Viscosity — The Most Important Lubricant Property
Viscosity is the measure of a lubricant's resistance to flow. Too low a viscosity at operating temperature = film breakdown, metal-to-metal contact, accelerated wear. Too high = excessive heat generation, energy waste, sluggish circulation.
Kinematic viscosity is measured in mm²/s (centistokes, cSt). The standard measurement temperature is 40°C for ISO grades. Industrial lubricant viscosity grades are standardised by ISO 3448 (ISO VG system):
| ISO VG Grade | Nominal viscosity at 40°C (cSt) | Typical application |
|---|---|---|
| VG 10 | 10 | Precision instruments, air tools, chain lubrication |
| VG 22 | 22 | Light spindles, small air compressors |
| VG 32 | 32 | Hydraulic systems (moderate pressure), vane compressors |
| VG 46 | 46 | Hydraulic systems (high pressure), screw compressors |
| VG 68 | 68 | Circulating systems, electric motor bearings (oil-lubricated), light gearboxes |
| VG 100 | 100 | General industrial bearings, reciprocating compressors |
| VG 150 | 150 | Medium-speed gearboxes, worm gears (small), enclosed gear drives |
| VG 220 | 220 | General industrial gearboxes — most common grade for enclosed spur/helical gearboxes |
| VG 320 | 320 | Heavy duty gearboxes, spiral bevel gearboxes |
| VG 460 | 460 | Very slow speed or heavily loaded gearboxes, worm gears (medium) |
| VG 680 | 680 | Slow-speed heavily loaded gears, large worm gear drives |
Oil vs Grease for Rolling Element Bearings
The choice between oil lubrication and grease lubrication for rolling element bearings:
| Factor | Grease preferred | Oil preferred |
|---|---|---|
| Speed (ndm value) | ndm < 500,000 mm·RPM | ndm > 500,000 mm·RPM (high-speed spindles) |
| Sealing requirement | Grease acts as a seal — low ingress environments | Sealed housing required if oil used |
| Heat dissipation | Normal operating temps < 80°C housing | High heat generation — circulating oil removes heat |
| Adjacent gearbox | — | If adjacent gearbox already uses oil, simplify with oil splash |
| Relubrication access | Remote regreasing systems (zerk fittings) easy to implement | Oil level gauge and drain required |
Base Oil Types
| Base oil type | Temperature range | Advantages | Limitations |
|---|---|---|---|
| Group I mineral (solvent refined) | −20 to +100°C | Low cost; widely available | Limited oxidative stability; high pour point |
| Group II mineral (hydrotreated) | −25 to +110°C | Better oxidation resistance than Group I; mid-range cost | — |
| Group III (VHVI / semi-synthetic) | −30 to +120°C | High VI (viscosity index); good low-temp flow | Compatibility check needed for seals |
| PAO (polyalphaolefin) — Group IV | −50 to +150°C | High VI; excellent oxidation resistance; long life | High cost; dissolves some seal materials — check seal compatibility |
| Ester — Group V | −40 to +180°C | Very high temperature; biodegradable options; high film strength | Higher cost; hygroscopic (absorbs water) |
Contamination Control
Particle contamination is the leading cause of premature bearing failure. The critical zone: particles in the 1–10 µm range (similar to the bearing raceway EHD film thickness) cause the most damage — they are too large to pass through the film and too small for most filters to catch efficiently.
ISO 4406 cleanliness code system (e.g., 17/15/12 = particles >4 µm / >6 µm / >14 µm per mL) guides contamination targets:
- Hydraulic servo systems: ISO 15/13/10 or better
- Standard hydraulic systems: ISO 17/15/12
- Industrial gearboxes: ISO 18/16/13
- Circulating bearing systems: ISO 17/15/12
Contamination control measures: breathing filters (desiccant breathers) on reservoir caps; offline kidney-loop filtration for critical systems; correct storage of lubricants (sealed drums, climate-controlled storage, clean transfer equipment); clean fill equipment; sample ports for oil analysis trending.
Oil Change Intervals
Oil change intervals should be based on oil analysis results rather than fixed time intervals wherever possible. Typical starting points:
| System type | Mineral oil (hrs) | Synthetic (hrs) | Trigger for early change |
|---|---|---|---|
| Industrial gearbox — closed, clean | 2,000–4,000 | 8,000–16,000 | Viscosity ±15%, TAN rise >1.5×, water >0.05% |
| Rotary screw compressor | 2,000–4,000 | 6,000–8,000 | Viscosity change, TAN, foam tendency |
| Hydraulic system | 2,000–4,000 | 8,000+ | Particle count, viscosity change, water ingress |
| Circulating oil bearing system | Annually or by analysis | 2× mineral interval | Oil analysis — TAN, viscosity, wear metals |