the clutch, which includes: • • •
Inspection of the friction surface (machined face) of the flywheel Inspection of the clutch plate, and, Inspection of the clutch cover, which is also known as the pressure plate.
We will replace any defective part or which is worn beyond the specified limit. In Lesson 7 of CAF 101 we studied clutches in general. We also studied the single plate dry clutch used in 4-wheelers. Here, before proceeding further, we will first study the construction of clutch used in Maruti car and also how it works in the car.
2.2
Construction
The clutch used in a Maruti 800 car is a diaphragm-spring clutch of a dry, single disc type as shown in Fig. 2.1 The diaphragm spring is a solid ring at the outer periphery, with a series of tapering fingers pointing inward. The clutch disc has six torsion springs and is mounted on the gearbox input shaft by a serration fit so that it can slide on it.
Fig. 2.1: Diaphragm clutch (a) Diaphragm spring. (b) Clutch disc.
The clutch cover is secured to the flywheel, and carries the diaphragm spring in such a way that the peripheral edge of the spring pushes the pressure plate against the flywheel, with the disc in between, when the clutch release bearing is held back. The clutch normally remains in the engaged position, unless disengaged. Pressing the clutch pedal down causes the release bearing to advance and push on the tips of the tapering fingers of the diaphragm spring, which then pulls the pressure plate away from the flywheel, thereby interrupting the flow of power from the flywheel through the clutch disc to the gearbox input shaft. The clutch release bearing along with the bearing retainer is mounted on the clutch fork with the help of two bearing retainer springs. The clutch release shaft is moved forward by the clutch release arm, which is pulled by the clutch cable when the clutch pedal is pressed down. All clutch mechanisms work in a similar manner irrespective of whether they are with a diaphragm spring or with coil springs and fingers.
2.3
Operation
The clutch is used during gear changing. To change the gears, the clutch pedal is pressed, the gear is shifted to the desired one and then the clutch pedal is released. On pressing the cutch pedal, leverage is applied and the clutch cable gets pulled, which, in turn, moves the clutch release arm and the clutch gets disengaged. When the foot is removed from the clutch pedal, the return spring makes the release arm return to its original position. The release arm of the clutch is mounted on the clutch housing and the movement of the release arm operates the pressure plate through the diaphragm spring and pulls it back so that the clutch plate underneath it becomes free and the clutch is disengaged. The clutch plate is pressed between two moving discs. Due to the relative motion between the clutch plate and the discs in the initial stages, the clutch lining wears out over a period of time. This leads to clutch slip, resulting in the full engine power not being transmitted. The clutch lining wears faster if the driver keeps his foot on the clutch pedal unnecessarily. Further, the torsion coil springs and the hub in the
clutch disc become loose over a period of time and the clutch becomes noisy. When either or both of these defects develop, the clutch needs overhauling.
Fig. 2.2: Floor pedals.
There are three pedals on the floor in the driver’s cabin as shown in Fig. 2.2 – the accelerator pedal, the brake pedal, and the clutch pedal. The left foot is always used to press the clutch pedal. At any time, either the accelerator pedal or the brake pedal is pressed down with the right foot. The left foot should not unnecessarily be kept on the clutch pedal and should be removed immediately after gear changing otherwise the clutch will wear out fast.
2.4
Dismounting
To remove the clutch from the engine for overhaul, the engine needs to be dismounted first and this is done following the procedure that was studied in Lesson 1 on engine dismounting. After removing the connections of the various systems, the engine is lifted along with the transmission with the help of a chain pulley block and placed on a rotary stand. The transmission is then removed from the engine, after unscrewing the bolts holding the clutch housing to the engine.
