Mechanical Engineering Dictionary-Friction and Bearings

Important Terms and their Meaning of Mechanical Engineering- Friction and Bearings:



ABSOLUTE VISCOSITY – Force per unit area required to move a surface at unit velocity, when it is separated by a fluid of unit thickness from a stationary surface.

ADDITIVES – Chemical compounds used to alter the characteristics of lubricating oils.

ANGLE OF REPOSE – The minimum inclination which a plane can have consistent with the body on it sliding down the plane by the force of gravity. It is the same as the friction angle.

ANTI CORROSION ADDITIVES – Chemical compounds added to lubricating oil to reduce or prevent the chemical reaction of acids (formed by the oxidation of oil) which destroy some bearing materials, such as lead in lead copper bearings, used for crankshafts and connecting rods of engines.

ANTI OXIDATION ADDITIVES – Chemical compounds added to decrease oxidation of the oil. These have a greater affinity for oxygen than does the oil.

API HEAVY DUTY TYPE OIL – Motor oil having oxidation stability, bearing corrosion preventive properties, and detergent-dispersant characteristics necessary to make it generally suitable for use in both high speed diesel and gasoline engines under heavy duty service conditions.

API PREMIUM TYPE OIL – Motor oil having the oxidation stability and bearing corrosion preventive properties necessary to make it generally suitable for use in internal combustion engines where operating conditions are more severe than regular duty.

API REGULAR TYPE OIL – Motor oil generally suitable for use in internal combustion engines under moderate operating conditions.

BALL BEARING – An antifriction bearing where the rolling elements are spherically shaped. Bearing consists of an inner and outer hardened steel races separated by a series of hardened steel balls.

BATH LUBRICATION – Lubrication system in which the bearing contains a space filled with oil, which is in contact with a portion of the journal.

BEARING – The part which transmits the load to the support and in so doing, takes the friction caused by the moving parts in contact. Area of the unit in which the contacting surface of a revolving part rests.

BEARING CAPS – On an engine, caps held in place by bolts or nuts which, in turn, hold bearing halves in place.

BEARING CRUSH – The additional height over a full half which is purposely manufactured in each bearing half. This ensures complete contact of the bearing back with the housing bore when the unit is assembled.

BEARING FAILURE – Failure of a bearing due to continued flexing of the bearing surface from the applied load.

BEARING OIL CLEARANCE – The space purposely provided between the revolving shaft and the bearing in which it rotates. Through this space lubricating oil can flow.

BEARING PRELOAD – Amount of static pressure exerted on a bearing or a set of bearings. Preload is usually adjusted by a threaded collar or shims.

BEARING PRELUBRICATOR – A special tank attached to an airline which supplies oil at a predetermined and maintained pressure to the engine lubricating system when the engine is not operating.

BEARING SPACER – A piece of tubing used between the two wheel bearing inner races to prevent unwanted bearing preload as the axle is tightened.

BEARING SPIN – A type of bearing failure caused by lack of lubrication which overheats the bearing while the crankshaft is still in place.

BEARING SPREAD – A purposely manufactured small extra distance across the parting faces of the bearing half in excess of the actual diameter of the housing bore.

BYPASS FILTER – An oil filter that constantly only filters a portion of the oil flowing through the engine or machine.

BOUNDARY FRICTION – The resistance to relative motion when one solid surface is caused to move tangentially over another, the surfaces being covered only by an adsorbed contamination film.

BOUNDARY LUBRICATION – Type of lubrication in which the two surfaces have between them a more or less complete layer of oil which is only, at the most, a few molecules thick.

BUSHING – A member that takes up space and usually allows movement at the attachment point. A one piece replaceable sleeve placed in a bore to serve as a bearing surface. Bearing for shaft, spring shackle, piston pin etc. A metallic or synthetic lining for a hole.

CARBON RESIDUE – Determined (canradson carbon test) by evaporating under specified test conditions, a known weight of oil and weighing the residue.

CENTIPOISE – A unit of viscosity of a fluid used in figuring pressure drop etc.

CENTRIFUGAL OIL SLINGER – Cup shaped centrifugal oil filter mounted to the end of the crankshaft. As the oil passes through the slingers, centrifugal force removes impurities that are heavier than oil.

