Lubrication, Wear and Gear Failures
Oils and greases, viscosity and film thickness, gear failure modes, inspection and a symptom table for noisy or hot drives.
Why lubricate
A lubricant does several jobs at once: it separates the moving surfaces with a film so that they wear little, it reduces friction (and so the heat and the loss of power), it carries heat away, flushes out wear particles, and protects against corrosion. Almost every failure of a gear, bearing, chain or belt-drive part is helped along by poor lubrication, contamination or overheating. The gears in this guide (Gear Strength and Design Calculations and Gear Types and Selection), the chains (Chains and Sprockets) and the bearings (Bearings and Bearing Selection) all depend on it.
Lubrication regimes: boundary, mixed, full film
Three regimes describe how the surfaces are separated. In boundary lubrication the peaks of the two surfaces touch, and only a thin chemical film (from additives) protects them: high friction and wear. In mixed lubrication the oil carries part of the load and the peaks touch part of the time. In full-film (hydrodynamic or elastohydrodynamic, EHL) lubrication the oil film is thicker than the surface roughness, there is no contact and wear is negligible. The film ratio λ = h ÷ composite roughness tells which regime applies: λ < 1 boundary, 1 to 3 mixed, > 3 full film.
Worked example: the film ratio of a gear contact
- Gear teeth with a roughness Ra = 0.4 µm on both flanks have Rq ≈ 1.25 Ra = 0.50 µm each, so the composite roughness is √(0.50² + 0.50²) = 0.71 µm.
- A typical EHL film in a steel gear contact is a fraction of a micrometer. With a film h = 0.5 µm: λ = 0.5 ÷ 0.71 = 0.71: in the boundary/mixed range, so the extreme-pressure (EP) additives are what protect the teeth.
- The film thickness varies with the viscosity to about the power 0.7: doubling the viscosity (for instance ISO VG 150 to 320) raises h by 20.67 = 1.59 to 0.80 µm and λ to 1.13. Finishing the teeth better (Ra 0.2 µm) does more: λ = 1.41 at the original film. Smooth, hard surfaces plus the right oil are the way to a long life.
- Start-up and low speed always pass through boundary and mixed lubrication, which is when most wear occurs: an oil with good EP and anti-wear additives matters, and a gear that is cold-started under load is at risk.
Choosing the oil: viscosity, base and additives
The oil is chosen by its viscosity grade (ISO VG, in mm²/s at 40 °C), its base and its additives. A higher viscosity gives a thicker film and better protection at low speed and high load, but more churning loss and heat at high speed and a worse flow when cold. The first rule is to follow the manufacturer’s recommendation for a gearbox; the table gives typical starting points (AGMA 9005, ISO 12925) for enclosed gears.
| Pitch-line speed | Typical ISO VG (mineral EP oil) | Notes |
|---|---|---|
| Below 5 m/s (slow, heavy) | 220 to 460 | Thick film for slow, heavily loaded gears |
| 5 to 15 m/s (general industry) | 150 to 220 | The usual range; 3.9 m/s for the pinion of the earlier example (50 mm at 1 500 rpm) |
| Above 15 m/s (fast) | 68 to 100 | Less churning loss; may need pressure or spray lubrication |
| Worm gears | 220 to 460, with a compounded or PAG oil | Sliding action needs a high-viscosity oil; avoid EP additives that attack bronze |
| Oil base | Features | Use |
|---|---|---|
| Mineral (refined petroleum) | Economical; limits at about 80 to 90 °C continuously | Most gearboxes |
| Synthetic hydrocarbon (PAO) | Wide temperature range, long life, low friction (a few percent better efficiency) | Hot or cold, long drain intervals |
| Polyglycol (PAG) | Very low friction, good for worm gears; not miscible with mineral oils | Worm drives; careful with seals and paint |
| Ester, silicone, food-grade (H1) | Special: biodegradable, high temperature, food contact | Food plants, environmentally sensitive sites |
- Additives: EP (extreme pressure, sulfur-phosphorus) and anti-wear types form a protective film at the boundary; rust and oxidation inhibitors, anti-foam and demulsifiers protect the oil. Do not mix oils of different bases or from different makers.
- Viscosity index (VI) measures how little the viscosity changes with temperature; a high VI (synthetics) suits a wide temperature range.
- Cleanliness: filtered oil of ISO 4406 class 17/15/12 or better lengthens the life of the gears and bearings greatly.
