The speed factor ndm: the first selection criterion
The ndm factor (speed in rpm × mean bearing diameter in mm) determines the speed regime and, with it, the most appropriate base viscosity and NLGI grade. At higher ndm, lower viscosity is needed so the grease does not generate heat through churning; at lower ndm, higher viscosity is needed to maintain the lubricating film under load.
Practical reference: ndm < 75,000 → NLGI 2-3; ndm 75,000-300,000 → NLGI 1-2; ndm > 300,000 → NLGI 1 or oil. Many high-speed bearing failures are due to using NLGI 3 where NLGI 1 should be used.
The correct quantity: neither more nor less
The amount of grease in a bearing should fill 30% to 50% of the internal free space. More quantity generates rotation resistance, heat, and pressure on seals. Less quantity leaves zones without a lubricating film.
For bearings in horizontal position at medium speed, the practical formula is: grease volume (cm³) = 0.005 × D × B, where D is the outer diameter of the bearing and B its width, both in mm. For vertical or high-speed bearings, reduce to 30% of free space.
An automatic greaser with calibrated volume is the most effective solution to eliminate human error in quantity. Cost: €15-50 per point. Saving: one bearing.
Relubrication intervals: how to calculate them correctly
Bearing manufacturers publish relubrication interval tables based on 70°C temperature, normal load, and dry environment. In industrial practice, these conditions are rarely all present simultaneously.
Temperature correction factor: for every 15°C above 70°C, halve the interval. At 100°C, the interval is 1/4 of nominal. At 120°C, 1/8. Correction factor for humidity or vibration: reduce by an additional 30-50%.
- 70°C → manufacturer's nominal interval
- 85°C → interval × 0.5
- 100°C → interval × 0.25
- 115°C → interval × 0.125
- High humidity → multiply by an additional 0.5-0.7
Thickener compatibility: the error that takes weeks to manifest
Mixing incompatible greases can destroy the structure of both. The result is a liquefied mass that abandons the bearing without protecting it. The problem is that the failure is not immediate: it may take 2-6 weeks to manifest, during which time the bearing operates without effective lubrication.
Before changing grease or supplier, always verify the compatibility of the new thickener with the residue that will remain in the bearing. If no compatibility data is available, clean the bearing completely before applying the new grease.
Warning signs in bearing lubrication
- Temperature above normal in the first few minutes after relubrication: over-greasing.
- Grease leaking through seals: over-greasing or grease too soft (low NLGI).
- Metallic noise or increased vibration: insufficient lubrication or degraded grease.
- Dark-coloured grease or with visible metallic particles: advanced wear, analyse.
- Rising temperature without apparent cause: beginning of failure, intervene.
Bearing lubrication is not about applying grease occasionally. It is about selecting the correct grease based on ndm and temperature, applying the correct quantity, respecting intervals corrected for actual conditions, and verifying compatibility before each product change. With these four factors controlled, a bearing can reach or exceed its nominal service life.
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