Liquid Nitrogen on Aluminum

By rlitman (***)

Date 01-17-2012 23:11

I don't think cryogenic temperatures will do much to aluminum.
This page has a neat video of aluminum (and copper and tin) being bent after a dip in liquid Nitrogen:
http://members.mrtc.com/anvk/cryogenics/cryogenics.html

Aluminum is used in space, BECAUSE it retains so much strength at cryogenic temperatures (it doesn't get brittle like most steel, and actually gets stronger; austenitic stainless being an exception).

By fschweighardt (***)

Date 01-18-2012 01:12

aluminum is fine at low temps, most new liquid nitrogen trucks have an aluminum inner vessel

By 803056

Date 01-18-2012 13:55 Edited 01-18-2012 14:03

Aluminum has a higher coefficient of thermal expansion than steel. If you are trying to shrink fit two components, it should work provided you take into consideration how much the part will contract at the temperature of the liquid nitrogen. You can heat the outside component to a couple of hundred degrees, while cooling the inner component. That will expand the outer component while causing the inner component to contract.

You have to consider how much dimensional variation will occur by cooling one component while heating the other. If the variation in the initial dimensions is too large, the technique will not work.

The amount of expansion is a simple calculation. The expansion is simply the coefficient of thermal expansion times delta "T" where delta "T" is the change in temperature from the initial temperature to the final temperature. The change in dimension is the sum of the initial dimension plus the expansion just calculated.

The values you will need for the calculations are as follows:
Coefficient of thermal expansion – Aluminum – 12.3 x 0.000001 inch/inch x degree F
- Steel – 7.3 x 0.000001 inch/inch x degree F
Boiling point of liquid nitrogen -195 degrees F

Expansion/contraction of aluminum in liquid nitrogen
Starting dimension x 12.3 x (delta T) x 0.000001= expansion, in this case the expansion results in a negative value indicating the part contracts

Final dimension of aluminum = starting dimension + expansion

Example: aluminum part at 75 degrees F is is 4 inches in length
4 x 12.3 x (Delta T) x 0.000001 ≈ expansion where delta T is -195-75=-270 degrees (reduction in temperature)
4 x 12.3 x (-270) x0.000001 ≈ -0.013 inch (negative sign indicates the part is contracting)

The final length = 4 + (-0.013) or 3.987 inches

Best regards – Al

By fschweighardt (***)

Date 01-18-2012 14:20 Edited 01-19-2012 02:17

As a small correction, liquid nitrogen is -320 deg F at atmospheric pressure

If you are doing a ring-shaped part, you get most of the dimensional change due to the "washer effect". if you cool a washer-shaped part, you can calculate the length along the centroid, apply Al's formula, and recalcualte the new diameter. If you want to get fancy, you can then factor in the through-thickness shrinkage as well, as it will contract toward the center of mass from both sides. If you have a piece of tube .100" wall thickness, and 1.000" in diameter through the centroid, (so 1.050 OD, and .950 ID) is pi" long, and then you cool it 400 degrees, the distance along the centroid is now 3.14159 X 12.3X10^-6*-400, which gives a length reduction of .0155, and a centroid diamter of .99508. now you figure the .100 shrinkage toward the centroid (.050 each side) as above, and you get an additional .000246 that the original ID gets smaller, and the original OD gets smaller as well. New ID is .945326. new OD is 1.044834. Wall thickness has gone from .100 to .099508.

tough to measure, but it is there

By rlitman (***)

Date 01-19-2012 16:05

If you have the micrometers or gauges that can give you accurate measurements to the nearest ten thousandth of an inch, then you've probably got enough data to run the calculations and see what temperature differential is required to get it to work.

Do you have any experience with liquid nitrogen. It has a number of hazards to be aware of.
If that is too difficult to deal with or procure (it's easy to get in larger quantities, but hard to get a small amount for just one shrink fit), then a common substitute is a bath in a mixture of chopped (or pellet) dry ice and acetone. That gets to -78C or -108F.

By fschweighardt (***)

Date 01-19-2012 19:52

All of my nonsense above is great theory, not sure how useful it is except for getting close on sizes before you try an actual fit. Nobody here is likely to measure .0002 on a part that is -300 deg or so. the mike freezes and gets to small, the part warms up in a hurry, and gets covered in frost pretty quick as well.

By rlitman (***)

Date 01-19-2012 20:32

Right, but if you measure to 0.0002 at room temperature, and know the interference of the fit, you can determine what temperature differential is required for a slip fit.
Then you'll know if a 300F differential is sufficient, or maybe if you can get away with less.
It won't be perfect (because in reality metals do not actually expand and contract at the same rate with temperature perfectly, and because the constants in the tables don't correspond to the alloy in question; i.e. pure aluminum vs 6061), but may be close enough.
However, if you're measuring to 0.002 (i.e. a vernier caliper, dial caliper, digital caliper, or similar 3/4 way reading caliper) you may not quite have the accuracy to be sure.

No, of course you can't really measure it hot or cold. Usually, once one part is hot, and the other cold, you want to get them together as quickly as possible, and can't waste time measuring, or it won't fit any more.

By fschweighardt (***)

Date 01-19-2012 20:36

Yeah, it will give some good ideas of how big stuff should be at room temp. I usually end up intentionally making one of the the pieces a little too big in one direction or the other and turn/bore it if I have to hit a tight dimension after everything is all put together

By gonzo

Date 01-26-2012 17:20

Thank you Mr Al .....
Your Instructions and informations is always helpfull
been doing this job that requires to be shrunk and fitted before welding
.....Thanks
Gonzo

By 803056

Date 01-26-2012 21:46

Happy to help.

