Inexpensive torque wrench for power panel lugs?

[fovea1959]

Looking for a 1/4" drive beam torque wrench for the power distribution board lugs (3.9Nm = 34.5 lb-in = 2.88 ft-lb). I've seen two stripped or busted lugs in the last year, and ain't repeating the experience with the new team...

Anyone seen a good source for one under $35? I don't want a cheap clicker, I don't trust em...

[JamesCH95]

Here is a beam-type you'd be looking at:

http://www.amazon.com/2955-Torque-Wr.../dp/B00004SQ3B

Here is the clicker-type I use on my race car, just for reference:

http://www.sears.com/shc/s/p_10153_1...block Type=G3

Both are +/- 4% accuracy.

For the un-initiated: the advantage is that the clicker-type will tend to prevent over-torquing and is largely 'idiot proof' because it doesn't require eye-balling the torque measurement. The clicker has the same repeatability with an inexperienced user as a beam-type does with a good user. Just zero it out when you're done and it'll last a long, long time.

[fovea1959]

thanks. I had zeroed in on the KD Tools 2955 and the Park Tools TW-1 for $35-$40, was hoping for cheaper. c'est la vie, still cheaper than a new power distribution board...

I agree about the clickers being easy to use, but I've read too many reviews of cheap clickers not clicking. Hard to break a beam wrench!

[tsaksa]

Has anyone tried using the torque driver settings on an ordinary cordless drill for tightening some of these connections? Many teams already use a cordless drill with a 3/8 or 1/4 inch socket drive adapter for speedy assembly/dis-assembly work. This would not be as accurate as a real torque wrench, but if it worked it would be handy and low cost.

[JamesCH95]

Originally Posted by tsaksa

Has anyone tried using the torque driver settings on an ordinary cordless drill for tightening some of these connections? Many teams already use a cordless drill with a 3/8 or 1/4 inch socket drive adapter for speedy assembly/dis-assembly work. This would not be as accurate as a real torque wrench, but if it worked it would be handy and low cost.

Decent idea in principal, terrible in execution in my experience. The clutch can't limit and torque from the kinetic energy of the chuck/bit/output shaft, so you'll wind up with different amounts of torque depending on how fast you run the drill. I.e. there is a larger torque resulting from running the drill in faster with the same clutch setting. It is also not calibrated to any known torque, so I think you'd need to verify the torque with a torque wrench anyway...

[Al Skierkiewicz]

The majority of failures I have witnessed were due to the use of the wrong hardware and/or the wrong size tool. The studs look like 1/4-20 but are not, they are metric.

[fovea1959]

Al: I've seen both failures, but the over-torque is the one that worries me. The big "NUTS ARE M6, NOT 1/4x20!" sharpied on the PDB had better take care of the other

my son snapped one off last year (while being careful, but no torque wrench). The studs do not seem to be a solid metal, the interior of the broken stud was quite porous (pot metal?).

We drilled and tapped the board to keep the robot operational while waiting for the replacement to arrive. I'm going to see if my old team can do that for a stripped one I have on the shelf so I can use it for testing and bench work, including working on the 2nd cRIO that I'll probably never be able to get

[Al Skierkiewicz]

Originally Posted by fovea1959

my son snapped one off last year (while being careful, but no torque wrench). The studs do not seem to be a solid metal, the interior of the broken stud was quite porous (pot metal?).

Sounds like a manufacturing defect. They are not pot metal. I have heard of some teams tapping the remaining block. However, there are 25 pins on the bottom that carry the current into the circuit board. Any repairs need to be performed with caution to maintain these paths. Heating of the block and board could result if any of the pins are drilled into and break the bond with the block.

[Richard Wallace]

To reinforce what Al said above:

Originally Posted by Power Distribution Board data sheet

(from p. 7 of the data sheet)

• The PD’s power input/battery shanks are Metric 6. A ¼” nut WILL strip these nuts. Use M6 nuts for the PD Shanks.
  
• The PD’s power input/battery shanks can shear if torqued more than 3.9 Nm. Do not over-tighten.

My team is lucky to have a sponsor that allows us to borrow this torque wrench. We treat it carefully and return it the next day.

Our torque target is 20 lbf-in. Any more just seems too close to the shear limit (3.9 Nm = 35 lbf-in) quoted above.

[fovea1959]

May have been a mfg defect, but since we didn't use a torque wrench, it would be a little hard to prove...

We did successfully drill and tap the block; it was fine for light bench testing...

[fox46]

Use a nut driver (screwdriver with a socket on the end) instead of a ratchet or wrench. It's an electrical connection. Just because it's a hexagon and you can put a ratchet on it doesn't mean it is meant to be tightened until you can't turn it anymore or a *click* tells you to stop. A little thinking here will easily substitute for a $35 torque wrench to tighten a couple nuts.