Interferance fit on bearings on 1/4 in router shaft


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?
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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 http://www.ntnamerica.com
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.
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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.
Tom wrote:

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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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