Tribology & Lubrication

Start here: ten words every newcomer needs

Overheard on the job

Real-sounding lines from the floor, translated into plain English.

• Spectro's clean but PQ jumped and the mag plug's got chunks on it, that box is spalling.

  TranslationThe lab's elemental readings look normal, but the magnetic debris index and the drain plug both show large steel particles, so a gear or bearing inside the gearbox is shedding fatigue flakes.

• Bull's-eye is a milkshake and the crackle test popped, send a bottle off for KF.

  TranslationThe sight glass shows milky, water-contaminated oil and a hot-plate check confirmed water, so a sample is going to the lab for an exact water measurement.

• RPVOT's under 25 percent and MPC is at 40, plan the change before varnish shows up in the servos.

  TranslationThe turbine oil has almost no oxidation life left and a strong tendency to form deposits, so replace it before sticky residue jams the control valves.

• Two shots with the ultrasound on it and it went quiet, any more and you're just overgreasing.

  TranslationTwo strokes of grease brought the bearing's ultrasonic noise down to normal; adding more would only overfill it and make it run hot.

• Servo loop is running 21/19/16, hang a kidney loop on it and swap that vent cap for a desiccant breather.

  TranslationThe hydraulic fluid is far dirtier than precision valves tolerate, so set up off-line filtration and fit a breather that filters and dries incoming air.

• Jug says full syn but the data sheet says Group III, so it's dressed-up dino oil.

  TranslationThe product is marketed as synthetic but is made from a severely refined mineral base oil rather than a true synthetic such as PAO.

• Somebody filled the vane pump with R&O instead of AW and now the cam ring is scuffed.

  TranslationA pump that needs antiwear oil got an oil with only rust and oxidation inhibitors, so its sliding parts lost protection and suffered welding damage.

• Pinion's got frosting in the dedendum but no pitting yet, step the oil up a grade and trend it.

  TranslationThe gear shows early micropitting below the pitch line with no visible craters, so switch to a thicker oil and keep inspecting it.

How surfaces rub, how oil films form and which regime a contact is running in.

• Asperityalso surface peak, roughness peak
  One of the microscopic peaks on a machined surface that makes first contact when the film gets too thin.
• Boundary lubricationalso boundary regime, thin-film lubrication
  Regime where surfaces touch through the oil and protection comes from chemical surface films, not fluid pressure.
• Coefficient of frictionalso COF, mu
  Ratio of the force resisting sliding to the normal load pressing two surfaces together.
• Dry startalso dry startup
  Machine start-up before oil has reached the contacts, when a big share of lifetime wear happens. Shop talk
• Elastohydrodynamic lubricationalso EHL, EHD lubrication
  Film regime in rolling contacts where huge pressure stiffens the oil and elastically flattens the metal.
• Hydrodynamic lubricationalso full-film lubrication, thick-film lubrication
  Full-film lubrication in which motion drags oil into a narrowing gap and builds enough pressure to float the load.
• Lambda ratioalso lambda, specific film thickness
  Oil film thickness divided by the combined roughness of the two surfaces, used to judge the lubrication regime.
• Mixed lubricationalso mixed-film lubrication, partial EHL
  Transitional regime where the load is shared between a partial fluid film and asperity contact.
• Oil wedgealso oil film wedge
  Shop name for the tapered, pressurized film of oil that lifts a shaft off a sleeve bearing. Shop talk
• Running-inalso break-in, wearing-in
  Early period when new contacting surfaces wear and smooth each other until they settle into a stable state.
• Stick-slipalso slip-stick
  Jerky motion where sliding parts alternately grab and jump because static friction beats sliding friction.
• Stribeck curvealso Stribeck diagram
  Plot of friction against viscosity times speed over load, showing the boundary, mixed and full-film regimes.
• Tribologyalso tribological engineering
  The science of friction, wear and lubrication at every contact where parts slide or roll.

What lubricants are made of, how they are graded and the nicknames techs use for them.