How the oil is applied
| Method | How | Limits |
|---|---|---|
| Splash (oil bath) | The gear dips into the oil and throws it around the case; keep the level so that the largest gear dips about 1 to 3 times the tooth depth (or the module) | Pitch-line speeds up to about 12 to 15 m/s; churning loss rises with speed |
| Spray or pressure feed | A pump sends oil through jets at the mesh, with a filter and cooler | High speed and power; needs a pump and a circuit |
| Drip or wick | Oil trickles onto the gears | Light duty, open gears |
| Grease (packed or applied) | A semi-fluid in the case or on open gears | Slow speeds, small gearboxes, no cooling |
| Oil mist | Air carries a fine oil mist to the bearings | Very high-speed spindles |
- Oil level: too low starves the mesh; too high churns the oil, heats it and wastes power. Check with the sight glass or dipstick, at rest and at the correct temperature.
- Breather and seals: a breather lets the case “breathe” as it heats and cools, and keeps out dust and moisture; fit a desiccant breather in humid places. A leaking seal is a warning sign.
- Magnetic drain plugs collect ferrous wear: inspect them at each oil change.
- Drain intervals: by oil analysis, or the maker’s hours (typically 2 000 to 8 000 hours for mineral oil, much longer for synthetics).
Temperature, oil life and cooling
Oil oxidizes faster when hot: a common rule is that the life halves for every 10 °C above about 60 °C. A mineral oil with a nominal life of 20 000 hours at 60 °C would last about 5 000 hours at 80 °C: a quarter. Keeping the oil sump under 80 to 90 °C is a design target. The temperature is set by the heat produced (the power loss of the gears and bearings) and the heat the case can reject.
Worked example: sump temperature
The gearbox loses 88 W (Gear Trains, Ratios and Efficiency). A housing area of 0.3 m² with a convective coefficient of 15 W/(m²·K) loses it at a temperature rise ΔT = 88 ÷ (15 × 0.3) = 20 K. In a 30 °C room the case is near 50 °C and the oil a little hotter: comfortable. At 2.2 kW with 90 percent efficiency the loss is 220 W: ΔT = 49 K, an oil temperature near 79 °C, which would call for fins, a fan or an oil cooler.
Grease for gears and bearings
- Grease is oil held in a thickener (lithium, lithium complex, polyurea, calcium sulfonate). The consistency is given by the NLGI grade (000 semi-fluid to 6 hard): 2 for most bearings, 0 or 1 for centralized systems and gearboxes.
- Base-oil viscosity matters as for oil: a thicker oil for slow, heavily loaded parts.
- Compatibility: do not mix thickeners without checking (lithium and polyurea, for example, can soften and leak).
- Quantity: grease in a bearing: 30 to 50 percent of the free space (see Bearings and Bearing Selection); too much heats. In a small gearbox a semi-fluid grease can stand in for oil.
- Open gears and chains use special adhesive, solid-filled lubricants that cling and resist throw-off.
Gear failure modes
| Mode | What you see | Cause | Remedy |
|---|---|---|---|
| Pitting (initial and destructive) | Small craters near the pitch line; may heal (initial) or spread (destructive) | Contact stress above the surface fatigue strength (see Gear Strength and Design Calculations) | Harder surface (case hardening), larger gear, better lubricant and finish |
| Micropitting (gray staining) | A matt gray zone on the flank; tiny pits under magnification | Thin film, rough surfaces, EP additive effects | Smoother surfaces, higher viscosity, micropitting-tested oil |
| Spalling | Large flakes of material from the surface | Case too thin, overload, subsurface fatigue | Deeper case, better heat treatment, lower load |
| Scuffing (scoring) | Welded and torn streaks in the sliding direction on the tip and root | Film breakdown at high speed, load and temperature | EP oil, lower temperature, smoother surfaces, tip relief |
| Abrasive wear | Fine scratches, a polished worn profile | Dirt and particles in the oil | Filtration, seals, cleaner assembly |
| Tooth breakage (fatigue) | A crack from the root fillet across the tooth; a beach-mark surface | Bending stress too high or repeated overload, a stress raiser | More module or face width, shot peening, a larger root radius |
| Overload breakage | A single, fast fracture, with a rough surface | A shock load or jam | Torque limiter, higher safety factor, check the application |
| Plastic flow (rippling, rolling) | Deformed profile, burrs at the tip | Surface yielding under a high load in soft gears | Harder material, lower load |
| Corrosion, fretting | Rust, red-brown deposits | Water, acid in the oil, standing idle | Dry oil, inhibitors, preservation during shutdown |
Inspection and oil analysis
Condition monitoring catches a problem before the failure. A regular sample of the oil, sent to a laboratory or tested on site, shows what is happening inside the gearbox:
| Test | Typical limit | What a change means |
|---|---|---|
| Viscosity | within ±10 to 15 percent of the new oil | Wrong oil, oxidation (thicker), fuel or shear (thinner) |
| Water | below 0.1 percent (1 000 ppm); keep under 500 ppm for bearings | Leaks, condensation, a failing seal or breather |
| Iron (wear metal) | a rising trend; typically alarm above 100 to 200 ppm in a gearbox | Gear and bearing wear |
| Copper | a rising trend; alarm above 20 to 50 ppm | Bronze bush or worm wheel wear, cooler leaks |
| Acid number (TAN) | rise of more than 1 to 2 mg KOH/g over the new oil | Oxidized oil: change it |
| Cleanliness (ISO 4406) | target 17/15/12 or better for gearboxes | Dirt ingress, poor filtration |
- Vibration monitoring (see Force, Load Cells and Vibration) shows gear mesh and bearing frequencies growing with damage.
