For mill gearboxes, the first oil change shall be performed at 500 running hours as a baseline. Under normal operating conditions, the recommended interval is 2,500–4,000 hours / 12 months for mineral oil, and 6,000–8,000 hours / 24 months for synthetic oil (whichever comes first). The interval shall be shortened by 20%–50% under harsh conditions, with oil condition analysis as the final judgment basis.
1. Baseline Oil Change Intervals (By Running Hours / Time)
| Equipment Stage | Recommended Interval for Mineral Oil | Recommended Interval for Synthetic Oil | Application Scenario |
|---|---|---|---|
| New unit / Overhauled | 500 hours (mandatory, to remove run-in metal debris) | 500 hours (mandatory) | New equipment commissioning, initial lubrication after overhaul |
| Normal continuous operation | 2,500–4,000 hours / 12 months (whichever comes first) | 6,000–8,000 hours / 24 months (whichever comes first) | Mills at normal temperature, medium load, clean environment (e.g. Raymond mills, ball mills) |
| Normal intermittent operation | 4,000–5,000 hours / 12–18 months | 8,000–12,000 hours / 24–36 months | Mills operating ≤8 hours per day |
2. Condition Adjustment & Special Requirements
2.1 Shorten Intervals for Harsh Operating Conditions
- High temperature (oil temperature ≥60°C), dusty or humid environment: shorten interval by 20%–30% (mineral oil: 1,500–2,500 hours / 6 months; synthetic oil: 3,000–5,000 hours / 12 months)
- Frequent start-stop and heavy shock loads: shorten interval by 30%–50% (mineral oil: 1,000–2,000 hours; synthetic oil: 3,000–6,000 hours)
- Outdoor / high-dust mills: prioritize shortening to 1,500 hours / 6 months, with enhanced filtration
2.2 Recommendations by Mill Type
- Raymond mill: first change at 500 hours, subsequent intervals 2,000 hours / 6 months (mineral oil), 5,000 hours / 18 months (synthetic oil)
- Ball mill (heavy-duty continuous operation): mineral oil 2,000–3,000 hours / 6–12 months, synthetic oil 5,000–8,000 hours / 18–24 months
- Vertical mill reducer: first change at 1,000 hours, subsequent interval 2,500 hours (mineral oil)
3. Oil Quality-Based Change Criteria (Priority Over Fixed Intervals)
Immediate oil change is required if any of the following indicators are out of specification (per NB/SH/T 0586-2024):
- Viscosity variation rate exceeds ±15%–20% (e.g. ISO VG220 oil <187 or >253)
- Total Acid Number (TAN) increase >0.5 mgKOH/g (severe oxidation)
- Moisture content >0.5%, emulsification / sludge formation, or mechanical impurities >0.5%
- Iron content >50 ppm with continuous upward trend (sign of gear/bearing wear)
- Abnormal rise in operating noise (above 85 dB), vibration acceleration >5 m/s², or abnormal oil temperature rise
4. Practical Operation Guidelines
- Oil Change Procedure: Drain oil when temperature drops to 40–50°C after shutdown, drain old oil completely and clean the filter; fill new oil to the marked level, avoid mixing different oils.
- Maintenance Suggestions: Inspect oil quality every 3 months, take oil samples for testing every 6 months; install desiccant breathers to reduce dust and moisture ingress.
- Special Notes: For mills idle for more than 12 months, oil must be changed before restart; extended interval by 30%–50% is allowed for mills with circulating lubrication systems.
5. Quick Decision Table
| Operating Condition | Mineral Oil Interval | Synthetic Oil Interval |
|---|---|---|
| New unit / Overhauled | 500 hours | 500 hours |
| Normal continuous (general mills) | 2,500–4,000 h / 12 months | 6,000–8,000 h / 24 months |
| High temperature / Dusty / Frequent start-stop | 1,500–2,500 h / 6 months | 3,000–5,000 h / 12 months |
| Trigger conditions for quality-based change | Viscosity ±15%, TAN >0.5, Fe >50 ppm, etc. | Same as above |
