# Compression fitting using Liquid Nitrogen



## Inferno (Mar 21, 2021)

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.


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## pacifica (Mar 21, 2021)

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_.


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## Inferno (Mar 21, 2021)

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??


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## pacifica (Mar 23, 2021)

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.


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## Inferno (Mar 23, 2021)

pacifica said:


> 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.


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## JimDawson (Mar 23, 2021)

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


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## rwm (Mar 23, 2021)

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


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## Inferno (Mar 23, 2021)

JimDawson said:


> 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.


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## Asm109 (Mar 23, 2021)

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.


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## JimDawson (Mar 23, 2021)

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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## Inferno (Mar 23, 2021)

JimDawson said:


> 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.
> 
> 
> 
> ...


Each wheel will be independent, and the holder of the wheel will need to be positional and adjustable. Some setups they could be a half inch apart, others (in the current planning stage) 22 inches apart. I have plans for up to 10 of these units. I figured half would be lefties, half would be righties.


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## rwm (Mar 24, 2021)

ASM brings up a good point. I assumed the bearing was a sleeve. If it is a ball bearing then you will deform the race with an interference fit that is too tight.
Robert


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## JimDawson (Mar 24, 2021)

Inferno said:


> 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?


Maybe 4140 for the axle plate.
O1 drill rod maybe for the axle
For 1/4 inch, 0.249 would give a tight shrink fit.  

I would look at a headless shoulder screw for the axle.  In fact a flat head shoulder screw would also work very well, Just turn down the flat head to just retain the bearing, could have nearly zero clearance.


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## Inferno (Mar 24, 2021)

JimDawson said:


> Maybe 4140 for the axle plate.
> O1 drill rod maybe for the axle
> For 1/4 inch, 0.249 would give a tight shrink fit.
> 
> I would look at a headless shoulder screw for the axle.  In fact a flat head shoulder screw would also work very well, Just turn down the flat head to just retain the bearing, could have nearly zero clearance.


I keep forgetting to add that most of my projects are a journey, not a destination. That comment isn't directed at anyone in particular.

Those are good metal recommendations. In fact, it gives me another idea if the first plan fails.
I don't know what headless shoulder bolts are. One thing I need to consider (the reason for the clip at the end of the shaft) is that I want to be able to remove these wheels from time to time.

Here's the wheel I'm planning with


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## JimDawson (Mar 24, 2021)

Inferno said:


> I don't know what headless shoulder bolts are.



Headless shoulder bolt 








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An ultra low profile shoulder bolt would work also, only 1/16'' head height.








						McMaster-Carr
					

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.




					www.mcmaster.com


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## 7milesup (Mar 24, 2021)

If there is room, I would rather do a shoulder bolt as Jim Dawson mentioned than a clip.  Clips can be a PITA.


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## Manual Mac (Mar 24, 2021)

Well, i’m glad there is more uses for liquid nitrogen.
I’m going to the Dermatologist to have some actinic keratosis frozen off of my face (again) with it in a couple days. With any luck it will not turn cancerous.
Just went to a welding supply with my neighbor the cow farmer, to buy a tank full with the stuff. He keeps bull semen in it.
Back in the day when I worked at the Caterpillar dealer, we used to keep bearing races etc. In the freezer over night then install them (hurry now) into transmissions housings etc.


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## Inferno (Mar 25, 2021)

JimDawson said:


> Headless shoulder bolt
> 
> 
> 
> ...


So this one would work but I'd want to recess the plate by about 2mm. My bearings are 7.12mm (aka .2811"). The axle on the bolt is 9.35mm. OR I could have a 2mm spacer. Not a big deal. 

But then what would I do with the 65 liters of liquid nitrogen? 

OK, I don't have the nitrogen yet. I have two 35 liter dewars though. No bull semen. YIKES!

No matter which way I go, I'm hell bent to try the nitrogen thing. I need smaller dewars though. 
I already bought the wheels, and the bearings are on the way. I also bought the reamers also. Five of them in increments of .2482-.2490
I'll probably never use them after this experiment but as I said, it's about the journey and I'd probably spend the money on something even more stupid if I didn't buy the stuff.


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## Boswell (Mar 25, 2021)

looking forward to how it works out. (as long as no one gets hurt)  Pictures at some point would be nice


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## Asm109 (Mar 25, 2021)

Inferno said:


> But then what would I do with the 65 liters of liquid nitrogen?


You don't have to do anything with it.  Like President Trump said about Covid.  "One day it will all just go away"  With LN2 that day will be coming up in a day or two at most.


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## Inferno (Mar 25, 2021)

Asm109 said:


> You don't have to do anything with it.  Like President Trump said about Covid.  "One day it will all just go away"  With LN2 that day will be coming up in a day or two at most.


You're not completely incorrect. The dewars I have will hold LN2 for up to 60 days. More if kept in a freezer.


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## rwm (Mar 25, 2021)

I used to get liquid nitrogen from Airgas in a metal thermos and kept the lid cracked in the trunk of may car. I doubt they will still allow that these days? Does anyone know? Has anyone done this recently? Are there other new requirements?
Robert


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## Inferno (Mar 25, 2021)

rwm said:


> I used to get liquid nitrogen from Airgas in a metal thermos and kept the lid cracked in the trunk of may car. I doubt they will still allow that these days? Does anyone know? Has anyone done this recently? Are there other new requirements?
> Robert


The local air gas supplier td me they have loaner dewars and said that a vacuum insulated container is all you need. He mentioned something about Styrofoam but by then I saw a squirrel.


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## pacifica (Mar 26, 2021)

Asm109 said:


> 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.


I agree, press fits can be problematic.Not only must tolerances be very  accurate, you need to know the metals you're working with and if bearings are involved one more difficult variable is added.


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## Inferno (Mar 28, 2021)

Proof of concept, step one.

I got the bearings in today and, rather than start the LN2 experiment now, I simply pressed one in to the wheel.
It went in nice and smooth. Smoother than the piston wrist pins I bought the hydraulic press for.
I have no doubt that this will work with Liquid Nitrogen also.

The bearing is shown flush but it's protruding about .012" on the other side. If this makes it all the way to a functioning system, I'll center the bearing in the wheel. Having a .006" clearance will be pretty good.

BTW, the "rusty" looking stuff is just the cosmoline I haven't cleaned yet


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