- Temperature, current and noise trends reveal slow changes: a rising oil temperature at the same load means more friction or less cooling.
- Visual inspection of the teeth through an inspection port or a borescope: look at the contact pattern, the pitting, scuffing and wear. Photograph and date it.
- Thermography finds hot bearings, a failing coupling and a blocked cooler.
- Record everything (oil type, amounts, dates, results): trends tell far more than single readings.
Symptom table for noisy or hot drives
| Symptom | Likely cause | Check |
|---|---|---|
| Gearbox runs hot | Oil level too high or low, wrong viscosity, overload, bearing preload, blocked cooling | Level, oil grade, load against rating, ambient and cooling |
| Whine at the mesh frequency | Tooth errors, misalignment, wear, a shaft deflecting | Contact pattern, alignment, backlash; helical gears are quieter (see Gear Types and Selection) |
| Rumble or growl, rises with speed | Bearing damage or lack of lubricant | Bearings by vibration; grease or oil supply |
| Clunking, rattling at low load | Backlash, loose coupling or key, torque reversal | Backlash, coupling, keys (see Shafts, Keys and Couplings) |
| Oil leaks | Worn or damaged seals, too much oil, a blocked breather | Level, breather, shaft finish under the seal |
| Metal particles on the magnet | Wear of the gears or the bearings | Amount and type of particles; oil analysis; stop if it is heavy |
| Frothy or milky oil | Water in the oil, overfilling, air ingress | Seals, breather, condensation; change the oil |
| Oil darkens and smells burnt | Oxidation from overheating | Temperature; change the oil and fix the cooling |
Related
- Gears and Power Transmission: the overview of this guide.
- Gear Strength and Design Calculations and Gear Types and Selection: stresses and gear types.
- Bearings and Bearing Selection and Chains and Sprockets: lubrication of bearings and chains.
- Workshop Safety: handling oils and lockout.
FAQ
How often should I change the oil in a gearbox?
Follow the maker, usually after the first 250 to 500 hours (running-in wear) and then every 2 000 to 8 000 hours for mineral oil, or longer for synthetics. Best is to base it on regular oil analysis and the operating temperature.
Can I mix different gear oils?
Avoid it: additives may react, and PAG and mineral oils do not mix. If you must top up in an emergency, use a small amount of the same grade, and change it as soon as possible.
What is an EP additive?
An extreme-pressure additive (typically sulfur and phosphorus compounds) that reacts with the metal surface under the heat of heavy contact to form a thin sacrificial layer, preventing scuffing in boundary lubrication. Some EP additives are not suitable for bronze (worm gears).
Why does my gear get pitting even though the oil level is right?
Pitting is a fatigue of the surface from the contact stress. The oil only helps: a heavier viscosity and a smoother surface help, but if the contact stress is above the material’s limit, the gear needs to be bigger, harder or case-hardened.
References
- AGMA 9005-E02, Industrial gear lubrication; ISO 12925-1 (lubricants for enclosed gear drives); ISO 6336 (calculation of load capacity, micropitting).
- Lubricant manufacturers’ gear-oil guides (Mobil, Shell, Klüber, Castrol); Stachowiak, G. W. and Batchelor, A. W., Engineering Tribology, Butterworth-Heinemann.
- Hamrock, B. J., Schmid, S. R. and Jacobson, B. O., Fundamentals of Machine Elements (EHL film thickness).
Last reviewed: October 2026. Viscosity grades, limits and intervals are typical; follow the gearbox maker’s instructions and the lubricant data.