10.3 Troubleshooting and Repairing Gearboxes
Key Takeaways
- Gear ratio equals the number of teeth on the driven gear divided by the number of teeth on the driving gear.
- Backlash is the clearance between mating teeth; too little causes binding and heat, and too much causes shock loading and noise.
- A plugged breather pressurizes the gear case as it heats and forces oil past the shaft seals, which is a common cause of chronic leaks.
- Pitting is surface fatigue on the tooth flank, scoring is adhesive damage from lubricant film breakdown, and tooth breakage usually starts as bending fatigue at the root.
- A tooth contact pattern taken with marking compound should be centered on the flank, and a pattern biased to one end indicates misalignment or shaft deflection.
Gear types and where they belong
| Gear type | Tooth form | Character |
|---|---|---|
| Spur | Straight teeth, parallel shafts | Simple and efficient; noisy, because each tooth engages all at once |
| Helical | Angled teeth, parallel shafts | Quieter and stronger from gradual engagement; produces axial thrust |
| Double helical / herringbone | Two opposed helices | Helical benefits with the thrust loads cancelling each other |
| Bevel / spiral bevel | Intersecting shafts, usually 90 degrees | Spiral bevel is quieter and stronger than straight bevel |
| Worm | Worm and worm wheel, shafts at 90 degrees, non-intersecting | Very high reduction in one stage; sliding contact makes heat and demands the right lubricant |
| Planetary (epicyclic) | Sun, planets, ring | High ratio in a compact, concentric package; load shared among planets |
Helical thrust is the practical consequence to remember: a single-helical gearbox must have a bearing arrangement that takes axial load, and that thrust bearing is often the first thing to fail when the unit is overloaded.
Worm gear drives run on sliding rather than rolling contact, so they generate significant heat and need a compounded or extreme-pressure gear oil of the specified type. Using an ordinary EP gear oil where a compounded oil is specified, or the reverse, can attack the bronze worm wheel.
Ratio
Output speed is the input speed divided by the ratio, and output torque is the input torque multiplied by the ratio, less losses.
Worked example. A 1,750 rpm motor drives an 18-tooth pinion meshing with a 72-tooth gear. The ratio is 72 ÷ 18 = 4:1, so the output runs at 1,750 ÷ 4 = 437.5 rpm with roughly four times the input torque.
For a compound train, multiply the ratios of the individual stages: a 4:1 stage followed by a 5:1 stage gives 20:1 overall.
Idler gears change direction of rotation but do not change the overall ratio, because the idler's tooth count appears in both the numerator and the denominator and cancels out.
Backlash and contact pattern
Backlash is the clearance between mating tooth flanks, and it is deliberate. It provides room for the lubricant film, for thermal expansion, and for manufacturing tolerance.
- Too little backlash: the teeth bind, the oil film is squeezed out, temperature climbs, and the gears scuff.
- Too much backlash: the drive takes up with a shock on every load reversal, producing hammering noise and accelerating fatigue at the tooth roots.
Measure backlash with a dial indicator against a tooth flank while the mating gear is held fixed and the measured gear is rocked, or with soft lead wire rolled through the mesh and then measured with a micrometer. Compare against the manufacturer's value; backlash generally increases with tooth size.
A tooth contact pattern check is the most informative single test on an open gear set or a repaired bevel drive. Coat several teeth with marking compound, roll the gears through mesh under light drag, and read the transferred pattern:
| Pattern | Interpretation |
|---|---|
| Centered on the flank, covering most of the face width | Correct |
| Biased to one end of the tooth | Misalignment, shaft deflection, or housing bore error |
| Concentrated at the tooth tips | Centers too far apart, or wrong tooth profile |
| Concentrated at the roots | Centers too close |
| Narrow band across the middle | Crowned teeth under light load, which may be normal |
Reading gear tooth failure
| Appearance | Mechanism |
|---|---|
| Pitting — small cavities on the flank, often near the pitch line | Surface contact fatigue; initial pitting may stabilize, progressive pitting does not |
| Spalling — larger flakes broken away | Advanced surface fatigue |
| Scoring / scuffing — radial scratches, welded-and-torn appearance | Lubricant film breakdown from overload, high speed, or wrong viscosity |
| Abrasive wear — uniformly worn, matte flanks | Contamination in the oil |
| Plastic flow / rippling — material pushed over the tooth tip or edge | Heavy load with a soft material or inadequate film |
| Tooth breakage at the root | Bending fatigue, frequently traced to shock loading or a stress raiser |
| Corrosion / etching | Water in the oil, or acidic oil degradation products |
Diagnosing the common complaints
Overheating. Check, in order: oil level (overfilling causes churning and heat just as surely as underfilling causes starvation), oil viscosity and type against the nameplate, the breather, the load against the service factor, the alignment of the input and output couplings, and the bearing condition.
Leaking. Most chronic gearbox leaks are not seal failures.
- Plugged breather. As the gearbox warms, the air in the case expands. If the breather cannot vent, case pressure rises and pushes oil past the shaft seals. Clean or replace the breather first.
- Overfilling. Oil above the intended level reaches the seal and is churned into a foam that leaks past it.
- Seal installed backwards or damaged on the shaft keyway. The lip faces the oil.
- Worn shaft at the seal contact area, which requires a wear sleeve or shaft repair, not a new seal alone.
Noise and vibration. A gear mesh generates vibration at gear mesh frequency, which is the number of teeth on a gear multiplied by that shaft's speed. A rising mesh-frequency amplitude with sidebands points to tooth damage or eccentricity. Broad-band growl usually means bearings. A once-per-revolution knock suggests a broken tooth or a coupling problem.
Wrong output speed. Confirm the actual ratio against the nameplate; a unit replaced with the wrong ratio is more common than anyone admits.
Repair practice
- Record as-found data before disassembly: backlash, endplay, shim thicknesses, contact pattern, bearing positions, and oil condition. Take an oil sample.
- Match-mark gears, shafts, and housing halves so the original orientation is restored.
- Keep shim packs identified by location; they set bearing preload and gear position.
- Inspect housing bores and shaft seats. A gearbox that ate a bearing has usually damaged the bore.
- Mount bearings per Chapter 7, and set endplay or preload to the manufacturer's value.
- Restore backlash and contact pattern and record them.
- Fill with the specified lubricant to the specified level — not to the top of the sight glass unless that is where the mark is — and fit a clean breather.
- Run in at reduced load where the procedure allows, monitoring temperature and listening, then take a baseline vibration reading.
A speed reducer has an 18-tooth pinion driving a 72-tooth gear, then a 20-tooth pinion driving a 60-tooth gear. What is the overall ratio?
A gearbox leaks oil from its output shaft seal. The seal has been replaced twice and leaks again within weeks. What should be checked before fitting a third seal?
A tooth contact pattern taken with marking compound shows contact concentrated at one end of the tooth face rather than centered across it. What does this indicate?