holding LCD to PCB: split-ring lock washer and flat washer on both sides or one? - Page 1
Forum section: electronics / general

author: aandrew
author_rank: Frequent Contributor
timestamp: July 19, 2016, 12:22:06 am
content: I've got a PCB with a standard HD44780 based LCD on one side.I've typically used four M2 screws with four 8mm standoffs and nuts, and have used split-ring lock washers along with flat washers on the nut side of the connection:screw head -- flat washer -- PCB -- standoff -- LCDPCB -- flat washer -- split washer -- nutIt's not failing me (high vibration environment) but I'm wondering if I'm applying this correctly. Some people absolutely hate on split ring lockwashers while others say they work fine. I tend to be in the latter camp since I don't recall ever having to tighten or replace one with any repairs that have come back, but if there's a better way I'm happy to learn.

author: rich
author_rank: Frequent Contributor
timestamp: July 19, 2016, 02:35:35 am
content: I'm in the camp that believes split washers are a waste of time if the fixing is solid and properly torqued. Go take a car to pieces and count how few split washers there are in that vibrating environment.For cases where you think the nut may rotate itself loose, most likely due to the two PCBs sliding around then use nylocs or thread lock.Just because you've not seen one come back loosened doesn't prove the split washer did a better job than omitting them entirely, unless you've got data to the contrary?

author: Ian.M
author_rank: Super Contributor
timestamp: July 19, 2016, 02:54:53 am
content: Its a known problem that epoxy/FR4 PCB material tends to cold flow under high local pressure. Therefore the spring washers are performing an important duty of maintaining acceptable clamping pressure even if the PCB gives a little bit. You need the flat washers to distribute the load and prevent the spring washers digging in so there is no way of simplifying this assembly.It sounds like its goode enough but if you think you need better, a drop of threadlocker on each nut might be a possible improvement. However, if it is assembled in a confined enclosure before the threadlocker is fully cured, vapours may haze a plastic display window. Another option would be to switch to Nylock nuts, keeping everything else the same.

author: charlespax
author_rank: Regular Contributor
timestamp: July 19, 2016, 09:24:04 am
content: What about a plasticsnap-in spacer?

author: SeanB
author_rank: Super Contributor
timestamp: July 19, 2016, 11:35:24 am
content: If you want better torque control and a lot better performance, plus no need for the flat washer and split washer, use a Belleville washer. This is very good in providing a controlled clamping pressure, and will spread the load out over the board evenly.http://www.bellevillesprings.com/belleville-washers.htmlThe supplier I have used before, very good quality and for the size I bought and the spec very cheap. There are clones available for non critical uses as well, but YMMV.https://en.wikipedia.org/wiki/Belleville_washer

author: aandrew
author_rank: Frequent Contributor
timestamp: July 20, 2016, 12:55:26 am
content: Wow, some great responses! Thank you everyone for your input!I was thinking of putting a dab of threadlocker on, and had thought of nylock nuts before as well. We used Belleville washers in my previous life (industrial power electronics), for exactly the reasons you gave too.Unfortunately the plastic standoffs I don't think would fly; these are control pads for construction equipment. I think the first good drop on to concrete would snap them.Thanks again everyone!

author: Monkeh
author_rank: Super Contributor
timestamp: July 20, 2016, 01:03:51 am
content: Nylon standoffs are remarkably durable. They might pop out if hit just right, though.

author: ConKbot
author_rank: Super Contributor
timestamp: July 20, 2016, 01:57:44 am
content: +1 to nylocks. There are better lock washers then split ones, but once they are loose they are loose. The nylock will stay on the nut even if it comes loose.I've also seen vibratite vc3 specified for high vibration applications(backshell on a connector in a bulldozer so probably similar vibration to your situation) It's a non hardening thread locker, performs similar to a nylock, low outgassing, and reusable a small amount of times.