Getting metal very cold

I want to put a steel shaft into an aluminium block so that it won't
move. This is clearly going to involve temperture differentials (hot
ali, cold steel). Ideally I'd warm the ali in the oven when Mrs P is
away and immerse the steel bar in liquid nitrogen. Problem is I have
no liquid nitrogen and I suspect that it'll be an expensive episode if
I use it. Are there cost effective ways to access liquid nitrogen or
alternatively what other ways can I get the steel very cold?
Suggestions welcomed
Thanks
Charles
Reply to
Charles Ping
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Charles I believe you can get some spray on gaer that freezes pipes so you can repair them without having to shut off and drain.
Farnell carry this stuff.
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Never used it so can't comment but perhaps the shaft in the freezer then a quick spray with this gear then a whack with a big shovel ?
John S.
Reply to
John S
Also available from sheds, plumbers suppliers, screwfix etc. for plumbing. If you are going down this route, stick the shaft in the freezer for a while first. That way you will need much less spray.
Dry ice might be another option, never looked for suppliers but it used to be used in theatres to make "fog". Don't think it is expensive.
Liquid nitrogen actually isn't all that expensive (ISTR about the same price as milk, last time I looked which was a long time ago). Any friends in university research departments, or in serious industrial laboratories? Most scanning electron microscopes also have a detector cooled by LN for elemental analysis. I've sneaked it home in a briefcase in an (industrial) glass thermos flask in the past, but with modern stainless steel thermos flasks it should be a doddle. Just keep the flask upright, and don't have the cap screwed down tight to prevent pressurisation as it evaporates.
If you are using LN you should get all the clearance you need without messing around with ovens.
Reply to
newshound
Charles,
With a hot oven at (say) 190C and a deep freeze easily going down to -25 surely that will be enough differential.
AWEM
Reply to
Andrew Mawson
Diameter and length of the components please plus what temperature will the finished assembly operate at?
This is a simple problem faced every day by anyone who fits valve guides into cylinder heads - me for example.
Most times heat isn't even necessary. Just a big hammer. I'll spec you the fit once I have the dimensions.
Reply to
Dave Baker
PS, you'll never need liquid nitrogen. You can get 4 thou per inch expansion of the aluminium just by popping it into the oven at 200C and that's way more than you'll ever need to stop the steel bar shifting.
Reply to
Dave Baker
It's a 5/8 shaft in a nominally 5/8 hole with a working temperature of possibly up to 200C
Therefore by my maths if the shaft and hole were both nominal sizes the hole would expand 2.8 thou and the bar 1.5 thou (loose by 1.3 thou) If I did the "oven/freezer" approach I'd get a shade over a three thou differential, leaving at working temp a fit of 1.7 thou. Is that enough to stop the shaft moving in the block when it's rotating at 6,000 rpm, with some drag on the alloy part through friction and loading? (it's a supercharger rotor).
Thanks
Charles
Reply to
Charles Ping
If the domestic oven is off limits 10 minutes in the direct output of a 2 or 3KW fan heater will do the trick.
Jim
Reply to
pentagrid
What's the engaged length? That's obviously critical to what the retaining friction forces are.
If there are torque loadings involved then you really need to know them and 200C sounds very high for something that isn't actually inside an engine. Even a cylinder head gets nowhere near that.
Reply to
Dave Baker
Reminds me of the time many years ago when I tried to replace the starter ring gear on a Ford flywheel. Heated the new ring to 300C and there was no way it would go on. In the end we had the ring at 300C and the flywheel in liquid nitrogen and the gear almost went on. Tried to persuade it just the last bit with a hammer and a chunk of flywheel cracked off! Some metals get pretty brittle when they are that cold. It turned out that there were two different flywheels in use on that particular engine, one with a replaceable ring gear the other not ? I found that out the expensive way.
Reply to
Norman Billingham
The average 10" flywheel ring gear goes on at about 10 thou, maybe 15 thou max, fit. Sounds like you were trying to get something about 1mm undersize on which I agree was doomed to failure. However a quick tweak of the flywheel o/d on the lathe would have enabled it to work. There's no such thing as a flywheel which can't have its ring gear changed.
Reply to
Dave Baker
Pretty well all carbon steels do!
Edwin Izod's prototype impact tester lives a short distance away from my office :-)
Mark Rand RTFM
Reply to
Mark Rand

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