Structural-mechanical fuse (load limiter)

[swearingen]

I'm looking for a pin-type element that is calibrated to resist a certain load, then shear. I expect the loads to be in the neighborhood of 10,000 lbs or so.

I have a threaded rod that is bracing a support holding up tubes that expand and contract due to high temperature. I don't want the expansion to break the wall I'm bracing against, and I want to be able to see that it has been overstressed.

Does anyone know of a commercially available item such as this? I can design it myself, but the margin of error will be quite a bit higher than a tested product.

Thanks


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[JStephen]

An intriguing idea.
Two problems I see right off are that mechanical properties for pins, etc., are usually tabulated as the minimum strength, and actual strength can run quite a bit over that, so you'd get a lot of uncertainty on the high side. Then secondly, properties are dependent on temperature, so if you potentially have a large temperature variation, that causes more uncertainty.
A linkage mechanism or use of Belleville springs might be alternate routes to consider.

 

[swearingen]

I'm aware of the pin properties being an issue, which is why I was looking for something that is calibrated specifically for this.

I'm familiar with Belleville springs, but what did you have in mind with the linkage mechanism?


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[Gumpmaster]

I performed a similar design, although it never got built.

Design based on a pin of a particular strength and then require testing of the actual pin material being installed. You can perform final machining to a particular diameter to match the design exactly (you can have a reduced diameter section which will control the capacity). Have a couple of spare pins made from the same material.

It has to be a machined, tested piece to be reliable. It's not too difficult to design or specify. Be sure to specify machining tolerances.

 

[rb1957]

that's the way we'd do it too ... careful machining and testing to verify.

like JS, I don't really see what you're trying to do ... if the thermal loads are anticipated why would you want a structural fuze ? what happens to your brace when the pin fails ?

I'd think that you could achieve the result you're looking for with a mechanism.

Quando Omni Flunkus Moritati

 

[JStephen]

I didn't have anything specific in mind with the mechanism, just thinking out loud, so to speak.
There should be quite a few different ways to arrange things to accomplish the purpose, as to how practical they are, that's a different issue.
See the attached sketch for an idea, though- weight stays put until tension exceeds a certain value, then either lifts up or flops on over depending on the amount of movement.

 

[swearingen]

I like the mechanism idea, however I need the fuse to work in both tension and compression.

The thermal loads are caused by the tubes binding on the supports. The support structure spans 18' horizontally and evidently is not stiff enough to overcome the friction before bending a little, which then allows the tubes to bind in their support holes. The idea is to brace the support, in both tension and compression, to a nearby wall with struts to stiffen it and force the tubes to overcome the friction and move over the support.

The issue is, if it binds even after we put the support rods in, it will pull the nearby wall and cause serious damage. The fuse would be placed on the rod, outside the wall, and would be able to be inspected on a daily basis for movement and/or breakage.

Unfortunately, this setup makes the Belleville washers impractical as well. Once they collapse, the rod could continue to move and pull the wall along with it. We need to know there's a problem inside where the tubes are without destroying the outside wall in the process.

Thanks for your replies...


-5^2 = -25 ;-)

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[BUGGAR]

Simpson or some company has a proprietary seismic joint with slip designed into it. I haven't explored this but maybe it will give you an idea.

 

[MikeHalloran]

Make the support more flexible in the direction of expansion, and make the tube support holes shorter, larger, and/or radiused, so that the support can bend as the tubes move, and the tubes 'roll without slipping' on the support.

OR, insert something like a rocker style bridge bearing...

OR, hang the tubes via cables from an overhead structure.



Mike Halloran
Pembroke Pines, FL, USA

 

[PMR06]

Have you looked into engineered pipe supports? There are "constant" hanger supports calibrated to specific loads. Unlike a spring support whose reaction increases with displacement, constants basically provide "constant" reaction (support) over a range of motion.

http://www.lisega.de/product-groups/constant-hangers-and-constant-supports.html

 

[swearingen]

The problem here is that this is an existing installation. Changing the support/guide scheme is not possible.

The extreme displacement that we've seen with the current setup was undoubtedly caused by "walking" of the support as the pipes heat cycled - the pipes pulled it a little, then contracted, then pulled a little more, and so on.

The holes are extremely hard to access, as the tubes are tightly packed and there are many of them. We are trying to do this without widening the holes (if at all possible).

Note that this support primarily acts as a wall and insulation support partition, not a pipe support per se. The pipes do rub on it, however.


-5^2 = -25 ;-)

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[dvd]

Here is an impractical and expensive idea. The Voith Safeset coupling works by "inflating" a thin walled steel bladder with hydraulic fluid which causes the bladder to grip a shaft or hub. The bladder is equipped with a shear tube that is sheared off when there is slip between components. It would be reasonably simple (and reasonably expensive)to apply this principle to linear motion also.

See this brochure

http://www.konaflex.fi/datafiles/us...nts/Voith_Turbo_Safeset_Ylikuormakytkimet.pdf

And a very short video

https://www.youtube.com/watch?v=hKxD6rhh_0U

 

[GregLocock]

You have looked at the shear pins used to hold engines onto passenger jets?

Cheers

Greg Locock


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[swearingen]

Neat concept, dvd, and no, I haven't considered aircraft engine shear pins.

I was, however, directed to this site:

http://sismocell.com/en/

This isn't perfect, but I think it could work in my situation. It's easy to see if it has failed and is easy to replace.

Thanks for all of the suggestions!


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