Gear Engineering

Start here: ten words every newcomer needs

Overheard on the job

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

• Pinion's frosting in the dedendum and lambda is under one at full load, that's micropitting, not wear.

  TranslationThe small gear shows a dull gray band low on its teeth, and the oil film is thinner than the surface roughness, so the damage is fine surface fatigue caused by poor lubrication rather than ordinary wear.

• Pattern's on the toe and high on the face, so add pinion shim and open the lash a couple thou.

  TranslationThe marking compound shows contact near the narrow end and the top of the tooth, so the pinion needs to move deeper into mesh and the backlash should be increased slightly.

• Sidebands around GMF at pinion speed, plus a ghost tone, so it's runout and a hobber problem.

  TranslationThe vibration spectrum shows the pinion is eccentric and also carries a tone left by an error in the machine that cut the teeth.

• Cut it green, carburize, then grind to Q12 with tip relief and a little crown.

  TranslationMachine the teeth soft, case harden them, then finish grind to a high accuracy class with profile and lead modifications.

• Nital etch shows burn at the root on two teeth, scrap them before they grenade a box in the field.

  TranslationAn acid etch revealed grinding heat damage in the tooth roots, so the parts must be rejected before they fail in service and destroy a gearbox.

• It's a lapped set, non-hunting, so keep the pair together and line up the timing marks.

  TranslationThe two gears were finished together and repeat the same tooth contacts, so they must be installed as a matched pair with their marked teeth meshed.

• Box whines on drive and howls on coast, TE's fine on the single-flank, so check the housing alignment.

  TranslationThe gearbox is noisy in both load directions, but the gears themselves tested accurate, so the likely cause is misaligned bores or deflection in the housing.

Spur, helical, bevel, hypoid, worm and planetary gears, and the boxes and axles built from them.

• Bevel gearalso straight bevel, miter gear
  A cone-shaped gear that transmits motion between intersecting shafts, usually at right angles.
• Boxalso gearbox, reducer
  Shop shorthand for a gearbox, reducer or transmission, as in the box is whining or pull the box. Shop talk
• Bull gearalso girth gear, main gear
  Shop name for the biggest gear in a drive train, usually the slow, heavy output gear. Shop talk
• Crash boxalso non-synchro box, unsynchronized box
  Shop name for an unsynchronized manual gearbox, where shifts grind unless engine and road speed are matched. Shop talk
• Dogsalso dog teeth, dog clutch
  Shop name for the chunky engagement teeth on a gear or sliding collar that lock a gear to its shaft. Shop talk
• Face gearalso crown gear, contrate gear
  A disk-shaped gear with teeth cut on its flat face, meshing with an ordinary spur or helical pinion.
• Gearheadalso precision gearhead, servo reducer
  A compact gear reducer that bolts to a motor face, and also a person who lives for machinery. Shop talk
• Granny gearalso granny low, creeper gear
  Shop name for an extra-low first gear used for crawling under heavy load, not for normal driving. Shop talk
• Helical gearalso helical
  A cylindrical gear whose teeth wrap at an angle around the blank, engaging gradually for quiet, strong running.
• Herringbone gearalso double helical gear, double helical
  A gear with two opposite-hand helical tooth sets side by side, so their axial thrusts cancel.
• Hypoid gearalso hypoid, hypoid set
  A spiral-bevel-like gear pair with the pinion axis offset from the gear axis, standard in vehicle drive axles.
• Idleralso idler gear, intermediate gear
  A gear placed between a driver and a driven gear to change direction or span distance, without changing ratio.
• Internal gearalso ring gear, annulus
  A gear with teeth cut on the inside of a ring, meshing with a pinion running inside it.
• Pinionalso drive pinion
  The smaller of two meshing gears, usually the driver in a speed reducer.
• Planetary gear setalso epicyclic gearing, planetary
  A compact gear arrangement of a central sun, orbiting planets on a carrier and an outer ring gear.
• Pumpkinalso third member, diff housing
  Shop name for the round differential housing in the middle of a drive axle, from its bulging shape. Shop talk
• Rackalso gear rack, toothed rack
  A straight bar with gear teeth along it, which turns a pinion's rotation into straight-line travel.
• Ring and pinionalso R&P, gear set
  Shop name for the hypoid final drive in a vehicle axle, named by its ratio, such as 3.73 or 4.10. Shop talk
• Spider gearsalso differential gears, spiders
  Shop name for the small bevel gears inside a differential that let the two wheels turn at different speeds. Shop talk
• Spiral bevel gearalso spiral bevel
  A bevel gear with curved, oblique teeth that overlap in engagement for smoother, quieter right-angle drives.
• Spur gearalso straight-cut gear, straight spur
  A cylindrical gear with straight teeth parallel to its axis, the simplest gear for parallel shafts.
• Wormalso worm screw
  A screw-shaped gear with one or more threads that drives a worm wheel at right angles for a large reduction.
• Worm wheelalso worm gear, wormwheel
  The gear driven by a worm, with throated teeth that wrap around the worm, usually made of bronze.

