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.

[de_]

Its not student training: The last one broke with a well trained student. We will not get a better student.

[Ether]

http://www.chiefdelphi.com/forums/sh...90#post1240690

Are these two problems related?

[de_]

Kevin, Where are you getting your data re other teams experiences re the stud ? Have you surveyed them ?

I have already stated I know exactly why how the current PDE ended up as is. So don't go there.

I can tell you 0 studs failed on the previous design (that was a $30 part). They were not brass.

[Kevin Sevcik]

Originally Posted by de_

Kevin, Where are you getting your data re other teams experiences re the stud ? Have you surveyed them ?

I have already stated I know exactly why how the current PDE ended up as is. So don't go there.

I can tell you 0 studs failed on the previous design (that was a $30 part). They were not brass.

Nothing scientific, just subjective impression based off the teams I work with here in Texas, and the lack of outcry on CD. If it was a very common problem, you'd see a lot more about it here. Look at the kerfuffle on the BAG Motors. By Paul Copioli's count, it's something that's affecting a very small percentage of motors, but there's several teams on here talking about it.

[FrankJ]

Here is a data point: We have six of these boards on various robots. We have one a broken stud. Relatively untrained student screwed on a 1/4-20 nut with an experienced mentor looking over her shoulder. Definitely operator error in our case.

We are now powering that PD board through a Wago connector for a test bed.

[FrankJ]

So where did it fail? Is it at the button of the stud where it transitions to the square? Does the nut turn freely on the stud? Was the stud bent or the threads damaged?

You are never going to get a grade 8 fastener in this application. You can teach the difference without going into metallurgy.

Occasionally even the best of us make mistakes. I would like to say it wasn't so.

[Richard Wallace]

The PD shanks are fragile, when compared against steel M6 screws. The PDB doc (here) states, on p.7, that the shanks can shear if torqued to more that 3.9 N-m. That's about four pounds of force at the far end of an eight inch wrench. Not nearly as much torque as is typically recommended for steel M6 screws -- about 10 N-m for Class 8.8 or 15 N-m for Class 10.9.

However, the OP clearly stated that the person with the wrench was a veteran electrical mentor guy. I agree this one is probably just bad luck. One of the best students on our team had similar bad luck a couple of years ago.