COEFFICIENT OF FRICTION – Ratio between the resistance due to friction in the direction of motion and the load carried normal to the plane of motion.

COLLAR BEARING – The collar which may be at any part of the length of the shaft, takes up the thrust produced along the axis of the shaft.

CRANKCASE DILUTION – Dilution of lubricating oil in the oil pan by liquid gasoline seeping down the cylinder walls. Accumulation of unburned gasoline in the crankcase.

CRITICAL SPEED – The limiting or critical speed corresponding to a given pressure is that speed at which surface irregularities may intervene and so lead to seizure.

DETERGENTS – Added to lubricating oils to improve the tendency of the oil to wash or cleanse the surface where oxidation products form, thereby resulting in a marked reduction in ring sticking, particularly in heavy duty service, and a marked improvement in cleanliness of pistons.

DETERGENT OIL – An oil which keeps particles and contaminants in suspension and has ability to neutralize acids resulting from combustion process.

DIRECT BEARING LUBRICATION – An oil injection system which feeds undiluted oil to two stroke cycle engine main bearings and rod big end bearings.

DRY SUMP-LUBRICATION – In this system, oil is gravity fed to supply side of oil pump from the remote oil tank. After the oil has been pumped through four stroke cycle engine, it is returned to the oil tank by return side of oil pump.

FILM LUBRICATION – Type of lubrication in which the two metallic surfaces are separated by a continuous film of liquid oil of measurable thickness which forces itself between them.

FIRE POINT OF OIL – The temperature at which the oil vapour on the oil surface ignites and burns for at least five seconds, when heated gradually, under specified test conditions.

FLASH POINT OF OIL – The temperature at which a momentary flash appears on the heated oil surface when a test flame is applied, under specified test conditions. It is a rough indication of the tendency of the oil to vaporize.

FOOT STEP BEARING – The bearing at the foot of a vertical shaft.

FORCE FEED LUBRICATION – A gear pump takes oil from the sump and delivers it to the distributor ducts which connect with all the main bearings and camshaft bearings, from where the oil reaches the various parts that need lubrication.

FORCE OF FRICTION – The least force acting parallel to the sliding surfaces of the bodies in contact, which will cause one body to slide over the other.

FRICTION – The force which acts between two bodies at their surface of contact so as to resist their sliding on each other, due to roughness of the surfaces. Also called FRICTIONAL FORCES.

FRICTION ANGLE – Angle which the resultant force makes with the normal to the plane when sliding begins. Also called LIMITING ANGLE OF RESISTANCE or LIMITING ANGLE OF REACTION.

FRICTION BEARINGS – Bearings having sliding contact between the moving surfaces. Sleeve bearings, such as those used in connecting rods, are friction bearings.

GEAR OIL PUMP – An oil pump with gear type teeth to move oil.

GREASE – The term is applied to a mixture of mineral oil with fats that have been saponified with an alkali. To this mixture fillers may or may not be added.

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

INNER RACE – In a bearing assembly, the part that connects a rotating shaft to the bearing.

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

LAWS OF FRICTION – The force of friction is (1) directly proportional to the pressure between the surfaces in contact, (2) independent of the extent of the surfaces in contact and (3) independent of the velocity of sliding.

LOAD MOTION – Rolling or sliding motion found at the bearing surfaces.

LUBRICANT – A fluid having two essential properties namely viscosity and oiliness, and used to reduce friction.

LUBRICATING FILM – A thin coating of lubricant (oil) which prevents contact between moving parts.

LUBRICATING SYSTEM – The system in the engine that supplies lubricating oil to the moving engine parts, to prevent actual contact between the moving surfaces.

LUBRICATION – Use of lubricant (oil or grease) to reduce friction.

MAIN BEARINGS – In the engine block, the bearings that support the crankshaft.

MICHELL THRUST BEARING – Type of tilting pad thrust bearing, the thrust supporting surface is divided into a number of segmental pads, which pivot on a radial edge at the back, and so automatically adjust themselves to the required slope.

MULTIGRADE OIL – An oil that retains its viscosity under varying temperature conditions better than a single weight oil.

ML OIL – Oil for light automotive service.

