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Unread 20-02-2012, 09:56
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PD Board Legality Question

So one of our students broke the screw terminal thing on the Negative side of the PD board yesterday where it connects to the battery. There is only a small amount of thread remaining. (not enough to mount the ring connector and nut at the same time. So we welded to nuts together to create a threaded sleeve, which we can attach to the remaining nub. We then screw in a short bolt to the top of this sleeve to hold the ring connector from the battery. My question is: Will this pass inspection? Ideally we dont want to spend ~$200 on a new PD board.
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Unread 20-02-2012, 10:04
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Re: PD Board Legality Question

Please see [R58] paragraph M.
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Unread 20-02-2012, 10:08
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Re: PD Board Legality Question

So would this be considered identical specifications as compared to prior to the repair. The electrical performance and specifications are the same, but the terminal is physically 3/4" taller now.
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Unread 20-02-2012, 10:11
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Re: PD Board Legality Question

I can only offer my opinions. For a definitive ruling, you must use the official FIRST Q&A.

But in my opinion you'll be fine.

It would be a good opportunity for a teachable moment for the students though. Don't break stuff 'cause it's expensive.
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Unread 20-02-2012, 10:19
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Re: PD Board Legality Question

Mathew,
Teams who have damaged the PD in this same fashion have found that a careful drill and tap of the post will give them back the use of the PD. I believe a 1/4-20 would be appropriate, although a 1/4-28 might be a better hold. It is not possible to replace the terminal post. It is force fit into the PD circuit board and is held in place by the 25 pins on the back of the post. (the wave solder is only secondary to the connection) When drilling you must be careful not to drill so deep that you break the contact with these pins. You must use a bottom tap to get threads all the way to the bottom of drilled hole. Be careful that chips from the drill and tap operation do not get into the electrical parts on the PD. I would also recommend that this PD be used only on practice bots in the future and that a replacement be obtained when you can.
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Unread 20-02-2012, 10:37
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Re: PD Board Legality Question

Any idea what depth we should shoot for with the drill & tap. I don't want to turn an ugly but working board into a pretty paperweight.
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Unread 20-02-2012, 10:46
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Re: PD Board Legality Question

Quote:
Originally Posted by matthewdenny View Post
So one of our students broke the screw terminal thing on the Negative side of the PD board yesterday where it connects to the battery.
Was he/she using a torque wrench?


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Unread 20-02-2012, 11:40
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Re: PD Board Legality Question

Quote:
Originally Posted by Ether View Post
Was he/she using a torque wrench?


Most common cause of breaking the terminals off is trying to thread a 1/4-20 nut on to the metric stud.
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Unread 20-02-2012, 11:41
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Re: PD Board Legality Question

The repair you are attempting is legal per the GDC, but I do not recommend it. That component carries several hundred amps for a few seconds at a time on a typical robot. An imperfection in the repair will add resistance, which will quickly translate to heat.

Personally, I would push this PD to practice bot status.
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Unread 20-02-2012, 11:48
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Re: PD Board Legality Question

Quote:
Originally Posted by Jeff Pahl View Post
Most common cause of breaking the terminals off is trying to thread a 1/4-20 nut on to the metric stud.
A torque wrench would have caught this, no?


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Unread 20-02-2012, 12:00
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Re: PD Board Legality Question

I've also implemented an emergency repair of this (on the positive side) using three 40 A breaker slots in parallel, with leads running back to the 120 A circuit breaker terminal. If I remember correctly, this was used on practice day, while a spare was being acquired from the spare parts desk, and installed after practice ended for the day.

(Erik would know if this will compromise the circuit-breaker fault indicators—but I didn't care about those, because several were defective already, and this whole PDB was going to be trash very soon.)

Last edited by Tristan Lall : 20-02-2012 at 12:02.
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Unread 20-02-2012, 12:02
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Re: PD Board Legality Question

Quote:
Originally Posted by Tristan Lall View Post
I've also implemented an emergency repair of this (on the positive side) using three 40 A breaker slots in parallel, with leads running back to the 120 A circuit breaker terminal.

(Eric would know if this will compromise the circuit-breaker fault indicators—but I didn't care about those, because several were defective already, and this whole PDB was going to be trash very soon.)
This is how I typically would do it, but it isn't legal for competition per the GDC.
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Unread 20-02-2012, 12:24
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Re: PD Board Legality Question

If you are able to get 2-3 turns on the remaining stud, what you are currently doing is the best technical solution. I am not a good one to say if it is legal. Strap the wire down so the stud is not taking any stress from the cable. A good idea in any case.

If you try the drill & tap method, I would go with a M6 (metric) tap to be consistent with the other post. For the final hole depth use a drill bit with the flattest angle you can find & a drill stop. A regular taper tap will probably be useless. Start with a plug tap & finish with a bottom tap. I would use a drill press to make sure everything is square. (Obviously do not use power when tapping, just turn the chuck by hand.) Find the most OCD person you can to do this.

When this happened to one of our boards, I just ran the negative to one of the Wago 40 amp terminals & demoted it to a test board. This would not be legal for competition
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Unread 24-02-2012, 23:56
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Re: PD Board Legality Question

Quote:
Originally Posted by Jeff Pahl View Post
Most common cause of breaking the terminals off is trying to thread a 1/4-20 nut on to the metric stud.
Quote:
Originally Posted by Ether View Post
A torque wrench would have caught this, no?


A good torque tool, yes. However, what 90+% of teams have, if they have any sort of torque tool at all, is only suitable for the lug nuts on the team transportation.

I would love to see a big warning label on the PD stating "CAUTION: METRIC HARDWARE"
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Unread 25-02-2012, 13:42
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Re: PD Board Legality Question

Quote:
Originally Posted by Ether View Post
A torque wrench would have caught this, no?
I don't think so Ether, there's anot a lot of torque difference between a tighened-down-properly metric nut and a mis-threaded 1/4-20 nut at about 3 or 4 turns. From there torque actually drops. It's just too easy of an error to make for some teams.
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