Increasing reliability of screw-down terminal blocks

Thread Starter

Paul of Alexandria

Joined Jan 28, 2015
9
I have a thermo-couple datalogger and amplifier that use screw-down terminal blocks (the clamping type) to hold the connection wires. They work fine in the lab, but can come loose when transported. Any thoughts on how to improve the reliability of these connections, without major modifications to the equipment, would be appreciated.
 

Yaakov

Joined Jan 27, 2019
3,736
Spring type terminals would be much better. Can you provide a photo of the existing terminals?
If there is enough room, you might be able to add a "jam screw" like a jam nut on top of an hex set screw so it can't back out. If the screw isn't loosening, then perhaps working out some serrations in the clamps would do it.
 

strantor

Joined Oct 3, 2010
5,694
I have a thermo-couple datalogger and amplifier that use screw-down terminal blocks (the clamping type) to hold the connection wires. They work fine in the lab, but can come loose when transported. Any thoughts on how to improve the reliability of these connections, without major modifications to the equipment, would be appreciated.
You should be using terminals designed for thermocouples, made of the same metals as the thermocouple wires. I assume that you are using them, hence the complaint. They suck. All the ones I've ever seen for thermocouples, suck. Never seen ones didn't suck. Sorry. Put a big sticker on the device in bright colors, advertising the requirement to tighten all thermocouple connections before use. Not sure what else to recommend. I'll be following this thread to see if anyone knows of thermocouple terminals that don't suck.
 

Yaakov

Joined Jan 27, 2019
3,736
Could you add split lock-washers under the screws?
Unfortunately, split lock washers don't actually work. Internal tooth lock washers can be a little effective if not overtightened, but split ones just compress into flat washers and don't prevent loosening.
 

strantor

Joined Oct 3, 2010
5,694
So all the millions of split lock washers in use are useless and you are the only one who knows this?
Hard to believe. :rolleyes:
:D

Got a belly chuckle out of me. I agree with you on the scale of bolts. On the scale of screws, @Yaakov 's argument might hold some water. I've removed some bolts that were secured with split lock washers, that took a layer of steel off when eventually untorqued, because the split was so well embedded like it's supposed to.
 

crutschow

Joined Mar 14, 2008
28,158
You can test it yourself with a torque wrench.
That's not a valid test.
The test would be to tighten two identical screws to the same torque, one with a washer and one without next to each other on the same substrate.
Then you subject them to vibration and see if there's a difference in when they loosen.
 

Yaakov

Joined Jan 27, 2019
3,736
That's not a valid test.
The test would be to tighten two identical screws to the same torque, one with a washer and one without next to each other on the same substrate.
Then you subject them to vibration and see if there's a difference in when they loosen.
https://www.boltscience.com/pages/Why_nuts_and_bolts_can_self-loosen.pdf

1626298787436.png
And there are many other articles and papers on this. The physics of self loosening is interesting, and while spot ring washers seem like they would work, they don't. There are alternatives. But castle nuts and anaerobic thread lockers are used where self lessening is a critical problem.
 

strantor

Joined Oct 3, 2010
5,694
How about using blue loctite on the terminal's screws? ... that should fix things ... just make sure no loctite spills into the terminal itself.

That might help, I never thought of that. It would definitely help in the situations where screws loosening is actually the problem, but I'm not sure that's actually the problem in many cases. Thermocouple terminals need to be either (1) made of the same metals as the thermocouple or (2) pass-through type where the ends of the thermocouple wires actually touch, and are compressed against each other.

In case #1, these alloys can (do) have poor mechanical properties and bend/deflect much easier than the stuff ordinary terminals are made of. So it might be less an issue of the screw getting loose and more an issue of the metal that it's screwed into deforming.

therm.png

In case #2, there must be some kind of huge and not-at-all-obvious engineering challenge to make these things more robust. Because all of them I've seen, I'll say it again, suck. I have no idea why. They're weak and they deform or strip out when you put just a moderate amount of torque on the screw. Made of stamped steel that just isn't thick enough.

Capture.JPG
 

hrs

Joined Jun 13, 2014
351

Yaakov

Joined Jan 27, 2019
3,736
That might help, I never thought of that. It would definitely help in the situations where screws loosening is actually the problem, but I'm not sure that's actually the problem in many cases. Thermocouple terminals need to be either (1) made of the same metals as the thermocouple or (2) pass-through type where the ends of the thermocouple wires actually touch, and are compressed against each other.

In case #1, these alloys can (do) have poor mechanical properties and bend/deflect much easier than the stuff ordinary terminals are made of. So it might be less an issue of the screw getting loose and more an issue of the metal that it's screwed into deforming.
Maybe it could be made in a composite way where the mechanical clamming is by something suited to that and electrically isolated from the contact for the ohmic connection.

Something like somewhat hardened steel with serrations to clamp and proper alloys to conduct.
 
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