GEAR FAILURES – SYMPTOMS, CAUSES, REMEDIES FAILURE TYPES DESCRIBED: - Broken Teeth - Pitting - Scuffing - Overheating - Foaming - Vibration - Unusual Noise - Wear – all types BROKEN TEETH Possible Causes: A. Fatigue 1. Load unevenly distributed 2. Sharp corner in tooth root 3. Notches in tooth root caused by improper filling or grinding 4. Overload 5. Torsional vibration 6. Bruises on teeth, eg. caused by dropping 7. Coarse martensitic structure in hardened steel 8. Blow holes in roots dedendum of cast teeth 9. Surface fatigue pits in dedendum of teeth. B Fracture 1. Repeated heavy loads 2. Hard foreign solids jammed in gear mesh Remedy: Temporary corrections may be made by cutting cut, broken and cracked sections until only sound teeth are left, the gear then being run at proportionately reduced load; by inserting pegs and building up new teeth on them by welding followed by reshaping. The only permanent correction for broken teeth, however, is to procure new hears without the faults listed, to have them correctly aligned and run with overloads limited by shock absorbing coupling. PITTING Symptom Possible causes Remedy a) Small widely scattered pits Surface fatigue at surface None: the pitting will on working surfaces asperities during initial cease to spread and running be of no consequence b) Pits concentrated at one end Concentration of load due Check alignment of of tooth on helix to slight misalignment, or shafts and casing for incorrect helix correction. distortion.The pitting may cease to spread c) Pits concentrated in regular Concentration of load on The pitting may cease Bands along tooth width surface undulations to spread after a while, but lapping or stoning may be advisable in bad cases. d) Pits concentrated along d)1) Faulty profile As above. pitch line 2) Excessive wear or Use oil of higher scuffing has produced viscosity or of higher a ridge along the pitch EP activity. line which becomes overloaded. e) Pits concentrated on e)1. Insufficient hardness The pitting will cease to dedendum surfaces of of softer gear or ex- spread if the opposing one gear cessive free ferrite in gear unaffected. If micro-structure relative possible, an increase in to loading oil viscosity could help. e)2. Excessive amounts of hard non-metallic in- clusions in the metal e)3. Initial surface finish too rough and oil viscosity too low e)4. Overloading, especially by shock torsional oscil- lations and heavy starting torque f) Pits concentrated on Insufficient hardness of both Reduce overloading, in- dedendum surfaces of gears relative to loading sert shock absorbing both gears, often when type of coupling be- distinct step at pitch line. tween source of shock and gear. If possible, increase viscosity of oil at operating temperature. Change to EP oil mainly to prevent scuffing of remaining contact areas. If possible, lap or stone teeth to improve surface and rub in dry MoS2. g) Pitting in case hardened Teeth too soft either because Reduce load gears. of accidental decarburis- ation or inadequate quenching h) Micro-pitting or “frost- 1. Nitrided gears: thin surface A 25mm layer (or less) ing” in surface hardened layer of weak brittle super- is generally harmless but gear. (Closely spaced rich nitrides (white layer). consider the removal of pits smaller than 0.1mm thicker layers by lapping across) or grinding. 2. Inadequate oil film Reduce oil and gear temperature or use more viscous oil. Note: Pitting of soft and through-hardened steel gears is so common that it cannot be counted as a failure. But if possible, it is prudent to correct, and detailed above. In some cases a fresh start may be made by turning round one or both gears and lapping them together as necessary. In severe cases where noise is excessive or reduced areas of contact cannot be prevented from scuffing, or there is danger of surface pitting initiating tooth breakage, it may be necessary to replace gears. SCUFFING Possible Causes Remedy a) Insufficient lubrication 1. Oil bath level too low Raise oil level so that gear dips 1-3 times tooth height when running. 2. Oil sprayer nozzles Check oil nozzles, clean as necessary, filter or change choked oil. 3. Pump suction strainer Check suction strainer, clean as necessary, filter or choked. change oil. 4. Bearings rob oil from Restrict oil supply to bearings. sprayer nozzles 5. Windage in high speed Increase oil pressure, re-position sprayer nozzles. gears deflects oil spray 6. Lubricant channels at low Use oil with low channel point. starting temperatures 7. Clearance between gears If possible, use lower viscosity oil. Increase and sides and bottom of clearance. casing too small: oil cannot flow back to bath b) Operating temperatures Reduce temperatures (refer separate section) Excessive c) Oil viscosity too low Use higher viscosity grade; improve cooling. (particularly soft or hardened gears). d) Lubricant has insufficient Use more active EP oil. EP activity (particularly hardened gears). e) Teeth have excessive Check with