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I am attempting to mount a device by screwing into ~2" of rubber (not sure what durometer or material, but it is decently firm, maybe 70 durometer). However, I am not sure what type of anchor to use (corkscrew, wall-anchor type, ground anchor, etc). Shown below are some examples:

enter image description here

What are the advantages/disadvantages of each for soft materials? I assume since the material is closest to a cork, a corkscrew would likely be best (assuming there is sound reasoning behind the design of this ageless device). The worst case scenario is if the fastener tears out, and pulls out a chunk of the material along with it. I would think a helix would minimize the risk of damage, however the lack of surface area might not provide the most shear strength/holding force.

Is there a guide, or published paper, discussing the tear-out strength of these fasteners in rubber/elastomers?

EDIT: As per the comments, here is some additional information: The rack (with measurement equipment on it), is ~100 lbs. Unfortunately, I do not have access to the inside plate, or other side of the rubber layer. This is also a temporary installation (hence screws), so I cannot bond to it (epoxy), which was my initial thought.

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  • $\begingroup$ If using a coarse screw such as that illustrated top-right, you might need to clear a hole for the core. The corkscrew mitigates this problem by effectively having no core. If you are able to clear out the core (not practical for wine-drinkers to have a small drill handy), then you may get a more robust connection with a more standard screw. $\endgroup$ – Jonathan R Swift Jan 9 '18 at 15:06
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    $\begingroup$ What is the "device"? How much load, mass, wind etc? Should you consider a "through" fixing with a large washer or plate on the inside? Is an inside plate possible? More details are necessary... $\endgroup$ – Solar Mike Jan 9 '18 at 15:23
  • $\begingroup$ @SolarMike The rack (with measurement equipment on it), is ~100 lbs. Unfortunately, I do not have access to the inside plate, or other side of the rubber layer. This is also a temporary installation (hence screws), so I cannot bond to it (epoxy), which was my initial thought. $\endgroup$ – dberm22 Jan 9 '18 at 17:39
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In tire rubber (~Shore 60A durometer), standard wood screws would work well. They are available, easy to install/remove, and will leave a smaller mark after removed. 2" is a pretty generous depth. While the pull out force will obviously not be as great as wood, it will be substantial. I would estimate over 100lbf tension on a single 2" wood screw; in shear it would easily be double that. Tension keeps the equipment cabinet from tipping away from the wall, and shear keeps it from sliding down the wall.

A corkscrew or expansion shield is more likely to pull out a chunk of material under excess load. While a wood screw would not hold as much load per fastener, it will most likely fail in just the area around the threads and strip out a hole a little smaller than its maximum diameter.

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  • $\begingroup$ Thanks. I am trying to quantify how much the screw could actually hold. Any references for your 100 lbf estimate? I agree a corkscrew would pull out a larger chunk compared to a woodscrew (if it does tear out), however, how would that change the load capacity? I would think a corkscrew might be able to bear more load. Thoughts? $\endgroup$ – dberm22 Jan 9 '18 at 20:31
  • $\begingroup$ You will have to do some testing with your actual material. You may be able to source some scrap if you can't test on the exact implementation. Pretty easy to test with a pull scale or filling a bucket with water and calculating the mass. Fasteners are typically something you don't need exact; just add more if necessary. Loads are not exact either; user handling, seismic, etc. Two wood screws would be much easier to install than one cork screw. $\endgroup$ – ericnutsch Jan 9 '18 at 22:02
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I recommend using a bracket as opposed to a single screw. This way you can separate the pull out stress from shear and avoid bending/ tilting of fastener which exacerbates the situation.

A bracket in the shape of letter H 1/8" thick, approximately 3" tall and 1.5" wide which has two pre-drilled holes on top and bottom flanges and two holes in the vertical web, ready to receive 1/8" by 2" long decking screws.

I venture to guess each bracket would support at least 200lbs weight.

You can start calculating the allowable loads by using the NDS American Wood Council equations by adjusting them to your rubber shear values!

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