Fan and blower bearings: arrangement, lubrication and failures
A fan is a big rotating mass on a shaft that stretches as it heats. The bearing arrangement has to hold the shaft in one place and let it grow in the other direction. Most fan trouble starts with dirt building up on the blades, which unbalances the wheel and shakes the bearings.
Typical duty
| Speed | Load | Temperature | Typical life |
|---|---|---|---|
| 600 – 3,600 rpm | Moderate radial, usually small axial | Ambient to 200 °C or more on hot gas fans | 40,000 – 100,000 h typical design target |
How the bearings are arranged
Double-inlet fan · two pillow blocks
Used when: Large fans with the wheel between the bearings
The fan wheel sits in the middle with a bearing on each side. One bearing is locked in its housing and holds the shaft’s position. The other bearing can slide along the shaft’s direction of growth. Both are self-aligning spherical roller bearings, which forgive a little misalignment between the housings.
Overhung fan · impeller outside the bearings
Used when: Single-inlet and smaller fans with the wheel on the end of the shaft
The wheel hangs off the end of the shaft like a flag on a pole. This tips the shaft in the bearing and creates a high sideways load on the bearing closest to the wheel. That is why the roller bearing is placed there.
Common failures and how to prevent them
| Failure | What you see | Why it happens | Prevention |
|---|---|---|---|
| Fatigue from unbalance Subsurface-initiated fatigue | Early spalling at the bearing nearest the wheel, high vibration at running speed. | Dirt build-up, erosion or a missing balance weight turns the wheel into a large rotating load. | Clean blades regularly, monitor vibration trends, rebalance after repairs. |
| Fretting under the sleeve or on the shaft Fretting corrosion | Brown-red powder, polished areas on the bore or shaft. | Loose fit of the adapter sleeve or bearing on a shaft that flexes, or insufficient locknut tightening. | Check the clearance reduction when driving the bearing up, tighten the nut to the specification, and lock it with the tab washer. |
| Overheating from too much grease or wrong grease Overheating & lubricant breakdown | Hot bearing housing, grease pushed out of the seals. | Over-greasing, or a standard grease on a hot fan. | Fill the housing only 30 to 50% full and use a high-temperature grease on hot fans. |
| Misaligned housings Misalignment & distorted seats (edge loading) | Off-centre wear track and uneven temperatures between the bearings. | Pillow blocks not aligned with each other, soft foot, or a distorted frame. | Align the two housings together, check for soft foot, and look at the frame for movement. |
| Dust ingress Abrasive wear | Scored raceways and gritty grease. | Worn seals in a dusty plant. | Replace seals regularly, fit a labyrinth with a grease purge, and keep the housing clean. |
| Skidding at the lightly loaded bearing Adhesive wear (smearing & skidding) | Smeared bands on the rollers or raceway on a lightly loaded bearing. | Roller bearing running below its minimum load at high speed. | Check the minimum load for the bearing and fit a smaller bearing or add preload if needed. |
Lubrication
Grease
A lithium-complex or polyurea grease with EP additives in NLGI grade 2 is common. Hot fans need a high-temperature grease. Do not mix thickeners.
Initial fill
Fill the bearing completely and 30 to 50% of the free space in the housing. More makes it hot.
Interval
Typically a few months at 1,500 rpm in clean conditions. Heat, dirt and humidity shorten it. The interval roughly halves for every 15 °C above 70 °C.
Sealing
- Labyrinth seals: Non-contact, with a grease-filled maze. Good for dusty plants and high temperature because nothing rubs.
- Felt and lip seals: Simple and cheap, but they wear and dry out. Replace at every overhaul.
- Heat slinger and cooling fins: On hot gas fans a slinger or finned disc on the shaft throws heat away before it reaches the bearing.
What to watch
- Vibration at running speed and its trend. Blade fouling shows up as a slow rise.
- Housing temperature at both bearings, and whether the floating end really floats after the fan has been hot.
- Gap at the floating end after a thermal cycle.
Questions
Which bearings are used in fans and blowers?
Double-inlet fan · two pillow blocks: Large fans with the wheel between the bearings. Overhung fan · impeller outside the bearings: Single-inlet and smaller fans with the wheel on the end of the shaft.
What are the common bearing failures in fans and blowers?
Fatigue from unbalance: Dirt build-up, erosion or a missing balance weight turns the wheel into a large rotating load. Fretting under the sleeve or on the shaft: Loose fit of the adapter sleeve or bearing on a shaft that flexes, or insufficient locknut tightening. Overheating from too much grease or wrong grease: Over-greasing, or a standard grease on a hot fan.
How should fans and blowers bearings be lubricated?
Grease: A lithium-complex or polyurea grease with EP additives in NLGI grade 2 is common. Hot fans need a high-temperature grease. Do not mix thickeners. Initial fill: Fill the bearing completely and 30 to 50% of the free space in the housing. More makes it hot.