2.5
Working
Inside the clutch housing, the movement of the clutch release bearing is operated by the clutch release lever. The input shaft, which is supported in the pilot bearing in the flywheel at one end, projects out from within the clutch release bearing. The clutch release bearing can also rotate, in addition to the axial movement. It exerts pressure on the diaphragm spring, which rotates with the flywheel, as it is a part of the clutch cover. When the clutch release bearing pushes the diaphragm spring finger tips inward,
Fig. 2.3: Clutch housing.
the outer periphery of the diaphragm spring, which is a solid ring, comes outward, bringing the pressure plate along with it. This removes the pressure from over the clutch plate. Until this time, the clutch plate bears on the flywheel and rotates with the flywheel since it is pressed between the flywheel and the pressure plate. The clutch consists of three components as shown in the Fig. 2.4 – the flywheel, the clutch plate and the pressure plate. The clutch facing, also known as the clutch lining, bears on the friction (machined) surface of the flywheel. The pressure plate is placed underneath the clutch cover. When the fingertips of the diaphragm spring are pressed, the pressure plate moves backward and releases pressure from over the clutch plate, which then becomes free and stops rotating even while the flywheel rotates. The clutch cover is connected to the flywheel with the help of six bolts.
Fig. 2.4: Clutch components (a) Clutch plate and flywheel. (b) Pressure plate.
Self-check Questions 1. Name the 3 main parts of a clutch. 2. Clutch used in Maruti car is ________________________________ 3. Clutch cover (pressure plate) rotates with the flywheel. State whether True/False 4. We press the clutch pedal to _______________ the clutch
2.6 •
Dismantling Lock the flywheel with the flywheel holding tool, as shown in the Fig. 2.5, to prevent rotation of the flywheel in counter-clockwise direction when the clutch-cover securing bolts are unscrewed.
The clutch-cover securing bolts are screwed in the flywheel. Fig. 2.5: Flywheel holding tool.
•
Remove the six bolts of No 12 size, which secure the clutch cover to the flywheel, and take the clutch cover and the disc off.
The clutch cover and the clutch disc are also called the pressure plate assembly and the clutch plate assembly, respectively. It is important that the bolts are unscrewed gradually and evenly. All the bolts on one side should not be removed together as this may cause the clutch cover assembly to distort. Also, there are two bolts of greater length and these are called reamer bolts. The reamer bolts are used to locate the clutch cover in place. •
Remove the six bolts of No 14 size, which secure the flywheel to the engine and take the flywheel off.
The input-shaft end bearing, which is also called the pilot bearing is removed from the flywheel next, using the bearing remover and the sliding shaft. • •
Insert the tool end in the pilot bearing inner race and tighten the tool bolt. Tighten the sliding shaft on the tool center bolt and strike with the sliding shaft weight, away from the pilot bearing.
When striking with the weight of the sliding shaft in this manner, the shaft should be kept perpendicular to the flywheel. •
Remove the sliding shaft from the bearing remover and the bearing from the tool by unscrewing the center bolt.
If a bearing remover is not available, push out the bearing with the help of a clutch center guide and hammer. •
Remove the clutch release bearing along with the release bearing retainer by removing the two bearing retainer springs from the clutch fork and the bearing retainer. The two bearing retainer springs are also called wire locks.
2.7
Inspection
The Maruti car uses clutch plates, which either have six torsion springs or four springs of a larger size. When a clutch plate is worn, the clutch facing has a glazed surface and the rivet heads are closer to the surface of the facing. The difference between a new and an old clutch plate can also be seen in the greater thickness of the clutch facing in a new plate. A burnt or glazed, that is, a glass-like surface of the facing can be reconditioned by grinding it with No 120-200 sandpaper. If the surface is in bad condition and beyond repair, it is recommended that the whole clutch disc be replaced. •
Check wear of the clutch facing, first visually and then by measurement.
If the clutch facing looks excessively worn or the rivets have come out of the lining or are very near to the surface of the lining, then the clutch plate assembly should be replaced. The clutch plate assembly should also be replaced if the lining is worn out unevenly. The torsion springs help cushion the shock of engagement. The hub has serrations in which the serrations of the clutch shaft, also called the gearbox input shaft, engage so that the power is transmitted from the clutch to the gearbox. In order for a clutch plate to be reusable, neither the springs nor the rivets in the clutch plate should be loose. The serrations should also not be excessively worn. •
Check for wear in the clutch lining by measuring the depth of each rivet head from the surface of the lining with a vernier caliper, after the clutch facing has passed visual inspection.
