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Questions about Liquid Shims

Questions about Liquid Shims

Questions about Liquid Shims

(OP)
Hi all,
I'm posting here since I believe this forum has more experience with liquid shims than the structural one. I'm working on a steel frame project (not aircraft but plant structure) where we are connecting large hollow rectangular girders with bolted connections using metal shim plates for full end bearing (see image). The design consultant has also specified 'expansion nodes' for this project (can't share images cuz licensing issues) due to which we are looking at a very high degree of gap variability across different connections - meaning that a significant amount of machining would be required if we were to use metal shims. To get around the cost and effort, we're looking for some good alternatives.

I came across this post (http://www.eng-tips.com/viewthread.cfm?qid=390155) where WKTaylor talks about liquid shims. I'm interested in these but have a couple of questions:

1. Do liquid shims show any significant levels of creep? Would it be large enough to affect the pretension in the bolts ?
2. Is the thermal expansion coefficient of liquid shims very different from that of steel? The steel frame could reach spot temperatures of around 85 degs (Celsius).
3. How much would the cost be (ballpark estimates are fine) in comparison to metal shims? The project is in Australia and we may need to import the liquid shim if it gets approved.

Thanks in advance !

http://files.engineering.com/getfile.aspx?folder=8...

RE: Questions about Liquid Shims

I will make a quick assumption, that you intend to use something like Devcon Plastic-Steel Putty; http://www.itwadhesives.com/brands/devcon/metal-re...
I've had some bad experiences with Devcon recently so I may be responding more negatively than necessary.

The first thing I see in the datasheet is that its modulus of elasticity is 1/35 of steel. So a 30mm layer of Devcon will deflect 5x more than the 100mm endplates being bolted together. You had better get some idea whether the compressive load applied by the bolt torque can be supported by the putty. Bolts are roughly 2" diameter? Are these bolts being torqued for "snugness" or because they have a structural tensile load to transfer? Is the joint subject to transverse loads? There's a lot of thinking needed before I'd have confidence using a liquid shim in any structural joint (not just yours) compared to just fitting a piece of sheet-metal, as your drawing already specified.

STF

RE: Questions about Liquid Shims

have you thought of using peel-able shims ?

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

RE: Questions about Liquid Shims

Before I scrolled down I was thinking 'this sounds like the reason peelable shims were invented'

So:

have you thought of using peel-able shims ?

RE: Questions about Liquid Shims

RE Your illustration...

An 'up-to-30-mm gap' between steel plates is NOT trivial. This configuration/gap screams 'insert a [or a very-few] solid [steel] filler plate[s], here'.

Of course, the gap would have to be measured and a custom filler-plate [filler-plate stack-up] would have to be 'made-to-fit' for each joint?

NOTE.
I suggest that You 'pre-manufacture' solid filler-plates of various thicknesses IF assembling many identical joints present like this. IF So the gap could be filled with a stack-up of 5-mm or plates of various other thicknesses/combinations. These fillers should be pre-finished for corrosion resistance [high-zinc epoxy primer].

Just out of curiosity, since Your illustration seem to show threaded rod-stock with double nuts, both side of the joint, inside of a hollow rectangular tube... How are You actually going assemble this joint??? I am still scratching my head on this aspect.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Questions about Liquid Shims

FE struct1
At 85C that is 185 F ,that is approaching the working limit of some of the room temperature cured epoxy putties, the Devcon Titanium filled putty will go to 300 F whilst the compressive strength is around 18000 psi at room temperature, this strength will fall as the material is re heated. So if you use this type of putty/liquid shim, you may be forced to post cure/temper the shims, to insure you do not, get any loss of preload in the bolted joints.
If your joints are parallel then pre cut shims from stock material ( sheet metal) are a better choice, you can generate a sequence of different thicknesses from stock material ( like pick up sticks.) to accommodate differences down to a couple of thousandths of an inch. Tapered gaps are where I think you are anticipating using liquid shims, again the point to bear in mind with epoxies is that they will cure ( solidify ) at one temperature, then if they are heated significantly above that cure temperature point, they will soften then re cure even harder than before. Tempering is the art of gradually increasing this temperature so that the material cures further without softening significantly.
Peelable shims are shimstock material glued together with a contact cement , or flexible epoxy glue, they are convenient, but relatively expensive, also some of them do not take kindly to being drilled or sheared. Some people have step peeled these shims in the past to produce a tapered shim, however I do not recommend the process, it is time consuming, and very often a machinist can produce a tapered shim on a milling machine in less time.
B.E.

You are judged not by what you know, but by what you can do.

RE: Questions about Liquid Shims

(OP)
Thanks for the replies guys! I'll try and answer your questions:

Quote (rb1957)

have you thought of using peel-able shims ?
I did think of them. Unfortunately the girders in the connection are actually inclined at various angles (probably should've added that in the original description or an image) and the shims required would likely need to be tapered. I understand that peelable shims are not the best to use when that's the case but if I'm wrong please do correct me.

Quote (WKTaylor)

Of course, the gap would have to be measured and a custom filler-plate [filler-plate stack-up] would have to be 'made-to-fit' for each joint?
This is pretty much the plan right now. The sub-contractor in charge of erection has already done what you suggested and we have several prefabricated shim plates of various sizes in stock. They will still probably need machining since the girders are inclined and its likely that some tapering in the shims will be required.

Quote (WKTaylor)

Just out of curiosity, since Your illustration seem to show threaded rod-stock with double nuts, both side of the joint, inside of a hollow rectangular tube... How are You actually going assemble this joint?
Haha I'm glad you caught that ! It's an exercise in money and time wastage to appease an architectural requirement. The girders would be fabricated with curtailments such that the web panels are recessed back to allow access for bolting. Once that is done the web plate will be put back and welded.

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