Gearbox Glossary: Essential Terms for Gear Repair and Gearbox Rebuild
Navigating the world of gear repair and gearbox rebuilds involves understanding key terminology. Whether you’re maintaining industrial equipment or planning upgrades, this glossary of essential terms will guide you through the process.
Abrasive Wear
Material removed from gear teeth or bearing surfaces by hard particles suspended in the lubricant, such as dirt, casting sand, or wear debris. It shows as fine scratches running in the direction of sliding and usually indicates contamination or inadequate filtration.
Addendum
The radial distance from the pitch circle to the top of a gear tooth. Addendum dimensions affect tip clearance and how far each tooth engages its mating member.
AGMA – American Gear Manufacturers Association
The association that publishes North American standards for gear design, rating, inspection, lubrication and quality. AGMA ratings and service factors are the common reference point for sizing industrial gear drives and evaluating them after failure.
Alignment
The positional relationship between shafts, gears, bearings and housing bores. Misalignment concentrates load on one end of the tooth face, which raises operating temperature and shortens the service life of the gearbox.
Annealing
A heat treatment that heats steel and cools it slowly to relieve internal stress and soften the material. Annealing improves machinability before gear cutting and is normally followed by hardening operations.
Axial Thrust
Force acting along the axis of a shaft. Helical, bevel, hypoid, and worm gearing all generate axial thrust that has to be carried by correctly selected and correctly set thrust bearings.
Backlash
The clearance between mating gear teeth is measured at the pitch circle. A controlled amount allows for a lubricant film and thermal expansion. Excessive backlash produces impact loading and positioning error.
Backstop
A one-way clutch fitted to a gear reducer that prevents the output shaft from turning in reverse. Backstops are standard on inclined conveyors and bucket elevators, where rollback under load would damage equipment.
Backlash Variation
Backlash variation is the difference in clearance between gear teeth over a full rotation. It affects gear smoothness and operational precision.
Base Circle
The circle from which an involute tooth profile is generated. Its diameter is determined by the pitch diameter and the pressure angle of the gear.
Bearing
The component supports a rotating shaft and carries radial load, axial load, or both. Industrial gearboxes commonly use tapered roller, spherical roller, cylindrical roller and ball bearings selected for load direction and shaft speed.
Bevel Gear
A gear with teeth cut on a conical surface, used to transmit power between intersecting shafts, most often at 90 degrees. Bevel teeth may be straight, spiral or hypoid depending on load and noise requirements.
Breather
A vent that allows air to move in and out of the housing as the oil heats and cools. Without a functioning breather, internal pressure forces oil past the seals. Desiccant breathers also limit moisture entry.
Bucket Elevator
A vertical conveyor that lifts bulk material in buckets attached to a belt or chain. Bucket elevator drives typically use a shaft mount reducer with a backstop to hold the loaded column when the drive stops.
Caburizing
A case-hardening process that diffuses carbon into the surface of low-carbon steel at high temperatures, followed by quenching. The result is a hard, wear-resistant case over a tougher core that resists tooth-bending fatigue.
Case Crushing
Subsurface fatigue in a case-hardened gear where the hardened layer cracks and separates from the core. It generally traces back to insufficient case depth or core hardness for the applied contact stress.
Case Depth
The depth of the hardened surface layer on a case-hardened gear tooth. Sufficient case depth is required to resist subsurface fatigue and case crushing under high contact stress.
Case Distance
The distance between the axes of two mating gears. Worn bearing bores or a distorted housing changes the effective center distance, which alters backlash and shifts the tooth contact pattern.
Concentric Inline Gear Drive
A gear drive with the input and output shafts on the same centerline. The arrangement suits installations where the motor and driven equipment sit in line and floor space is limited.
Contact Pattern
The area of the tooth flank that carries load during mesh, usually revealed with marking compound. The size and position of the pattern show whether gear alignment, bearing settings and tooth geometry are correct.
Contact Ratio
The average number of tooth pairs in mesh at any moment. A higher contact ratio distributes load across more teeth and produces smoother, quieter running, which is one reason helical gears are quieter than spur gears.
Continuous Duty
Operation at rated load for extended periods without scheduled rest intervals. A drive rated for continuous duty has to shed heat steadily at or below its thermal rating.
