ChipsAhoy wrote: ↑Tue Aug 16, 2022 7:48 am
I think we have pretty much come up with a good interference fit at a one thousand per inch of shaft diameter.
Now....
If I can freeze my axles, and adequately heat my wheels & hubs... what would then be a PREFERRED interference? By doing this I have reduced the mechanical resistance to mating the parts. So am I safe to assume that tighter is a better quality fit?
In other words....
Is the one thousandth chosen as a happy medium to enable the mechanical process of pressing on, or is it the best fit for the final product in the long run?
I ask these questions not just to stir discussion, but we are looking around the corner here at assembling some larger diameter wheels on some larger diameter axles. And the more information we can gleen the better decision we can make.
Scotty
Well, one thing you can do is make a search on this board as there has been many postings on shrink fits
I have done thousands of shrink and press fits at work and you need to know this magic number for steel/iron ( 5 ball63)
To see the growth of the PART per degree F, you multiply 5 ball 63 by the diameter.
example, for a 1 inch shaft you get .0000063 x 1.00 = .0000063"
The easy way now, is to relate it to 100 degree increments, so multiply that by 100.
You get .00063. that is the change in size for a 1" part for every 100 degrees
DIFFERENTIAL
All dimensional changes are relative to ambient temperature.
So if you cool the male to -38 F, it shrinks .00063"
If the female is heated to 168, it grows the same amount and they would be close to size to size for a .001 interference fit. BUT !
Shrink fits change temperature VERY fast, so going over the 168 to about 400 will only give you .001 clearance at time of fit.. too close, unless your guide method is perfect.
I would opt for a hotter temp like 600. the steel will start to turn dark purple to light blue then move fast, and make sure you have some stop of some sort to stop at the right location. marking the parts and holding to the mark is hard when under a rush
have a physical stop if possible when shaft penetration depth is important. Also the entire wheel needs to be heated evenly
Your .001" interference fit per inch is a good number .
When you mix metals for interference fits ( other than steel and cast iron ) , all numbers and conditions change , so Stainless,bronze,aluminum are a whole lot different and approaches to those fits are another animal
Lastly, size for size, a shrink fit has twice the holding power of a press fit for the same interference number---assuming the fits have not been fitted previously
Rich
