Posted by Sherman in Idaho [108.162.245.23] on Sunday, July 10, 2016 at 22:08:26 :
I have occasion to press a 2.2497 OD ball bearing into a ~2.2467 hole in a cast iron housing. This seemed like a mighty tight fit to me, especially because the ID was likely smaller than I measured. I googled "bearing press fits" and the like and came up with dozens of pages full of arcane mathematical formulas and tables that referred to the different kinds of fits by combinations of upper and lower case letters and numbers, almost all of it in metric. This was all way too much for me, and I thought surely somebody must have figured his out long ago and put it into human-readable form. Then I remembered I still had my dad's 1938 edition of Kent's Mechanical Engineer's Handbook (Design and Shop Practice). Sure enough there was a simple section with a handy table giving 8 classes of fits (each described in a sensible way)and all measured in inches. Turns out a "class 7" fit was the tightest fit that won't exceed the stress limits for cast iron (nice that they tabulated that common situation that rather than making me go back and find the properties of cast iron to plug into a formula). Turns out the limit for the tightest fit is 0.00189 at that diameter, so my .003 is definitely way too tight and will crack the cast iron or bind up the bearing. But so nice to be able to look it up in a simple table rather than get a Master's degree in mechanical engineering just to be able to understand the tables. It's 80 years old, but I still find I pull Kent off the shelf more than any other reference book.
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