Corrosion
Chemical or electrochemical deterioration of gear and bearing surfaces, most often caused by water in the oil, acidic lubricant breakdown, or an aggressive process environment. Corrosion pits can later act as fatigue initiation sites.
Coupling
A device connecting two shafts to transmit torque while accommodating limited misalignment. Coupled arrangements require a base and periodic alignment checks. Shaft mounted drives remove the coupling between the reducer and the driven shaft.
Dedendum
The radial distance from the pitch circle to the bottom of the tooth space. Dedendum is normally greater than addendum so the mating tooth tip clears the root of its partner.
Diametral Pitch (DP)
DP measures gear tooth size based on the number of teeth per inch of pitch diameter. It helps determine the size and spacing of gear teeth.
Double Helical Gear (Herringbone Gear)
A gear with two opposing rows of helical teeth, often separated by a central gap. The opposing helix angles cancel axial thrust, which allows high load capacity without large thrust bearings.
Efficiency
The ratio of output power to input power in a gear drive, stated as a percentage. Losses come from tooth friction, bearing friction, seal drag and oil churning. Helical and bevel stages run more efficiently than worm stages.
EP (Extreme Pressure) Additive
A lubricant additive that forms a protective chemical film on metal surfaces under high contact pressure and sliding. Hypoid and worm gearing often require EP oils, though sulfur-phosphorus additives can attack bronze components at elevated temperatures.
Epicyclic Gearing
A gear arrangement in which one or more gears orbit a central gear while meshing with it. Planetary gear sets are the most common epicyclic configuration in industrial drives.
Exchange Unit
A rebuilt gearbox of the same make, model, and ratio supplied in trade for a customer’s used or failed unit. MAYDAY exchange units carry a 12-month warranty from the date of installation.
Face Width
The length of a gear tooth measured parallel to the gear axis. Greater face width raises load capacity, but it also demands closer alignment to keep contact across the full tooth.
Failure Analysis
Systematic examination of damaged components to determine the root cause of a gearbox failure. The findings guide repair decisions and identify the operating or installation conditions that need correction.
Fatigue
Progressive cracking of a material under repeated cyclic stress below its ultimate strength. In gearboxes, fatigue drives tooth root cracking and bearing raceway spalling.
Filleted Root
The curved surface joining the tooth flank to the root of the tooth space. Fillet geometry controls bending stress concentration, and most tooth breakage originates at the fillet.
Fretting Corrosion
Wear at tight-fitting contact surfaces subject to small oscillating movement, such as bearing fits, keyways and splines. It produces reddish-brown oxide debris and can lead to loose fits and cracking.
Frosting
A mild form of micropitting that gives tooth flanks a dull gray, etched appearance. Frosting often appears during run-in on case-hardened gears and may stabilize or progress depending on load and oil film thickness.
Gear Grinding
A finishing process that removes material from hardened gear teeth with an abrasive wheel to achieve accurate profile, lead and spacing. Grinding corrects heat treat distortion and improves surface finish.
Gear Ratio
The ratio of input speed to output speed, equal to the number of teeth on the driven gear divided by the teeth on the driving gear. A higher reduction ratio lowers output speed and raises output torque in proportion, less efficiency losses.
Gear Reducer
A gearbox that lowers rotational speed from the prime mover while increasing output torque. Most industrial gearboxes are reducers, and the terms gearbox, gear drive and gear reducer are used interchangeably in the field.
An enclosed assembly of gears, shafts, bearings, seals and lubricant that transmits power between a prime mover and driven equipment while changing speed, torque or the direction of the shaft.
Complete disassembly, cleaning, inspection and restoration of a gearbox, with worn or damaged components repaired or replaced before reassembly and testing to OEM specifications.
A motor and gear reducer supplied as one integrated unit. A gearmotor reduces the mounting space and alignment work compared with a separate motor, coupling, and reducer.
Hardness (BHN)
Resistance of a material to indentation and wear, reported for gears on the Brinell (BHN) or Rockwell C (HRC) scale. Tooth hardness governs contact fatigue capacity and wear resistance.
Controlled heating and cooling used to change the microstructure and mechanical properties of gear steel. Common gear heat treatments include through hardening, carburizing, induction hardening and nitriding.
A right-angle gearbox that combines one or more helical stages with a spiral bevel stage. The arrangement delivers high torque capacity and efficiency in a compact envelope for conveyors, mixers, extruders, and process lines.
