HOPPER COOLED SYSTEM – A non (external) circulating system. The cylinder has an open water jacket of considerable volume.

HALF AXLE DRIVE SHAFTS – Two in number, transmit the driving torque from the final drive and differential unit to the driving road wheels.

HANDLING – The ease of manoeuvring a vehicle without slipping or skidding.

HARSHNESS – Bumpy ride produced by a stiff suspension.

HEEL – Anchor end of a brake shoe.

HOTCHKISS DRIVE – A rear suspension with open propeller shafts with two or three universal joints. Braking torques are transferred to the frame through links, control arms or leaf springs.

HORN RELAY – A relay connected between the battery and horns, when energized by closing of the horn button, it connects the horn to the battery.

HYDRAULIC BRAKE – A brake system using hydraulic fluid, piston and cylinders to provide extremely high pressure for break application.

HYDRAULIC CLUTCH – A clutch that uses hydraulic pressure to actuate the clutch. Used in heavy duty equipment and where the engine is away from the driver’s compartment so that it would be difficult to use mechanical linkages.

HYDRAULIC CONTROL VALVES – A system of valves that senses driving conditions and automatically shifts the transmission.

HYPOID GEARS – Drive pinion and ring gears whose shape allows them to mesh off centre.

HOT SMOKE – The smoke that consists of unburned carbon particles (0.5 to 1.0 micron in diameter) and other solid combustion products. This may be light gray to black smoke.

HORN GATE – A curved gate in the shape of a horn arranged to permit entry of molten metal at the bottom of the casting cavity.

HOT TEARS – Cracks in castings formed at elevated temperatures, usually by contraction stresses.

HALF LIFE – The characteristic time required for half of the nuclei of a radioactive species to disintegrate spontaneously.

HALF VALUE LAYER – In radiation, the thickness of absorber that will reduce the intensity of radiation to one half. It is useful in estimating radiographic exposure.

HIGH CARBON STEEL – Steel that has more than 0.6% carbon.

HIGH SPEED STEEL – Alloy steel (alloying elements being tungsten, chromium, vanadium, cobalt and molybdenum) which retains its strength and hardness at red heat, and is thus suitable for cutting tools which reach high temperatures in use.

HINDU MINIUM – A high strength aluminium alloy containing, in addition to aluminium, magnesium, iron, titanium, copper, nickel and silicon, which after heat treatment has a strength exceeding that of mild steel.

HOT ROLLED STEEL – Steel rolled to shape while being heated to the plastic condition.

HOYT METAL – Commercial grade of white metal used for bearing purpose.

HYPER EUTECTIC ALLOY – Any binary alloy whose composition lies to the right of the EUTECTIC on an equilibrium diagram and which contains some eutectic structure.

HYPO EUTECTIC ALLOY – Any binary alloy whose composition lies to the left of the EUTECTIC on an equilibrium diagram and which contains some eutectic structure.

HYDRODYNAMIC LUBRICATION – A condition that occurs when a film of oil is constantly maintained between moving parts.

HEATING VALUE OF A FUEL – The heat liberated by the complete and rapid burning of a fuel per unit weight or volume of the fuel. Also called calorific value of the fuel.

HIGHER HEAT VALUE – A standard recommended by the ASME, the higher heat value of a fuel includes the heat value of the hydrogen in the fuel. The heating value indicated by a fuel calorimeter.

HIGH TEST GASOLINE – A term referring to the octane rating of a fuel. A high test fuel has a high octane rating.

HOGGED FUEL – Wood that has been chipped and shredded, usually by a machine called a “hog”.

HYDROCARBON – A compound made of elements of hydrogen and carbon atoms. Gasoline, diesel oil are blends of different hydrocarbons refined from crude oil.

HEAT EXCHANGER – Device which uses some of the heat in the turbine exhaust gas to preheat the air entering the combustion chamber. This reduces the fuel supply for a given required temperature increase. Also called REGENERATOR.

HEAT EXCHANGER THERMAL RATIO – Ratio of the actual heat picked up by the compressed air in the heat exchanger to the maximum possible heat that could be absorbed. Also called EFFECTIVENESS OF HEAT EXCHANGER.

HARDENING – Process of increasing the hardness of a ferrous alloy by austenitizing and quenching, also the process of increasing the hardness of some stainless steels and non-ferrous alloys by solution heat treatment and precipitation.

HARD SURFACING – Arc welding a surface layer usually at strategic spots on a part, using an electrode having a suitable alloy content for the purpose. Also known as SURFACING and FACING.

HEAT TREATMENT – Operation or combination of operations, involving the heating and cooling of a metal or alloy in its solid state with the object of changing the characteristics of the material.

HOT ROLLING – Surface layers of noble metals are often hot rolled (or inlayed) on less noble metals.

HEAT DAM – In a piston top land a groove cut out to reduce the size of the heat path, allowing the piston skirt to run at lower temperature.

HEAT LAND RING – A compression ring having the cross-sectional shape of the letter L, used as top ring.

HELICOIL – A rethreading device to repair worn or damaged threads. It is installed in a retapped hole to bring the screw thread down to the original size.

