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Imagine a screw that has a hole drilled through the center with a counterbore, a skinnier screw is then placed in the hole with threads in the opposite direction. The threaded hole would have the deeper portion correspond to the skinnier screw.
Now in order to turn it you need a special tool that
can turn both heads in opposite direction at the same time. This would be a very good tamper resistant screw as you couldn't slot it, or grab it with pliers like a normal torx bit or what not. IT could also never back out from vibration or torque as it would tighten down one of the threads.
[link]
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// could also never back out from vibration or
torque as it would tighten down one of the
threads//. |
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Consider an ordinary screw. Thermodynamically,
the system sits in a local
minimum, adjacent to two other local minima,
one corresponding to an increment of unscrewing,
the other to an increment of screwing. The
energy barrier to unscrewing is slightly lower, isn't
it? So
prolonged
mechanical noise of sufficient amplitude will,
eventually, back the screw out. |
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Now consider your two concentric screws.
Because for each of them, the energy barrier will
be slightly lower for unscrewing than for screwing,
there will be a (perhaps narrow) range of
mechanical perturbations from vibration noise,
sufficient to unscrew one screw, a little, but
insufficient to tighten the other screw. Wait long
enough, and your double turn screw will back out
from vibration. |
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Or else I'm missing something, and if so, what is it? |
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God rot every inventor of "tamper resistant screws"
that force me to go find a special kind of bit or a
special tool. Why not just make it a square style
metric backwards-threaded wing stripped-head nut-
double phillips-
flathead-torque diamond star?! Or just use rivets and
welds |
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The thing is, screws are cheap. Non-removable lok-
tite is fairly cheap. This screw is at least twice, and
more like 5-10 times more expensive than a simple
screw. |
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Imagine a screw that has a hole drilled through the center with a counterbore. You should have stopped right there. |
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This is essentially two screws, one inside the other, each independently vulnerable to the weaknesses you are trying to protect against, provided you work them in the correct order. |
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If the skinnier screw were put down the centre of the outer screw, could the outer screw not still be removed by itself leaving just the skinny screw down the hole and the entire thing not very securely fastened? |
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What you could do is use the inner screw as a wedge,
tightening the outer screw's threads against the wall.
No need to oppose the threads, though. |
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I think this could work as a self-tapping screw pair
with the right tools. |
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Detractors: c'mon, this is the HB! + |
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Or else I'm missing something, and if so, what is it?
screws aren't a simple manner of energy states
with "unscrewing" representing a lower state" |
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similar to a knot, you can make the act of
tightening a screw increase the energy required to
initiate unscrewing. Think of a zip-tie that once
tightened takes more tensile force to loosen. |
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If you have screws in the opposite direction the
act of vibrating or spinning one screw would apply
friction to the other tightening it down. You
could also accomplish this with some fancier
geometry such that both screws apply tension to
the other screws threads, essentially increasing
the friction needed to turn either one, therefore
torque would have to applied to both screws
simultaneously in order to release that friction. |
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[tatterdemalion], I think [metarinka] is correct. While the main text does basically describe one screw inside another, the key point is that the inner screw is longer. So its opposite threading connects it to the workpiece differently from the way the outer screw connects to the workpiece. |
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If they were both the same length, then you could remove both screws together because there is no RELATIVE motion between the two screws when turning them together. But with the longer inner screw connected to the workpiece, the outer screw is certainly locked in place. The inner screw must be removed first (and inserted last). |
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I dunno 21 quest... depends what you torque the
screw too. If the outter screw is torqued to 30+ lb/ft
and has smooth walls, then you wouldn't be able to
hold it with pliers with enough force to turn. |
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In my mind these were countersunk screws. so
grabbing with plies wouldn't work, and depending on
the design of the head, you could make it so backing
out the inner one wouldn't work, you would have to
spin both parts at once. |
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Are these for everyday 'tamper resistant' applications or a
special type of superscrew for high security applications? I
need to know how determined Jim is to get at whatever's
behind the panel before I work out how he's going to tear
this thing apart. |
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Unless you're from a place that uses vastly different nomenclature than here, you're actually talking about bolts, not screws. Although you seem to be referring to blind holes rather than through-holes... |
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What sizes are we talking about here? Drilling and threading a hole in a bolt that itself is under say 10mm is going to be a technological nightmare. I forsee snapped middle bolts aplenty. |
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If the only proposed advantages to this idea are tamper proofing and security from loosening, then why not just use huck bolts? |
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Did I mention how much I despise this idea? |
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// If the only proposed advantages to this idea are tamper
proofing and security from loosening, then why not just use
huck bolts? // |
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Because hucks A) already exist and B) aren't nearly as
problematic. |
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Maybe we need a diagram, but I'm pretty sure vernon is right in that no matter how it is built, the two screws would be able to turn independatly of each other. |
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Will double turn screws somehow be protected against being bent? |
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Suppose the outer screw loosens anti-clockwise and the inner screw loosens clockwise. You won't need needle-nose pliers to hold the outer screw while removing the inner screw; the outer screw is already tight, and is going to be hard to move in a clockwise direction. So, if you turn the inner screw clockwise, the outer screw, far from tightening, will just sit where it is while the inner screw is removed. |
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[metarinka], how could you design the head so that // backing out the inner one wouldn't work // ? |
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