2 x 2 spacers under the frame set far enough apart not to
throttle the airstream. Fan RPM was erratic, as if the
fan was designed without thrust bearings.
How about flipping it 180?
If you set it on edge the speed remains constant?
Interesting idea with the thrust bearings. Ball bearings shouldn't
matter they can support some axial loading without a problem - and the
impeller is usually very light.
Sleeve bearings might be another matter. Most of the fans I've
disassembled had ball bearings - but the ones with sleeves make little
or no provision for thrust loading - a bronze washer or two or the
really cheap ones a few plastic washers.
One bearing is sometimes visible under a paper or plastic label on the
stationary part of the housing. You can look to see what you have, if
that's the case.
Sleeve bearings are nice when you want a quite fan - and they
frequently outlast ball bearings - but they should be mounted on edge.
They are rare in fans
And you are talking about Radio Shack . . . their parts aren't always
the highest quality. You'd be better off with a mail order surplus
parts outlet.