Belleville washers for keeping busbars on cells terminals?

BiduleOhm

Electronics Sorcerer
So, I saw the thread talking about using belleville washers instead of springs for cell compression (NB: I prefer the springs for that as otherwise you'll need to stack a lot of washers to have a low enough k and high enough usable travel) and posting about busbars on another thread when I had the idea to merge the two.

Here's the idea: M6 terminals are somewhat fragile and you can easily strip their threads if you don't pay attention. Also, as with any screwed terminal connection, thermal cycles and metals malleability (and vibrations in a mobile application) have the tendency to loosen the connection over time.

So by replacing the standard washer by a belleville one between the nut and the busbar you give yourself more margin before stripping the threads (NB: it doesn't increase the max torque, but it gives you more travel to realise your mistake before it's too late) and you have something compliant to mitigate or even negate the causes of the connection getting loose over time.

Good idea? bad idea? other?
 

Hedges

I See Electromagnetic Fields!
Here's a vendor with various compression forces listed:


6mm, 87 to 218 pounds to flatten.
1/4", 70 to 1350 pounds

Stacking arrangements can tune the force:


Looks like "compressed height at working load" is about half way between "height" and "thickness" (i.e. flat), and working load is around half of force to flatten.

Seems like it ought to do what's intended.
 

HaldorEE

Photon Sorcerer
Got results from my research to report.

I found a stackup of McMaster Carr spring washers that will do what I think is needed.

My criteria is to limit force at 100% SOC to 15 PSI. I have read that these cells expand between 0.7mm to 0.8mm from 0% to 100% SOC. I am working with 4 cell stacks in each compression frame. This totals up to 3.5mm of travel so that became my goal.

I am using 5/16" x 18 hardware and 18 TPI = 0.058 "/turn:

0.058 "/turn x 25.4 mm/" x 2.5 turns = 3.683 mm of travel.

I ordered and tested a number of different spring washers from McMaster Carr, and one the worked for me is this one.


What I found is that a stack of 12 of these disc springs in series <><><><><><> resulted in a force of 100 kg when compressed with 2.5 turns of the nut. This is starting with the nut finger tight (close to 0 kg). I then turned the nut 2.5 turns and my load cell read ~ 100 kg (+/-5 kg).

I am going to declare victory and that is how I am going to build my compression cells. I could keep working to see if I can refine these results, but I am pretty happy with this. When I build my compression cells I will test a set of 4 cells in compression with the load cell and measure what the peak load is at 100% SOC, but that is for another day.

Here is a pic of my test setup.

PXL_20210120_022059386.jpg


F.Y.I. Avoid stainless steel hardware like the plague. I started testing with a stainless steel bolt and flange nut and after a half dozen cycles the nut galled on the bolt so bad I had to cut if off my fixture. I am really glad I have a band saw.


PXL_20210120_010350337.jpg
 

HaldorEE

Photon Sorcerer
P.S. Forget using a torque wrench to set compression. I was seeing tremendous non-repeatability (+/-50%) when trying to use a torque wrench to set the pressure. Springs are very repeatable. I am getting +/-5% variation from one test cycle to the next.

And I finally found a use for that old welding coupon. Makes a nice base plate for a load cell.
 

fafrd

Solar Addict
P.S. Forget using a torque wrench to set compression. I was seeing tremendous non-repeatability (+/-50%) when trying to use a torque wrench to set the pressure. Springs are very repeatable. I am getting +/-5% variation from one test cycle to the next.

And I finally found a use for that old welding coupon. Makes a nice base plate for a load cell.
Excellent work!

Do you think the variation would be reduced if the threads were oiled/greased?
 

Hedges

I See Electromagnetic Fields!
Excellent work!

Do you think the variation would be reduced if the threads were oiled/greased?

I recall looking up torques before. Lubricated, force for a given torque was increased, but I don't think variability was decreased.
Friction varies too much. So a spring gauge works better.
Older engines used torque for head bolts, but some newer ones are just pre-loaded with a lighter torque then stretched a prescribed amount by some number of turns.

Maybe the guys assembling the San Francisco Bay Bridge erector set (kit came from China, but bolts not included, to be obtained locally) should have tried that. Broke a third of them.


1611120730843.png
 

HaldorEE

Photon Sorcerer
Correction. I would use a torque wrench when tightening the bus bar hardware. I found this bicycle torque wrench that should work well for the purpose.


My comment was about using a torque wrench for compressing cells. I saw a lot of posts from people claiming that is how they were going to do it. Just use threaded rod and nuts, then torque the nuts using a torque wrench. I tried that without a spring on my test load cell and the results were all over the place (+/-50%).
 

noenegdod

Solar Addict
This kind of ended a couple weeks ago but Im really interested. The pressure ratings of the springs McMaster-Carr and lee spring offer all seem a bit light. (176lbs at working load and 218 at flat) This is based on reading somewhere on this forum that the 30-40 inchlbs of torque for the m6 terminal bolts results in somewhere around 700 lbs of "force". Does anyone know if it is acceptable to tighten them down flat to the desired torque spec and if during heating/cooling cycles it does loosen a little the washer will maintain a margin of pressure?
 

Bob B

Photon Sorcerer
I ordered some of these but haven't implemented them yet. They are 620 lbs working and 884 flat.

BellevilleWasherBusbar.png
 

noenegdod

Solar Addict
I was trying to avoid jumping up to 1/4" ID washers but it is looking more and more like that will be the only option. Are you going to use a metric flat washer on top or just apply the nut straight to it?
 

