How does a clutch work and why do we need it to change gears smoothly?
In: Engineering
The whole reason you have gears is so they can be different. In LOW gear, the engine’s middle RPM (say 3000) is 20 miles/hour. In a MIDDLE gear the same engine RPM might be 40 miles/hour. When you are going 20 in LOW, you shift to MIDDLE to continue to accelerate. But your massive car is still going 20 miles per hour. In MIDDLE, this is 1500 RPM. If you try to connect an engine running 3000 RPM to a gear going 1500 RPM, stuff’s gonna break. A clutch is a special friction disc system that allows the two to come to a common speed efficiently. There is some other coordination involved in shifting, using your right foot to slow the engine when the clutch is pressed in to remove the load. Beginners tend to do this badly, causing jerky driving.
A clutch is the intermediate between the engine’s output and the transmission’s input. It’s essentially a hjgh-friction surface that mates to both components, and transmits rotational force.
When you shift gears, the engine needs to be momentarily disconnected from the transmission by pushing the clutch because their speeds will become desynchronized as the transmission shifts into a new gear. When the clutch is released, the friction surfaces “grab” onto the transmission input and the two resynchronize back to normal driving speed.
Without a clutch, shifting would have to happen with the engine spinning at exactly the right RPM for the desired gear before shifting- this is known as [floating gears](https://en.m.wikipedia.org/wiki/Float_shifting).
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The problem is a combustion engine can’t start turning from stop – that’s why it needs a starter motor… but we need to be able to connect it to the wheels of a car that is stopped without grinding the metal of the gears together and allowing us to do so gradually for that acceleration. If the weight of the car were introduced to the engine instantly it would just force the engine to slow down so much it would stop – this is stalling the engine.
So how do we get the car moving when what we have is an engine that can’t be stopped? Gears alone can’t solve that specific problem.
The answer is to introduce another part – the clutch. It tolerates slipping – it can move at a different speed from the engine and partially transfer the power. So we can use the clutch to introduce engine power gradually while allowing the engine to spin faster than the gears during that starting-from-a-stop phase.
The clutch is a big round disc with material on it like what your brakes are made of. It connects to the transmission/gearbox facing the engine with that brakepad material facing the engine. The engine has a big round disc of its own facing the clutch which is what the engine is turning, and the clutch is push into this engine disc by a strong spring-like mechanism. Like brakes they lock up when the pressure is at maximum and they’re as good as bolted together most of the time, spinning together. However the clutch pedal, when pressed, pulls the clutch disc away from the engine. The pressure on the clutch pedal is that spring. As you release the clutch pedal the clutch disc is pressed back into the engine. When the clutch is partially down there’s partial pressure onto the engine, like when you’re applying the brakes gently to stop – rubbing together causing a force to be applied.
To prevent grinding gears when moving the gear lever around, press the clutch pedal all the way down so the clutch disc is fully separated from the engine. Now with minimal weight on the gears you can move between them without issue. Release the clutch slowly so the friction between the clutch and engine will make the engine slow down a little bit – add gas here to help compensate – and the gears will start slowly turning, hence turning the wheels.
Now… in theory you don’t need to press the clutch pedal to change gears while moving. If the engine speed is exactly correct for the current car speed, then the gear lever will go into gear without fuss even if you don’t touch the clutch pedal. In practice there is very little room for error here and a mistake will cause metal to grind. Disconnect the engine from the gearbox while changing gears to make the procedure safe, and then reconnect the engine and gears with the clutch. Let it take the speed difference – that’s what it’s designed to do. Seriously, never do this.
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