BACKLASH: The distance (play) between two movable components.

BACK PRESSURE: A pressure exerted contrary to the pressure producing the main flow.

BAFFLE: A device which slows down or diverts the flow of gases, liquids, sound, and so forth.

BALANCED VALVE: A valve in which the fluid pressure is equal on both sides (the opening and closing directions).

BALL BEARING: A bearing that uses steel balls as its rolling element between the inner and outer ring (race).

BALL CHECK VALVE: A valve consisting of a ball held against a ground seat by a spring. It serves to check the flow or to limit the pressure of a liquid or substance.

BAROMETER: An instrument which measures atmospheric pressure.

BBDC: Abbreviation for before bottom dead center.

BDC: Abbreviation for bottom dead center. The position of a reciprocating piston at its lowest point of travel.

BEARING: A mechanical component which supports and guides the location of another rotating or sliding member.

BEARING CLEARANCE: The distance between the shaft and the bearing surface.

BELL BOOK: An official record of engine orders received and answered.

BIMETALLIC: Composed of two metals with different rates of expansion, which curve to a greater, or lesser, extent when subjected to temperature changes.

BLEEDER: A small cock, valve, or plug that drains off small quantities of air or fluids from a container or system.

BLOCK DIAGRAM: A diagram in which the major components of a piece of equipment or a system are represented by squares, rectangles, or other geometric figures, and the normal order of progression of a signal or current flow is represented by lines.

BLOWBY: Exhaust gases that escape past the piston rings.

BLOWER: A low-pressure air pump, usually a rotary or centrifugal type of pump, that supplies air above atmospheric pressure to the combustion chambers of an internal-combustion engine.

BLUEPRINTS: Copies of mechanical or other types of technical drawings.

BOILER: Any vessel, container, or receptacle that is capable of generating steam by the internal or external application of heat. The two general classes are fire tube and water tube.

BOILER BLOW PIPING: Piping from the individual boiler blow valves to the overboard connection at the skin of the ship.

BOILER DESIGN PRESSURE: Pressure specified by the manufacturer, usually about 103% of normal steam drum operating pressure.

BOILER FEEDWATER: Deaerated water in the piping system between the deaerating feed tank and the boiler.

BOILER LOAD: The steam output demanded from a boiler, generally expressed in pounds per hour (lb/hr).

BOILER REFRACTORIES: Materials used in the boiler furnace to protect the boiler from the heat of combustion.

BOILER WATER: The water actually contained in the boiler.

BONNET: A cover used to guide and enclose the tail end of a valve spindle.

BORE: A cylinder hole or the inside diameter of a cylinder or hole.

BOSCH METERING SYSTEM: A metering system with a helical groove in the plunger which covers or uncovers ports in the pump barrel.

BOTTOM BLOW: A procedure that removes suspended solids and sludge from a boiler.

BOTTOM DEAD CENTER: See BDC.

BOURDON TUBE: A C-shaped hollow metal tube that is used in a gauge for measuring pressures of 15 psi and above. One end of the C is welded or silver-brazed to a stationary base. Pressure on the hollow section forces the tube to try to straighten. The free end moves a needle on the gauge face.

BOYLES LAW: The volume of any dry gas varies inversely with the applied pressure, provided the temperature remains constant.

BRAKE HORSEPOWER (bhp): The usable power delivered by an engine.

BRAKE MEAN EFFECTIVE PRESSURE (bmep): Mean effective pressure acting on the piston, which would result in a given brake horsepower output if there were no losses from friction, cooling, or exhaust. Bmep is equal to mean indicated pressure times mechanical efficiency.

BRAKE THERMAL EFFICIENCY: Ratio of power output (in the form of brake horsepower) to equivalent power input (in the form of heat from fuel.

BRAZING: A method of joining two metals at high temperature with a molten alloy.

BREAKER POINTS: Metal contacts that open and close a circuit at timed intervals.

BRINE: (1) A highly concentrated solution of salt in water, normally associated with the over-board discharge of distilling plants. (2) Any water in which the concentration of chemical salts is higher than seawater.

BRITISH THERMAL UNIT (Btu): A unit of heat used to measure the efficiency of combus-tion. It is equal to the quantity of heat required to raise 1 pound of water 1F.

BRUSH: The conducting material, usually a block of carbon, bearing against the commutator or sliprings through which the current flows in or out.

BTDC: Abbreviation for before top dead center.

BULL GEAR: The largest gear in a reduction gear train-the main gear, as in a geared turbine drive.

BUS BAR: A primary power distribution point connected to the main power source.

BUSHING: A renewable lining for a hole in which a shaft, rod, or similar part moves.

BUS TRANSFER: A device for selecting either of two available sources of electrical power. It may be accomplished either manually or automatically.

BUTTERFLY VALVE: A lightweight, relatively quick acting, positive shutoff valve.

BYPASS: To divert the flow of gas or liquid. Also, the line that diverts the flow.