Tribology Calculator
Calculate ASTM-style viscosity index, FAG/SKF grease relubrication hours, and ball/roller bearing friction torque. Lubrication and maintenance design tool.
Introduction
The Tribology Calculator bundles three friction-and-lubrication tools a maintenance or design engineer reaches for constantly. Mode one computes Viscosity Index from kinematic viscosity at 40 °C and 100 °C, telling you how stable an oil's film is across temperature. Mode two estimates grease relubrication intervals for rolling bearings from bore diameter, speed, and operating temperature using the FAG/SKF speed-factor method. Mode three converts bearing load into friction torque so you can size drive motors and estimate heat generation. Together they answer the three questions that dominate bearing upkeep: which lubricant, how often, and at what running cost.
How it Works
Viscosity Index follows the ASTM D2270 Dean–Davis procedure, computing the L and H reference viscosities from kv100 and interpolating your kv40 between them. The grease interval implements tf = K × (14×10⁶ ÷ (n√d) − 4d) hours, where n√d is the speed factor, with K = 1.0 for ball bearings and 0.5 for roller bearings; the base interval is then halved for every 15 °C above 70 °C operating temperature. Friction torque uses M = μ × P × d ÷ 2 with default coefficients of 0.0015 (ball) and 0.0020 (roller).
Usage Scenarios
- Building a PM schedule: converting each bearing's bore, speed, and housing temperature into a concrete grease interval instead of copying a generic monthly default.
- Comparing two hydraulic oils whose datasheets quote different kv40/kv100 pairs — the computed VI shows which one holds viscosity better through a cold-start-to-hot-run cycle.
- Explaining why a gearbox bearing running at 85 °C needs regreasing twice as often as its 70 °C twin (one 15 °C halving of the base interval).
- Estimating drive sizing margin: summing friction torque across a conveyor's bearing stations to check motor torque reserve at startup.
- Auditing an overheating spindle: a high speed factor n√d shrinks the computed interval enough to reveal the existing schedule was starving the bearing of grease.
FAQ
What does a higher Viscosity Index actually mean?
Less viscosity change with temperature. A VI above roughly 95 indicates the oil thins slowly as it heats, holding film thickness through warm-up — critical for machines that cycle between ambient and operating temperature.
Why do roller bearings get half the grease interval of ball bearings?
Line contact in rollers churns and shears grease harder than ball point contact, so the method applies K = 0.5 for rollers versus 1.0 for balls at the same speed factor.
How strongly does temperature affect relubrication?
Above 70 °C the interval halves every 15 °C, reflecting accelerated base-oil oxidation. A bearing at 100 °C gets only a quarter of its 70 °C interval.
Is the friction torque estimate adequate for motor sizing?
It captures steady-state rolling friction well. Add seal drag, lubricant churning at high speed, and startup breakaway allowances before finalizing drive selection.