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Old 08-24-2016, 09:17 PM
foamcar foamcar is offline
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Default How to shrink a tube

The question is basic, but I'm afraid the answer will be advanced way beyond my skill set. But, worth a try. This is a 356 Porsche front axle beam, about 2" O.D. and 1/8"-3/16" wall thickness. Someone tried to pry out a press fit needle bearing and looks to have stretched the outer end of the beam. I have inserted a socket of 51.8mm diameter to use as a dolly and got the end round again. But the old bearing is a slip/slop fit, not a press fit. I think the bearing is a little undersize at 51.9mm as new is slightly over 52mm. I have ordered a new one but don't expect it to be a press fit. On the 356 Registry forum I guy suggested possibly shrinking the end with an external burnishing tool. Never heard of one of those, but looked it up and appear pretty expensive unless I can find someone local who has one and is willing to take a chance trying it. Any ideas on how to shrink the end of this tube?

distorted axle tube 1.jpg

sorta round again.jpg
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Old 08-24-2016, 09:42 PM
JimRussell JimRussell is offline
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Phil,

I have not done this, but I think it will work. As I understand heat shrinking, the heated (expanded) metal forces itself into the unheated metal. When the heated metal cools and contracts it can't return to it's former surface area so it shrinks the surface. What I would do is heat the tubing a small stripe at a time front to back. Cool each area before going to the next. That should reduce the diameter of the tubing. If you heat the whole tube you wouldn't have the cool metal for the hot metal to force itself into. It would be worth a try on a piece of scape tubing.
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Old 08-24-2016, 10:00 PM
foamcar foamcar is offline
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Thanks Jim. Nice low tech that I could do. Good point about a strip at a time, and to practice. Now to find some practice tube.

Phil
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Old 08-25-2016, 12:19 AM
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Joe Hartson Joe Hartson is offline
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Phil, Do you have a way to accurately measure the ID of the tube? Would like to know how round the ID is and is there any variation from the open end to the depth of the bearing, taper. Looks like the tube is bored and there is a shoulder at the end of the bore. There are several ways to fix your problem depending on how much clearance you have to reduce.

I am assuming that you need to press an outer race in for the needle bearing. If the clearance is only a few thousands you could use a loctite product if there is little or no taper. Knurling the ID is another option and evenly compressing to OD is also possible. Heat shrinking is also a possibility as Jim stated but the amount of shrink is hard to control.

Need to know what the condition and size of the ID to suggest what would be the best solution.
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Old 08-25-2016, 06:55 AM
Bluchip Bluchip is offline
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Couldn't you simply use a close fitting (to final desired OD) 2 piece shaft collar arrangement? Find a slightly undersized mandrel to support ID. Heat stub tube and quickly slip the shaft collar over and tighten. You can grind down the split mating surface of the collar to allow for spring back of parent material. Repeat process with collar mate surface clocked at different positions about the stub tube to assure roundness. Of course, if you have the facilities, you can make such a collar from steel plate of whatever proportions you need for strength. If choosing a collar, try the strongest materials and high grade fasteners as it may take some force.
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Old 08-25-2016, 07:34 AM
foamcar foamcar is offline
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Joe
I have tools to measure. Here is data on the O.D. of the upper and lower tubes. The diam. variation of the upper is .014" and lower .058". Have not measured since I made it rounder. I will use a bore gauge to see what the I.D. is now. The inside of the tube appears to have been machined, as you can see a step back about 40mm. Below is pic of bearing.

data.jpg

repair tool.jpg

original needle bearing.jpg

Eric
The shaft collar looks like a great idea if I can find one close. Beats what I tried below.
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Old 08-25-2016, 07:48 AM
Oldnek Oldnek is offline
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The way I have shrunk pipe around that wall thickness, is to find a heavy walled water pipe or steam pipe larger than your outside diameter.
Drill a hole in the end of the pipe and weld a nut to it so you can screw a bolt of the same thread into it. Same principle as a adjustable tent pole.
Fit that over your stretched pipe and tighten the bolt, work your way round a couple of times, and it will shrink you pipe.

Tool. copy.jpg
If you go to far stretch it the same way you made it round.
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Old 08-25-2016, 09:21 AM
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Phil, The ID condition and size is the only thing that matters, you need to support the bearing OD evenly and stop it from being able to turn in the bore.

When you get the ID measurements post them. Again I am interested in taper of the ID and how much variation there is in the OD of the tube.
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Old 08-25-2016, 12:50 PM
bobadame bobadame is offline
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If you have access to a lathe, make a solid steel slug the same diameter as the bearing outside diameter. Drill and tap the center so that you can pull it back out with a bolt. Insert that in to the bore. Tap the lump down against the slug. At this point you could use a Loctite product called Bearing Mount and be done. Or you could reduce the diameter a little at a time with a torch, either tig or gas until you get the fit you want. Do this with the slug in place. If the slug becomes stuck, tap around the outside of the tube to stretch it back out a bit.
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Old 08-25-2016, 01:04 PM
foamcar foamcar is offline
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Wow! Thanks for all the great advice. I had ordered 2 new 20mm needle bearings to pick up tomorrow in Saginaw, on my way down to Ann Arbor(MI). They also stock a 2.25" shaft collar, so will pick that up also. Am heading to my grandson's place of employment open house Sat. He just finished auto tech at a local community college and wants me to be there. So, will probably get the better I.D. measurements when I get back Sunday P.M. or Monday. I like the pipe idea also. If the shaft collar does not work, will give it a try, but need to find right size pipe > 2.25".

Phil
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