The screws, bolts, studs, nuts, etc are widely used in various machines and structures for temporary fastenings. These fastenings have screw threads, which are made by cutting a continuous helical groove on a cylindrical surface. If the threads are cut on the outer surface of a solid rod, these are known as external thread. But if the threads are cut on the internal surface of hollow rod, these are known as internal threads.
1. Helix: It is the curve traced by a particle, while describing a circular path at a uniform speed and advancing in the axial direction at a uniform rate.
2. Pitch: It is the distance from a point of a screw to a corresponding point on the next thread, measured parallel to the axis of the screw.
3. Lead: It is the distance, a screw thread advances axially in one turn.
4. Depth of thread: It is the distance between the top and bottom surfaces of thread is known as crest and root of thread.
5. Single-threaded screw: If the lead of screw is equal to its pitch, it is known as single threaded screw.
6. Multi-threaded screw: If more than one thread is cut in one lead distance of a screw, it is known as multi-threaded screw. In double-threaded screw, two threads are cut in one lead length. In such cases, all the threads run independently along the length of the rod. Mathematically,
Lead = Pitch x Number of threads
7. Helix Angle: It is the slope or inclination of thread with the horizontal. Mathematically,
tan α = lead of screw / circumference of screw