MM OIL – Oil for medium or average automotive service.

MS OIL – Oil for severe automotive service.

MULTI WEIGHT OIL – Type of oil that provides adequate lubrication at both high and low temperatures.

MULTIPLE VISCOSITY OIL – An engine oil which has a low viscosity when cold (for easier cranking and a higher) viscosity when hot (to provide adequate lubrication).

NEEDLE BEARING – Antifriction bearing of the roller type, the rollers are very small in diameter (needle sized). Rollers have a length over four times greater than their diameter.

NEEDLE LUBRICATOR – A needle which rests on a journal and exposed at top to the oil in an inverted glass bottle or reservoir, causes oil to flow slowly onto the journal, due to the vibration set up during shaft rotation.

OIL – A liquid lubricant derived from crude oil used to provide lubrication between moving parts.

OIL CLEANER – The filtering device through which oil passes, which filters dirt and dust from the oil.

OIL CONSUMPTION – Oil pumped into the combustion chamber of an engine by the piston and rings and burnt there in a given time.

OIL CONTROL RINGS – The lower ring or rings on the piston of an engine, designed to prevent excessive amounts of oil from working up into the combustion chamber.

OIL COOLER – A small radiator through which the oil flows to lower its temperature.

OIL DILUTION – Dilution of oil in the crankcase of a SI engine, by leakage of liquid gasoline from the combustion chamber past the piston rings.

OIL FILTER – Unit in the lubrication system through which oil passes to remove any impurities from oil. It may be paper, wire screen or rotor designed to keep oil clean.

OIL LEVEL INDICATOR – The indicator usually called the dipstick, that can be removed to determine the level of oil in the crankcase of an engine or machine.

OIL PAN – The detachable lower part of the engine made of sheet metal, which encloses the crankcase and acts as an oil reservoir.

OIL PASSAGES – Holes and passages drilled in the block and in the engine parts through which oil flows.

OIL PRESSURE GAUGE – An instrument used to measure and indicate the amount of oil pressure in the lubrication system.

OIL PUMP – In the lubricating system, the device that delivers oil from the oil pan to the various moving engine/machine parts.

OIL PUMP BLEEDING – Removal of air from the supply line and the pump in a two stroke cycle oil injection system.

OIL PUMPING – Passing of oil past the piston rings into the combustion chamber because of defective rings, worn cylinder walls etc.

OIL RESERVOIR – That space in the base of the oil separator where oil is accumulated prior to its return to the pump.

OIL SEAL – A seal placed around a rotating shaft or other moving part, to prevent passage or escape of oil.

OIL SEAL AND SHIELD – Two devices used to control oil leakage past the valve stem and guide into the ports or the combustion chamber.

OIL SPILL RING – Collars or oil throw rings put on the rotating shaft, prevent oil leakage along the shaft, utilizing the increase of centrifugal force with the increase of diameter.

OIL STRAINER – A wire mesh screen placed at the inlet end of the oil pump pickup tube to prevent dirt and other large particle from entering the oil pump.

OUTER RACE – In a roller/ball bearing assembly, the stationary part that supports the rotating load on the bearing.

OXIDIZED OIL – Oil that has been chemically combined with oxygen as a result of excessive heat, oil agitation, and exposure to combustion contaminants.

PAD LUBRICATION – A part of the bearing surface upon which there is no pressure is dispensed with, and its place is taken by a soft pad, which is kept saturated with lubricant.

PICKUP SCREEN – A screen in the oil pan that prevents any large particles from entering the oil pump.

PIVOT BEARING – Pivot portion (flat or conical) which is on the end of a shaft, that takes up the thrust produced along the axis of the shaft.

PLAIN BEARING – A one or two piece assembly consisting of a hard metal backing piece with a softer metal mated to it.

PLUNGER OIL PUMP – An oil pump that uses a piston or plunger and check valves to move oil.

POUR POINT OF OIL – The temperature at which the oil will not flow when cooled under specified conditions, due to formation of wax.

PREMIX – Two stroke cycle engine lubrication is provided by oil, mixed with fuel in the fuel tank.

PRESSURE RELIEF VALVE – A spring loaded valve used to regulate the pressure in the lubrication system.

