Hi Wayne,
It took some digging, but I finally found out what material the washers were made from. It's Dupont Zytel® 101 NC010 - PA66 nylon. It has a Tg of only 60 C.
http://www.materialdatacenter.com/ms/en/Zytel/DuPont/Zytel%C2%AE+101+NC010/84f2fe69/839#page31724
At the end of wave pre-heat the screw/washers are seeing anywhere from 100-130 C. as measured by a Fluke IR thermometer. The boards get waved in a tin/lead process and I only have bottomside pre-heat on my wave.
It's no wonder the screws lose torque after wave, thanks for pointing me in the right direction. Now we just need to convince our customer... :^)
Steve
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Sunday, July 6, 2014 6:26 AM
To: [log in to unmask]
Subject: Re: [TN] Screws losing torque by themselves
Hi Steve-
I love the video. I've been trying to tell the mechanical engineers I've worked with about the problems with the helical washers (sometimes stupidly referred to as "lock" washers!). This video assists with developing an intuitive feel for what's going on. The helical washers do have their place, but not in a high vibration environment, and virtually always in addition to a thread locking system of some type (double nut or chemical adhesive).
The root of your problem is in the understanding of what Glass Transition Temperature means. It means that above that temperature, the material will deform PERMANENTLY if placed under a load. Nylon has an extremely low Tg, but even most PC board materials top out at about 170C.
So you're asking us why, when you put a clamp on some plastic material and take it way above its Tg, it seems to deform, thus reducing the clamp pressure?
Wayne
________________________________________
From: TechNet [[log in to unmask]] on behalf of Yuan-chia Joyce Koo [[log in to unmask]]
Sent: Thursday, July 03, 2014 5:28 PM
To: [log in to unmask]
Subject: Re: [TN] Screws losing torque by themselves
Nylon washer might made of nylon 66, that would be lost of mechanical strength during your wave temperature. see attached. IMO. (double check your datasheet for the washer for thermal properties. fyi.
http://www.plastic-products.com/spec1.htm
joyce
On Jul 3, 2014, at 4:19 PM, Steve Gregory wrote:
> Hi all,
>
>
>
> Thought I might try to catch you before you left for your 4th of July
> holiday. We have a strange little issue that we're trying to figure
> out that's been happening with some torqued hardware on a board that
> we build.
>
>
>
> The assembly has this heat-sink/bracket deal with four TO-220 devices
> put together before we stuff the board and wave solder it.
> The bracket is aluminum and we first put down a SIL pad, then the
> TO-220 devices, three of them get nylon shoulder washers, then a split
> lock washer, then a nut. The three devices with the nylon shoulder
> washers get torqued to 3.4 in/lb., and the one without the shoulder
> washer gets torqued to 6 in/lb. We tighten and torque from the screw
> side, and hold the 3/16ths nut stationary.
>
>
>
> Then the whole bracket assembly is stuffed into the board along with
> the rest of the parts, then the board is wave-soldered and cleaned.
> Any touch-up is done, then there are certain parts and points on the
> board that is RTV'ed and cured, then the board is shipped.
>
>
>
> Our customer has been finding that when they get the boards and they
> check that the hardware is torqued, they can get ¼ to sometimes a ½ a
> turn on the screws with the shoulder washers before the torque driver
> clicks, and of course we're getting zinged for not torqueing the
> hardware. The ones without the shoulder washers are not near as bad,
> but they tell us that they have been finding a few that they can turn
> a little before the torque driver clicks.
> We've got new torque drivers and they're set correctly. We even sent
> our drivers to our customer so they could check them out and compare
> them with the drivers that they have. They were pretty much identical.
> We use Lindstrom torque drivers, so they're not some cheap knock-off
> brand.
>
>
>
> Here's a picture of the bracket and screws we're having most of the
> trouble with:
>
>
>
> http://stevezeva.homestead.com/Bracket-Heatsink.jpg
>
>
>
> I've checked them here, and I find the same thing. I know they were
> torqued properly when the brackets were assembled, but when I check
> the boards that have been wave soldered and cleaned, I find I can get
> ¼ turn one the screws with the nylon shoulder washers before the
> torque driver clicks.
>
>
>
> I've always thought that when you used a screw and nut with a split-
> washer it would hold it's torque, but then I found this:
>
>
>
> http://www.boltscience.com/pages/helicalspringwashers.htm
>
>
>
> So now I'm not sure. Has anybody ever run into this issue? Is it
> normal to have things "relax" like this after they've been torqued?
>
>
>
> Steve
>
>
>
>
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