A cylindrical gear with teeth cut at an angle to the shaft axis. Teeth engage gradually along the helix, which lowers noise and vibration and raises load capacity. The angled teeth also generate axial thrust.
Helix Angle
The angle between a helical tooth and the axis of the gear. Larger helix angles increase contact ratio and smoothness while also increasing axial thrust on the bearings.
Hobbing
A gear cutting process in which a rotating cutter called a hob generates tooth profiles as the workpiece rotates in time with it. Hobbing is the most common method of cutting spur, helical and worm gearing.
Hollow Output Shaft
An output bore that slides directly over the driven shaft and is retained with keys, bushings or a shrink disc. The hollow shaft is what allows a shaft-mounted drive to operate without a base or coupling.
Housing
The case that encloses the gears, shafts, bearings, and lubricant. The housing holds the bearing bores in a precise relationship, so cracks, distortion or worn bores directly affect gear mesh and bearing life.
A spiral bevel type gear in which the pinion axis is offset from the gear axis. The offset increases tooth contact and allows a larger pinion, at the cost of higher sliding velocity and stricter lubrication requirements.
IEC Motor
An electric motor made to International Electrotechnical Commission dimensional standards, using metric frame and flange dimensions. Many inline gear drives accept IEC motors directly or through an adapter.
Induction Hardening
A surface-hardening process that uses an electromagnetic field to heat the tooth surface rapidly before quenching. It hardens selected areas with limited distortion and is often applied to large gears.
Internal Gear
An internal gear has teeth cut inside the cylinder. It meshes with an external gear (pinion) and is used in various gear arrangements.
Involute Profile
The tooth curve traced by unwrapping a taut line from the base circle. Involute teeth maintain a constant velocity ratio and tolerate small variations in center distance, which is why they are standard for industrial gearing.
Keyway
A slot machined into a shaft and mating hub to hold a key that transmits torque. Worn, rolled or cracked keyways caused by fretting or a loose fit are a common finding during reducer inspection.
Lead
The axial distance a helical tooth or worm thread advances in one complete revolution. Lead error causes uneven load distribution across the face width and shifts the contact pattern toward one end of the tooth.
Lip Seal
A radial shaft seal with a flexible lip held against the shaft by a garter spring to retain oil and exclude contaminants. Dual-lip designs add a second lip specifically for contamination control.
Load Sharing
The distribution of torque among multiple planet gears or gear meshes. Accurate manufacturing, along with floating or self-aligning carriers, keeps the load equalized so no single planet carries a disproportionate share.
Low-Cycle Fatigue
Fatigue failure that occurs after a relatively small number of high-stress cycles, typically from repeated overloads or peak starting loads. The 1991 AGMA paper in MAYDAY’s technical library examines low-cycle bending strength in gear teeth.
Lubrication Failure
Loss of an adequate oil film between contacting surfaces, caused by low oil level, incorrect viscosity, contamination, oxidation, or excessive operating temperature. Scuffing and bearing damage follow quickly.
Mechanical Rating
The power a gear drive can transmit based on the strength and durability of its gears, shafts, bearings, and housing. It is evaluated separately from the thermal rating, and the lower of the two governs selection.
Micropitting
Surface fatigue produces very small pits a few micrometers deep, giving the flank a gray, stained appearance. It develops under thin oil films on case-hardened gears and can progress into pitting and loss of profile.
Module (MOD)
The metric measure of gear tooth size, equal to pitch diameter in millimeters divided by the number of teeth. Module is the metric counterpart to diametral pitch, where module equals 25.4 divided by DP.
Mounting Position
The orientation in which a gear drive is installed, such as horizontal, vertical or inclined. Mounting position determines oil level and which bearings receive splash lubrication.
NEMA C-Face
A motor mounting standard from the National Electrical Manufacturers Association that uses a machined face with threaded bolt holes and a pilot diameter, allowing a gear drive to bolt directly to the motor.
Nitriding
A surface hardening process that diffuses nitrogen into steel at relatively low temperature. It produces a thin, very hard case with minimal distortion, which suits gears that cannot be ground after hardening.
Oil Analysis
Laboratory testing of a gear oil sample for viscosity, water content, wear metals, additive condition and particle contamination. Tracking results over time identifies developing gear or bearing wear before it becomes a failure.