If the depth is found to have reached the service limit, that is, 0.5 mm in any hole, the clutch disc assembly should be replaced. The standard rivet head depression is 1.2 mm. • •
Check the backlash in the plate serration fit between the clutch disc and the gearbox input shaft. Check the backlash on the periphery of the clutch disc by mounting the disc on the transmission input shaft and measuring the backlash with a dial indicator while turning the disc back and forth. A mark should first be made on the periphery of the lining for ease of checking.
The disc assembly should be replaced if the backlash is found to exceed the service limit of 0.5 mm. •
Inspect the torsion springs and ensure that they are in good condition. If any spring is found to be loose or broken, replace the clutch plate.
•
Inspect the clutch cover for loose diaphragm spring rivets. If the rivets are found to be loose, replace the clutch cover assembly, otherwise, it will make a rattling noise when the clutch pedal is pressed. Inspect the tips of the tapering fingers for wear. If they are worn excessively, replace the cover assembly.
•
The release bearing exerts a push on the tips of the tapering fingers to disengage the clutch. • • •
Inspect friction surface of pressure plate. Inspect the surface of the flywheel that contacts the clutch disc, for any wear or damage. It should be free of any deep lines or grooves. Remove the clutch release bearing and compare it with a new one. The release bearing should spin freely without any play.
If the release bearing sticks, rattles or makes abnormal noise when spun and turned by hand, it must be replaced. If the condition of the bearing seems to be acceptable, it may be reused after lubricating it with a few drops of oil. • •
Check the bearing retainer lock wire condition. Inspect the bearing retainer for wear at the point where the clutch fork pushes it. If it looks excessively worn out, it should be replaced.
•
Inspect the input shaft end bearing. If the input shaft end bearing sticks rattles or makes abnormal noise when spun and turned by hand, it should be replaced.
Self-check Questions 5. A burnt or glazed facing can be reconditioned by ______________________ 6. The torsion springs _________________________. 7. Depth of rivet head from the surface of the lining should not be less than ________
2.8 •
Assembly Tighten the six bolts of No 14 size, which hold the flywheel to the crankshaft flange, gradually and evenly, first by ring spanner and then by torque wrench to the specified torque.
To prevent the rotation of the flywheel, the special tool, that is, the flywheel holder must be installed before fully tightening the bolts to the specified tightening torque of 4.0 to 4.5 mkg. • • •
•
Install the pilot bearing in the flywheel using the bearing installer and hammer. Strike the installer with hammer slowly till the bearing takes its seat properly in the hole in the flywheel. Locate the clutch disc and the clutch cover for installation on to the flywheel using the clutch center guide, while taking care to ensure that the clutch plate is installed correctly, that is, the side marked ‘flywheel’ should be towards the flywheel. Tighten the six bolts, of No. 12 size of the clutch cover, to specification. Use the flywheel holder to prevent rotation of the flywheel while tightening the bolts. Tighten the bolts flange gradually and evenly first by ring spanner and then by torque wrench to the specified torque.
The bolts on one side should never be tightened all at once. The tightening torque for clutch cover bolts is 1.8 – 2.8 mkg. When mounting the clutch cover onto the flywheel, the two reamer bolts should be tightened in proper place.
• •
• • • •
•
2.9 • • •
2.10
Pull the clutch center guide out from the flywheel through the clutch assembly and remove the flywheel holding tool, after the bolts have been tightened. Apply grease to the fork-end of the clutch release shaft and the bearing retainer inner surface and add a few drops of oil to the bearing if the old one is being fitted again. Install the bearing retainer and the clutch release bearing. Insert the two clutch bearing retainer springs to attach the retainer with the fork and install the bearing retainer on to the clutch shaft fork. Apply SERVOGEM-3 grease to the gearbox input shaft serrations. Install the clutch release arm on the clutch release shaft in such a way that the punched mark on the clutch release arm is shifted towards the front side by one notch from the punched mark on the clutch release shaft. Install the clutch release shaft return spring in the release lever.
