Friction clutch and the force required

Question

I recently learned the basics of automotive clutches, more specifically friction clutches. Here is a simplified version of their operation explained to me (the car starts at rest): The engine and the transmission are connected via friction plates. When the clutch is pressed down, the plates are separated. When to clutch is released slowly, the plates press harder together and the friction between them causes the car, via transmission, to gain speed.

But what confuses me is this: The car weights many tonnes, and the engine spin very fast. When the plates are coming into contact with each other, they are slipping until the car begins to move. Isn't this causing a huge amount of heat to be produced? And most of all, how are the friction plates manufactured so that they don't "smoothen out" very quickly, reducing the friction coefficient between them? I understand that they do wear out and need to be replaced sometimes, but considering the high torque and speed of the engine, you would think they'd wear extremely quickly.

Also, isn't the force needed to press the plates together enormous? Considering the large mass of the car, how kind of mechanism is used inside to car the press the plates together? And how can a person then, using only one foot, be able to again separate these plates? I assume the clutch pedal is connected to some kind of hydraulic system to amplify the force?

I've Googled a lot and watched several Youtube videos on the matter but they all simply show that friction holds the plates together and pressing the clutch releases them.

Answer 1

To answer your questions:

1. Yes, a lot of heat gets produced. Most of it will sink into the flywheel, but the clutch does get very hot. That's why it's good to keep the revs low when feathering the clutch. Don't pull away revving more than 1200rpm. Better to keep it at 1000, also with a petrol car. That may require some practice, though. It's not hard at all to burn the clutch and flywheel. Your flywheel then will look like this:

If you'd run your fingers across it, you'd feel a crater-like surface. Chips of metal just melted and sprung off the flywheel, that's how hot it can become. You'd get this when trying to pull away with a caravan on a steep hill for too long, or when your clutch is incorrectly tuned (issue on older cars). The glue/resin in the clutch can melt and cause a glazed surface on the clutch, reducing its friction.

It's not that hard to get a car moving. Try to push a car when it's in neutral, one person is adequate to get it moving. And the clutch has it much easier, being helped by a transmission and a final gearing; after the clutch, the transmission with a gearing of about 3 to 4 (in 1st) and a differential also with a gearing of about 3 to 4 is used to amplify torque. So the torque of the engine is amplified roughly 12 times.

2. They achieve smooth operation by carefully designing the lever ratios in the pedal and clutch arm, and by using diaphragm springs. Older clutches used conventional springs, newer clutches since the 60s use diaphragm springs. Those are springs with a progressive-like behaviour. With older coil spring clutches, you feel a constant force when depressing the clutch pedal. It kind of tires your leg in traffic jams. With modern ones, you feel a 'wobble' in the pedal. At the end of its travel, the spring is pushed with much greater force. It makes it possible to use minimal travel and relatively weak leg strength to control massive amounts of force. I won't elaborate about that here, but you could ask about it in a separate question.

3. It's not a problem if the clutch has a smooth surface, better even. It won't wear down as quickly. If it's flat, that doesn't mean it doesn't create friction. It gets pushed hard against the flywheel and pressure plate, creating enough friction for operation. The material of a clutch is designed to withstand wear by friction, and it gets used relatively little. The brakes are used far more and are loaded much more, and thus they wear out much quicker.

4. Yes, the force is indeed great. Consider a car that uses a clutch with a mean radius of 12cm, and the width of the padding on the clutch is 3cm. That gives us a surface of about 0.023m2. Assume the clutch has to transmit 100Nm of torque. That means the force on the clutch padding will be about 833N.

   If our clutch has a friction coefficient of 0.8, we need to push on the clutch with a force of $F=833/0.8=1042N$ That's more than 100kg. In reality, the clutch has to be able to withstand much more. We're lucky to have levers and hydraulics.

5. To clean up your main confusion: The pedal travel is relatively long with roughly 25cm, considering the pressure plate only backs away a few mm. The force of your leg is indeed amplified by a lever and hydraulics, the same way you're able to jack up the car with your arms. If your foot travels 25cm, and the clutch only travels 2.5mm, that means the travel is reduced a factor of 100. It also means the force is amplified by a factor of 100. 1kg of pressure by your leg translates in 100kg force on the clutch.

Answer 2

Clutches do indeed carry quite lot of force but when you pull away from stationary the correct technique is to use fairly low revs and blend the clutch in quickly but smoothly. In this case the clutch is only slipping for a second or so and the torque involved is fairly small. If you are more aggressive or slip the clutch for long periods you can indeed wear it out very quickly.

The friction surface is also typically quite large and or large diameter which provides a mechanical advantage, reducing the torque seen by the friction plates.

When changing gears the synchromesh in the gearbox matches the engine speed to the road speed pretty closely so the forces involved are much less.

The clutch also bears on the flywheel which makes a pretty effective heat sink.

The friction plate material itself is usually a composite of some sort, not dissimilar to brake pads and may include ceramics, glass fibre, various resins and synthetic fibres such as Kevlar. the material is developed such that it retains its friction properties as it wears.

As to the force required, yes the normal force between the two plates is pretty big this is achieved by a combination of hydraulics and mechanical leverage in the linkages. Note that the travel in a clutch pedal is usually quite long while the actual distance moved by the plates may just be a few mm, again it is a very similar situation to the braking system.

When the clutch is engaged it is held together by springs, pressing the clutch pedal pushed the plates apart against those springs. Also the reason that it is a pedal at all is that extending your leg can provide quite a lot of force, trying to depress the clutch with your hand is more difficult than you expect.

Some cars, especially older ones with big engines (eg older Jaguars and Land Rovers) do have somewhat heavy clutches which can become difficult in situations like stop-start traffic where you frequently need to fully depress and hold the clutch.