Overhung Load
A radial force applied to a shaft at a distance from its supporting bearing, typically from a sprocket, sheave or pinion mounted on the shaft extension. Published overhung load ratings limit how much side load a gearbox shaft can carry.
A gear drive in which the input and output shafts run parallel, usually through helical gear stages. The configuration provides high load capacity and efficiency for conveyors, pumps, fans, and heavy process equipment.
The smaller of two mating gears and usually the driving member. The high-speed pinion in a multistage reducer turns at motor speed and therefore accumulates the most load cycles in the unit.
Pitch Diameter
The diameter of the theoretical pitch circle at which mating gears roll together without slipping. Pitch diameter, along with tooth count, defines the ratio and center distance of the gear set.
Pitting
Surface fatigue damage in which small pieces of material break out of the tooth flank under repeated contact stress, leaving craters. Initial pitting may stabilize after run-in, while progressive pitting continues until tooth profile and capacity are lost.
Planet Carrier
The member that holds the planet gear shafts and transmits output torque in a planetary gear set. Self-aligning carriers help equalize load sharing among the planets under shock and overload conditions.
Planet Gear
A gear that meshes with both the sun gear and the ring gear while orbiting the center of a planetary set. Using several planets divides the transmitted torque among multiple meshes.
A gearbox made up of a central sun gear, orbiting planet gears and an outer ring gear. Sharing load across multiple planet meshes gives planetary units high torque capacity in a compact, coaxial package.
A Falk product line from Regal Rexnord consisting of concentric planetary drives for severe service. Features include self-aligning planet carriers, hardened wear-resistant gearing, solid or hollow output options and the capacity to absorb substantial shock loading.
Plastic Deformation
Permanent reshaping of tooth surface material under heavy contact load, producing rolled edges, ridges, or rippling on the flank. It appears most often on softer through-hardened gears exposed to repeated overload.
Pressure Angle
The angle between the line of action and a line tangent to the pitch circle, commonly 14.5 or 20 degrees in industrial gearing. A higher pressure angle strengthens the tooth while increasing separating force on the bearings.
Quenching
Rapid cooling of heated steel in oil, water, polymer or gas to lock in a hard microstructure. Quenching is followed by tempering to restore toughness and relieve the stresses it creates.
Rebuild
Restoration of a gearbox to OEM specifications by reconditioning or replacing worn components, then reassembling, aligning, and testing the unit. A rebuilt gearbox returns to service at a fraction of replacement cost.
A gearbox that turns the direction of power flow 90 degrees between perpendicular shafts, using bevel, hypoid or worm gearing. It allows engineers to redirect torque within tight equipment layouts.
An internal gear with teeth cut on the inside of a cylinder. In a planetary set the ring gear meshes with the planet gears and is commonly held stationary in the housing.
Root Diameter
The diameter measured at the bottom of the tooth spaces. Root diameter, together with fillet geometry, determines the section carrying bending load at the base of the tooth.
Run-In
A period of controlled operation on a new or rebuilt unit during which mating surfaces conform and the contact pattern settles. An oil change after run-in removes the wear debris generated in this phase.
Scuffling (Scoring)
Adhesive damage that occurs when the oil film breaks down and tooth surfaces momentarily weld, then tear apart. It leaves rough, torn marks running in the direction of sliding and points to high temperature, high sliding speed or insufficient oil film.
Seal
The component that retains lubricant inside the housing and keeps contaminants out at shaft openings and joints. Persistent oil leakage at a seal often indicates a worn shaft surface, a blocked breather or bearing wear.
Service Factor
A multiplier applied to the nominal load when selecting a gear drive, accounting for shock, duty cycle and daily operating hours. A service factor of 1.5, for example, calls for a drive rated at 1.5 times the application load.
A helical gear drive that mounts directly on the driven shaft through a hollow bore, held against rotation by a torque arm. The design removes the base, coupling and alignment points required by a foot-mounted arrangement.
A shaft mounted speed reducer used on conveyors, screw conveyors, bucket elevators and crushers, commonly driven by a V-belt and fitted with a backstop. Installation depends on correct shaft fit, torque arm angle, belt tension and lubrication.
A sudden load well above the normal running load, produced by jams, hard starts, material surges or reversals. Crushers, mills and shredders require drives selected with service factors that account for shock.
