Fan bearing load calculation (overhung impeller)
Work out the load on each bearing of an overhung centrifugal fan from the impeller mass, the shaft geometry, the drive pull and the pressure, then check bearing life. The Rotec AI worked template does every step.
What loads a fan bearing
- Impeller weight hangs beyond the nearer bearing, so it acts like a lever and loads that bearing with more than the weight itself.
- Unbalance from dust build-up, erosion or repair adds a rotating force in proportion to the weight and the square of speed.
- Drive pull: a coupling adds a small radial force; a belt adds a large one.
- Shaft and coupling or sheave weight sit between and beyond the bearings.
- Thrust from the pressure across the fan pushes the shaft along its axis.
The method in five steps
- Torque and drive force. Torque in N·m = 9,550 × power in kW ÷ speed in rpm. For a belt, the tangential force is 60,000,000 × P ÷ (π × D × n) newtons and the radial belt load is a type factor k times that. For a coupling use the coupling maker's radial force, or the empirical rule F = constant × √(P/n).
- Impeller force. Weight F1 = mass × 9.81. Unbalance F2 = a constant (about 0.9 is a common rule of thumb) × F1. Impeller force on the shaft Fm = reduction factor × (F1 + F2).
- Reactions by moments. With bearing A nearest the impeller and B on the drive side, take moments about each bearing. The reaction at A is Fra = (Fm × (a + b) + Qs × d − (F + Qc) × c) ÷ b, and at B Frb = (Qs × (b − d) + (F + Qc) × (b + c) − Fm × a) ÷ b. Here a is impeller to A, b is A to B, c is B to the coupling or sheave, d is the shaft weight position from B, Qs the shaft weight and Qc the coupling or sheave weight.
- Direction. The drive pull can act in any direction, so check them all. The reaction can be much larger one way than the other, and the load on B can reverse.
- Thrust. Fa = π ÷ 4 × inlet diameter² × pressure difference ÷ 10⁶ with diameter in mm and pressure in pascals gives newtons. This is a conservative estimate.
Worked example
Coupling-driven fan, 500 kW at 2,980 rpm, impeller mass 400 kg, shaft 750 N, coupling half 500 N. Geometry: a = 207 mm, b = 436 mm, c = 207 mm, d = 218 mm. Inlet 450 mm, pressure 40,000 Pa. Unbalance constant 0.9 and reduction factor 0.75.
| Quantity | Value |
|---|---|
| Torque | 1,602 N·m |
| Coupling radial force F | 2,294 N |
| Impeller weight F1 / unbalance F2 | 3,924 N / 3,532 N |
| Impeller force Fm | 5,592 N |
| Reactions with the coupling force down (A / B) | 7,295 N / 1,841 N |
| Reactions with the coupling force up (A / B) | 9,473 N / 4,925 N (load reverses) |
| Thrust Fa | 6,362 N |
| Life of bearing A (22224 E, thrust on A, shock factor 1.1) | about 84,000 hours |
The lesson: with a realistic impeller mass the load on the impeller-side bearing is several times higher than a quick weight-only estimate, and the load on the other bearing swings through zero. Check both directions.
Check the inputs
- Impeller mass. Take it from the drawing. A guess is the largest source of error.
- Pressure. 1 mm water gauge is 9.81 Pa. Standard single-stage fans of this class are usually under about 1,000 mm water gauge (9.8 kPa); a figure of tens of kilopascals needs checking.
- Speed. Pressure and thrust go with the square of speed, so recheck the loads after any speed change.
Do it in Rotec AI
The licensed Load Calculator has two worked templates (a free preview shows the layout and an example) for overhung fans, one for a coupling drive and one for a belt drive. They take the geometry, sweep the drive force through every direction, work out the thrust, and give the equivalent load and life of both bearings from the tool's own bearing tables. More templates are being added.
Related tools and guides
Questions
How do I calculate the load on the bearings of an overhung fan?
Take moments about each bearing using the impeller force, shaft weight, drive pull and their distances, then add the thrust from the pressure on the inlet. Check the drive pull in every direction. The worked template does these steps for you.
Why is the bearing next to the impeller so heavily loaded?
The impeller hangs beyond that bearing, so its weight and unbalance act through a lever arm. The reaction at the nearer bearing is the impeller force times (a + b) divided by b, which is always more than the impeller force.
Can the load on a fan bearing reverse?
Yes. On an overhung fan the far bearing can be pushed up by the impeller and pulled down by the drive. Check the bearing for load in both directions and keep enough load on it to avoid skidding.
How do I find the thrust on a fan?
Multiply the pressure difference across the fan by the inlet area: thrust in newtons = π ÷ 4 × inlet diameter in mm squared × pressure in pascals ÷ 1,000,000. It is a conservative estimate.
Results are starting points from standard engineering practice. Confirm critical selections with the bearing or lubricant maker.