• Base oil groupalso API group, Group I to V
  Five-group system that sorts base stocks by saturates, sulfur, viscosity index and chemical type.
• Cosmolinealso rust preventive, RP
  Genericized name for the thick, waxy rust preventive smeared on stored parts, spares and weapons. Shop talk
• Dino oilalso dino, mineral oil
  Joking name for conventional mineral oil refined from crude, used to set it apart from synthetics. Shop talk
• Esteralso synthetic ester, polyol ester
  Synthetic or natural base oil made from acids and alcohols, valued for solvency, film strength and biodegradability.
• Flash pointalso COC flash, open cup flash
  Lowest temperature at which an oil gives off enough vapor to ignite briefly when a flame passes over it.
• ISO VGalso VG, ISO grade
  ISO viscosity grade, the number on an industrial oil that names its kinematic viscosity in cSt at 40 C.
• Molyalso moly grease, MoS2
  Shop name for molybdenum disulfide, the black solid lubricant, or for any grease that contains it. Shop talk
• PAGalso polyglycol, polyalkylene glycol
  Polyalkylene glycol, a synthetic base oil prized for low friction and clean running but hostile to most other oils.
• PAOalso polyalphaolefin, Group IV
  Polyalphaolefin, the synthetic hydrocarbon base oil built from alpha-olefins and classed as Group IV.
• Pour pointalso PP
  Coldest temperature at which a chilled oil still moves in its test jar, a lab benchmark for cold flow.
• Solid lubricantalso dry lubricant, dry film lubricant
  Dry film material such as graphite, molybdenum disulfide or PTFE that cuts friction without a liquid.
• Synalso synthetic, full syn
  Clipped shop word for synthetic oil or grease, as in syn gear oil or full syn. Shop talk
• Turbine oilalso R&O turbine oil
  Highly refined, lightly additized circulating oil built to last for years in steam, gas and hydro turbines.
• Viscosityalso vis, KV40
  A fluid's resistance to flow, the lubricant property that sets how thick an oil film can build.
• Viscosity indexalso VI
  Number showing how little an oil's viscosity changes with temperature; higher means flatter.

The chemistry blended into base oil to fight wear, oxidation, rust and deposits.

• Additive packagealso additive pack, adpack
  Pre-blended mix of performance chemicals that a blender adds to base oil to make a finished lubricant.
• Antioxidantalso oxidation inhibitor, AO
  Additive that slows the chemical aging of oil by intercepting the radicals and peroxides that drive oxidation.
• AWalso antiwear, AW hydraulic
  Antiwear: additives or oils that build protective films to cut wear under moderate boundary contact.
• Detergentalso metallic detergent, overbased detergent
  Metal-based additive that keeps hot engine parts clean and neutralizes acids from combustion.
• Dispersantalso ashless dispersant
  Ashless additive that keeps soot, sludge and oxidation products suspended so they drain out with the oil.
• EPalso extreme pressure, EP additive
  Extreme pressure: additives or oils that react with metal under heavy load to prevent welding and scuffing.
• R&Oalso R and O, rust and oxidation oil
  Rust and oxidation inhibited: a lightly additized oil with no antiwear or EP chemistry.
• Tackifieralso tackiness agent, tack additive
  Polymer additive that makes a lubricant sticky and stringy so it clings to chains, open gears and slides.
• Viscosity index improveralso VII, VI improver
  Polymer additive that thickens oil more when hot than when cold, raising its viscosity index.
• ZDDPalso ZDP, zinc dithiophosphate
  Zinc dialkyldithiophosphate, the classic antiwear and antioxidant additive in engine and hydraulic oils.

Thickeners, consistency grades and the ways grease behaves, mixes and fails.

