TORQUE FOR NUTS ON PANEL SWITCHES ETC - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: February 11, 2013, 08:29:54 am
content: I've been asked to put torque settings on nuts that are holding switches onto panels and glands onto the back of boxes.The supplier of the switch recommends 15pounds but that is only 1.6Nm which surely is ridiculous and is the same torque required to turn the switch so I'd think it risks loosening the switch again ?I'm being advised 4-6 Nm for the M12 thread on the cable gland........Usually we'd know what torque to apply but these are thinner nuts and the "bolt" is hollow plus I'm guessing it is not exactly bolt grade material.Any ideas ?

author: Kremmen
author_rank: Super Contributor
timestamp: February 11, 2013, 09:13:21 am
content: Don't these switches have a key in the frame / slot in the hole to prevent slippage? That would remove the need for friction lock. And if there is a star washer under the nut then you don't need a lot of torq to hold the nut and switch together. 1.6 Nm is not much but could be enough. OTOH a bit more would hardly break it.

author: Simon
author_rank: Unknown
timestamp: February 11, 2013, 09:15:33 am
content: well we are talking an M10 thread, yes there is a D washer and tab but you still have to think vibrations etc, I've never even seen a wrench that can do 1.6Nm (I think we have down to 4 or 6 and they'll need no end of adapters to get the crow foot on - that in turn will increase the min torque).

author: Kremmen
author_rank: Super Contributor
timestamp: February 11, 2013, 10:13:25 am
content: Sure, but M10 in itself doesn't say much. Usually these nuts are more like a washer than a true nut so any machine shop practices as to tightening torque won't apply. I was thinking vibrations as well and there are 2 things; firstly the mass is miniscule so the actual forces will be low and secondly, if you are really worried about shaking the nut loose, the traditional practice of fixing with a drop of blood red nail polish will do the trick every time. Easy to apply, easy to break if needed and haven't seen it fail yet.

author: Simon
author_rank: Unknown
timestamp: February 11, 2013, 10:17:29 am
content: WELL we have figured out that our lowest torque wrench is 4Nm and this should be not too tight so I've had to leave it at that. Yes our concern was that the nut is very thin and it will have loctite 222 on it anyhow.

author: ConKbot
author_rank: Super Contributor
timestamp: February 11, 2013, 12:27:53 pm
content: Quote from: Simon on February 11, 2013, 09:15:33 amQuote from: Simon on February 11, 2013, 10:17:29 amI was gonna say threadlocker is a better solution for vibration then torquing it more, but I see you got that covered already.If the application of loctite is a production issue, then something like vibra-tite VC-3 (http://www.vibra-tite.com/vc3.php) can be applied beforehand to the threads, and the switches can be kept that way, rather then having to apply the locker at the time of installation. Obviously torque specs will change with the viscous thread locker on it.

author: robrenz
author_rank: Super Contributor
timestamp: February 11, 2013, 12:43:42 pm
content: I suggest your company reconsider and buy a smaller range torque wrench. Most switches are very thin wall under the thread form and the "v" thread form is a very effective stress concentrator along with the key slot. Also some switches use plated brass in that threaded portion which is much lower strength than steel. The nut even though thin has a greater shear area at the base of the threads than the "bolt" section because of its much greater circumference so the "bolt" is likely to fail first. You are almost tripling the max torque recommended by the switch manufacturer. Are you going to not spend $150.00 on a Torque wrench and risk even one field product failure that will cost many times that amount?This is equivalent to putting a 16V capacitor in a circuit that needs at least a 40V and saying its OK because it works at the moment and we don't have the right tool to install the 40V.This ended up sounding like a scolding but that is not my intent, I just don't have time to rewrite it

author: Simon
author_rank: Unknown
timestamp: February 11, 2013, 12:46:44 pm
content: Unfortunately I work for a bunch of idiotscan't put it plainer than thatwe are supposed to be an engineering company but will never go the extra mile to do it right and bodge it instead. we prefer to use an assumption that suits us as fact rather than the actual fact that we don't like.

author: robrenz
author_rank: Super Contributor
timestamp: February 11, 2013, 01:04:07 pm
content: If that is the case then go back to tightening it by hand with the threadlocker. I tested and 15 inch*Lbs is a firm tightening with a regular handle nut driver. About how tight a experienced tech would tighten it anyhow. I can only get to about 30 inch pounds with one hand on the nut driver connected to the torque wrench. That gives you an idea of how excessive the 4Nm is. Hand your boss a nut driver connected to the torque wrench set for 4Nm and tell him to get there with one hand and does he really want to do that to the switches.

author: Simon
author_rank: Unknown
timestamp: February 11, 2013, 01:07:22 pm
content: Quote from: robrenz on February 11, 2013, 01:04:07 pmHe's the technical one, I'm the idiot, when I told him that I'd done a test and our minimum of 4 was quite adequate he said make it 5not my problem, I'm the one that get's paid fuck all and he's the one in charge. I give up worrying about other peoples screw ups for the sake of being able to say "they took a decision"

author: FJV
author_rank: Regular Contributor
timestamp: February 11, 2013, 06:48:22 pm
content: Maybe a stupid question, but 15 Pound-force foot or 15 Pound force inch?It does make a difference between 20.33Nm and 1.69Nm.http://www.unitconversion.org/unit_converter/torque.html

author: Simon
author_rank: Unknown
timestamp: February 11, 2013, 06:49:59 pm
content: it's pound/inch, 20 Nm would definitively damage it.

author: FJV
author_rank: Regular Contributor
timestamp: February 11, 2013, 07:28:06 pm
content: Do you have a link to s datasheet?

author: Simon
author_rank: Unknown
timestamp: February 11, 2013, 08:57:08 pm
content: http://www.cole-switches.com/pdfs/s3600.PDF

author: FJV
author_rank: Regular Contributor
timestamp: February 11, 2013, 10:20:41 pm
content: Calculating the whole thing using the formulas of my text book takes a bit too long for me to bother.However I found a Bourns potmeter that also uses 3/8" 32 threads per inch UNEF mounting thread.And their datasheet says 1.7...2.0 Nm Mounting (torque on bushing)http://www.bourns.com/pdfs/385126.pdfWhich is probably the same as what they meant with 15 inch pounds mounting ferrule strength in your data sheet.It is also of equal value (1.69 = ±1.7).They are using a bloody vague terminology to describe maximum nut tightening torque, though.

author: FJV
author_rank: Regular Contributor
timestamp: February 11, 2013, 10:54:52 pm
content: When you write:Mounting (Torque on Bushing)...1.7-2.0 N-m (15-18 lb.-in.) maximumIs 1.7 Nm the maximum, or 2.0 Nm, or is it somewhere in between 1.7 and 2.0 (we just don't know)?