Best regard - Al

By DaveBoyer (*****)

Date 01-20-2012 03:00

For what it might be worth, We put 3" diameter bronze bushings in dry ice, and they would drop into a bore that was .003" interferance at room temperature.

The issue is to have everything lined up so it goes in without binding, as metal to metal contact transferes heat quickly, and the bushing is no longer "shrunk".

By CLH1978 (**)

Date 01-20-2012 17:12

Great stuff. You guys are cool.

By 803056

Date 01-20-2012 17:31 Edited 01-20-2012 17:49

I have a quick question or comment regarding which temperature to use when calculating how much contraction will take place when using the liquid nitrogen.

Do you use the temperature of liquid nitrogen or the boiling point of liquid nitrogen?

I used the boiling temperature of the liquid nitrogen in my calculations because it is boiling when you place the part into the liquid. That is, when you pour the liquid nitrogen into an open container at room temperature (70 degrees) the liquid is boiling and undergoes a phase change from liquid into gas. If the surrounding ambient temperature was at -300 degrees F, the liquid nitrogen would not boil, nor would it transform into a gas.

Consider water, if it is in a container at room temperature, it does not boil. While some evaporation may take place, it is not transforming into a gas (steam) in any quantity. Once the water is heated to its boiling temperature, the transformation to steam is very rapid as heat is added (latent heat of fusion). I would expect the same is true in the case of liquid nitrogen. Except in this case, room temperature is well above the boiling temperature of nitrogen, i.e., 75 degrees is well above the -195 degrees required to bring nitrogen to a boil. The temperature of boiling water stabilizes at 212 degrees unless the container is closed and pressure builds. In a similar manner, if the container is open to the atmomsphere, the temperature of the boiling liquid nitrogen will stabilize at -195 degrees unless it is in a closed container, in which case pressure will start to increase.

The use of the incorrect temperature will either under estimate the amount of contraction or over estimate the amount of contraction that will take place.

OK, I see the error of my ways. My first source listed the boiling point of nitrogen as -195 degrees F, not -195 degrees C. It appears that the -320 degrees F is correct. Thanks for showing me my error. I just checked with a different source and saw the temperature error. The embedded graph says it all. Again, thanks for point out the temperature error. Oops!

Best regards - Al

NitrogenVapPressvsTemp20-Jan-2012.jpg (182k)

By fschweighardt (***)

Date 01-20-2012 21:19

I wondered if you had F and C mixed up somehow. Its just like boiling H2O, it shouldn't be hotter than 212F/100C. Liquid nitrogen can actually be a good bit warmer than the -320 if it is under pressure of `500 psi, it could be a "warm" as -235 F

By G.S.Crisi (****)

Date 01-20-2012 17:16

Flexible couplings used to drive rotating equipment (e.g., pumps and compressors driven by electric motors or steam turbines) are split into two halves, one of which is fixed onto the pump shaft and the other onto the motor shaft.
The holes in both flexible coupling halves are a few hundreds of a millimeter smaller than the pump and motor shafts diameters. So, the flexible couplings halves must be heated up to be put onto the shafts. When they cool down, they shrink and press the shaft, making a pefect joint.
The flexible coupling halves are heated into a lubricating oil bath, plain lubricating oil, the one that you buy at the service station. SAE 40/40W is fine. It can be heated up to some 200 degrees Celsius into a bucket. When it's hot, the coupling half is submerged into it until the desired temperature has been reached. Then, it's taken off the oil bath and inserted onto the shaft.
This is the method I've used dozens of times back in my days of erector engineer. Does it help you in solving your problem? Make calculations using all the information Al has given in his posting and check it out.

Take into account that handling liquid nitrogen is a dangerous task that requires experienced people; it's not a task for who doesn't know how to do it, or knows theoretically how to do it but has never done. Liquid nitrogen burns human flesh like fire does.

Giovanni S. Crisi
Sao Paulo - Brazil

By Milton Gravitt (***)

Date 01-20-2012 23:29

Thanks for everyone's help and I will keep all the info for use in the future. The planer over this job finely talk to the customer and got a waver to machine to fit. I think that was the easy way to fix the problem and the cheapest. We've have work on this job on an off going on a couple of years and this has always been trouble it looks good on paper but don't work out in the real world. The aluminum tubing don't run the same all the time.

M.G.

By DaveBoyer (*****)

Date 01-21-2012 03:10

One shop I worked in used a food fryer to heat oil for shrink fits. The thermostat controlled the temperature.

My cousin froze a small part of 2 of His fingers working with a cyrogenic liquid [don't remember what]. He said it got cold so fast He never felt the "cold" sensation. He was lucky, and had no permanant damage.

By rlitman (***)

Date 01-23-2012 19:35

Heh, that fryer idea is a pretty good one.
I know some people who work with cryogenic liquids (both nitrogen and helium, for MRI superconducting magnets), and the dangers of those liquids are probably worse than what the fryer can do.

You can actually safely pour liquid nitrogen over your fingers, so long as you don't "cup" it with your hand. The heat from your hand causes the liquid to boil so rapidly that a nitrogen gas layer insulates your skin from the liquid.
Still, I know one guy who had an accident with the stuff (although admittedly, it was liquid helium, which is many times more dangerous), and for weeks he had to get around with hands bandaged to look like basketballs.