optimal Metal-PVC duct connection under stress

[mark mcfarlane]

I need to make two connections between 6" S&D PVC and Nordfab 6" machine adaptors. The metal Nordfab duct OD is 2-3mm too large to fit inside the 6" S&D. Close, but not quite.

For those not familiar with the Nordfab machine adaptor, it's basically a straight 6" pipe about 6" long, rolled on one end to connect to the upstream Nordfab quick-connect duct, and a straight pipe on the other end that will go into the S&D that runs under my slab.

One of the connections will have a rather large vertical stack of metal pipe, Wyes, and blast gates, maybe 40 lbs of stuff but all vertical force.

The second connection has a 90 degree coming out of it, then a wye, then a couple blast gates, ... so most of the force will be lateral rather than vertical.

What do y'all think the best way is to make these joints. Two ideas are to

(1) crimp the metal, pound it in and screw/tape it. I don't currently own a crimping tool.
(2) heat the PVC with a heat gun (I have one) and push in the the metal duct, screw/tape it.

Whatever I do, I'm hoping to do it Sunday afternoon. Could make a trip to the HW store and buy a crimper, or given that the diameters are so close, maybe needle nose pliers could be used. On one end of the OVC pipe the crimp will be against the air flow, but I could out some aluminum tape inside the joint to smooth things over a bit...

I haven't tried heating PVC pipe to stretch it, but have read of others in this forum doing so successfully. I can't afford to damage the in-slab PVC, but I have about 10" sticking out of the ground, so I could mess up a little and cut off a few inches and start over ....

[Bill Dufour]

Do you have a lathe? If you do I would make a tapered nose plug to expand the pvc. taper one end, then a straight section of the correct diameter to match up to the other pipe. Could you use a fernco? For the vertical no reason not to hang a vertical piece of plumbers tape from the ceiling and attach it to each fitting in turn. Or maybe every other fitting would be enough.
Whn I had to expand a fernco fitting to go over a rusty iron sewer pipe, three feet underground, I used a expanding pneumatic pipe plug, cherne plug, to expand the rubber for a few minutes then quickly remove it and slipped it in place easily. The rubber shrank back down pretty quick and the hose clamp has kept it in place for two years so far. I did buy a 6" cherne plug to use with PVC dust pipe. All I have used it for so far is to plug one run.

[Lane Hardy]

I do not have a lathe, however I needed to make an odd size PVC fitting to connect to a shop vac hose the hose was larger than any standard fitting available some where near 2 1/4 inches but nothing would fit that I could find. A Fernco would work but the plastic vac hose would crush under the pressure of a hose clamp so that was not an option.
My solution! Pop another top! Oh just kidding.
I do have a drill press, I glued several pieces of pine to make a 2 1/2 blank almost a cylinder. I drilled a hole down the center. Threaded a section of all thread with jam nuts on both ends. I chucked the all thread into the chuck. The bottom end I used a chunk of red oak and drilled a shallow hole more like a divit to act as a bearing / bottom stop for the all thread rod. (I filed the sharp edges down) added a drop of machine oil. I then used the drill press as a lathe,
true the bearings on a DP were not intended for side loads and the Morse taper does not like side loads. Go easy and try not to hog out a lot of wood and it work. Not ideal but it work. I used a rasp rather than turning tools that do have.

I made a tappered chunk of wood smaller than the ID of the Plactic shop vac Hose and the other end larger than the shop vac hose. This would serve as a mold for the 2 inch PVC pipe adapter. One side smaller than 21/4 male end, the other larger than 2 1/4 female end.

I cut off a section of 2 inch schedule 40 PVC pipe heated with a heat gun, on the larger end I slid the hot playable PVC over the mold this stretched the PVC to the desired Diameter. On the other with the PVC still on the tappered mold.
heatef the end that I needed to make smaller that 2.00 inches I used hose clamps to compress the diameter.

on another fitting I made to tie a dyer vent to a metal duct I had a similar issue both pipes were very thin ducting that would not support a pipe clamp. I cut a four inch section of a 4 diameter pvc pipe. I then cut a 1/4 inch slit in the PVC pipe I compressed the pvc with my hands and the pipe uslide into the two pipes and gave me solid piece to tighten a hose clamp around.
cheap coupling!

[David L Morse]

... I can't afford to damage the in-slab PVC, but I have about 10" sticking out of the ground, so I could mess up a little and cut off a few inches and start over ....

One way to avoid damage to the PVC pipe is to avoid directly interfacing it to the Nordfab fitting. Instead, use a PVC coupling for that. This approach keeps the end of the pipe clean and lets you do the work at a bench instead of on the floor. I think it also flips the mismatch, the metal is now too small instead of too big and you have a gap to fill.

If you prefer the tight fit of a heat formed joint a coupling might be useful there, allowing you to work on a short length of PVC rather than the precious stub. I haven't tried it yet but I've read that boiling water is a good way to heat the PVC.

As a first attempt at this I would probably try a short length of PVC inserted in a coupling then placed in 3 or 4 inches of boiling water. With the coupling end out of the water there's a place to grip and it's less likely to go out of round. It would probably help to first chamfer the inside of the pipe to make it easier to start the stretch.

[mark mcfarlane]

Thanks Bill an Lane for the ideas. FYI, I don't have access to a lathe. I can try local HW stores for Fernco adaptors. I really want to fire this thing up today, been waiting too long.

[mark mcfarlane]

Great idea David, thanks. With a half dozen wraps of duct tape I'd probably have a decent fit of the metal inside the coupler. That would at least work very well for the vertical-stress joint. Now I just need to find 6" S&D couplers, which might mean waiting until Monday.

[Andy Giddings]

Would a Fernco rubber coupling work? I used some on my 6 inch S & D when I had odd size connections to make and wanted some flexibility yet a strong connection

[Jim Becker]

If you roll some sheet metal and place it inside the Nordfab component, will that slip into the plastic?

[mark mcfarlane]

The Nordfab OD is already larger than the ID of the PVC. Adding another layer would make the fit worse. Sorry this wasn't clear.

[Bill Dufour]

I have been known to go to the grocery store with my tape measure. I found the tall pineapple cans make a nice fit for 4" dust piping. My wife eats the pineapple and I get a empty can that fits We are both happy. I think the #10 cans may be close for 6" duct work? I do not remember.
Bill

[mark mcfarlane]

For there record, tapering the PVC pipe inside a tad with a half round file, followed by 2-3 minutes of heat gun touching (at an angle) the PVC pipe and moving in circles worked like a charm for the joint that had a vertical stress. I was able to easily insert 3" of the machine adaptor into the S&D PVC without any crimping and it held fairly tight.

My other joint is going to have so much lateral stress on it I'll probably need to build a wood brace from the floor.

tree.jpgvjoint.jpg

Sorry about the annoying rotation, that keeps happening for some reason.

[Jim Becker]

A roll of sheet metal on the inside of the Nordfab would be smaller...but if it's still too big to fit in the PVC, the point is moot.