 A screw is a combination of two simple machines: an inclined plane wrapped around a wheel and axle.     When you turn a screw into a piece of wood, the wheel provides a force multiplication to turn the axle. Then the screw threads act as an inclined plane, forcing the wood fibres apart, and pulling the screw inwards. The steeper the angle of the plane (the greater the pitch of the threads), the more force you'll have to apply, ... but the screw will go in quickly. Alternately, the shallower the angle of the plane (the smaller the pitch of the threads), the less force you'll have to apply, ... but the screw will go in slowly. The force applied by the threads can also be used for other purposes. Screws can be used to drill holes, and if contained within a pipe, can be used to lift things, such as grain (in an auger), or water (with a device invented by Archimedes several thousand years ago). |