Most of the screws are characterized by a helical ridge and are commonly used as fasteners; square-threaded screws are often employed in machines to transmit power or motion between parts. Consider a jack with a square-threaded screw carrying a load. When a force is applied at the handle, the screw turns, and the load is raised. The screw can be considered a cylindrical shaft with an inclined square ridge wrapped on it. The distance between the crests of two adjacent threads is the pitch, and the distance the screw advances in one revolution is known as its lead. When the thread is unwrapped by one revolution, it resembles an inclined ramp with an angle. The thread's inclination angle is called the lead angle and is expressed in terms of the mean thread radius and lead. Since frictional force between surfaces is independent of the contact area, the screw is represented as a block over the thread of the jack. Angle between the reaction force and the normal to the thread is the angle of friction.