Clutch

As shown above, a clutch is made up of the following components

l  Clutch disk

l  Pressure plate assembly

l  Control mechanisms to engage or disengage the clutch

Clutch assembly is mounted onto the flywheel as shown below. The input shaft of the transmission is connected to the engine through splined hub on disk plate.



Two methods of controls over clutches are available

When driver presses pedal, the fork is activated to push the release bearing onto the diaphragm finger and disengage the clutch. When the pedal is released by the driver, diaphragm will again push the pressure plate to hold disk plate firmly against the flywheel.

When the clutch is engaged, the disk plate is held firmly between the pressure plate and the flywheel. As the disk plate rotates so will the input shaft of transmission, because they are connected together by the splined hub. When diaphragm is pressed by the release bearing, the pressure plate is pulled back and released the disk plate therefore disengaged the clutch.

1.         Mechanical Control – this includes cable and linkage control. They are very similar. Cable control seems to be more common, because cable is more flexible.

2.         Hydraulic control

In both of these controls, release fork is connected to the clutch pedal with either cable or hydraulically .

Above is my basic understanding of how a clutch of a manual transmission works.

In our lab we studied the clutch of the transmission with the following specification

TOYOTA 5 SPEED MANUAL TRANSAXLE

MODEL C50, C51, C52 AND C150

CABLE CONTROLLED CLUTCH

Listed below is what we did 

1.         External inspection on Pedal free play and height

2.         Disassembly

3.         Visual inspection of clutch components

4.         Assembly

External inspection on Pedal free play and height

This procedure should always be carried out when looking for a fault in a clutch before any major work is carried out. Incorrect Pedal height and free play could be cause of faulty clutch operation as described below. Pedal height should always be checked first.

Pedal Height

This is the distance between the pedal when it is against the pedal stop and the floor pan. It determines how far the cable will be able to move therefore how far the release bearing will push the diaphragm fingers. Specification is 165mm +/- 0.5mm.

If pedal height is too much, the diaphragm or clutch mechanism could be damaged and result faulty clutch. If it is too small then the clutch might not be able to fully disengaged because release bearing wont push the diaphragm far enough to release the disk plate.

Adjustment:

This is done by tighten or loosing the Pedal Stop screw.

Pedal free play

Depress the pedal until resistance is felt. This determines how far the pedal could be depressed before the release bearing will start to push on the diaphragm. The specification is 15 – 20mm.

If it is too much the clutch would be able to fully disengage because diaphragm is not pushed far enough by the release bearing. On the other hand if it is too small then the release bearing will always push on the diaphragm therefore clutch is never fully engaged.

Adjustment:

This is done by adjusting the adjusting nut at the clutch end.

LAB RESULTS:

Pedal Height:                    165mm

Pedal Free Play         18mm

External adjustment is not needed because both Pedal height and free play are within specification.

Disassembly

The clutch is disassembled in the following order

1.         Transmission is supported before trying to remove the transmission to prevent distortion of the disk plate.

2.         Bolts that holding the transmission are removed. Cable control is disconnected

Transmission is removed and clutch assembly exposed.

Identification mark is made on the pressure plate cover and flywheel to insure correction installation later on. This will help to preserve balance.

Loose the holding bolts on opposite progressively. This prevents distortion of the cover.

Aligning tool is used to prevent disk plate from dropping.

Notice the side of disk plate where it faces the flywheel for later installation, in this case the left photo.

Component inspection

We inspected the following components listed in the chart below for damage, wear and sign of overheating and made conclusion wether or not they are still servicable. Everything associated with a transmission need to be precise. Most of these parts are rotating and will take a lot of force, if they are damaged or worn out not only they might fail to work but also damage other component. For example if clutch disk is worn out then it may slip when engaged and cause itself to be more quickly destroyed. This might also damage the pressure plate because they are all rotating at very high speed.

Damage – bent, distortion or crack etc.

Wear – score, dents or excessive wearing

Overheating – when a component is overheated will normally have a kind of dark blueish color.

Conclusion:

Flywheel has no noticeable sore or wear. It’s run out is tested with DTI – 0.1mm which is within specified 0.15mm. No oil contamination or sigh of overheating. This shows that flywheel is working properly. It won’t cause clutch disk to slip or do any damages.

Pressure plate assembly has no noticeable sore or wear. The pressure plate facing is free of damage, contamination or heat mark. The diaphragm finger is not worn or damaged. The assembly casing is in good condition. Pressure assembly should be working properly. Diaphragm finger should be able to lift the pressure plate when pressed. The pressure plate should be able to engage or disengage the clutch plate properly without slip or damage.

Clutch plate assembly facing is free of oil contamination or physical damage. The hub slid smoothly on the input shaft. Springs are in good condition. However, clutch plate is worn out. The rivets on the disk facing is almost at same level as the facing itself. The clutch disk needs to be replaced.

Release bearing is rotating freely without noise, this means it is working fine.

Release fork has no noticeable damage or wear, it is fines as well.

Release linkage has no noticeable damage or wear, it is fines as well.

Spigot bearing is working fine, spigot shaft rotate freely in the bearing with no side movements.

Spigot shaft has no sign of damage or wearing and is in good condition.

Clutch housing is in good condition without damage or wearing.

This clutch is free of oil contaminations. This type of clutch is dry clutch. When oil gets onto the facing between clutch disk, pressure plate or flywheel, the clutch won’t be able to engage properly. Slip disk will cause damage to these components and could cause the clutch eventually to fail.

After inspection other than the fact that new clutch disk is needed, there is no problem with this clutch. The reason for the disk to wear out is probably due to time. No further investigation is needed at this stage. (Please note for any faults found, the reason caused these faults must be find before we could start to fix the fault. We have to trace to the root of the problem and try to correct everything from there.)

Reassembly

This is done in exact reverse order of disassembly.

Make sure the facing of the disk plate is correct. Put clutch assembly onto a aligning tool and lace the whole clutch back on to the flywheel according to the identification mark. Aligning tool will not only support the weight of the clutch so it wouldn’t distort the clutch disk but also allow the input shaft goes smoothly into the spigot bearing therefore connect to the engine.

Put on the holding bolts. Tighten them progressively in opposite positions. Final tightening is done with torque wrench – 18 nm.

Remove the aligning tool and put the transmission back to the engine and secure it with bolts. Grease should be applied to release fork pivot, ball, ends of the fork and release bearing before connecting the transmission back to the engine.

Reconnect cable control and checked everything worked alright.