shrink-fitting with heat/cold

[Don Krag]

How much do y'all typically oversize/undersize sections when shrink fitting with heat? What diameter sections were you using? How hot do you typically get the hot piece? How "easy" do they go together?

I have a 6" section that is sitting at 0.012" over sized. I'd really hate for it to get stuck only halfway in. I typically heat hot hot piece up to about 900 *F (very faint red in dark light conditions) and cool the other piece down in a cooler. For section on the order of 2" - 3" I have ~0.004 - 0.006 oversize and have to use a hammer to hammer in the sections.

Anyone want to share their experiences/methods?

[GGaskill]

The rule I have used is .001" plus .001" per inch of diameter.  So a 6" sleeve on a 6" shaft would be .007" smaller than the shaft.  The expansion ratio of steel that I use is 6x10^-6 inches per degree F per inch diameter so 900° F on 6" would give about .029" expansion, assuming an ambient temperature of 100° F for easy calculation.

Work fast!

[Rickk]

I used to work in an aircraft engine plant. The would heat up turbines in hot oil, and put the shafts in horizontal fridges filled with dry ice. Dry Ice is something like -65C, which is an extra 50 or so C below what you can get in a regular freezer.

[AaronH]

At a place I used to work at we did a lot of shrink/press fit. Mostly .500 to 1.500 diameter and 1.250 to 4 or 5 inches long. When I started there they kept liquid nitrogen on hand. They'd pour some into a Styrofoam cup and let the part soak in there for a few minutes. We used the nitrogen so little we'd end up filling our can everytime we needed to press something. Liquid nitrogen boils at around -320 degrees F.  We found that we could heat the outer piece in a small oven to over 400 degrees. So, we would get more expansion. I think partially due to the fact that we could let the outer piece heat soak in the oven longer and get a better temperature transfer than we could with the nitrogen. Nitrogen is scary stuff anyway and I don't think anyone was sad to see it go. When you use the heat method you have to be somewhat aware of what you are doing and what you want as an end result. Anything over 500 degrees can alter the structure of the steel and change it's characteristics. So, if you are working with heat treated material, or you don't want it heat treated when you are done you better stay below 500 degrees. Note: this is more to do with self hardening steels like tool steels and such. Low carbon isn't as a big deal. Whatever method used you have to move quick. As soon as the heated (or cooled) piece touches the other it will rapidly shrink (or expand) and get stuck. However, we noted that the heating method was more forgiving with time (probably due to the greater dimensional change). We had a 10 ton hydraulic press so generally if something didn't make it all the way it wasn't a problem. We did discover though that when you drop the smaller piece into the heated hole you better actually push it down and hold it. Otherwise, when it hits the bottom it will bounce and then stuck on the rebound.

We also used the .001 per inch rule. Works pretty good, especially anything 2" diameter and under.

Aaron

[Don Krag]

Thanks for the info!!

Looks like I'm well within the range on 6" then. Speed is going to be the killer on this one. There's just no quick way to cram a 40lb section into a 170 lb section. I'll have three of my brothers-in-law in town next week, plus a few friends...so hopefully I won't have to work too hard. They actually started carrying dry ice in the grocery stores here, so I can cool it down with that. Hopefully, being large masses, they'll hold temp well enough for us to get things where they need to be. If something does go wrong, I know where some 30-40 ton presses are.

Aaron,
These are all normalized, so temp isn't much of an issue. I certainly wouldn't want to go over the critical/austenizing temp for either piece...but that would probably take over an hour in the furnace anyway! So why didn't y'all like the LN2? I'd love to have some here for ulta-low quenching of some of my high alloy knives, but it's too cost prohibitive! Of course, last year they had a 500lb cylinder do a rocket impression on campus. A lab was using a "repaired" cylinder and had plugged the vent to "avoid losing all that LN2 in our heat". Idiots. The bottom blew out along a deteriorated weld seam and launched the cylinder straight up.