PRESSURIZED FEED OIL SYSTEM – A type of engine lubricating system that makes use of an oil pump to force oil through tubes and passages to the various engine parts requiring lubrication.

PROFILOMETER – Instrument used to measure the surface irregularities.

RING LUBRICATION – Two loose rings carried by the journal rotation, being driven by frictional contact with the journal, dip into an oil bath and carry oil to the top of the journal so as to flow over the surface of the journal through oil grooves in the bearing, and finally return to the bath below.

ROLLER BEARING – An antifriction bearing consisting of hardened steel inner and outer races, separated by hardened steel rollers, which are two or three times as wide as their diameter.

ROLLING FRICTION – The force resisting the motion when a body rolls on a surface.

ROTARY OIL PUMP – An oil pump which uses an inner and outer rotor to move oil.

SCHIELE’S PIVOT – A pivot bearing on a shaft, designed to give uniform wear in the direction of the axis with uniform pressure, the coefficient of friction being assumed to be constant.

SCRAPER RING – A piston ring that assists in compression and wipes off the excess oil from the cylinder walls.

SEAL – A material, shaped around a shaft, used to close off the operating compartment of the shaft, preventing oil leakage.

SEIZING – The phenomenon in which the metal surfaces adhere and then tear owing to relative motion, caused by the attraction between molecules on opposite sides of the common surface between two elements of a bearing.

SERVICE RATING – For lubricating oils used in engines, a designation that indicates the type of service for which the oil is best suited.

SHIM (bearing) – A strip of copper or similar material used under a bearing cap, for example, to increase bearing clearance.

SKIMMING – Using a machine to remove oil or scum from the surface of the water.

SLUDGE – Accumulation in oil pan, containing water, dirt and oil, sludge is very viscous and tends to prevent lubrication.

SPLASH FEED OIL SYSTEM – A type of engine lubricating system that depends on the splashing of oil for lubrication of moving parts. The engine is enclosed, and the crankcase contains oil into which the cranks dip and splash as they rotate, throwing the oil over the various bearings.

STATICAL FRICTION – Friction at starting from rest, and this is greater than the friction of motion and depends on the hardness of the bodies and the length of time during which they have been in contact.

SUMP – A system for storing oil, either in the crankcase (wet sump) or in a separate tank (dry sump).

SYNTHETIC OIL – Oil made from material other than petroleum.

SYPHON LUBRICATOR – The unit in which the oil stored in a cup above a journal, is delivered slowly to the bearing through a wick which acts as a syphon.

TAPERED ROLLER BEARING – An antifriction bearing where the rolling element is similar to a roller bearing, however it has a taper along its axis, allowing it to accept axial loads in one direction.

THICK FILM LUBRICATION – Lubrication arrangement wherein there is no metal to metal contact. Also called PERFECT LUBRICATION.

THIN FILM LUBRICATION – Type of lubrication that occurs when the condition of bearing operation reduce the thickness of the oil film sufficiently to make the rubbing of the bearing surfaces a near possibility. Oiliness of the lubricant is important in this type of lubrication.

THRUST BEARING – In the engine, the main bearing has thrust faces
which prevent excessive endwise movement of the crankshaft.

VALVE STEM SEAL or SHIELD – A device placed on or surrounding the valve stem to reduce the amount of oil which can get on the stem and thereby work its way down into the combustion chamber.

VARNISH – A build up of oxidized oil or fuel.

VISCOSITY – Resistance of oil to flow. The thickness of oil is determined by its rate of flow, a thick oil has a greater viscosity than a thin oil. Unit of viscosity is centipoise.

VISCOSITY INDEX – A measure of the change of viscosity with temperature of an oil compared with two reference oils having the same viscosity at 210°F one of naphthenic base and the other of paraffinic base.

VISCOSITY INDEX IMPROVERS – Additives added to an oil, to reduce the change of viscosity with increase in temperature.

VISCOSITY RATING – An oil classification based on the time it takes for an amount of oil to flow through a capillary tube, under specified conditions, the higher the rating, the thicker the oil.

WARNING LIGHT – A light on the instrument or control panel that turns on to warn of low oil pressure.


WORK OF FRICTION – Product of the frictional force and the distance through which it moves.


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