An interference fit holds a bearing in its housing or on its shaft because the part is slightly larger than the hole it goes into, by up to 0.02 mm for the light press fits the carrier gives. At that scale the fit is only as good as the measurement behind it, a pressed bushing’s bore closes by up to the interference, and a hardened bearing must never be heated past the temperature it was tempered at.
Part of Bearings and Bushings
How tight a bearing or bushing must fit its housing and shaft, how to get it in without damage, and what the fit does to the bore.
A Press Fit Moves The Bore, Not Just The Part
Bearings and Bushings lists the fits as: housing bore equal to the bearing outside diameter plus 0.00 to 0.02 mm, described as a light press fit; shaft equal to the bearing inside diameter minus 0.00 to 0.02 mm, with a bushing turning freely on its shaft; and for ball bearings the inner race pressed on the shaft with the outer race sliding into the housing. Read literally, a housing bore of “outside diameter plus 0.02” is bigger than the bearing, which is a clearance, not a press. The wording in the source is ambiguous, so this article treats a press fit as the bearing outside being larger than the bore by 0 to 0.02 mm, and flags the literal wording as unresolved.
The fit does more than hold the part still. Pressing a thin-walled bushing into a smaller bore squeezes it, and its inside diameter shrinks by up to the amount of interference. The bore you cut before pressing is not the bore you have after.
What You Need
- A micrometer or bore gauge that resolves the 0.02 mm window (Micrometers)
- A press or a screw to push the bearing in straight, and oil on the seat
- Heat for the housing if the fit is tight
- A record of the clearance before and after (see Fitting and Alignment)
Method: Fit Without Forcing
Step 1: Measure both parts at several points, and write the numbers down.
Step 2: Work out the interference as bearing outside diameter minus housing bore.
Step 3: Heat the housing to expand it, not the bearing. Steel grows 0.012 mm per metre per degree (Precision Measurement). A 50 mm bore therefore grows 0.012 x 0.05 = 0.0006 mm per degree: 33 degrees closes a 0.02 mm interference, 100 degrees opens 0.06 mm. The coefficient is for steel; a bronze or cast-iron housing has its own, not given here.
Step 4: Do not heat a hardened bearing past its temper temperature. The carrier tempers balls at 150°C and races at 200°C; heating a finished bearing past those softens the steel, and grease or tallow in it melts far lower.
Step 5: Press in square, in one movement. Never force a bearing that will not slide in; it means the fit is too tight.
Step 6: Re-measure a pressed bushing’s bore and, if it has closed, finish it in place.
Worked Example
A bushing bored for a 50 mm shaft with 0.060 mm clearance is pressed into a housing with 0.02 mm interference. If the bore closes by the full interference, the clearance becomes 0.060 - 0.020 = 0.040 mm, below the 0.05 mm minimum of the 0.05-0.10 mm window. Boring with a clearance of 0.075 mm leaves 0.075 - 0.020 = 0.055 mm: inside the window but with almost no margin. The sound plan is to press the bushing in first and bore it in place.
Measurement: with a micrometer accurate to +-0.003 mm, the interference is a difference of two readings, so the worst-case error is 2 x 0.003 = 0.006 mm. Against a 0.02 mm window that is 30% of the window.
Common Mistakes
- Heating the bearing instead of the housing. It softens hardened steel.
- Treating “light press fit” as a number you can hit by feel. Measure.
- Boring before pressing. The pressed bush closes.
- Hammering to seat. It dents races and cracks brittle rings.
Safety
Hot housings and a loaded press
A housing hot enough to expand burns skin; use tongs and let it cool before handling. A press or screw stores force: guard it, and keep hands out of the line of a slipping part. Never heat grease-packed or tallow-soaked parts near flame.
What’s Next
- Learn next: Shaft Alignment - the bearings now need to line up
- Combine with: Boring and Finishing - sizing the bore for the clearance you want