Well from my perspective I would look more towards the nylon shouldered 
washers are relaxing when the boards are subjected to the heat of wave 
soldering.   If the nylon is not specified then you may find more 
consistent torque with fiber washers, little more expensive but may 
stand the stress better.    Check with Keystone electronics for the 
shouldered fiber washers.    The other thing is re torque after 
soldering.

pat goodyear
Former control tech.

On Thu, Jul 3, 2014 at 1:19 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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