On a vehicle with a manual gearbox, a single-plate, dry clutch with a central diaphragm spring is fitted.
The driving plate (cover assembly) is connected to the engine flywheel by six bolts.
Three pins are pressed into the flywheel and centre the clutch cover.
A diaphragm spring, stamped from spring steel sheet, is fitted in the driving plate cover.
In the free state, the spring has the form of a truncated cone with radial slots running from the inner edge of the spring.
The slots in the spring form eighteen fingers, which are elastic release levers.
Owing to the elasticity of the fingers, the diaphragm spring creates a more uniform pressure on the clutch pressure plate, which promotes smooth engagement and disengagement of the clutch.
At three points, the clutch cover is connected by elastic steel plates to the pressure (driving) plate.
The cover, together with the diaphragm spring and the pressure plate, forms a non-detachable assembly (also called the clutch cover assembly), which is balanced on a test bench.
If the cover assembly fails, it is replaced as a whole.
Replacement of the cover assembly is necessary in the event of severe circumferential wear of the diaphragm spring fingers and if the ends of the fingers are not at the same level.
We also replace the cover assembly if the pedal effort decreases when disengaging the clutch (and, correspondingly, the working travel increases), which indicates heavy wear of the pressure plate surface or "settling" of the diaphragm spring.
The driven plate with a spring torsional vibration damper is located on the splines of the gearbox input shaft between the engine flywheel and the pressure plate.
The torsional vibration damper damps vibrations arising from dynamic loads in the transmission and from uneven engine operation.
Two friction linings of the plate are riveted on both sides to a spring plate, which in turn is riveted to one of the two damper plates.
The spring plate has a wavy shape.
When the clutch is engaged, the friction linings compress the spring plate, which promotes smooth clutch engagement.
The plate hub is mounted between the damper plates.
Damper springs are fitted in the slots of the hub and the damper plates.
The damper plates are connected by support posts.
In the plate hub, opposite the support posts, there are cut-outs that allow the hub to turn within certain limits relative to the damper plates, while compressing the damper springs.
This makes it possible to reduce dynamic loads in the transmission when starting off and changing gears.
On the driven plate hub, in a plastic housing, there is an idle damper, which serves to eliminate knocking in the gearbox when the engine is idling.
The driven plate is replaced if its axial run-out in the area of the friction linings exceeds 0.5 mm, if it is oil-contaminated, cracked, scored, or if the linings are evenly worn, if the riveted joints are loose, and also if the distance between the working surface of the lining and the rivet head is less than 0.2 mm.
The slave cylinder of the hydraulic drive is made as a single unit with the clutch release bearing and is located inside the clutch housing.
The slave cylinder is attached to the clutch housing.
The moving part of the slave cylinder is connected to the bearing, which can move along the gearbox input shaft.
A spring (enclosed by a corrugated rubber boot) located between the cylinder and the bearing race constantly presses the bearing against the diaphragm spring of the clutch cover assembly.
Therefore there are no clearances in the clutch mechanism and no adjustment is provided.
The release bearing is of the sealed type.
Grease is contained inside the bearing.
The clutch hydraulic drive consists of a master cylinder and a slave cylinder, connected by a pipeline.
The working fluid is poured into a reservoir mounted on the master brake cylinder.
The clutch master cylinder is connected to the reservoir by a rubber hose.
The master cylinder is secured to the front panel, and its rod is connected to the clutch pedal.
To bleed the clutch hydraulic drive, a nipple closed by a rubber cap is located on the plastic terminal of the hydraulic drive pipe.
Clutch disengagement occurs as follows.
When the pedal is depressed, pressure of the working fluid is created in the hydraulic system of the clutch release drive.
The fluid pressure is transmitted to the slave cylinder piston, which moves the clutch release bearing along the axis of the gearbox input shaft.
The bearing presses on the fingers of the diaphragm spring of the pressure plate.
The spring, deforming, moves the pressure plate away; as a result, the pressure plate ceases to press the driven plate against the flywheel.