Spalling
Severe fatigue damage in which large, irregular flakes separate from a tooth flank or bearing raceway. Spalling generally develops from advanced pitting or subsurface cracking, and the affected component has to be replaced.
Speed Increaser
A gear drive arranged to raise output speed above input speed, with a corresponding reduction in torque. Speed increasers appear in compressor, pump, and generator drivetrains and are serviced much like reducers.
Spiral Bevel Gear
A bevel gear with curved, angled teeth that engage gradually along the tooth. The gradual engagement supports higher loads and quieter operation than straight bevel gearing at comparable size.
Spline
A series of teeth machined along a shaft that mate with matching internal teeth in a hub. Splines transmit high torque and can allow axial movement between the connected parts.
Spur Gear
A cylindrical gear with teeth cut parallel to the shaft axis. Spur gears are simple to manufacture and produce no axial thrust, though they engage abruptly and run louder than helical gears at speed.
Strip Report
A document issued after a gearbox is disassembled and cleaned, recording the condition of each component, the identified cause of failure, and the recommended repairs. The strip report forms the basis for the repair quote.
Sun Gear
The central gear in a planetary set, meshing with the surrounding planet gears. The sun gear is frequently the input member and sees the highest rotational speed in the unit.
Tempering
Reheating quenched steel to a controlled temperature below its critical point to reduce brittleness and relieve stress. Tempering trades a small amount of hardness for a large gain in toughness.
Thermal Rating
The power a gear drive can transmit continuously without exceeding its allowable oil temperature. When the thermal rating falls below the mechanical rating, cooling fans, oil coolers or a larger unit are required.
Through Hardening
A heat treatment that hardens the full cross section of a gear blank to a uniform hardness, typically lower than a carburized case. Through-hardened gears can often be cut after heat treatment, which lowers manufacturing cost.
Tooth Breakage
Fracture of a complete tooth or a major portion of one, caused by bending fatigue at the root fillet or by a single overload. Fatigue fractures show progression marks, while overload fractures leave a rough, fibrous surface.
Torque
Rotational force, expressed in pound-feet or newton-meters. A gear reducer multiplies input torque in proportion to its reduction ratio, less the losses in the gear meshes and bearings.
Torque Arm
The link that anchors the housing of a shaft-mounted reducer to a fixed structure, reacting output torque so the unit does not rotate with the shaft. Its length and angle also set V-belt tension on many installations.
Torque Density
The amount of torque a drive transmits relative to its size and weight. Planetary gearing achieves high torque density by dividing load across multiple planet meshes instead of a single pair of teeth.
A Falk product line from Regal Rexnord of concentric helical gear drives for medium and heavy duty service. The line covers ten sizes with ratios from 1.5:1 to 985.3:1 and several motor mounting arrangements.
A Falk product line from Regal Rexnord of concentric inline gear drives designed for NEMA C-face and IEC motors. Features include dual-lip sealing, fully supported high-speed pinions and continuous duty ratings.
Measurement and interpretation of vibration signals to identify gear mesh defects, bearing damage, misalignment or imbalance. Vibration limits drawn from ISO standards are also used to verify a unit after rebuild.
A lubricant’s resistance to flow, specified for gear oils by ISO viscosity grade. Correct viscosity maintains oil film thickness at operating temperature. Oil that is too thin allows metal contact, while oil that is too thick raises churning losses and heat.
Worm
The threaded driving member of a worm gear set, resembling a screw. The worm slides across the wheel teeth as it turns, which is the source of both the high reduction capability and the heat generated.
A right-angle gearbox using a worm and bronze worm wheel to achieve high reduction in a single stage. Sliding contact lowers efficiency and generates heat, so lubrication condition and operating temperature are primary service checks.
The toothed wheel that meshes with the worm, usually made of bronze so the softer material wears in preference to the hardened steel worm. Wheel wear is measured to judge remaining life during inspection.
Understanding these terms empowers you to communicate effectively with gear repair and gearbox rebuild professionals like MayDay. Whether you’re discussing maintenance needs or planning upgrades, familiarity with these terms ensures your equipment operates efficiently and reliably.
Partnering with experienced professionals who understand these terms ensures your gear systems perform optimally, minimizing downtime and maximizing productivity.