JOUNCE – A compression load on the springs as the space between the frame and the axle is reduced.

JOUNCE BUMPER – A rubber bumper used to absorb shock during full suspension system movement.

JOURNAL – In a shaft, the hardened polished area that is used with a bearing.

JET PROPULSION – The unit in which the gas turbine is designed to produce just sufficient power to drive the compressor and the exhaust gases from the turbine are then expanded to atmospheric pressure in a propelling nozzle to produce a high velocity jet.

JACKETS – The water jackets that surround the cylinders through which the coolant passes.

JO BLOCKS – These refer to Johansson precision gauge blocks.

JACOB’S CHUCK – Gear operated drill chuck of three jaw type for use on drilling or milling machines and lathes.

JACOB’S TAPERS – Standard system of tapers which are used for drill chucks and spindles.

JIG – A device that locates and holds a workpiece and guides the cutting tool.

JIG BUSH – Hardened steel cylindrical guide, made a press fit into the body of the jig for a drill or reamer to pass through.

JIG PLATE – Flat steel plate provided with a number of bushed holes and used to locate the drill when drilling a component.

JOURNAL BOX – A housing or support for a bearing which contains the journal of the shaft.

JOURNAL – The part of a shaft or axle that has been machined or finished to fit into a bearing.

JIG BORING MACHINE – An extremely accurate machine tool having a precession spindle and a table to hold the work. The table and spindle are movable with built in measuring devices. Used for spacing and producing precision holes in tools, dies, and small lot production where extreme accuracy is required.

JACK – Device used to lift loads through a short height some 0.8 to 1 m used widely in repair and erection jobs, e.g., screw jacks, rack and lever jack’s rack and pinion jacks and hydraulic jacks.

JOGGLING – Bending operation necessary for making a lap joint, yet keeping the top surface flush.

JET – A calibrated passage in the carburettor through which fuel flows.

JAW STAY – A round bar having jaws forged at one end and a flat plate at the other inclined at the proper angle for riveting to the boiler shell.

JET CONDENSER – A closed chamber within which exhaust steam comes in direct contact with a spray or jet of cold water and is condensed.

JOULE’S LAW OF INTERNAL ENERGY – The internal energy of a given quantity of a gas depends only on the temperature of the gas. As temperature changes, the internal energy also changes.

JOINT – Junction of members or the edges of members which are to be joined or have been joined.

JOINT EFFICIENCY – The ratio of the strength of a joint to the strength of the base metal expressed in per cent.