• Bleedalso oil bleed, oil separation
  Separation of oil from a grease, either as a storage problem or as the intended release of oil to a bearing.
• Calcium sulfonatealso cal sulf, calcium sulfonate complex
  Grease thickener based on overbased calcium sulfonate, known for water resistance and built-in EP protection.
• Channelingalso channelling
  Grease pushed aside by rolling elements so it sits in shoulders beside the track instead of churning.
• Cone penetrationalso worked penetration, pen
  Depth a standard cone sinks into grease in five seconds, the basic measurement of consistency.
• Cup greasealso lime soap grease, calcium grease
  Shop and farm name for plain calcium soap grease, the kind fed from screw-down grease cups. Shop talk
• Dropping pointalso drop point, DP
  Temperature at which a heated grease turns fluid enough to drip from a test cup.
• Greasealso lubricating grease
  Semi-solid lubricant made of oil held in a thickener sponge, usually with additives.
• Grease compatibilityalso grease mixing, compatibility
  Whether two greases can be mixed without losing consistency, dropping point or stability.
• Lithium complexalso Li complex, lithium complex soap
  Grease thickener made from a lithium soap plus a complexing acid, the all-purpose bearing workhorse.
• NLGI gradealso NLGI number, grease grade
  Number from 000 to 6 describing grease stiffness, based on cone penetration after working.
• Polyureaalso urea grease, diurea
  Non-soap grease thickener with long high-temperature life, standard in sealed electric motor bearings.
• Soupalso souped out, soupy
  Grease that has broken down or been mixed with an incompatible type and turned runny. Shop talk
• Thickeneralso gellant
  Solid structure in a grease, usually a metal soap or polyurea, that holds the base oil like a sponge.
• Water washoutalso water washout resistance, washout
  Percentage of grease washed out of a running test bearing by a water jet, a measure of water resistance.

Bench tests and used-oil lab work that rate lubricants and read machine health.

• Analytical ferrographyalso ferrography, ferrogram
  Microscope study of wear particles pulled from oil by a magnet, used to identify how a machine is wearing.
• Copper strip corrosion testalso copper corrosion test, copper strip
  Test that rates how badly an oil or grease tarnishes a polished copper strip, checking for active sulfur.
• Crackle testalso hot plate test, hot plate crackle
  Quick screen for water in oil: put a drop on a hot plate and listen for crackling. Shop talk
• Demulsibilityalso water separability, D1401
  How quickly an oil sheds water and settles into clean layers after being churned with it.
• Elemental analysisalso spectrometric analysis, spectro
  Spectrometric measurement of wear metals, contaminants and additive elements in oil, in parts per million.
• Four-ball EP testalso four-ball EP, weld load test
  Four-ball test that raises load in ten-second steps until the balls weld, rating extreme pressure capacity.
• Four-ball wear testalso four-ball, 4-ball wear
  Bench test that spins one steel ball against three clamped balls in a lubricant and measures the wear scars.
• FTIRalso IR, infrared analysis
  Fourier transform infrared spectroscopy, an oil test that fingerprints oxidation, contamination and additive loss.
• ISO cleanliness codealso ISO code, ISO 4406
  Three-number code for how many particles above 4, 6 and 14 microns are in a millilitre of fluid.
• Karl Fischer titrationalso KF, KF water
  Chemical titration that measures water in oil down to about 10 parts per million.
• Live zonealso live zone sampling
  Spot in a circulating system where oil is moving and well mixed, the right place to pull a sample. Shop talk
• MPCalso MPC Delta E, varnish potential test
  Membrane patch colorimetry, a test that measures varnish-forming material in turbine and hydraulic oils.
• Oil analysisalso used oil analysis, UOA
  Lab testing of in-service oil samples to judge oil condition, contamination and machine wear.
• Particle countalso PC, particle counting
  Count of solid particles per volume of oil, sorted by size, used to track dirt and wear debris.
• PQ indexalso PQI, particle quantifier
  Magnetic measure of total ferrous debris in an oil sample, including large particles that spectrometers miss.
• RPVOTalso RBOT, bomb test
  Rotating pressure vessel oxidation test, which rates how much oxidation life is left in an oil.
• TANalso acid number, AN
  Total acid number: milligrams of potassium hydroxide needed to neutralize the acids in one gram of oil.
• TBNalso base number, BN
  Total base number: the alkaline reserve left in an engine oil to neutralize combustion acids.

How bearings, gears and lubricants fail, and the names for the damage you find on the parts.