The engine crankshaft and the gearbox input shaft can then rotate independently of each other.
When the clutch pedal is released, the bearing returns to its initial position; the diaphragm spring again begins to press on the pressure plate, which in turn presses the driven plate against the flywheel — as a result, torque transmission is resumed.
Bleeding the clutch hydraulic drive
We bleed the clutch hydraulic drive to remove air from it after depressurisation when replacing the clutch master or slave cylinder, pipes and hoses, as well as when removing the master brake cylinder or the reservoir of the brake and clutch hydraulic drives.
To bleed the clutch hydraulic drive, a nipple closed by a rubber cap is located on the plastic terminal of the pipe.
The adapter, onto which the pipe terminal is fitted, is not secured in the clutch housing; therefore, for convenience of bleeding, we recommend making a retaining washer for the adapter from a metal washer (1.0—1.2 mm thick, 23 mm outside diameter and a 10.5 mm hole).
Before bleeding, we check the fluid level in the reservoir. If necessary, we top up the fluid.
We install the made retaining washer in the adapter groove (for clarity, it is shown with the pipe terminal removed).
We remove the protective cap from the bleed nipple.
We fit a transparent hose onto the nipple, the other end of which we lower into a container partially filled with working fluid so that the free end of the hose is immersed in the fluid.
An assistant depresses the clutch pedal several times and holds it depressed.
We press the spring retainer of the pipe terminal.
We move the adapter pipe terminal by 4—6 mm.
Some of the brake fluid and air are thereby displaced into the container.
Air bubbles are clearly visible in the container with fluid.
After the fluid stops flowing into the container, we fit the pipe terminal fully onto the adapter.
The assistant can then release the clutch pedal.
We repeat this operation until air bubbles stop coming out of the hose.
We remove the hose and fit the protective cap onto the nipple.
During bleeding, we monitor the fluid level in the reservoir and top up the fluid if necessary.
Possible clutch faults and remedies
Cause of fault (Remedy)
Incomplete clutch disengagement (clutch drags):
- Reduced full travel of the clutch pedal — repair the clutch release drive;
- Distortion of the driven plate (face run-out) — replace the driven plate;
- Irregularities on the surfaces of the friction linings of the driven plate — replace the driven plate;
- Loosening of rivets or breakage of the friction linings of the driven plate — replace the driven plate;
- Seizing of the driven plate hub on the splines of the gearbox input shaft — clean the splines, coat with LSTs-15 grease. If the cause of seizing is crushing or wear of the splines, replace the input shaft or the driven plate;
- Air entering the hydraulic system — bleed the system;
- Fluid leakage from the master or slave cylinder of the clutch release drive — replace the master or slave cylinder;
- Loosening of the rivets securing the pressure spring — replace the clutch cover with pressure plate assembly;
- Skewing or distortion of the pressure plate — replace the clutch cover with pressure plate assembly.
Incomplete clutch engagement (clutch slips):
- Excessive wear or burning of the friction linings of the driven plate — replace the driven plate assembly;
- Damage to or seizing of the clutch release drive — remedy the faults.
Jerking during clutch operation:
- Seizing of the driven plate hub on the input shaft splines — clean the splines, lubricate with graphite grease;
- Breakage or reduced elasticity of the damper springs of the driven plate — replace the driven plate;
- Seizing in the clutch release drive mechanism — replace the deformed parts;
- Excessive wear of the friction linings of the driven plate — replace the driven plate;
- Loosening of the rivets of the friction linings of the driven plate — replace the driven plate;
- Damage to the cover assembly — replace the clutch cover assembly.
Increased noise when disengaging the clutch:
- Wear, damage or grease leakage from the clutch release bearing — replace the bearing.
Increased noise when engaging the clutch:
- Breakage of the plates connecting the pressure plate to the cover — replace the clutch cover assembly.
Clutch slip can easily be detected by the tachometer.
If, while driving, on sharply pressing the accelerator pedal the engine speed rises sharply, then drops slightly and the vehicle begins to accelerate, the clutch needs repair.
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