Manual clutch.
A clutch is used to match the speed of two rotating systems; in the case of a car, this can mean the engine and the transmission.
Why do we need this? - in most vehicles, the engine is required to spin constantly, but the transmission may need to stop: viz, at traffic lights. Clearly, there's a need for a system which allows the rotation of both systems to be matched as the operator wishes, and prevents the vehicle's engine from stalling when the vehicle is at rest. Various systems have been devised to achieve this end- the clutch is just one, but a testament to its effectiveness is that it has been around for almost as long as the car has. All clutches operate via the effects of friction.
A clutch can be controlled manually or electronically- I'll detail the manual variety, as it is a bit easier to explain. Manual clutches are usually controlled by a footpedal, but any controller can suffice.
In simplified form, what happens is this:
The flywheel, a heavy disc of metal, is connected directly to the engine. When the engine is running, the flywheel is rotating. The clutch plate, a thin disc of 'friction material'- typically asbestos- is connected directly to the transmission, and faces the flywheel. Both are usually protected by some form of housing. The clutch plate's distance from the flywheel can be anything from an inch or so away, to zero distance- crucially, this distance can be controlled by some form of actuator, connected to the aforesaid controller. With this arrangement, we end up with a system where the operator can control the separation between the two systems- anywhere from fully engaged- where all engine power is directed to transmission- to fully disengaged- the engine can rotate independently of the transmission. This makes for a simple, effective means of controlling how much engine power ends up at the wheels.However, the system is not without pitfalls-
-This arrangement can generate intense heat, hence both sides need to be made of heat-hardened materials. However, it is still possible to burn out either side by 'dumping' the clutch- lifting the pedal too abruptly- or 'riding' the pedal- maintaining light pressure and thus separation between plate and flywheel.
-Over time, friction takes its toll on the linings, and can wear them out, requiring renewal of some parts.
-Both sides need to be in precise alignment, or the system will be liable to inadvertent vibrations- this can be felt as a shuddering or juddering through the clutch pedal on some vehicles.
Caveats aside, clutches are fairly simple to look after, and, if set up properly at installation, can last for a surprising length of time, depending on your driving style. I've had clutches which last longer than the car! But for maximum lifespan-
-Don't allow any oil or other fluid on the lining.
-Check your hydraulic fluid or cable distance as recommended by your car's manufacturer.
-Don't ride that pedal, hoss!
Basic clutch diagnostics
By its nature, the clutch is capable of transmitting some meaningful info to you, through your right foot, or whatever you're controlling it with. Here's some stuff to be aware of:
-Clutch slip. There's a test you can try to check the effectiveness of your clutch's operation. Find a reasonable hill, and get your speed around about 30-40 miles per hour. Be in a high gear- and floor your accelerator. If the engine revs suddenly rise, with no sign of meaningful acceleration- it can be a sign of clutch trouble. On some cars, this can just mean adjusting your clutch cable to allow the clutch plate to 'bite in' more, or checking that the hydraulic fluid reservoir is adequately filled- on others, it may indicate a worn plate which will definitely need renewing. This can also be caused by fluid- generally oil- on the lining itself. Again, this will indicate that the plate needs changed- and the source of contamination will need checked and stopped too.
-Clutch judder. If you can feel a 'thumping' or 'oscillation' through your clutch pedal, specifically at low speeds or when changing gear, this usually means that the clutch plate is misaligned.(It can mean a problem with engine mounts too: for simplicity, we won't go into that here.) This often occurs after the plate or other components have been renewed, if proper care has not been taken to check alignments. In these cases, the plate can often be realigned, if it hasn't been damaged- if it happens on a car that has an older clutch, it is worth getting it checked out to find out what's caused the judder/ misalignment in the first place. Worst case scenario is a damaged flywheel, which will mean that it, and probably the clutch assembly too, will need replaced. Bad luck Stirling.
-Funny noises. Tricky. Mechanics love drivers whos cars have funny noises- they can spend ages faffing about and charging you money. In the case of the clutch- as with humans, little creaks and whines are a common thing with age, not much to worry about there. However, if you hear a 'crunchy whine' when the engine is turning and you've got your foot on the clutch pedal, it can mean that the release bearing is beginning to disintegrate. This needs changed- get it done.
Don't try this at home?
Fortunately, clutch renewal is well within the scope of the home mechanic, depending on ability and model of car. FWIW- although you may have correctly identified the offending part on your car, most stores sell the entire set of replaceables: usually the clutch plate, a thrust plate, and possibly one or two other gewgaws, as a clutch kit, and will be loath to sell you the part solus; bow to their wisdom, and replace all the wearable stuff while you've taken the trouble to unbolt everything. And now, by way of example, here's my experience of clutch replacement on two diametrically-opposed cars:
-Austin Mini. A cinch. Clutch and housing are easy to reach- clutch can be removed with no special tools, clutch kits are easy to get and cheap to buy- £80 UKP,and you and a patient friend can do it in an afternoon if you're careful about alignment.
-MCC Smart. Don't even think about it. Inaccessible clutch, automation...yeecchhh.
In all matters of home automotive repairs, your best friends are going to be a Haynes or equivalent manual, an owner's club, and being chummy with the parts guy (they are invariably male) at your local repair store.
So you see, clutches can be fun!