Module, pitch, pressure angle, addendum and the modifications that shape a working tooth.

• Addendumalso addendum height
  The radial height of a gear tooth above the pitch circle, normally one module for a standard tooth.
• Base circlealso base diameter
  The circle from which the involute tooth profile is unwound, smaller than the pitch circle.
• Circular pitchalso CP, transverse circular pitch
  The arc distance from one tooth to the same point on the next tooth, measured along the pitch circle.
• Crowningalso lead crowning, lead crown
  A slight barrel shape along the tooth face so contact stays centered despite misalignment and shaft deflection.
• Dedendumalso dedendum depth, root depth
  The depth of a tooth space below the pitch circle, slightly larger than the addendum to give tip clearance.
• Diametral pitchalso DP, P
  The inch measure of gear tooth size: number of teeth per inch of pitch diameter, larger number for finer teeth.
• Helix anglealso beta
  The angle between a helical gear's teeth and its axis, along with the hand, right or left.
• Involutealso involute curve, involute profile
  The curve traced by the end of a taut string unwound from a circle, the shape of almost every gear tooth flank.
• Modulealso m, metric module
  The metric measure of gear tooth size: pitch diameter in millimeters divided by the number of teeth.
• Pitch diameteralso reference diameter, PD
  The diameter of the pitch circle, the imaginary rolling circle on which tooth size and gear ratio are based.
• Pressure anglealso PA, profile angle
  The angle between the line of tooth contact force and the tangent to the pitch circle, usually 20 degrees.
• Profile shiftalso addendum modification, x coefficient
  Moving the cutter in or out from the standard position, making teeth fatter or thinner without changing pitch.
• Tip reliefalso profile relief, tip modification
  A slight removal of material near the tooth tip so each tooth enters mesh smoothly under load.
• Tooth thicknessalso circular tooth thickness, arc tooth thickness
  How thick a gear tooth is along the pitch circle, the dimension that is thinned to create backlash.
• Undercutalso tooth undercut, undercutting
  A notch cut into the base of small-pinion teeth when the generating tool removes part of the working involute.

How mating teeth share load, how much play they need, and the whines, howls and rattles they make.

• Backlashalso lash, circumferential backlash
  The play between mating teeth: how far one gear can rock before its teeth contact the other gear's teeth.
• Center distancealso CD, operating center distance
  The distance between the axes of two meshing gear shafts, set by the housing bores.
• Clunkalso driveline clunk, lash clunk
  A single heavy knock when a drive takes up its backlash on a load reversal, such as on and off the throttle. Shop talk
• Contact ratioalso profile contact ratio, transverse contact ratio
  The average number of tooth pairs in contact at once, which largely sets how smoothly and quietly a pair runs.
• Drive sidealso loaded flank, drive flank
  The tooth flank that carries load when power flows in the normal direction; the opposite flank is the coast side.
• Gear ratioalso ratio, reduction
  The number of teeth on the driven gear divided by those on the driver, setting speed reduction and torque gain.
• GMFalso gear mesh frequency, tooth mesh frequency
  Gear mesh frequency: tooth count times shaft speed, the frequency at which teeth enter mesh.
• Howlalso axle howl, gear howl
  Shop word for a loud, droning gear noise, usually from a vehicle axle with a worn or badly set ring and pinion. Shop talk
• Hunting toothalso hunting ratio, hunting tooth ratio
  A tooth combination with no common factor, so every pinion tooth eventually meets every gear tooth.
• Lashalso backlash, gear lash
  Shop clipping of backlash, the free play between mating gear teeth, as in set the lash or check lash. Shop talk
• Line of actionalso path of contact, pressure line
  The straight path along which the point of tooth contact travels, tangent to both base circles.
• Meshalso gear mesh, in mesh
  The engagement of two gears' teeth, or the zone where they meet; also the verb for gears engaging.
• Rattlealso gear rattle, neutral rattle
  A clattering noise from unloaded gears bouncing back and forth inside their backlash. Shop talk
• Slopalso play, free play
  Shop word for too much play in a gear train or drive, felt as free movement before anything turns. Shop talk
• Transmission erroralso TE, loaded transmission error
  The tiny difference between where the output gear actually is and where a perfect gear pair would put it.
• Whinealso gear whine, gear noise
  A steady, pitch-changing tonal noise from gears, rising with speed and caused by tooth meshing errors. Shop talk

