8.8 : Frictional Forces on Screws

Screws are characterized by a helical ridge known as a thread wrapped around a cylindrical shaft. They are commonly used as fasteners to hold objects together or to transmit power and motion in machines. One type of screw that is particularly useful for transmitting power is the square-threaded screw.

A jack with a square-threaded screw is a mechanical device used to lift heavy loads by applying a force at its handle. When the force is applied, the screw turns, raising the load. The screw can be considered a cylindrical shaft with an inclined square ridge or thread wrapped around it. This thread has specific characteristics that define its functionality.

The distance between two adjacent threads on a screw is called the pitch. This measurement is critical, as it determines how closely the threads are packed together. Another essential characteristic of a screw is its lead, which refers to the distance the screw advances in one complete revolution.

When the thread of a screw is unwrapped by one revolution, it resembles an inclined ramp with a specific angle. This angle of inclination, known as the lead angle, can be expressed in terms of the mean thread radius (the average distance from the center of the screw to the centerline of the thread) and the lead. The lead angle is a crucial parameter, as it affects the efficiency of the screw and the amount of force required to turn it.

To understand the mechanics of a square-threaded screw, it is helpful to consider the frictional forces that act on the screw. Since the frictional force between two surfaces is independent of the contact area, the screw can be represented as a block over the thread of the jack. The angle between the reaction force (the force exerted by the thread on the block) and the normal to the thread (the direction perpendicular to the thread surface) is called the angle of friction. This angle is determined by the materials in contact and the lubrication used between them.

The friction between the screw and the load it carries plays a crucial role in the efficiency and functionality of the square-threaded screw. The frictional forces must be overcome to turn the screw and raise the load. However, these forces also provide stability, preventing the screw from turning and the load from falling even when the force is removed from the handle. This self-locking feature is particularly useful in applications where the screw must hold the load in place without continuous input force.

Due to their high efficiency and self-locking capabilities, square-threaded screws are often employed in machines to transmit power or motion between parts. Examples of such applications include jacks, clamps, vises, and various presses. These screws are designed to withstand heavy loads and provide smooth, controlled motion.