Hi Wayne-

I don't understand your comment on "double locking mechanisms". One is
obviously the nylon patch, but if the other is a lock washer that seems to
run against your other comment that "As a CM, I flag designs that apply a
lock washer...."

Thanks,

Wayne Thayer

On Thu, Aug 30, 2018 at 11:23 AM Wayne Showers <[log in to unmask]>
wrote:

> The best hardware setup I have used (WABTEC and GETS, end of train cars,
> so vibration is an issue) was unplated holes with a pad, SEMS hardware with
> a nylon patch applied (double locking mechanisms).  Fastenal offers this
> service, but I am not sure if they sub-contract it out or not.
> The unplated hole was nice to avoid the potential for hardware scraping,
> the pad was nice for easier hole identification (operator aid), and the
> SEMS has all the right 'stack-up,' so it saves on having multiple part
> numbers to track down.
> Cost differential of a screw, lock, and flat versus the SEMS was around 3
> cents on average (small bump in price, but well worth it).  In helicopter
> applications, I am still required to strap the hardware, but have never
> (Knock on wood), had any hardware come loose.
>
> As a CM, I flag designs that apply a lock washer directly to the PCB
> whether there is a pad or not.  I can usually get the customer to update to
> a SEMS.
>
> The other advantage of the unplated hole with a pad is that if at any
> point a Press-Fit PEM(R) or other hardware is applied, any cracking that
> occurs (it always does), the copper pad limits the extent of the cracking.
>
> Recommendation, have an unplated hole with a pad.  Reduce the pad diameter
> if needed, but the flexibility and advantages outlined, I feel, outweigh
> not having a pad on the PCB.
>