Jacket: Usually part of a term for either heating or cooling the stuffing box, known respectively as the heating jacket and the cooling jacket.
Jet: A powerful stream of fluid emanating from an opening.
Jet pump: Jet pumps are commonly found in residential water supplies and use the principle of the venture to produce a low pressure area that acts as a suction for the intake of water/fluid and allows the pump to lift a fluid.
Jig: A guide used in manufacturing to allow a machine tool to accurately cut pieces repetitively.
Jockey pump: A second, smaller pump found in sprinkler systems. It is connected in parallel to the main fire pump and serves to maintain the pressure in the sprinkler system so that when a fire occurs and a sprinkler activates, lowering the pressure in the system, this will be detected by the controller, and the fire pump will be switched on.
Joint: The locations where two pieces are joined together either by welding or a series of flanges and nuts and bolts.
Joule: A unit of measurement of energy. Defined as the energy required to apply a force of one Newton through a distance of one metre.
Journal: The proportion of the main shaft that is supported by a plain bearing.
Journal bearing: A plain, sleeve or sleeve-shaped bearing used to support a shaft.
Journal box: A casing or housing that contains the journal and the journal bearing.