Compression fitting using Liquid Nitrogen

[Inferno]

I have a project that I'm designing in my head. I've purchased a couple components that I am sure I can fit together using liquid nitrogen, rather than press fitting.
I have a bearing that has a .750 OD which will fit into a 19mm ID piece. .002 size isn't worrisome in those dimensions. I'll freeze the bearing and it should compression fit nicely when it comes back to temp.

Now I need to think about the axle for the bearing. The ID of the bearing is .250"
I want to compression fit the axle to a vertical plate. I am imagining the .250" axle, dropped in nitrogen, but don't know how much it will shrink. Yes, I know different metals have a different shrink ratio which is part of where I'm hung up. I don't know what type of metal (steel alloy) I want to use for the vertical plate or axle.
I also don't mind heating the vertical plate to expand it a little while fitting the axle.

For the metal plate, I'd have the initial part waterjet cut and then do the final machining. It can't be so hard that I can't machine it with my Smithy and the HSS reamer(s) I will buy but mild steel won't work at all.

So, I guess my questions are:
What would be the best alloy to use for the axle plate?
What would be the best alloy to use for the axle itself?
What size ream would I need for a .250" axle using Nitrogen compression using the axle alloy from above?

Yeah, I can research all this. I'm not asking anyone to do the research work for me. Just opening a discussion.

 

[pacifica]

Multiply temp change by 7.2x10-6, or .0000072,if your liquid nitrogen is -320 and room temp is 80 my calculation is a little less than .003" movement per inch of steel. Here's a chart: https://www.engineeringtoolbox.com/thermal-expansion-metals-d_859.html . Seems like you would want some clearance so the axle doesn't expand halfway through, and you want to work fairly quickly. How do I know a piece can get stuck halfway and can ruin the part getting it out? Been there done that or Bin there Dump that.

 

[Inferno]

I can't make heads or tails out of that chart. Unless it's straight forward, this part of math is why I went into printing instead of metal manufacturing.
Having a number of .003" per inch doesn't give a lot of room on a .250" shaft. I can pretty reliably heat the axle plate to 400 degrees which could double the .003" per inch to get to .00125" of variance on the .250" axle.
Using a .2485" reamer would seem like a quick fit, meaning make it fit quick or start over. Also available is a .2487" and .2490"
Maybe I should buy all three sizes??

 

[pacifica]

Some alternatives are : Knurl end of shaft and press fit, use loctite retaining compound 638(for close fits). Not sure how much strength you require and what facilities you have for pressing.

 

[Inferno]

Some alternatives are : Knurl end of shaft and press fit, use loctite retaining compound 638(for close fits). Not sure how much strength you require and what facilities you have for pressing.

The part is going to take a beating. I'm toying with a press fit (I have a 12 ton press) and then welding the back side then grinding it flush.
That's if the nitrogen idea doesn't pan out. I bought 4 sizes of reams so I can sneak up on a tight fit.

 

[JimDawson]

Is there a reason that you couldn't use a shoulder screw for the axel? That way it just screws into the plate.

 

[rwm]

I have done this type of interference fit. I have used dry ice on one part (much easier to get) and heated the other.
Robert

 

[Inferno]

Is there a reason that you couldn't use a shoulder screw for the axel? That way it just screws into the plate.

That was my first thought. unfortunately that adds distance between centers. Any reduction in clearance between the "wheels" is an improvement over what I am currently using though. Right now my limit is 5/8". I'd like to get it to 1/4" center to center on the wheels.

 

[Asm109]

You need to find out how much press fit on the outer ring the manufacturer of the bearing recommends. The bearing ring is thin and most of the interference in the press fit just translates into a shrinking of the ID of the ring.
Bearings are made with a bit of clearance between the rings and the balls. something like .0002-.0005 inches.
A two thousandth press will close up all the clearance and the bearing will NOT turn freely, it will go bump, bump, bump

Using temp extremes to make up for lousy machining tolerances will not get you a well running machine.

 

[JimDawson]

Do the wheels need to be independent of each other? Headless shoulder screws are available, also low profile heads are available, would likely take up less space than the clip.

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