A fan bearing failed at 9 months in a food plant. The fan ran at 1450 RPM, 24/7, in a warm area (40°C ambient). The bearing was a 6206 with a metal shield, greased at the factory. The maintenance crew greased it every 6 months with a multipurpose lithium grease from a gun. The bearing seized with the grease cooked to a hard cake. The grease was the wrong grade for the speed and temperature. The bearing wasn’t the problem.
The three grease specs that matter
Bearing grease has three numbers worth understanding before you pick a tube.
1. NLGI consistency grade. This is the grease’s stiffness, measured on a scale of 000 to 6. NLGI 2 is the standard for most ball and roller bearings — thick enough to stay in the bearing, soft enough to flow into the rolling elements. NLGI 1 is softer, for cold conditions or long grease lines. NLGI 3 is firmer, for high-speed bearings or vertical shafts where you don’t want the grease running out. The multipurpose grease the crew used was NLGI 2 — that part was fine.
2. Base oil viscosity. The grease is a sponge of base oil with a thickener. The base oil viscosity (ISO VG number) is what actually lubricates the rolling contact. The rule of thumb: base oil viscosity should be at least 13 cSt at the operating temperature for a ball bearing. A 6206 at 1450 RPM with a 1000 N radial load needs an ISO VG 32-68 mineral oil. Multipurpose greases are typically ISO VG 100-150 — too thick for a high-speed fan, which makes the grease work harder and heat up.
3. Operating temperature range. The grease’s temperature rating is the drop-point (where the thickener melts) and the continuous operating limit. Standard lithium grease runs -20°C to +120°C. In the 40°C ambient, the bearing runs at about 65°C surface temperature — well within lithium’s range. Temperature wasn’t the killer directly. It was the combination of the wrong base oil and over-greasing.
Why the grease cooked
The crew greased the shielded 6206 with a grease gun every 6 months. A shielded bearing (metal shield, no grease fitting) doesn’t need external greasing — the factory charge is designed to last the bearing’s life. The crew’s grease gun forced grease past the shield and packed the bearing. The bearing cavity filled. The rolling elements churned the grease, which heats it. Hot grease thins and leaks out past the shield… or in a sealed area, the churning heat cooks the grease into a cake. The 6206 was a metal-shielded bearing with no drain — the grease had nowhere to go. The churning heated the grease past 90°C, the oil separated, and the bearing seized.
The right grease for the fan
The replacement bearing went in with the factory charge and a written “do not regrease” note on the maintenance schedule. If regreasing had been necessary (an open bearing with a fitting), the correct grease would be a lithium complex NLGI 2 with ISO VG 68 base oil, rated for the 5000 RPM speed class. The multipurpose lithium grease in the gun was a truck wheel bearing grease — fine for slow shafts, wrong for a 1450 RPM fan.
The grease selection rule for electric motors and fans: use an electric-motor-rated grease (like a polyurea NLGI 2 with ISO VG 32-68). Polyurea grease handles the higher speeds, resists oil separation, and lasts longer between relubes. It costs 2-3x the multipurpose lithium. For a $30 bearing that fails at 9 months costing $200 in downtime, the $40 grease cartridge is the cheaper answer.
The relube interval formula
The regreasing interval for a bearing is a function of speed, size, and temperature. A practical formula (from the bearing manufacturers’ consolidated data):
t = 1000 × (d/D) × (10^7 / n) × (1 for T<70°C, 0.5 for 70-90°C, 0.25 above 90°C)
where d is the bearing bore in mm, D is the outside diameter in mm, and n is RPM. For a 6206 (d=30, D=62) at 1450 RPM, 65°C:
t = 1000 × (30/62) × (10^7 / 1450) × 1 = 1000 × 0.484 × 6897 = 3338 hours
That’s about 4.5 months at 24/7 operation. The fan should have been regreased at 4 months — with the right grease, and only a small amount (about 20% of the free space). The crew greased it at 6 months with the wrong grease and too much of it. Both mistakes compounded.
The grease amount rule
For a bearing with a grease fitting, the relube amount is about 20-30% of the free space in the bearing housing, applied with the bearing running (grease spreads by the rolling action). Over-greasing is worse than under-greasing — the churning heat is the number one cause of premature bearing failure. A 6206 in a pillow block takes about 10 grams. Two pumps from a standard grease gun (which delivers 5-8 grams per pump) is enough. The crew’s “one good pump until it squirts out the seal” was the opposite of the rule.
The sealed bearing alternative
For a fan that runs 24/7 with minimal access, the better answer is a sealed bearing (2RS) with a factory lifetime charge. The seal keeps the grease in and the contamination out. The life is about 20,000 hours at the fan’s load — 2.5 years at 24/7. The shielded 6206 needed the maintenance crew to get the relube right, which they didn’t. The sealed version removes the human error. The next fan used sealed bearings and the maintenance schedule doesn’t touch them.
The fan bearing failed from the wrong grease and too much of it. Multipurpose lithium from a gun, pumped into a shielded bearing that didn’t need it, at the wrong interval. Match the base oil viscosity to the speed, use electric-motor-grade grease, relube at the formula interval, and 20% of the free space is plenty. Or buy a sealed bearing and take the maintenance decision away entirely.
