In April of 2004 I purchased a Eagle America 196-9645 45 piece bearing
survival kit. I have not had a reason to use the 1/4 inch pattern
bearings until now. Today I attempted to use the 1/4 inch bearings and
they appear to have an interference fit on nearly all the shafts of my
router bits with the exception of only 2 or 3. The bearing ID measures
0.2490 to 0.2495. Most of my router shafts measure 0.249 as close as
my calipers can measure. The diameter of the router bits that work to
the ones that don't vary by such a small amount that my calipers (which
have a precision to only 0.0005) are unable to show the difference
between the shafts that work and those that don't. This tells me that
the ones that work vs. the ones that don't vary by only a few 10ths of
a thousanth. The same problem seems to be true with the 1/2 inch
router bit shafts. I measured the 8 mm bearings and they appear to be
7.98 mm. (Actually, I have no use for the 8mm bearings, but they came
with the kit). I have no problems with the 3/16 inch bearings that go
on the top of the router bits.
I wonder if these 1/4, 1/2 & 8mm bearings are designed for an
interference fit? Has anyone gotten one of these kits or a similar
kit? What are your experiences?
Bearings used for a rotating shaft application will normally have an
interference fit on the shaft and slip fit in the housing. Each
bearing, depending on size will have a tolerance range which on bores
will range from plus 0 to minus ??. These are normally measured in
microns and range from 8 microns on up. If you want more info on shaft
and housing fitting practices go to NTN web site
Then click on Technical Support, then Download NTN Ball and Roller
Bearing Catalog - then look around page A-52 (actually there is a lot
more info preceding that) and you can find all sorts of info on shaft
and housing fits. Good luck.
I downloaded the catalog. Thanks. A lot of good info there. However,
the application of using these bearings for guiding router bits seems
somewhat specialized. Because my bearings came with stop collars, the
intention seems that they should be loose fit on the shaft. If I used
the guidelines strictly, you would think the fit should be tight.
However, the application is intermittent and if the stop collar bears
against the inner race well enough that it does not allow it to easily
rotate, it would not seem to result in a significant wear problem.
I called Eagle America and the seem to feel it should be a loose fit
and want me to send the bearing kit back. I will order the replacement
kit and cross ship. My only reservation is that I don't want to find
that the new kit has the same problem. The only way to know is order
it and find out unless someone in the group has the experience to know
what I will experience with the new kit.
If some are tight and others are loose, it sounds like you have a
fairly broad tolerance range. I would be concerned with a loose fit as
the shaft could slip on startup. If they are a little tight, you can
set the bearing on a light bulb for a few minutes then see if it will
slide on the shaft (it will have an interference fit for removal).
Good luck with the new set.
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