Serial port lock nuts always coming loose - Page 1
Forum section: electronics / projects

author: tkamiya
author_rank: Super Contributor
timestamp: October 15, 2019, 11:34:08 pm
content: I think one of very annoying thing on PCs are those DB<blah> connectors having lock nuts coming loose. I can't tighten them too much or otherwise the thread on bolt or nut will break.I had number of VGA connectors and serial ports come loose. Are there anything out there that won't come out like these? Long time ago, I've seen a rectangular piece used in place of stud/screw combination but I have not seen them recently.Someone help before I go "nuts".....

author: dom0
author_rank: Super Contributor
timestamp: October 15, 2019, 11:35:50 pm
content: Loctite

author: hendorog
author_rank: Super Contributor
timestamp: October 15, 2019, 11:50:26 pm
content: I never screw them in as long as they are a tight fit. Occasionally they fall out but that is better than having to unscrew the bastards while lying awkwardly under a desk.

author: tkamiya
author_rank: Super Contributor
timestamp: October 16, 2019, 12:52:37 am
content: Locktite is nice but I need to be able to take it apart when I need to.

author: sokoloff
author_rank: Super Contributor
timestamp: October 16, 2019, 12:56:29 am
content: Loctite 242 ("blue Loctite") easily comes apart with hand tools.Loctite 271 ("red Loctite") is intended to be permanent, but on threads this small, also tends to be removable without excessive heat.

author: coppercone2
author_rank: Super Contributor
timestamp: October 16, 2019, 01:39:55 am
content: there is also lesser known and weaker purple loctite.

author: duak
author_rank: Super Contributor
timestamp: October 16, 2019, 02:50:58 am
content: The screw & standoff connector retention system is (IMHO) kind of marginal with a number of weak links - something will loosen or break or strip or ...First of all, get rid of the split lock washers & replace them with star washers. If not, just ditch them. Seehttps://engineerdog.com/2015/01/11/10-tricks-engineers-need-to-know-about-fasteners/Loctite can work well, but the combined nut and standoff can spin together - but at least the nut won't be floating around inside the equipment. Some heat on the Loctite makes it easier to remove it if needed.

author: tkamiya
author_rank: Super Contributor
timestamp: October 16, 2019, 04:19:11 am
content: FUNNY! I was at that exact site studying it.I replaced my regular nut with flair nut with serrated washer. Much stronger! Now it's a lot more secure. I don't think I need locktite them anymore. I'll put it to use and see. I have a lot of older broken HP gear. I take them apart for parts but I keep all the screws and nuts. They are often very high quality stainless steel type. That's where my flair nut came from.

author: duak
author_rank: Super Contributor
timestamp: October 16, 2019, 04:59:04 pm
content: Stainless hardware is good because it doesn't rust. It does have problems though. Unless you're talking about special hardened fasteners, It's not actually stronger than ordinary mild steel. It's also quite soft so It can also gall or stick to some metals, in particular to itself. Here's an article on it - I agree with with most everything except for using WD-40. WD-40 is mostly solvent with a bit of something that dries up and gets sticky. A teeny bit of grease or wax on the external threads can really help. For the internal nut and standoff, galling is an asset - no need for Loctite!https://www.boltdepot.com/fastener-information/materials-and-grades/thread-galling.aspxSome retention hardware is plated brass. Brass doesn't gall easily but is softer and weaker than steel. In a torque competition with a steel screw, most of the time the brass standoff's threaded section will break or the internal threads will strip before the screw's will.

author: dom0
author_rank: Super Contributor
timestamp: October 16, 2019, 05:35:10 pm
content: Things that have been retracted for screw locking because they don't really work:- DIN 127, DIN 128, DIN 6905, DIN 7980: Split and spring rings-- these are generally considered to aid loosening of fasteners- DIN 137, DIN 6904: Split and spring washers-- DIN 137 style washers work quite well for connections on softer materials. For example, for screwing down a PC board to a standoff a DIN 137 washer will probably not come loose. These are/were pretty common for exactly that purpose in industrial electronics.- DIN 6797, DIN 6798, DIN 6907 Toothed and serrated washers-- DIN 6797 work to a limited degree on soft materials (e.g. PC boards)DIN 6796 Belleville washers work for fastening things in general, but would not work in this application since you are applying an external torque. These washers are completely unsuited for that. A toothed/serrated washer will appear to work at first but if you put a bunch of torque cycles on this they'll loosen rather quickly.So in terms of mechanical locking only locking nuts, and north lock (DIN 25201) or crimping/riveting would work in this application.That's why I recommended Loctite. It actually works and isn't speciality hardware.

author: magic
author_rank: Super Contributor
timestamp: October 16, 2019, 05:39:35 pm
content: One thing which will never be retracted for screw locking because it works:a pair of nuts locked against each other

author: dom0
author_rank: Super Contributor
timestamp: October 16, 2019, 07:53:37 pm
content: That works as long as the tension between the nuts is much greater than the tension on the bolt. The usual two-wrench technique ensures this, normally...

