Question

friction does not depends upon surface area..then why multiplate clutch is better than single plate clutch?

Answer 1

The force of friction depends upon the normal force, i.e. the force that is pressing the two surfaces together.

But you have to remember that if an object is not accelerating in a direction, the forces in that direction must be balanced. So if gravity is pushing on object down onto the ground, the ground must also be pushing up with an equal force. Otherwise the object would fall into the ground!

In a multiplate clutch, multiple plates are pushed together. But this that between each plate there is a pair of equal and opposite forces!

In my drawing I used 4 plates, which means 3 surfaces where the force transfers from one plate to the next. So the total friction is 3x more than a system with just 2 plates.

The important distinction is that by stacking the plates, you can get more friction by transferring the force through more surfaces. But if you just made the plates bigger, but kept the force the same, friction would not change.