Fit a bearing in order: confirm the shaft journal and housing bore are sound, press the bearing in without forcing it, align the shafts to within 0.1 mm per metre of span, then run it in at reduced load while watching its temperature. A bearing that will not slide in has the wrong fit, so heat the housing or chill the bearing rather than hammering it.
Part of Bearings and Bushings
The steps between a finished bearing and a bearing that lasts: preparing the seats, pressing it in, aligning the shafts, and running it in.
Most Early Bearing Failures Are Fitting Failures
A well-made bearing fails early when it is fitted badly. Misaligned bearings wear rapidly and generate heat, and a bearing forced into a seat that is too tight is damaged before it turns. A shaft whose journal is grooved or ridged will destroy a new bearing in hours, so it must be ground or turned smooth first (Plain Bearings).
Fitting is also where you record the numbers that wear assessment needs later. A new bearing’s clearance, written down on the day it goes in, is the baseline against which every future inspection is read.
What You Need
- The finished bearing, shaft and housing, measured (Micrometers)
- A straight edge, a string line, and ideally a dial indicator
- Heat for the housing, or cold for the bearing, for a tight fit
- Oil or grease ready before the first turn
Method: Fit In Order
Step 1: Inspect the seats. Journal smooth and round, housing bore clean and the right size. Measure and write the sizes down.
Step 2: Fit the bearing with the right interference (Interference Fits). Never force a bearing: if it will not slide in, the fit is too tight. Heat the housing to expand it, or cool the bearing to shrink it.
Step 3: Align (Shaft Alignment): a straight edge across the bearing housings, a dial indicator on the shaft turned by hand to read runout, or a string line along a long span. The target is parallel to within 0.1 mm per metre of span.
Step 4: Lubricate before the first turn (Lubrication Systems).
Step 5: Run in under reduced load for at least an hour, watching temperature and sound (Lubrication and Failures).
Step 6: Record the baseline: clearance, date and running temperature. These are the numbers Wear Assessment and Replacement compares against.
Worked Example
The alignment target is a parallelism: 0.1 mm per metre is an angle of 0.1 / 1,000 = 0.0001 radian, about 0.006 degrees. The carrier also quotes an angular misalignment of under 0.5 degrees for ball bearings and up to 2 degrees for self-aligning bushings.
Over a metre, 0.5 degrees is 1,000 x tan(0.5 degrees) = 1,000 x 0.00873 = 8.7 mm, and 2 degrees is 1,000 x 0.0349 = 34.9 mm. So 0.5 degrees is 87 times the 0.1 mm per metre target. The two numbers are not rivals: the target is what you aim for and what makes the bearing last; the angular figure is the upper limit the carrier sets, not an aim. A shaft aligned only to the tolerance runs, but wears.
Common Mistakes
- Hammering a bearing home. It dents the race or closes the bore.
- Fitting a new bearing on a worn shaft. The journal destroys it.
- Aligning by eye. Use a straight edge, string or indicator.
- Not recording the new clearance. Later you cannot tell how much has worn.
Safety
Heating, pressing and lifting
A heated housing burns skin, and a pressed bearing can slip a tool. Use tongs, a press with a guard, and lift heavy shafts with slings, not hands. Fit and align with the drive locked out so the shaft cannot turn while your hands are on it, and know where the stop is and who can reach it.
What’s Next
- Learn next: Interference Fits - how tight is tight enough
- Combine with: Shaft Alignment - the checks in more depth