Joint, Machine-Part

any of the fixed or movable joints between machine parts specified in the design of the machine or of the machine’s subassemblies. In machine building, a machine-part joint is usually understood to mean a fixed joint, with movable joints being exclusively determined by the kinematics of the machine.

Joints permit the disassembly of machines into subunits and individual parts, simplify the technological processes of manufacturing and assembling machines, facilitate repairs and the rebuilding and replacement of parts, and allow for the transport, assembly, and installation of machines. Complex machines have a multitude of joints, mainly because of the number and range of the parts involved; for example, there are approximately 5,000 parts in a coal combine, more than 16,000 in an automobile, and approximately 1.5 million, with 400,000 names, in a rail-structural mill. Joints must be not only strong but also tightly sealed when connecting pipes, vessels, and apparatus containing liquids or gases.

Fixed joints are classified as either temporary or permanent. Temporary joints permit the parts to be assembled and disassembled repeatedly without damage; permanent joints are designed to connect parts in such a way that there is no possibility of disassembly.