The first thing any auto aficionado thinks of when they hear the word “clutch” is the third pedal necessary for operating any manual transmission. However, clutches are actually very useful mechanical systems that can be found in all kinds of devices like drills, chain saws, and even yo-yos.
Clutches have two rotating shafts, one of which is typically driven by a motor or pulley while the other actively drives another device. For example, the clutch inside a drill is driven on one side by a motor and in turn drives a drill chuck on the other side. The clutch connects the two spinning shafts so that they can either be locked together and spin at the same rpm or remain connected in some way despite spinning at different rpm’s.
In a vehicle context, a clutch comes in handy because the engine must spin constantly in order to function, but most drivers appreciate a good stop every now and then during their voyage from point A to point B. In order for the wheels to stop without killing the engine, the wheels and the engine need to be temporarily disconnected. Additionally, because engines have a certain range of rpm in which torque and horsepower are at their maximum, it helps to have gears that allow for that same rpm range to accomplish different speeds as needed. The clutch allows for a spinning engine to smoothly engage with a non-spinning transmission, and it allows for cars to switch between gears.
A clutch functions by manipulating the friction between a clutch plate and a flywheel. The clutch plate is connected to the transmission, while the flywheel is connected to the engine. When you don’t push down on the clutch pedal, springs push a pressure plate against the clutch disk which in turn pushes against the flywheel. That means that the transmission input shaft is locked to the engine and they will both spin at the same speed.
When you engage the clutch pedal, you disengage the clutch from the spinning engine by applying pressure that ultimately pulls the pressure plate away from the clutch disk.
This is an elegant system, but it is limited in terms of how long it will last. If the clutch isn’t used gently enough, the friction material on the clutch disk can wear down until the clutch begins to slip and eventually doesn’t transmit any power from the engine to the wheels.
The clutch wears when the clutch disk and the flywheel are spinning at different speeds. When they are spinning at the same rpm, they don’t rub against each other because they are spinning in synch.
A clutch can also stick, which causes major problems for drivers. Sticking can happen as a result of a broken or stretched clutch cable, a leaky or defective slave and/or master clutch cylinders, air in the hydraulic line, misadjusted linkage or mismatched clutch components.
This may come as a surprise, but automatic transmissions also contain a clutch; in fact, they contain several. The clutches on automatic transmissions are put into motion through using pressurized hydraulic fluid.