Power Distibution Board - Broken stud

[Jon Stratis]

Originally Posted by de_

If you only install 1 board a year and a mentor does it as opposed to a random student, then sure you are less likely to have a problem.

But in our team, random students installed at least 5 boards on various desktop and test & competition robots this year alone (many when I am not there).

What is the probability a student will realize the stub is brass and its tensile strength is 19K PSI versus the grade 8 fasteners we use which is 150K psi. Zero. We don't teach metallurgy in our high school. We don't own a torque wrench. What student could possibly recognize the metric nuts Zero - looks just like a 1/4 nut. Standard 1/4" 20 nut fits on the stud and tries quite happily to cut a thread into the soft brass. That is how a student trashed our first PDE.

This is a $190 hidden mine waiting to explode, especially in a dynamic high student to mentor ratio environment. Count your self lucky if you have not had it happen yet.

The particular student in this case has install a lot of PDEs over the years. He's installed a lot of brass pneumatic fittings in aluminum with no issues. He has no history of brute force. He used a small wrench. So don't blame the student.

Our robot is probably 50% over budget this year and now we should be buying a new PDE and a new Digital side cars as both did not survive even a minute in our student environment. Is it the students fault or is it a design fault? I can't find fault with the students.

What I will do between trying to get a robot built and buying replacement parts is try to find the time to make a warning label for future PDEs that warns the studs are brass and metric and will break 7.5 times easier than a grade 8 bolt. Something FIRST might consider. I will also send a link of this post to FIRST.

It really comes down to student training. You don't have to go into too much detail or talk about metallurgy. We have a small bag in our electrical kit labeled "PDB Nuts" with spares of the correct nuts in it. The students on the electrical team know its there, and we make it a talking point every year that it takes different nuts than the rest of the robot. When we go over all of the electrical components, we stress that they need to make things tight, but they can't crank down on them like they would on frame members - plastic will crack, and even if it doesn't it will compress and damage circuitry. Our students do all the install work for us, and we have never had a problem. We typically install 2-4 of them each year, as part of practice robots, demo's, bench-top test boards, etc.