HORSE POWER (hp) – A measure of the mechanical power, or the rate at which work is done. One horse power equals 4500 m.kg of work per minute.

HEIGHT GAUGE – Marking out or measuring tool designed on the lines of a scribing block, but capable of more accurate duty.

HERMOPHRODITE CALIPER – A caliper having one leg similar to that of an inside caliper, while the other leg is pointed like a divider leg.

HOOK RULE – A steel rule with a projecting piece or hook at one end at right angles to the edge of the rule.

HALF NUT – A mechanism attached to the apron of a lathe and operated by a lever. The lever opens and closes a nut that has been split lengthwise so that the two halves of the nut can be closed upon the lead screw when threads are being cut. Also called SPLIT NUT.

HAND VICE – Small vice used for holding light articles such as pins, screws etc., especially during heat treatment.

HAND WHEEL – Any of the wheels found on machine tools for moving or positioning parts of a machine to hand feed, as the tailstock handwheel on a lathe.

HARDIE HOLE – Square hole in the face of an anvil which forms the location for the hardies and other tools used by the smith, such as small swages, fullers and cutters.

HEAD STOCK – An assembly containing the headstock spindle and the mechanism for driving it which is fastened permanently to the left end of the lathe bed.

HOLDING BLOCK – Fixture designed to hold small or thin gauge parts which cannot readily be held in a vice.

HAND WHEEL – Any of the various wheels found on machine tools for moving or positioning parts of the machine by hand feed, as the tailstock handwheel on a lathe.

HALF MOON KEY – A fastening device in a shape somewhat similar to a semicircle.

HELICAL GEAR – A gear in which the teeth are cut at some angle other than a right angle across the gear face.

HELICOIL – A thread insert used to repair worn or damaged threads. It is installed in a retapped hole to bring the screw thread down to original size.

HELIX – The curve formed by a line drawn or wrapped around a cylinder which advances uniformly along the axis for each revolution, as the thread on a screw or the flute on a twist drill. A helix is often called a spiral in the shop.

HELIX ANGLE OF A THREAD – The angle made by the helix of the thread at the pitch diameter line with a line at right angle to the axis.

HEXAGONAL NUT – A nut having six sides and shaped like a hexagon.

HEAT EXCHANGER – A device used to cool a fluid by transferring heat.

HOIST – Appliance or machine for lifting heavy material, also called lifting tackle.

HYDRAULIC GOVERNOR – A governor using fluid to operate the fuel control.

HALF-LIFE – The characteristic time required for half of the nuclei of a radioactive species to disintegrate spontaneously.

HARDENABILITY – The property that determines the depth and distribution of hardness in a ferrous alloy induced by heating and quenching.

HARDNESS – The property of metal to resist being permanently deformed. This is divided into three categories, resistance to penetration, resistance to abrasion and elastic hardness.

HOT SHORT – Brittleness in hot metal. The presence of excess amounts of sulphur in steel causes hot shortness.

HYDROGEN EMBRITTLEMENT – A condition of low ductility in metals resulting from the absorption of hydrogen.

HAMMER WELDING – Uniting by heating two pieces of mild steel or wrought iron to a soft malleable condition and hammering them together on an anvil.

HEADING – A metal gathering or upsetting operation. Originally used for production of screw and rivet heads in cold working process.

HONING – Process whereby a mirror finish is given to important bearing surfaces by using hones or abrasive tools that normally employ both rotary and longitudinal motion.

HOOKER PROCESS – It is usually a cold extrusion process and is commonly employed for the production of small, thin walled copper and aluminium seamless tubes and small cartridge cases.

HOT PRESSING – Forming or forging tough metals such as alloy steel at high temperatures.

HOT PRESSURE WELDING – In this, bonding of two materials is brought about by application of high pressure and production of mutual deformation at temperatures below the melting range of either. Bonding results from interlocking of the atoms of each piece with some additional interlocking introduced by diffusion.

HOT ROLLING – A process of forming metals between rolls in which the metals are heated to temperatures above the transformation range.

HYDROJET – A manufacturing process in which a material is cut by a high pressure jet of water often containing an abrasive material to enhance cutting action.

HELICAL MOTION – Motion of a body in which each point in the body describes a helix. Helix is the locus of a point which rotates about an axis at a fixed distance and at the same time moves parallel to the axis.

HELICAL GEARING – A type of spur gearing in which although the axes of the shafts are parallel, the teeth are cut on helices instead of straight across the wheels parallel to the axis.

HIGHER PAIRS – Types of kinematic pairs, namely, two elements generally have line or point contact and the pair must be force closed in order to provide completely constrained motion.

HUNTING OF GOVERNOR – The governor is said to hunt if the engine speed is caused to fluctuate continually above and below the mean speed.

HYPOCYCLOID – The locus of a point on the circumference of a circle which rolls inside a circular arc without slipping.

HALIDE LEAK DETECTOR – Operates on acetylene to detect vapour leaks of halogen refrigerants.

HEAT OF RESPIRATION – Heat given off by cargo.

HELIUM LIQUEFIER REFRIGERATOR – The complete system for liquefying helium.

