A plain bearing runs in one of three regimes: boundary, where metal touches through a thin oil film, mixed, where the film is partial, and hydrodynamic, where the shaft floats on oil and wear nearly stops. Which regime it reaches depends on speed, load, oil thickness and temperature together, so any load or speed limit quoted for a bearing is only true for the oil and the cooling it was quoted with.
Part of Bearings and Bushings
How an oil film forms between shaft and bearing, what destroys it, and how to read the regime a bearing is in from how it behaves.
The Shaft Passes Through All Three Regimes At Every Start
At rest the shaft sits on the bearing with no oil between the high points: that is boundary lubrication. High friction, high wear, acceptable only for slow or intermittent motion. As the shaft turns it drags oil into the narrow wedge-shaped gap between shaft and bearing, and the pressure that builds in the wedge starts to lift the shaft. Partway through that process the film is partial, which is mixed lubrication, and most bushings spend their working lives there. Once the film fully separates the surfaces the shaft floats and friction and wear are at their minimum: hydrodynamic lubrication.
Three things move a bearing between regimes. More speed drags in more oil, so the film thickens. A thicker oil carries a thicker film. More load squeezes the film thin. The film grows with speed and oil thickness and shrinks with load, which is why a heavy shaft that never gets fast enough sits in mixed lubrication permanently.
Heat Turns A Working Film Into A Failing One
Oil thins as it warms. A thinner oil carries a thinner film, a thinner film lets more metal touch, and more contact makes more heat. That loop is how a bearing that is fine at one speed seizes at a higher one. The lubricant’s own limit sets where the loop starts. The lubricant table in the carrier article gives:
| Lubricant | Temperature limit |
|---|---|
| Tallow | 40°C |
| Lard oil | 50°C |
| Castor oil | 80°C |
| Mineral oil | 120°C and up |
Lubrication gives tallow a melting range of 40-50°C, so the figures agree. The consequence is that one bearing at one load and speed has different safe limits on tallow and on castor oil. A load or speed rating is therefore never a property of the bearing alone.
What You Need
- A bearing with a known diameter, length and clearance
- A lubricant with a known temperature limit (see Lubricant Types)
- A way to carry heat away: a ring oiler or drip oiler moves oil through the bearing and takes heat with it; grease only conducts it slowly (see Bearings and Shafts, which says grease suits lower speeds where a dynamic film cannot form)
Method: Read The Regime From The Behaviour
Step 1: Set the clearance. Radial clearance is about 0.001 x shaft diameter for a well-lubricated bearing, so the diameter difference is about 0.002 x D. A 25 mm shaft therefore takes 0.05 mm and a 50 mm shaft 0.10 mm, which matches the 0.05-0.10 mm the carrier gives for a bronze bushing. Slow, intermittently loaded bearings can run looser, at 0.002-0.003 x D radial.
Step 2: Keep the grooves out of the load zone. The film pressure builds where the load presses, and a groove there breaks the wedge. Grooves belong where the film is not working.
Step 3: Run in under reduced load. After a rebuild run at reduced load for at least an hour while watching temperature and sound (see Lubrication and Failures).
Step 4: Diagnose by symptom.
- Squeal or chatter on starting that clears as speed builds: boundary to mixed, normal.
- Hot, harsh and getting worse at steady speed: the film is failing; stop.
- Warm but steady, quiet, no metal in the oil: mixed or hydrodynamic, healthy.
- Runs well slow, seizes when speed rises on the same load: heat is outrunning the cooling, not a material fault.
Worked Example
A steel shaft of 50 mm diameter runs in a bronze sleeve with 0.10 mm diametral clearance. Suppose the shaft runs 40°C hotter than the sleeve. Steel grows 0.012 mm per metre per °C (the figure in Precision Measurement), so the diameter grows by 0.012 x 0.05 x 40 = 0.024 mm.
That is 24% of the clearance gone before the load is considered. The 40°C difference is an assumed case, not a measurement, and the 0.012 figure is for steel only. The point is the direction: the part that heats first, the shaft, closes the gap the oil needs.
Common Mistakes
- Choosing the oil for the load and forgetting the temperature. A tallow-fed bearing above its 40-50°C range loses its oil and runs dry.
- Setting clearance tight to reduce wobble. Too little clearance leaves no room for the film; too much lets the film break and the shaft run off-centre.
- Cutting an oil groove through the bottom of the bearing. It dumps the film pressure where the load is.
- Reading a quiet bearing as healthy without checking temperature. A bearing can run silent and still be a few degrees from losing its film.
Safety
Hot oil and hot housings burn, and a running shaft does not forgive reaching
Do not test a bearing by hand while the shaft is turning. Stop the machine, name the stop and its coast-down time, and let the housing cool before touching it. Hot tallow and oil also ignite: keep a rag-soaked or overfilled housing away from a forge or furnace.
What’s Next
- Learn next: Lubrication Systems - getting oil to the bearing and heat away from it
- Combine with: Bronze Bushings - the usual sleeve this theory applies to