Bearing Housing Fit: How to Tolerance the Bore
Bearing housing fit depends on which ring sees the rotating load. How to pick an ISO 286 bore tolerance, when a press is needed, and what grade to ask for.
A 47 mm ball bearing and a plate with a hole in it. The bearing housing fit you put on that hole decides whether the bearing presses in cleanly, binds, or slowly spins in its seat.
How Do You Tolerance a Hole in a Plate for a Press-In Bearing?
Pick an ISO 286 hole tolerance from the bearing maker's fit tables, based on which ring turns relative to the load. If the outer ring sits still under a load that doesn't rotate, it often doesn't need a press fit at all.
An r/engineering user asked almost exactly that. Their bearing had a 47 mm OD toleranced +0/-0.011 mm, and they'd picked N6 almost at random. One reply confirmed an N6 hole at that size runs from -0.028 to -0.012 mm, and said that for small ball bearings they usually go K7, N7 or P7, depending on the application. Another asked if it only locates or also carries thrust, since locating may need just the barest press.
Bearing housing fit starts with the load
SKF's guide to selecting fits sorts it by load. A ring under a rotating load needs an interference fit, or it creeps on its seat and the seat wears. A ring under a stationary load normally doesn't creep and doesn't need one.
On a typical motor shaft, the inner ring turns with the load and takes the interference on the shaft. The outer ring sees a stationary load, and SKF's seat-tolerance table lists H7 for that in steel or cast iron housings. A wheel hub on a fixed axle flips it around. Now the outer ring turns relative to the load, and the same table steps through M7, N7 and P7 as the load gets heavier.
Housing material moves things too. SKF says housings made of something other than steel or cast iron may need different fits because of strength and thermal properties, and NTN's fit guide says light-alloy housings normally need a tighter fit than its tables list.
Don't go tighter just to be safe. Too much squeeze takes up the bearing's internal clearance, and one reply suspected exactly that behind past failures.
What it means for the machined bore
The grade matters as much as the letter. At 47 mm, an IT7 hole (the 7 in K7 or N7) has a 0.025 mm band and an IT6 hole has 0.016 mm. Our CNC page lists tolerances down to ±0.01 mm, a 0.02 mm band, so a grade 7 bore is wider than the tightest band the page lists. IT6 is tighter than that. If you need one, ask us when you quote.
Reaming vs boring covers how to finish a bore like this. If one comes out small, here's how to judge whether it can be saved. Dowel bores follow different logic, covered in slip fit vs locational fit.
Whatever bearing housing fit you pick, write the fit and the limits together on the drawing, like Ø47 H7 (47.000/47.025). The shop can't misread that.
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