Are the nylon washers responsible for cold flow
creep (sometimes called cold flow) is the tendency of a solid material to move slowly or deform permanently under the influence of mechanical stresses
Mike Bogden
Engineer
From: Steve Gregory <[log in to unmask]>
To: [log in to unmask]
Sent: Thursday, July 3, 2014 3:19 PM
Subject: [TN] Screws losing torque by themselves
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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