Judge a bearing’s wear by comparing its measured clearance with the figure recorded when it was fitted, and treat noise, a housing too hot to hold for about 3 seconds, visible shaft wobble and metal particles in the oil as signs to act. A mill step socket worn more than 3 mm deeper than its original depth is past use, and a slow bushing loosened beyond about 0.004-0.006 times the shaft diameter is outside even the loose clearance window.
Part of Bearings and Bushings
Reading a bearing’s condition from sound, heat, play and the oil, and deciding whether to adjust, replace or stop.
A Recorded Baseline Turns Guessing Into Measuring
Bearings fail gradually and give warning. Bearing Replacement lists the signs: increased noise, a housing too hot to touch, visible shaft wobble and metal particles in the lubricant. A bearing that runs to failure rarely fails gracefully; it overheats, seizes, and may damage the shaft.
Signs tell you something is wrong. A measurement tells you how much, and only if you have a number to compare it with. The clearance, date and running temperature recorded at fitting are that number (Fitting and Alignment).
What You Need
- The recorded baseline for this bearing
- A dial indicator, or feeler leaves, and a bar to lift the shaft
- A micrometer and inside calipers or bore gauge
- A clean cloth and a little oil to inspect for metal particles
Method: Assess A Bearing
Step 1: Listen and feel. Knocking or rumbling means the clearance is too large or the bearing metal has worn away (Plain Bearings). A housing that cannot be held for about 3 seconds is overheating.
Step 2: Measure radial play. With the drive locked out, lift the shaft slightly and read the movement on an indicator. That total movement is the clearance.
Step 3: Measure axial play on thrust bearings. More than 0.010 inch (0.25 mm) in most applications warrants inspection (Thrust Bearings).
Step 4: Check the shaft journal. Grooves or ridges mean the shaft must be ground or turned smooth before a new bearing goes on, or the new bearing dies in hours.
Step 5: Compare with the baseline and the windows. Decide: adjust (split bearing shims, Split Bearings), replace the bearing, or replace and recondition the shaft.
Step 6: Record what you found and the new baseline.
Worked Example
A 50 mm shaft: a well-lubricated bearing is fitted at about 0.001 x D radial clearance, 0.05 mm radial or 0.10 mm diametral, and slow intermittent bearings can run looser at 0.002-0.003 x D radial (Bearings and Shafts). The looser window is therefore 0.10-0.15 mm radial, or 0.20-0.30 mm diametral, which is 0.004-0.006 times the shaft diameter (twice the radial figures, divided by 50 mm).
The indicator shows 0.28 mm of total play. That is inside the loose window but at its top end, and nearly three times the 0.10 mm baseline, so for a slow shaft it is time to plan the replacement or take shim out. For a fast shaft it is already beyond what the film can bridge: replace.
For the mill step socket the rule is different: it is judged by depth. A socket worn more than 3 mm deeper than its original depth changes the mill geometry and must be replaced.
Common Mistakes
- Replacing the bearing and ignoring the shaft. A rough journal ruins the new one.
- Waiting for failure. A seized bearing costs a shaft.
- Having no baseline. Then “a bit loose” is an opinion.
- Quoting a wear limit without the speed. The loose window is for slow bearings only.
Safety
Never assess a turning bearing by hand
Lock out the drive before lifting the shaft or feeling a housing, and block any load a vertical shaft carries. Name the stop, who can reach it, and the coast-down time. A bearing hot enough to smoke is shut down, not investigated.
What’s Next
- Learn next: Bearing Replacement - a worked mill case
- Combine with: Lubrication and Failures - why the film failed in the first place