Cutting, hardening and finishing processes, from hobbing green blanks to grinding carburized teeth.

• Carburizingalso case carburizing, carburize
  A heat treatment that diffuses carbon into the surface of low-carbon steel gears, then quenches for a hard case.
• Case depthalso effective case depth, ECD
  How deep the hardened surface layer of a gear tooth goes, usually measured to a set hardness such as 50 HRC.
• Gear grindingalso form grinding, generating grinding
  Finishing hardened gear teeth with abrasive wheels to correct heat treat distortion and reach high accuracy.
• Gear shapingalso shaping, Fellows shaping
  Cutting gear teeth with a reciprocating, gear-shaped cutter that rolls with the blank as it strokes.
• Greenalso green cut, soft
  Shop word for a gear in its soft, machined state before heat treatment, as in cut green, finish hard. Shop talk
• Grind burnalso grinding burn, temper burn
  Heat damage to hardened gear teeth from aggressive grinding, which softens or rehardens the surface.
• Hard gearingalso hardened and ground, case-hardened gearing
  Shop term for case-hardened, ground gears, as opposed to through-hardened soft gears that are cut to finish. Shop talk
• Hobalso gear hob, hob cutter
  A gear cutter shaped like a worm with gashes across its threads, used on a hobbing machine.
• Hobbingalso gear hobbing, hob cutting
  Generating gear teeth with a rotating worm-shaped cutter that turns in step with the blank.
• Induction hardeningalso induction hardened, IH
  Hardening gear teeth by heating them locally with an induction coil and quenching immediately.
• Lappingalso gear lapping, lapped set
  Running a matched bevel or hypoid pair together with abrasive compound after heat treat to refine contact.
• Nitridingalso gas nitriding, plasma nitriding
  A low-temperature surface hardening that diffuses nitrogen into alloy steel gears with almost no distortion.
• Power skivingalso skiving, gear skiving
  Cutting gear teeth with a gear-like cutter spinning at a crossed axis angle in continuous high-speed rotation.
• Scallopsalso feed marks, generating flats
  Shop word for the small ridges a hob leaves across and along each tooth flank, visible as a fine texture. Shop talk
• Shot peeningalso peening, shot peen
  Blasting gear roots with hard steel shot to build compressive stress and raise bending fatigue strength.
• Superfinishingalso isotropic superfinishing, ISF
  Polishing ground gear teeth to a mirror-like finish, chemically assisted, to cut friction and surface fatigue.

Accuracy grades, gear checker traces, composite tests and the contact pattern work of setting gears up.

