Maintenance, Lubrication and Energy/Consumption Calculations
Maintenance questions cover maintenance strategy, lubrication selection and energy or cost calculations. Reliability numbers and kWh costs are common.
Maintenance and reliability
- Preventive: scheduled servicing before failure. Predictive: condition-based checks such as vibration, thermography and oil analysis. Breakdown: run to failure, acceptable only for non-critical, cheap items. Total productive maintenance builds operator-level care and overall equipment effectiveness.
- MTBF = total operating time ÷ number of failures. MTTR = total repair time ÷ number of repairs. Availability = MTBF ÷ (MTBF + MTTR).
- Lubrication: oil for high speed and cooling, grease for sealed, slow or high-load points; viscosity grade is ISO VG for industrial oil and SAE for engine oil; too much grease churns and overheats.
Energy calculation
Energy in kWh = kW × hours × load factor. Cost = kWh × tariff. For a motor, use input power, not nameplate output alone.
Worked example
A 15 kW motor runs 8 h/day, 25 days/month at an average load factor of 0.6. Energy = 15 × 0.6 × 8 × 25 = 1,800 kWh/month. At ₹8.50/kWh, energy cost = ₹15,300. If a variable-speed drive and correct belt tension cut average load factor to 0.5, energy = 1,500 kWh and saving = 300 kWh = ₹2,550/month. Simple payback = installation cost ÷ monthly saving.
Traps
- Using rated power as actual power.
- Confusing MTBF with service life, or MTTR with total downtime including spare-part waiting.
- Mixing kWh (energy) with kVAh (apparent energy) or kW (demand).
- Over-greasing bearings.
- Ignoring no-load losses when estimating savings.