Screws
Like a wedge, a screw is a simple machine that is related to the inclined plane. A screw can be thought of as an inclined plane wrapped around a cylinder. This spiral incline forms the threads of a screw. When you twist a screw into a piece of wood, you exert and input force on the screw. The threads of a screw act like an inclined plane to increase the distance over which you exert the input force. As the threads of the screw turn, they exert an output force on the wood, pulling the screw into the wood. Friction between the screw and the wood holds the screw in place. The closer together the threads of a screw are, the greater the mechanical advantage. This is because the closer the threads are, the more times you must turn the screw to fasten it into a piece of wood. Your input force is applied over a longer distance. The longer input distance results in an increased output force. Think of the length around the threads as the length of the inclined plane, and the length of the screw as the height of the inclined plane. The ideal mechanical advantage of a screw is the length around the threads divided by the length of the screw. This is shown in the equation below.