The hardened metal used in bearings is much like tempered glass, very strong yet it has it's weaknesses. Just like the glass will shatter if lightly tapped on its edge with a hard object, hardened metal can break easily if forced the wrong way--in this case spread out a little bit. It doesn't take much. If you can't get the bearing on by hand, there's an issue with either the bearing size or the shaft--which is likely the problem.
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