How does a clutch work?

For Bangalore professor, Dr. Dinesh K. Anvekar — seeing is believing. That is why he makes teaching aids from scratch. Here we learn the workings of a clutch in a car   

Jari Lehtinen

We met Dr. Dinesh K. Anvekar at Bangalore’s Mini Maker Faire last month. This model (shown in the video) was built by him entirely from scratch and is used to teach the working of a clutch to engineering students.

According to the definition, a clutch is a mechanical device that engages and disengages the power transmission. Let’s simplify that. In a car, you need a clutch because the engine spins all the time, but the car’s wheels do not. When getting a car to start moving, one needs a higher torque and a lower speed. But as you start moving, you will continue to do so with a reduced torque. As you go higher in gears, the speed increases and torque reduces.

A clutch plays an important role in bringing about this change in function.

A clutch connects and disconnects two rotating shafts (drive shafts orline shafts). One shaft is typically attached to an engine or other power unit (the driving member) while the other shaft (the driven member) provides output power for work.
The clutch allows engine power to be applied gradually when a vehicle is starting out and interrupts power to avoid gear crunching when shifting. Engaging the clutch allows power to transfer from the engine to the transmission and drive wheels. Disengaging the clutch stops the power transfer and allows the engine to continue turning without force to the drive wheels. To understand how a clutch works, we first need to understand who the players are and how the whole shebang works.

So let’s look at the basic components: the flywheel, clutch disk, pressure plate, throw-out bearing and linkage.

Its main function is to transfer engine torque from the engine to the transmission. The flywheel also has teeth along the circumference, allowing the starter motor to contact when turning the engine over.

The clutch disc is basically a steel plate, covered with a frictional material that goes between the flywheel and the pressure plate. In the centre of the disc is the hub, which is designed to fit over the spines of the input shaft of the transmission. When the clutch is engaged, the disc is “squeezed” between the flywheel and pressure plate, and power from the engine is transmitted by the disc’s hub to the input shaft of the transmission.

When the clutch is disengaged (pedal depressed), an arm pushes a release bearing against the centre of the diaphragm spring which releases the clamping pressure.

The outer part of the pressure plate, which has a large friction surface, then no longer clamps the driven plate to the flywheel, so the transmission of power is interrupted and gears can be changed.

Some cars have a hydraulically operated clutch. Pressure on the clutch pedal inside the car activates a piston in a master cylinder, which transmits the pressure through a fluid-filled pipe to a slave cylinder mounted on the clutch housing.

It’s not a stupid question. Yes, automatic cars may not need you to change gears manually but it has clutches, too. In fact, there are clutches in many things you probably see or use every day: Many cordless drills have a clutch, chain saws have a centrifugal clutch and even some yo-yos have a clutch.

Too much jargon? This video should clear things out.

Why does an engine stall?
First let’s see what is happening inside your car when it starts to make that awful grinding noise.

Engine A has the clutch pedal depressed. This causes the clutch plates to separate which in turn disengages the engine from the gearbox allowing you to change gear. You can depress the clutch pedal as quickly as you wish.

Engine B has the clutch pedal fully released which in return joins the clutch plates and making a connection from the gearbox to the engine. When the clutch pedal is brought up too fast, it causes the plates to slam together abruptly which it when you stall the car.

Bringing the clutch pedal up slowly allows the plates to join smoothly which allows the car to pull away. For more tips on driving without clutch catastrophes, read here.



Photo credit: Jari Lehtinen

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