How does a clutch throwout bearing work?

The clutch release bearing, also known as the release fork bearing, works as part of the clutch operating mechanism and is responsible for the clutch release action when the driver operates the clutch pedal. The following is the detailed working process:

1. Connection and release mechanism: When the driver depresses the clutch pedal, through a series of mechanical or hydraulic transmissions (depending on the type of clutch manoeuvring system of the vehicle), the release bearing is pushed against the release lever of the clutch.

2. Release lever action: The force of the release bearing is transmitted through the release lever, which is usually connected to the pressure plate. The release lever is designed so that when the bearing is pushed forward, it pushes the pressure plate backwards, thus reducing the pressure on the disc. 

3. Separation of the platen from the follower disc: As the platen moves, it is no longer pressed tightly between the follower disc and the flywheel, allowing the friction between the follower disc and the flywheel to be reduced to virtually nil, which interrupts the transfer of power from the engine to the gearbox, allowing the driver to shift gears without affecting the power output of the vehicle.

4. Self-aligning design: In some modern designs, the release bearings may incorporate a self-aligning structure that ensures even compression of the release fingers, even when not perfectly concentric, reducing wear and noise.

5. Return and engagement: When the driver releases the clutch pedal, the release bearing is returned to its original position by means of a return spring or other mechanism, and the pressure plate is again pressed against the driven disc, restoring contact with the flywheel and allowing power transmission to continue.

6. Lubrication and Cooling: The release bearing requires good lubrication to minimise wear during operation, and some designs use a fully sealed construction and high temperature grease to increase its life and aid in circulating the lubrication through the centrifugal force.

The operation of the throwout bearing is a vital part of the clutch system, ensuring smooth starts and gear changes.

How do you diagnose a throwout bearing?

Diagnosing the condition and potential failure of the throwout bearing (often referred to as the ‘release bearing’) in a clutch usually involves a series of inspection and testing steps. The following are some common diagnostic methods:

I. Visual Inspection

1.1 Inspect the bearing surface:

Observe the bearing surface for signs of wear, cracks, rust or grease leakage.

These signs usually indicate that the bearing may have been damaged or is wearing out.

1.2 Inspect bearing housings and fixtures:

Ensure that the bearing housing and fixtures are not loose or damaged.

Loose or damaged fixtures may cause the bearing to operate erratically.

ii. operation check

2.1 Clutch operation:

Without starting the engine, operate the clutch pedal several times to feel the pedal travel and resistance.

If the pedal travel is too long or the resistance is too great, it may be an indication of a worn or damaged release bearing.

2.2 Sound check:

When operating the clutch, listen carefully for unusual sounds such as metal rubbing or squeaking. One of the most common symptoms is a grinding or chattering sound when you press the clutch pedal. This noise is often heard when the clutch is engaged or disengaged. If the noise persists even though the clutch is fully depressed and the car is in neutral, this is a strong indication of a worn throwout bearing.

III. Function Test

3.1 Testing clutch disengagement:

Start the engine, place the vehicle in neutral and gently depress the clutch pedal.

Observe if the engine speed drops while feeling the feedback from the clutch pedal.

If the clutch does not fully disengage, the release bearing may be damaged or improperly adjusted.

3.2 Check bearing rotation:

Near the release bearing mounting position, try to rotate the bearing manually.

Feel if the bearing rotates smoothly and if there is any snagging or abnormal resistance.

3.3 Clutch pedal feel:

Changes in clutch pedal feel, such as increased resistance or a spongy feel, can suggest a problem with the clutch system, including the throwout bearing.

3.4 Clutch engagement:

Observe for poor clutch engagement or slippage, which could be due to a bearing failure that is not releasing the pressure plate properly.

3.5 Driving Diagnostic Test:

Drive the vehicle and note if there is any difference in clutch operation or if the noise changes under different loads or speeds. If the noise disappears when the clutch is fully depressed but returns when it is released, it is likely to be a throwout bearing.

IV. Professional Inspection

4.1 Vibration analysis:

Use vibration analysis instruments to perform vibration tests on the bearing.

Analyse the frequency and amplitude of the vibration signal to identify potential problems within the bearing.

4.2 Infrared thermal imaging:

Use infrared thermography to check the temperature distribution of the bearing.

Areas of abnormally high temperatures may indicate friction or damage to the bearing.

4.3 Mechanic Inspection:

Due to the location of the throwout bearing, a mechanic may need to perform a more thorough inspection. They may use a stethoscope-like tool to listen directly to the bearing or remove transmission fluid to check for metal fragments, which may be an indication of bearing wear.

V. Other Considerations

5.1 Lubrication:

Check the lubrication of the release bearing.

Lack of lubrication or poor lubrication may result in bearing wear and damage.

5.2 Replacement intervals:

Inspect and replace the release bearing periodically according to the vehicle manufacturer's recommendations.

Follow the manufacturer's maintenance schedule to ensure bearing reliability and durability.

5.3 Preventive Maintenance:

Regular inspections and early detection are key. If you notice any of these symptoms, it's best not to delay, as continued use can lead to further damage to the clutch assembly.

Remember that diagnosing a throwout bearing usually requires specialised knowledge and experience. If you are unsure, it is advisable to consult a professional mechanic to avoid misdiagnosis or further damage to your vehicle.

In summary, diagnosing a throwout bearing in a clutch involves a number of inspections and tests. By using a combination of these methods, the condition of the bearing can be accurately assessed, and potential problems can be identified and dealt with in a timely manner to ensure the proper function of the clutch and the safe operation of your vehicle.