Bob B

Photon Sorcerer
I haven't decided for sure yet. I have some brass hardware on the way including some fender washers. I will probably experiment with putting the belleville washer on top of the fender washer ... but will be doing some experimenting. I don't think the 1/4 is that much larger than the m6 .... I think it is something like m6.5.
I have a bar type torque wrench and will be comparing what torque to how flat the washer goes.

Wish I had one of those pressure gauges like HaldorEE has.
 

noenegdod

Solar Addict
Yes, a strain gauge would be a nice toy to have laying around!

The ID in a 1/4" washer is .533mm larger than the ID in a 6mm washer. Not a lot but is definitely noticeable. My current thinking was the same as you in that I was going to put a fender washer, belleville but also put a 6mm on top to ensure the nut doesnt tip into the ID of the belleville.

I do have to pick up a torque wrench. Probably going to go with the bar type suggested in a few other threads.
 

fafrd

Solar Addict
I haven't decided for sure yet. I have some brass hardware on the way including some fender washers. I will probably experiment with putting the belleville washer on top of the fender washer ... but will be doing some experimenting. I don't think the 1/4 is that much larger than the m6 .... I think it is something like m6.5.
I have a bar type torque wrench and will be comparing what torque to how flat the washer goes.

Wish I had one of those pressure gauges like HaldorEE has.
This thread has really mangled the subject of Belleville Washers for securing busbars with the subject of Belleville washers for 300Kgf fixtures.

Earlier today, I started a new thread in the best lock washers for securing busbars and lugs to grubscrews here: https://diysolarforum.com/threads/best-lockwashers-for-m6-grubscrews.18004/

My thoughts on using a Belleville washer to secure my grubscrews is I want a much larger diameter than 6mm to distribute the force over the widest area possible.

I’d want at messy an M10 washer and would position it cone-up then cover it with a inch string M6 fender washer to compress to ~35inch-lbs with a torque wrench.

If you trust the rating of your Belleville washer, you could also use mm of compression to calibrate 35 inch-lbs (since a full turn = 1mm of compression.

You’d probably want a washer that delivers 35 inch-lbs of force at no more than 50% compression (and possibly as little as 25%).

Has anyone done the conversion of how many pounds of force is needed on an M6 thread to equal 35 inch-lbs of torque?
 

fafrd

Solar Addict
Yes, a strain gauge would be a nice toy to have laying around!

The ID in a 1/4" washer is .533mm larger than the ID in a 6mm washer. Not a lot but is definitely noticeable. My current thinking was the same as you in that I was going to put a fender washer, belleville but also put a 6mm on top to ensure the nut doesnt tip into the ID of the belleville.

I do have to pick up a torque wrench. Probably going to go with the bar type suggested in a few other threads.
As I just posted, I’d do the opposite.

Belleville washer directly on lug/busbar, cone side up, and largest diameter you can get up to 10mm to distribute force over terminal surface area.

M6 fender washer on top of cone so nut has a close-fitting area to apply force.

And torque nut down for ~35 inch-lbs of torque or use mm of compression if you trust your Belleville specifications.
 

Bob B

Photon Sorcerer
Sorry you think we have mangled this thread .... I would much rather have as much information about a particular subject in one thread instead of having multiple ones of them going..

I'll be careful not to come over an mangle your thread.
 

noenegdod

Solar Addict
This thread has really mangled the subject of Belleville Washers for securing busbars with the subject of Belleville washers for 300Kgf fixtures.

Earlier today, I started a new thread in the best lock washers for securing busbars and lugs to grubscrews here: https://diysolarforum.com/threads/best-lockwashers-for-m6-grubscrews.18004/

My thoughts on using a Belleville washer to secure my grubscrews is I want a much larger diameter than 6mm to distribute the force over the widest area possible.

I’d want at messy an M10 washer and would position it cone-up then cover it with a inch string M6 fender washer to compress to ~35inch-lbs with a torque wrench.

If you trust the rating of your Belleville washer, you could also use mm of compression to calibrate 35 inch-lbs (since a full turn = 1mm of compression.

You’d probably want a washer that delivers 35 inch-lbs of force at no more than 50% compression (and possibly as little as 25%).

Has anyone done the conversion of how many pounds of force is needed on an M6 thread to equal 35 inch-lbs of torque?
I have no interest in lock washers for the busbars. There are better ways.

There is no reliable way to deliver a "force equal to 35 ftlbs". Your thinking about it wrong. Torque on a fastener is at best, an unreliable easiest way to "consistently" apply a load. Your not trying to apply 35 ftlbs. Your trying to apply a force, using applied torque, which is inherently inconsistent but is the best we can easily do. The most accurate way is to have a spring with a known rate (belleville washer) and load it through to a distance.

If by cone side up you mean the concavity of the washer is facing away from the cell and then you apply a fender washer on top, I think you are ending up introducing substantial error into the system by doing that. The fender washer of unknown and probably unrepeatable "spring rate" will deform. You will be in no mans land.
 

HaldorEE

Photon Sorcerer
This kind of ended a couple weeks ago but Im really interested. The pressure ratings of the springs McMaster-Carr and lee spring offer all seem a bit light. (176lbs at working load and 218 at flat) This is based on reading somewhere on this forum that the 30-40 inchlbs of torque for the m6 terminal bolts results in somewhere around 700 lbs of "force". Does anyone know if it is acceptable to tighten them down flat to the desired torque spec and if during heating/cooling cycles it does loosen a little the washer will maintain a margin of pressure?
The working rating of the springs I used is 218lb (which is at 60% compressed). I am using a stack in each corner and compressing them with the cells at 100% SOC to the max pressure I want to see which works out to just under 90 kg. Over 3 mm of travel I see the pressure vary from 14 PSI to 9 PSI. I tested my setup with a load cell.


PXL_20210120_022059386.jpgW10_1_Turn.jpg
 
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