Remounting Assemble the housing onto the engine so that the release bearing bears on the diaphragm spring and the input shaft is pushed in pilot bearing. Tighten the bolts to join the housing to the engine. Mount the engine along with the transmission in the car.
Maintenance
There are some maintenance services that may have to be performed when a car is being serviced, even when overhauling the clutch is not required.
2.10.1 Clutch Pedal Height • •
Adjust the height of the clutch pedal with the help of the clutch pedal stop bolt so that the pedal is at the same level as the brake pedal. Tighten the lock nut after adjustment.
2.10.2 Clutch Release Arm Play •
Check the clutch release arm play by moving the release arm with your finger. If the play is not within the specification of 2 to 4 mm, adjust it by turning the adjusting nut.
If the clutch cable has been replaced with a new one, the clutch pedal should be depressed and released about ten times before checking the clutch release arm for play. •
Tighten the clutch release arm nut between 1.0 and 1.6 mkg torque.
Self-check Questions 8. To prevent rotation of the flywheel when tightening bolts which hold the flywheel to the crankshaft flange, ______________________________. 9. We locate the clutch disc and the clutch cover for installation on to the flywheel using the _________________ 10. clutch release arm play should be within _____________
2.11
Assignments
2.11.1 Class Assignment 1. Discuss why clutch lining (facing) wears and how to prolong its life. 2.11.2 Home Assignment 1.
2.12
Take an old clutch plate and tell whether it can be re-used. Give justification for your answer. Also try and re-condition it.
Summing Up
The clutch helps transmit power or stop power transmission from the engine to the gearbox and the wheels. Over a period of time, the clutch lining wears out and the clutch needs to be dismantled for repair or replacement. We have studied the complete procedure of removal, dismantling, inspection, assembly and maintenance of the clutch in this lesson. In the next lesson we will begin with the transmission components.
2.13
Possible Answers to Self-check Questions
1. friction surface (machined face) of the flywheel, the clutch plate, and the clutch cover, which is also known as the pressure plate 2. diaphragm-spring type dry, single plate clutch 3. True 4. disengage 5. grinding it with No 120-200 sandpaper 6. cushion the shock of engagement 7. 0.5 mm 8. flywheel holding tool 9. clutch center guide 10. 2 to 4 mm
2.14
Terminal Questions
1. 2. 3. 4.
2.15
Describe the construction of the clutch used in Maruti 800 car. Describe the working of the clutch used in Maruti 800 car. Describe the inspection procedure of the clutch Give the points of clutch maintenance
References
1. Maruti 800 Service Manual
2.16
Glossary
Clutch Coupling that connects and disconnects a shaft from its device while the drive mechanism is running Clutch center guide Special tool which helps centering the clutch plate hub with the pilot bearing so that clutch shaft is properly aligned Clutch cover Also called pressure plate Clutch lining Ring or segments of friction material riveted or bonded to each side of the clutch driven plate. Clutch plate Friction plate of the clutch assembly Clutch release bearing Thrust bearing which presses on the fingers of the pressure plate to disengage the clutch Clutch fork Fork, attached to the clutch thrust bearing at one end and connected by rod, cable or hydraulic mechanism to the clutch pedal at the other. Diaphragm spring clutch Type of clutch in which the clamping or gripping load is applied by a diaphragm spring. Flywheel Heavy disc bolted to the engine crank shaft; it acts as an energy reservoir and smoothes out the individual power impulses of the pistons. Flywheel holding tool Special tool used to hold the flywheel when loosening its bolts Pilot bearing A small bearing in the center of the flywheel end of the crankshaft, which carries the forward end of the clutch shaft Serrations Saw teeth like grooves which prevent relative motion between two mating parts Torsion spring Spring which takes torsional load