Thread locking - Stainless steel into PVC

I have a 1/2" x 20 UNF bayonet style electrical connector threaded into PVC.
The PVC is the standard gray material bought as a round bar off the shelf.
Mechanical constraints prevent using a locking nut or radial set screw for retention.
Does anyone have any suggestions as to what chemical means might be useful to lock or bond the (probably stainless) metal connector to the PVC to prevent unscrewing?
Thanks.
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I can't tell you what to use, but I can tell you what NOT to use -- Loctite. It attacks most thermoplastics.
--
Ed Huntress

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On Mon, 11 May 2015 08:43:56 -0400
<snip>

And I'll second that from experience. Loctite would be a big mistake...
--
Leon Fisk
Grand Rapids MI/Zone 5b
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On 11-May-15 9:03 PM, Leon Fisk wrote:

Thanks, I didn't know that. I was thinking of one of the Sika urethane adhesives perhaps.
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On Monday, May 11, 2015 at 8:23:10 AM UTC-4, snafu wrote:

No experience, but I would try the pvc pipe adhesive.
Dan
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On 11-May-15 9:03 PM, snipped-for-privacy@krl.org wrote:

Hi Dan, I thought of that too, but figured it would simply soften the PVC which would then re-harden after the solvent has evaporated. It's an easy test so I should give it a go. Thanks.
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On Monday, May 11, 2015 at 9:10:21 AM UTC-4, snafu wrote:

It might work better if the threads are rough rother than nice smooth surfaces.
Dan
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On Mon, 11 May 2015 06:34:04 -0700 (PDT), " snipped-for-privacy@krl.org"

Boat/Marine supply stores have tons of sealants that bond to multiple materials including PVC and stainless. You might consider Dow Corning 795 - I have used it to bond plexiglass windows to stainless hatches. It probably would work with PVC but check the materials sheet.
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On 12-May-15 9:47 PM, William Graves wrote:

I'll check it out - gave me an idea - I might look at Sika 291, its a urethane adhesive that I *know* sticks very well to PVC & stainless.
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Part of the question here is whether you want to *permanently* lock those threads, or if you want to be able to disassemble the joints in the future. The commercial thread-lockers, including both Loctite acrylics and the commerical epoxy beads used in production, are calibrated to keep a tight joint and to tolerate a lot of vibration, but also to have a low breakaway shear strength that allows the joint to be unscrewed when necessary. Loctite, for example, comes in different degrees of breakaway shear strength for different jobs.
If you don't care about future disassembly, ordinary household two-part epoxy should do the job. The amine hardener is pretty active, however; I've never checked to see which plastics are safe with it, but it should be easy to Google. Unlike the epoxy beads made specifically for thread-locking, household epoxies are not made for specific breakaway shear strength.
If you do want to be able to disassemble, I'd go for one of the gooey pipe-thread sealants. I've used a liquid Teflon-containing sealant for decades, and it both locks joints tightly and allows joints to be unscrewed. I have them all over my house, in plumbing and (hot water) heating, and they work perfectly. I have some of them on metal-to-PVC threaded joints and I've never noticed any deterioration of the PVC.
You application doesn't sound particularly demanding. It should be easy to find some sealant that has decent joint strength without permanently binding the joint, and without destroying your PVC. Read the labels on the tubes in some hardware-store plumbing departments. They'll probably answer the PVC question.
--
Ed Huntress

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On 13-May-15 10:09 PM, Ed Huntress wrote:

The epoxy is another route worth looking at. We already use an epoxy with the same PVC ans it bonds quite well. It will actually break away some of the PVC if you try to chip it off too fast. The main worry is the connector is a bayonet style and we have concerned end users may over tighten or not push inward when undoing them and force the connector to unscrew rather than disconnect.
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    [ ... ]

    [ ... ]

    Hmm ... from the description, snipped a few posts upthread, the connector sounds like a BNC. And it is common for those to have a D-shaped threaded stem, so it will be non-rotating in a punched hole in sheet metal. If so, then the solvent (not really an adhesive, I think) would soften the hole and allow it to deform partially into the flat of the 'D', so that would likely work.
    As for the material of the connector -- assuming it is a real BNC, it is likely a copper alloy, plated with either nickel for the low grade ones, or with silver for the really high quality ones.
BTW    The BNC is also supplied with a square flange with four corner     holes for screws, so you could drill and tap the holes for those     in the PVC if that would work better for you.
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On 14-May-15 10:36 AM, DoN. Nichols wrote:

Hey Don, it's a large bayonet style connector. It has half a dozen conductors but uses a bayonet fitting about 2x the diameter of a BNC to lock & seal. Judging by the price and the look of the thing I'm pretty sure its a custom made connector housing.
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    [ ... ]

    [ ... ]

    Oh -- multi-pin. Glass (light green) seal holding the pins, or dark brouwn soft rubber?
    Three bayonet ears instead of two?
    If so -- it sounds like one of the MS series connectors, which come both with solder cups (and a rubber strainer which is held in place by screwing onto the back of the connector body -- usually with a metal ring inside the shell), and with crimp-on pins, which are inserted from the back of the connector into the soft rubber pin separator/supporter.
    If glass seal, and bare metal finish, it is for soldering onto the housing of hermetically sealed things like aircraft instruments.
    But the thread at the back sounds like the ones made for as screw-on back shell -- with or without a cable clamp, depending. If so, that *should* have the rubber strainer through which each wire passes, and which is then squeezed down by the back shell and ring, to make a water-tight connection.
    They used to be refrred to as "Bendix" connectors, but the line is now produced by Canon -- and some by Deutch and other companies. All at painful prices. :-)
    If 6 pins, it should have moulded into the rubber separator some number endig nin "-6" -- the number being the diameter of the mounting hole in 16th of an inch IIRC. Maybe 32nds of an inch. I would have to dig back into the catalog to be sure. The whole shell should have a number staring with MS-??? which defines the style and shell size.
    Good Luck,         DoN.
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On 17-May-15 9:45 AM, DoN. Nichols wrote:

No its not the Amphenol MS series. It's a 2 pin bayonet with a 1/2NF threaded back end. The internal connector parts look like the small Lemo stuff. It comes with flying leads potted in to the back end.
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On Monday, May 11, 2015 at 9:10:21 AM UTC-4, snafu wrote:

Tell an Electrical Supply store that you need to see their NMC fittings.
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If it has a hex area above the thread, perhaps you could plastic-weld some more PVC material up around that to keep it from unscrewing.
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On 11-May-15 9:57 PM, Larry Jaques wrote:

Good idea - unfortunately its cylindrical.
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Then I'd try epoxy.
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snafu wrote:

Take the connector out. Mill a slot up one side to half of the depth per width (1/8" wide slot Mill 1/16" deep). Install the connector and use a drill bit to "transfer" the slot to the PVC and finish the hole to size. (1/16" in the connector 1/16" in the ABS). Now take a chunk of PVC welding rod or ??? that is 1/8" and apply some PVC glue and insert it into the hole. Once it cures the connector won't move. Basically a linear locking spline. Same idea as used on key lock thread inserts. However instead of using the steel pins those use you have PVC. That way if you ever did need to remove it you could heat the connector and turn it to shear the PVC spline.
--
Steve W.

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