author: gibbled
author_rank: Regular Contributor
timestamp: October 16, 2019, 08:50:30 pm
content: Slight marring of the male threaded portion works well, just a squeeze with a pair of pliers is sufficient.

author: boffin
author_rank: Unknown
timestamp: October 16, 2019, 09:28:44 pm
content: Quote from: dom0 on October 15, 2019, 11:35:50 pmor, just as effective, a bottle of nail polish. Just dab the thread before putting it in. Just enough stick to stop them from coming loose, and cheap cheap cheap

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 16, 2019, 09:56:40 pm
content: Quote from: magic on October 16, 2019, 05:39:35 pmThis brings up the interesting case of using a regular nut plus a thin jam nut. The correct assembly order isn't what most people think.

author: coppercone2
author_rank: Super Contributor
timestamp: October 17, 2019, 06:13:23 am
content: is the dsub connector correct for repeated connection cycles is the more interesting question.

author: cs.dk
author_rank: Unknown
timestamp: October 17, 2019, 06:26:47 am
content: Quote from: magic on October 16, 2019, 05:39:35 pmNord-Lock does not agree. See this vibration test; The clamping force goes down pretty quick.

author: coppercone2
author_rank: Super Contributor
timestamp: October 17, 2019, 07:15:32 am
content: nord lock washers on a computer part will make someone have a heart attack

author: Fred27
author_rank: Unknown
timestamp: October 17, 2019, 07:42:45 am
content: Nord-Lock does look impressive, but I wonder how the other mechanisms would have worked without lubricating the bolt first.

author: tautech
author_rank: Super Contributor
timestamp: October 17, 2019, 08:08:15 am
content: Quote from: Fred27 on October 17, 2019, 07:42:45 amWouldn't be a fair test without lube because lube makes the torque setting accurately repeatable.Further, these types of lubes/anti-seize are commonly used in industry in permanent exterior installations.

author: cs.dk
author_rank: Unknown
timestamp: October 17, 2019, 08:08:55 am
content: Quote from: coppercone2 on October 17, 2019, 07:15:32 amI know that's not a job for Nord-lock. I just replied to the "a pair of nuts locked against each other" - Which won't work on a serial port either.In my previous work we used Nord-lock for the knives in wood chippers. You don't wanna knife to come loose in there!I don't think the bolts are lubricated makes much difference.

author: Electro Detective
author_rank: Super Contributor
timestamp: October 17, 2019, 08:44:32 pm
content: Quote from: boffin on October 16, 2019, 09:28:44 pmBeat me to it

author: boffin
author_rank: Unknown
timestamp: October 17, 2019, 11:12:36 pm
content: Quote from: Electro Detective on October 17, 2019, 08:44:32 pmI once went to the discount store, bought a $1 bottle of nail polish (with handy brush applicator too) in the ugliest colour I could find (bright orange), and my girlfriend at the time still pinched it from my workbench. <sigh>now I just grab a bottle of my wife's polish from the bathroom when she isn't looking.

author: DBecker
author_rank: Frequent Contributor
timestamp: October 18, 2019, 01:10:22 am
content: Quote from: cs.dk on October 17, 2019, 06:26:47 amNice video. Great production values. I'm almost convinced. But what's going on 2:08? How did that upper lock nut spin less than the lower nut? Was there an optical illusion? Was the lock nut collapsing? Doesn't that indicate incorrect assembly torque?

author: tautech
author_rank: Super Contributor
timestamp: October 18, 2019, 04:42:01 am
content: Quote from: DBecker on October 18, 2019, 01:10:22 amThink about it. When one nut is torqued onto another the bottom nut is effectively loosened in that its threads no longer bear upon the bolt thread with the same force as the lock nut is forcing it forward off the threads.I've found that when using lock nuts you need go back and tighten the lower nut with an open end spanner and then hold it while you bring the top nut tight on it again. It's all a bit of a science getting the best from fasteners.

author: sokoloff
author_rank: Super Contributor
timestamp: October 18, 2019, 07:33:32 am
content: Even so, in order to vibrate loose, the top nut would seem to have to turn more not less. It’s true that the clamping force on the through bolt might be reduced, but the local strain to jam the nuts together should still preclude the top nut turning less than the bottom one.

author: duak
author_rank: Super Contributor
timestamp: October 18, 2019, 05:36:52 pm
content: FWIW, Polylocks are available for automotive valve adjustments. They are easier to adjust and lock than a jam nut combination. Easy to implement with threaded standoff and a setscrew. Complete overkill for serial port locknuts.For the absolute best removeable fastener retention, you can't trust friction. You have to use something like castle nuts and cotter keys or safety wire.