HERMETIC COMPRESSOR – The compressor unit in which the motor and the compressor are manufactured as a single self contained unit housed within a casing, the electric motor is in contact, therefore with the refrigerant.

HIGH PRESSURE CUTOUT – A pressure operated switch which stops the machine on the rise of pressure to a level approaching danger, and usually has to be reset by hand.

HIGH SIDE FLOAT – Metering system which locates the float and needle valve on the high pressure side of the refrigeration system.

HOLD OVER PLATES – Containers that hold the eutectic and provide refrigeration.

HUMIDITY – Moisture in air.

HUMIDISTAT – Operating control which reacts to variation in humidity.

HEAT RANGE – A term used to describe the ability of a spark plug to carry away heat. Plugs with longer nosed insulators take longer path and time to carry heat off effectively.

HEMISPHERICAL COMBUSTION CHAMBER – A combustion chamber shaped like a round dome, allowing use of large valves placed opposite each other in the chamber.

HIGH SPEED CIRCUIT – The circuit in the carburettor that supplies fuel into the air passing through the air horn during, medium and high speed, part to full throttle operation.

HIGHEST USEFUL COMPRESSION RATIO – is the compression ratio at which a fuel test engine can be operated without detonation with any mixture strength or with any ignition timing, at a speed of 1500 rpm.

HOLED PISTON – A condition caused by severe detonation or pre-ignition, where a hole is eaten or burned through the piston crown, as a result of extreme heat and pressure.

HOT PLUG – has a longer heat path, hence it runs at a much higher temperature than a cold plug.

HEATING SURFACE IN A BOILER – Fire side area in a boiler exposed to the products of combustion. The area consists of the surface area of tubes, fire boxes, shells, tube sheets and projected area of headers.

HORIZONTAL RETURN TUBULAR BOILER – One so arranged that the products of combustion after passing along the length of the shell, return in an opposite direction through the tubes, before passing up the stack.

HOT WATER HEATING BOILER – A hot water heating boiler is used for space water heating, with the water returned to the boiler.

HOT WATER SUPPLY BOILER – A boiler which supplies hot water to be used externally to itself for washing, cleaning etc.

HEAT ENGINE – A device which transforms heat energy into mechanical energy and render the latter available for doing useful work. It is merely an energy transformer.

HELICAL FLOW TURBINE – A single pressure, multiple velocity stage machine designed for helical flow.

HEAT – A form of energy and is due to the motion of the molecules of which all substances are composed. Unit of heat is kilocalorie.

HEAT OF COMPRESSION – Increase of temperature brought about by compression of air or air fuel mixture.

HEAT OF CONDENSATION – The heat that is removed per kilogram of vapour to cause it to condense. It has the same numerical value as the heat of vaporization.

HEAT ENGINE – It is an energy transformer. It transforms heat energy into mechanical energy and render the latter available for doing useful work.

HEAT PUMP – is a device which extracts heat from low temperature surroundings and sends it to a high temperature body, while operating in a cycle. Heat pump maintains a body or system at a temperature higher than the temperature of the surroundings, with the work supplied to it.

HEAT SINK – A means for disposing of unwanted heat, usually by using it to increase the temperature of water, which is then run to waste.

HEAT SOURCE – Supplier of heat to the working agent of a heat engine-a fraction of the heat supplied being changed into work.

HEAT TRANSFER – Movement of heat energy from one place to another (warmer to cooler portion).

HETEROGENEOUS SYSTEM – is a system which is made of more than one phase.

HOMOGENEOUS SYSTEM – is a system consisting of a single phase.

HYPERBOLIC PROCESS – is one in which a gas is heated in such a way that at any instant its pressure multiplied by its volume remains constant.

HUB – The center part of a wheel.

HYDRAULIC DAMPING – A conversion of oil resistance to heat energy to create a force that opposes input motion.

HYDRAULIC DISC BRAKE – A brake system that exerts pressure through brake fluid to press brake pads against a disc to slow or stop the vehicle.

HYDRO-PLANING – A phenomenon of driving when water builds up under the tyre tread, causing it to lose contact with the road. Slowing down will usually restore normal tyre contact with the road.

HY-VO-CHAIN – A very strong chain made of toothed plates positioned side by side and held together by pins. Advantage of this chain is great strength and quiet operation.

HAND SHIELD – A protective device, used in arc welding, for shielding the eyes, face and neck. A hand shield is equipped with a suitable filter plate and is designed to be held by hand.

HEAT AFFECTED ZONE – The portion of the base metal that has not been melted, but whose mechanical properties or microstructure have been altered by the heat of welding, brazing, soldering or cutting.

HELMET (eye protection) – Device designed to be worn on the head to protect eyes, face and neck from arc radiation, radiated heat, spatter, or other harmful matter expelled during arc welding.

HOT PRESSURE WELDING – Process that produces coalescence of metals with heat and application of pressure sufficient to produce macro-deformation of the base metal. Vacuum or other shielding media is used.

HOTWIRE WELDING – Arc welding process in which a filler metal wire is resistance heated by current flowing through the wire as it is fed into the weld pool.