Introduction

All machines are composed of many parts, which are joined together by some type of joint. The joint may be permanent like riveting or welding, or detachable using bolts and nuts. The latter provides the advantage that the parts can be separated, whenever required, without damaging the joint. Bolt and nut are mostly used for such joints. Standardization of such widely used components is necessary for interchangeability.

A bolt or screw is a type of fastener having a helical ridge wrapped around a cylinder, known as an external thread. The cylindrical portion of the screw from the underside of the head to the tip is known as the shank; it may be fully threaded or partially threaded (Plate 16.1).

Some threads mate with a complementary thread, known as an internal thread of a nut, which is a machine part having internal threads formed into it. Other screw threads are designed to cut a helical groove in a softer material as the screw is inserted, for example, wooden screw. The most common use of screws is to hold objects together. Majority of bolts are tightened by clockwise rotation, which is termed a right-hand thread. Screws with left-hand threads are used in exceptional cases, for example, the left side pedal of a bicycle has a left-hand thread.

Often bolts have a head, which is a specially formed section on one end that allows it to be turned or driven. Common tools for driving bolts are spanners and screwdrivers for screws. The head is usually larger than the body of the bolt, which keeps the bolt from being driven deeper than its length and provides a bearing surface.