Gearbox repair: signs of failure, the causes behind them, and what an industrial gearbox rebuild includes

A failing gearbox shows a change in gear noise, a rise in oil or bearing temperature, oil leaks, foaming oil or rising vibration. One maker's operating instructions for industrial helical and bevel-helical units list daily checks of oil temperature and unusual noise. The instructions require the drive to be switched off at once when an irregularity is found.^[1]

The same instructions set the maximum oil sump temperature at 90 °C for mineral oil and 100 °C for synthetic oil. The temperature is measured at full machine performance after running in. The instructions require an immediate shutdown above it.^[1]

The fault table in the same instructions links each sign to its usual causes. A change in gear noise points to damaged gear teeth, excessive bearing play or a defective bearing. A higher temperature at the bearings points to a wrong oil level, old oil or a defective bearing. Foaming points to water in the oil, old oil or mixed oils.^[1]

Rising vibration is the sign a survey catches before the noise does. A route of vibration analysis is therefore the usual early warning. ISO 20816-9:2020 sets how vibration of enclosed gear units from 10 kW to 100 MW and 30 to 12 000 r/min is measured and classified. The standard leaves the dedicated techniques for judging gear condition outside its scope.^[3]

Bearings are the part that fails most often in a US public study of wind turbine gearbox damage, followed by the gears. A US Department of Energy summary covers 750 confirmed wind turbine gearbox damage records from 2009 to August 2015. In those records, bearings caused 76% of failures and gears 17.1%. Other parts made up the rest.^[2]

Lubrication is behind many bearing and gear faults. One maker's instructions name old oil, contaminated oil, water in the oil and the wrong oil level as causes of high temperature and foaming. The instructions state that each 10 K rise in oil temperature halves the oil's service life.^[1]

Misalignment between motor, gearbox and driven machine loads the bearings and the tooth flanks unevenly. After a gearbox goes back in, laser shaft alignment removes that cause. Loose fastening bolts show up as loud noise at the mounting. The maker's table lists that fault with retightening to the specified torque as the remedy.^[1]

Oil analysis shows water, particles and oil breakdown before the gearbox is stripped, and gives the repair a measured starting point. One maker's instructions call for testing the water content at about 400 operating hours and at least once a year after that.^[1]

The same instructions say the sample must be taken while the gear unit runs, downstream of the filter. A fresh sample of the same oil is sent for reference. A laboratory's confirmation that the oil is still usable can replace a fixed oil change.^[1]

Particle counts are often reported as a cleanliness code. ISO 4406:2021, written for hydraulic fluid power, specifies the code that defines the quantity of solid particles in the fluid. Ask the laboratory which code and edition its report uses, so successive samples compare.^[4]

A gearbox rebuild includes cleaning, a full strip, an assessment of every part, replacement of what is worn or damaged, reassembly and a run test. A South African gearbox repairer described its process in MechChem Africa's January-February 2025 issue. The steps are wash, dimensional scan, strip, shot blast and a condition assessment of each component. A quotation then lets the client choose between a new unit and a repair.^[6]

A quote for an industrial gearbox rebuild should list these items separately:^[6][1]

• the strip-down, cleaning and inspection, with a written strip report and photographs;^[6]
• bearings, radial shaft seals and gaskets to be replaced, with the bearing makers and designations;^[1][6]
• the gears: reused, reworked or replaced; the repairer quoted in MechChem Africa replaces the complete matching gear set rather than one or two wheels;^[6]
• shafts and housing: measurements against the original fits, and any machining, sleeving or a new housing;^[6]
• bearing play set and the tooth contact pattern checked, with ISO/TR 10064-4:1998 as the reference for contact pattern checking;^[1][5]
• fresh oil of the grade on the rating plate, and a run test with the oil temperature recorded;^[1]
• the warranty period and what it covers.^[1]

The strip report should show the condition of every part, the measured wear, the cause of the failure and the repair proposed. The buyer can then check the quote against the evidence. In practice, a useful report names the root cause rather than only the failed part. A new bearing fails again if the oil, the alignment or the load that killed the old one stays the same.^[1][6]

Ask for photographs of tooth flanks and bearing races. Ask for the bearing clearances and shaft and housing fits, measured against the original dimensions. Ask for the oil and debris found in the sump, and a note on any earlier repairs. The repairer in MechChem Africa warned that every time a housing is machined its tolerances grow. A welded-in bush or sleeve can also affect how precisely the gears mesh.^[6]

Repair when the strip report shows the housing sound and the parts obtainable. Replace when the housing is beyond tolerance, the parts cannot be had or the repair price approaches a new unit's. The repairer in MechChem Africa prepares its quotation precisely so the client can decide between purchasing a new unit and repairing the existing one.^[6]

Lead time often decides it. The same article reported new gear sets made abroad within three to four weeks of the scan being sent. The article also reported a replacement housing cast locally for a coal mine conveyor. The original maker no longer had stock anywhere in the world.^[6]

No public price list exists for industrial gearbox rebuilds. The price follows the gearbox's size and type and how many gears and bearings need replacing. Whether gears must be made, housing work, transport and the run test also move the price.^[6][1]

South Africa's machinery regulations require the employer or user to keep machinery safe rather than to repair gearboxes in a set way. The General Machinery Regulations, 1988 require machinery to be "installed, operated and maintained in such a manner as to prevent the exposure of persons". The exposure in question is to "hazardous or potentially hazardous conditions or circumstances" (regulation 3(1)(a)).^[7]

The Driven Machinery Regulations, 2015 apply to the operation, repair, maintenance, inspection and testing of driven machinery (regulation 2). The same regulations require every shaft, gear, coupling and similar revolving part to be securely fenced or guarded (regulation 3(a)). A part that is as safe by its position or construction is excepted. The coupling guard therefore goes back on before the drive restarts.^[9]

A draft to replace the General Machinery Regulations was published for public comment on 22 August 2025 (Notice 6532, Government Gazette 53210).^[8]