• Abrasive wearalso abrasion, three-body wear
  Wear caused by hard particles or hard surface peaks plowing and cutting through a softer surface.
• Adhesive wearalso adhesion wear
  Wear where touching asperities weld together and tear, moving metal from one surface to the other.
• Electrical flutingalso fluting, washboarding
  Washboard pattern on bearing races caused by electric current arcing through the rolling contacts.
• False brinellingalso false brinell
  Dents in bearing races caused by vibration while the machine sits still, not by impact.
• Frettingalso fretting corrosion, fretting wear
  Wear from tiny oscillating slips between clamped parts, leaving reddish-brown oxide debris.
• Frostingalso frosted, gray staining
  Shop name for the dull, satin-gray sheen of early micropitting on gear teeth or bearing races. Shop talk
• Gallingalso cold welding
  Severe adhesive wear where sliding metals seize and tear, raising lumps of transferred metal.
• Glitteralso metal in the oil, sparkles
  Visible sparkling metal flakes in drained oil or a cut-open filter, a red flag for serious wear. Shop talk
• Micropittingalso gray staining, grey staining
  Fatigue damage made of tiny pits, about 10 to 20 microns deep, that turns gear and bearing surfaces dull gray.
• Milkshakealso milky oil, mayo
  Oil turned milky or creamy by emulsified water, seen in gearboxes, hydraulics and engines. Shop talk
• Pickupalso metal pickup, metal transfer
  Metal transferred from one sliding surface onto another, seen as smeared lumps after adhesive wear. Shop talk
• Pittingalso macropitting, fatigue pitting
  Rolling contact fatigue that leaves visible craters on gear teeth or bearing races.
• Scuffingalso scoring, scuff
  Sudden, severe adhesive damage where the oil film fails and sliding surfaces locally weld and tear.
• Spallingalso flaking, spall
  Flaking of large chunks from a bearing race or gear tooth after fatigue cracks below the surface link up.
• Varnishalso lacquer, oil deposits
  Thin, sticky, amber-to-brown deposit from degraded oil that coats valves, bearings and coolers.
• Wipealso wiped bearing, wiping
  To smear the soft babbitt lining of a plain bearing when the oil film fails, ruining the bearing. Shop talk

Day-to-day lubrication work in the plant, from grease guns and sight glasses to lube routes.

• Babbittalso white metal, babbit
  Soft tin- or lead-based alloy that lines plain bearings so they tolerate dirt and brief contact.
• Bull's-eyealso sight glass, bullseye
  Round oil-level sight glass on a gearbox or bearing housing, with a center dot or line marking correct level. Shop talk
• Desiccant breatheralso desiccant filter breather, breather
  Vent filter packed with drying beads that cleans and dries the air a gearbox or tank breathes in.
• Grease monkeyalso grease jockey
  Slang, sometimes fond and sometimes dismissive, for a mechanic or whoever is stuck with grease duty. Shop talk
• Kidney loopalso kidney loop filtration, offline filtration
  Off-line filtration circuit that continuously pumps reservoir oil through a filter and back. Shop talk
• Lube routealso lube PM, oil route
  Scheduled walk through the plant to grease, top off and inspect a set list of lubrication points. Shop talk
• Lube tagalso lube ID tag, lube label
  Color- or shape-coded label at a fill point that tells techs exactly which lubricant goes in. Shop talk
• Lube techalso lube man, oiler
  Lubrication technician, the person who owns the lube routes, oil sampling and lube room in a plant. Shop talk
• Mag plugalso magnetic plug, chip detector
  Magnetic drain plug that collects ferrous wear debris and is checked at oil changes as a quick wear gauge. Shop talk
• Overgreasingalso over-lubrication, overlubrication
  Pumping more grease into a bearing than it needs, causing churning, heat and blown seals.
• Packalso hand pack, packing
  To fill a bearing by hand, forcing grease between the rolling elements before assembly. Shop talk
• Purgealso purging, flush
  To pump fresh grease into a bearing until the old grease is pushed out of the relief port or seal. Shop talk
• Running hotalso hot bearing, running warm
  Shop phrase for a bearing, gearbox or motor operating above its normal temperature. Shop talk
• Shotalso pump, stroke
  One full stroke of a manual grease gun, the everyday unit for how much grease goes in. Shop talk
• Top offalso top up
  To add oil to bring a gearbox, sump or reservoir back up to its correct level. Shop talk
• Ultrasound-assisted greasingalso acoustic lubrication, ultrasonic greasing
  Greasing a bearing while listening with an ultrasonic detector and stopping when the noise level drops.
• Weepalso weeping, seep
  To leak very slowly, leaving a damp oil film around a seal or joint rather than actual drips. Shop talk
• Zerkalso grease fitting, grease nipple
  Everyday name for the small ball-check grease fitting that a grease gun coupler snaps onto. Shop talk

Frequently asked questions

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