• AGMA quality numberalso Q number, A number
  A grade that sets how accurate a gear's teeth must be, on either the Q-number or the A-number scale.
• Bluingalso marking compound, Prussian blue
  Shop word for marking compound brushed on gear teeth to read the contact pattern after running. Shop talk
• Contact patternalso tooth contact pattern, tooth bearing
  The imprint left on gear teeth by marking compound after running, showing where and how the teeth touch.
• Double-flank testalso double-flank composite test, radial composite test
  Rolling a gear in tight mesh against a master gear and recording how the center distance varies.
• Fingernail testalso nail test, fingernail check
  Shop check of dragging a fingernail across a tooth flank to judge whether surface damage has real depth. Shop talk
• Gear checkeralso gear analyzer, gear measuring machine
  Shop name for a gear measuring machine that traces profile, helix, pitch and runout, and for its operator. Shop talk
• Ghost frequencyalso ghost tone, ghost noise
  A gear noise tone that matches no tooth count in the box, left behind by errors in the cutting machine.
• Heel and toealso heel, toe
  The outer, wide end (heel) and inner, narrow end (toe) of a bevel or hypoid gear tooth.
• Helix deviationalso lead error, lead deviation
  How far a tooth's actual path across the face width departs from the intended helix, including crowning.
• Measurement over pinsalso MOP, over pins
  Measuring across precision pins or balls placed in opposite tooth spaces to check tooth thickness.
• Pinion depthalso pinion position, depth
  How far the hypoid or bevel pinion sits along its axis toward the gear, set with shims behind its bearing.
• Pitch deviationalso index error, spacing error
  Errors in tooth spacing, either from one tooth to the next or accumulated around the gear.
• Profile deviationalso profile error, involute error
  How far a tooth's actual shape from root to tip departs from the intended involute and modifications.
• Roll checkalso roll test, master gear check
  Shop name for rolling a production gear against a master gear to catch size errors, runout and nicks. Shop talk
• Runoutalso radial runout, TIR
  How much the teeth wobble radially relative to the gear's axis, measured with a probe in each tooth space.
• Set the patternalso setting up gears, gear setup
  Shop phrase for adjusting pinion depth and backlash on a bevel or hypoid set until the contact pattern is right. Shop talk
• Single-flank testalso single-flank composite test, transmission error test
  Running a gear against a master at fixed center distance and measuring rotational error on one flank only.
• Span measurementalso base tangent length, span
  Checking tooth thickness by measuring across several teeth with a disc micrometer.
• Tracealso chart, profile trace
  The printed plot of a tooth's measured profile or helix from a gear checker, read against tolerance bands. Shop talk

How gear teeth pit, scuff, spall, crack and break, and the shop words for what comes out in the oil.

• Edge loadingalso end loading, end contact
  Tooth load piling up at one end of the face width because of misalignment or deflection.
• Frostingalso frosted, gray staining
  Shop word for the dull, frosted gray patches on gear teeth that turn out to be micropitting. Shop talk
• Fuzzalso drain plug fuzz, magnet fuzz
  Shop word for the fine metallic paste that collects on a magnetic drain plug from normal gear wear. Shop talk
• Grenadealso grenaded, blew up
  Shop slang for a gearbox or axle destroying itself all at once, scattering broken teeth and bearing parts. Shop talk
• Micropittingalso gray staining, grey staining
  Very fine surface fatigue pits, too small to see one by one, that give tooth flanks a dull gray look.
• Pittingalso macropitting, pitting fatigue
  Surface fatigue of gear teeth that breaks out small craters, typically on the flank just below the pitch line.
• Scuffingalso scoring
  Sudden adhesive damage where tooth surfaces weld and tear apart because the oil film broke down.
• Spallingalso spall, spalled
  Large, irregular flakes broken out of a tooth surface, usually where pits have grown and joined.
• TIFFalso tooth interior fatigue fracture, TFF
  Tooth interior fatigue fracture: a crack that starts deep inside a case-hardened tooth, not at its surface.
• Tooth breakagealso broken tooth, tooth fracture
  A gear tooth cracking off, most often by bending fatigue starting in the root fillet on the loaded side.

Stresses, rating factors and oil film concepts used to size gears and keep them alive.

• Bending stressalso root stress, tooth bending stress
  The tensile stress in the root fillet of a loaded tooth, which decides whether teeth break off.
• Contact stressalso Hertzian stress, surface stress
  The compressive stress where two tooth flanks press together, which governs pitting life.
• Face load factoralso load distribution factor, KH beta
  A rating factor that raises calculated stress to cover uneven load distribution across the face width.
• Flash temperaturealso Blok flash temperature, flash temp
  The brief spike in surface temperature where teeth slide over each other under load, used to predict scuffing.
• Lambda ratioalso specific film thickness, lambda
  Oil film thickness divided by combined surface roughness, showing how well the lubricant separates the teeth.
• Run-inalso break-in, running-in
  Operating new gears at reduced load at first so surface high spots wear smooth before full duty.
• Service factoralso SF
  A multiplier applied to the required power when picking a gearbox, to cover shock and duty severity.

Frequently asked questions

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