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Load indicating washer
4

Load indicating washer

Load indicating washer

(OP)
I am looking for a load indicating washer that can give that signal more than once. With most of those: you crush the bumps (or it "squirts") and that's it. If it loses the pre-tension....you won't know it.

Does anyone know someone who makes a washer like that? Thanks.

RE: Load indicating washer

None that I am aware of aside from a Skidmore-Wilhelm testing apparatus.

In my experience if there was ever a worry about losing pretension in the bolt then some positive locking mechanism should be used to ensure pretension is maintained (locktite, lockwire, cotter pin, etc.) is better than trying to monitor the pretension.

What's the application?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: Load indicating washer

(OP)

Quote:

In my experience if there was ever a worry about losing pretension in the bolt then some positive locking mechanism should be used to ensure pretension is maintained (locktite, lockwire, cotter pin, etc.) is better than trying to monitor the pretension.


You are going to have relaxation in the materials (especially if we are talking it being embedded in concrete)......with or without a cotter pin.

Quote:

What's the application?

I got a client that I am always doing equipment foundations for. I typically call for pre-stressing the anchor bolts. (To a % of yield.) Since pre-stress gets lost over time......and they don't do a good job of monitoring this.....they wanted to know if there is such a product. (Where they can do a drive-by check.) I sure couldn't think of any.

RE: Load indicating washer

maybe there'd be a nasty rattle sound (if the bolts lost preload) ?

agree with above ... if preload is important, design a means to ensure it is retained.
maybe "just" witness marks (to show the nut has rotated/loosened) ?

another day in paradise, or is paradise one day closer ?

RE: Load indicating washer

(OP)

Quote:

maybe there'd be a nasty rattle sound (if the bolts lost preload) ?

Only problem is: they typically don't notice that rattle until it has created some issues. (Not the best run organization on the planet.)

Quote:

maybe "just" witness marks (to show the nut has rotated/loosened) ?

I've thought of that myself....even seen it tried....but the nut doesn't have to rotate for it to lose pre-tension.




RE: Load indicating washer

4
Under the bolt head, stack a belleville washer and then a flat washer.

bolt head

flat washer

belleville washer (cone pointing up)

flange being fastened


Choose the spring rate of your belleville washer such that it is not flattened until your critical preload value is reached, if possible.

When the assembly has the correct preload applied, the flat washer and belleville washer will both be flat. There will be no gap between the two at their OD. If the preload in the fastener falls below the critical value, the belleville washer will deform toward its free-state shape, and there will be a gap between the two washers on the OD, which is very easy to inspect for visually. Since the spring rate of the belleville washer is known with a reasonable level of precision (assuming you purchase high quality washers) you can also get a reasonable estimate of remaining preload by measuring the edge gap using feeler gauges.

RE: Load indicating washer

(OP)
Thanks jgKRI!

RE: Load indicating washer

Use an ultrasonic thickness gage and measure the bolt length after tightening. Measure periodically to check for elongation.

RE: Load indicating washer

Neat. Some spirited Googling turned up this: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Load indicating washer

Quote (KootK)

Neat. Some spirited Googling turned up this: Link

Great find!! I use this technique from time to time- just added that excel sheet to my digital library.

Quote (Ron)

Use an ultrasonic thickness gage and measure the bolt length after tightening. Measure periodically to check for elongation

Can you use ultrasonic measurement to get accurate readings on anchors embedded in concrete?

My knowledge on ultrasonic measurement is limited to the fact that it exists.. does it still work when the target is embedded in media other than air?

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