manufacturer. addendum height. f) Teeth do not have suf- Reduce undulation by stoning, lapping or shaving. ficient contact because of Check alignment, check that casing is not distorted undulations or misalignments. by uneven settling of foundations, check that bearings are not worn. g) Teeth are not adequately Apply tip relief by shaving or stoning. relieved. h) Full load applied before Use active EP oil to prevent further scuffing while gears have been adequately further running in makes surfaces smoother. run-in. i) Both gears have high Use active EP oil or higher viscosity grade. chromium content, and are unhardened. Note: Provided scuffing has not roughened the surface unduly, and specially with helical and spiral bevel gears, once the basic cause of scuffing has been eliminated, further running, especially with an EP oil, will correct the problem. OVERHEATING Possible Causes Remedy a) Oil level too high in bath Adjust oil level when running to correct level; one to three times the tooth height. b) Oil viscosity too high Change to lower viscosity grade. c) Speed too high for bath Change to spray system. lubrication. d) Too much oil sprayed too Direct oil spray more in advance of contact. close to ingoing mesh of high speed gears. e) Cooler dirty Check and clean as necessary, oil and water slides. f) Cooler inadequate Change cooler for larger size or switch to cooler water supply. g) Heat radiated from Interpass radiation shields. Surroundings i) Air flow over gearbox Increase ventilation of surroundings air space. Inadequate j) (Worm gears): unsuitable Change to HVI mineral oil, or preferably, polyglycol lubricant type synthetic oil. FOAMING Possible Causes Remedy a) Gear dips too deeply into Adjust oil bath level when gear running to 1-3 times oil bath tooth height. b) Air leaks on suction side Remake joints, including pump gland, on suction side. of circulating system c) Oil cascades down vertical Rearrange return lines to allow smooth flow into tank return lines and down into at oil level. reservoirs. d) Oil contaminated by grease, Renew oil charge. jointing compound, etc. VIBRATION Possible Causes Remedy a) Defective bearings or Check bearing and coupling and replace as necessary. Couplings b) Shafts misaligned Check that casing is not distorted, realign shafts. c) Gears out of balance Check balance, if necessary, re-balance. UNUSUAL NOISE Possible Causes Remedy a) Defective rolling element Check these parts and replace as necessary. Bearing b) Defective oil pump As above c) Defective coupling As above e) Tooth surfaces excessively Stone or lap teeth surfaces and rub in MoS2 powder. pitted or roughened WEAR Symptom Possible causes Remedy Wear of softer member only Surface finish of harder Stone or lap teeth to a finer of gear pair. member too coarse finish Wear of harder member only Abrasive dirt in oil Change oil or pass it through of gear pair becoming embedded in fine filter. Fit air filter on soft gear vents. Wear at low speed Oil film too thin Use oil of higher viscosity Wear at high speed with Excessive friction caused Reduce overloads and over- discoloration and loss of by overload, overspeed, speed, increase backlash, hardness lack of backlash or faulty improve oil flow and lubrication distribution Plastic flow of teeth a) Combination of soft Reduce shock loads, use surfaces with pronounced material and repeated shock higher viscosity oil, re- fin at tooth tips loads leading to separation duce backlash, change to of gear teeth with re-contac harder gear material. insufficiently damped by oil film b) Very heavy even loads Reduce loads, increase (rolling) gear surface hardness. Rippling Excessive surface friction Use hypoid type full EP at low speeds. oil. Bulk plastic flow of teeth Very severe overheating due Repair teeth or renew especially at middle of to failure of oil supply gears. Eliminate cause of tooth width oil supply failure. Install alarms so that unit can be quickly stopped in event of oil supply failure Cracks on the tooth surface Overheating during grind- Check with manufacturers of hardened gears, often in ing, incorrect heat treat- net-like pattern ment, or both Indentations in gear teeth Hard foreign matter in Thoroughly clean system, system, usually swarf, check heater surfaces for occasionally carbonised carbon formation, clean oil from highly rated heaters and reduce surface heaters in system. temperatures as necessary. Longitudinal cracking and Case is too thin and core Check with manufacturer. flaking in case hardened too soft so that the surface gears. has collapsed under load. Red-brown spots on surfaces Attack by corrosive Clean spots with emery of case hardened gears. substances such as salts stick. Clean, flush and re- from hardening process. fill lubrication system. Red-brown contact marks on Fretting due to vibration Arrange for the gears to teeth together with red-brown while under stationary be flushed with oil and powder in nominally stationary load. slowly rotated. gears