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. >