Types of gears and their lubrication needs
- Helical gears: rolling contact with moderate sliding. Accept mineral or synthetic EP oils. Viscosity according to AGMA or ISO standard.
- Bevel and hypoid gears: more sliding than helical. Require more aggressive EP. Sensitive to micropitting.
- Worm gears: pure sliding contact. Require specific formulation without active sulphur (bronze wheel protection).
- Open gears: no housing or oil retention. Require adhesive greases or oils with high adhesive content.
- Planetary gears: high speeds and loads in confined space. Frequently require PAO synthetic oils.
The AGMA standard and how to use it to select viscosity
AGMA standard 9005-F16 establishes recommended viscosity based on gear pitch line speed and operating temperature. The AGMA table classifies oils in EP grades (0 to 8) which correspond approximately to ISO VG 46 (EP 0) to ISO VG 1500 (EP 8).
For a helical gear reducer with moderate pitch line speed (3-10 m/s) and operating temperature 60-80°C, AGMA grade 4-5 (ISO VG 150-220) is usually the standard recommendation. Higher speed = lower viscosity; higher load = higher viscosity.
Always prioritise the reducer manufacturer's specification over generic tables. Manufacturers know the tolerances, materials, and design of their equipment and publish the optimal viscosity in the manual.
Micropitting: the most silent gear failure
Micropitting is a progressive failure mode in hardened steel gears that creates a matt appearance on the tooth flanks. It is caused by surface fatigue under boundary lubrication conditions — when the oil film is insufficient to completely separate surfaces.
Conventional mineral oils with sulphur-phosphorus EP additives can increase the risk of micropitting in hardened steel gears. PAO synthetic oils with modern low-friction EP packages offer better protection against micropitting.
Real drain intervals in industrial reducers
- Mineral oil in standard reducer: 4,000-6,000 hours or 1-2 years.
- HC mineral oil in reducer with temperature <80°C: 6,000-8,000 hours.
- PAO synthetic oil in reducer with analytical monitoring: 10,000-20,000 hours.
- Always change oil if analysis indicates viscosity out of specification, elevated TAN, wear metals on upward trend, or water contamination.
- Oil analysis is the only tool that allows the interval to be safely extended.
Industrial gear lubrication is not complicated, but it requires the right decisions at three moments: initial selection (type, viscosity, base), in-service maintenance (level, temperature, analysis), and drain interval (based on analysis, not just calendar). With those three points well managed, a reducer can exceed its nominal service life.
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