Fan bearing hot from lubrication and cooling: the grease, the oil and the heat path
Shallow oil level, blocked feeds, wrong lubricant for hot gas, pumped-out grease and the habit of over-greasing.
Check the lubrication system first
On oil-lubricated housings check the oil level (is the reservoir too shallow?), the oil pick-up ring (missing or damaged?), and any pumps, piping, filters and flow in a circulation system. On grease-lubricated housings check the fill, the relubrication quantity and interval and whether the feed point is blocked. If lubricant is leaking from the housing, see the leakage article.
Relubrication practice
- Tag grease nipples with the right product name and use a separate grease gun for each grease type.
- Adapt relubrication intervals to the level of dust, moisture, temperature and other conditions, and calculate them with an interval tool.
- Use the recommended quantity: over-greasing makes heat by churning and damages seals.
- If a bearing seems hot, check temperature and trend before adding more grease.
Wrong lubricant choice
Hot gas fans test a lubricant hard. If the lubricant is not in the maker's documents, check that the one in use has similar properties to the one recommended, and compare the actual running conditions with the design conditions. If the speed is too high for the lubricant, a bearing for higher speed or thin-film lubrication may be needed, such as a hybrid (ceramic ball or roller) bearing.
Not enough heat removal
High air velocity near the housing can suck grease out of a bearing; differences in pressure across the bearing can pump it out. Remedies:
- Clean the oil filters and heat exchangers.
- Fit a cooling disc on the shaft or another cooling device if the housing sits in hot gas.
- Fit a balancing vent pipe from the labyrinth on one side of a split housing to the seal on the other to equalise the pressure.
- If the problem persists consider a change from grease to oil lubrication, and a different housing.
Hot gas along the shaft
Hot gas that leaks from the impeller along the shaft to the bearing needs a better casing seal and a cooling disc.