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Double Turn Screw

 Screw that has to be turned both ways
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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.

—metarinka, Jun 04 2011

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       // could also never back out from vibration or torque as it would tighten down one of the threads//.   

       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.   

       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.   

       Or else I'm missing something, and if so, what is it?
—mouseposture, Jun 04 2011
   

       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
—Voice, Jun 04 2011
   

       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.
—MechE, Jun 04 2011
   

       Imagine a screw that has a hole drilled through the center with a counterbore. You should have stopped right there.
—zeno, Jun 07 2011
   

       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.
—tatterdemalion, Jun 07 2011
   

       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?
—kdmurray, Jun 07 2011
   

       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.
—RayfordSteele, Jun 07 2011
   

       I think this could work as a self-tapping screw pair with the right tools.   

       Detractors: c'mon, this is the HB! +
—csea, Jun 08 2011
   

       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"   

       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.   

       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.
—metarinka, Jul 06 2011
   

       [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.   

       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).
—Vernon, Jul 07 2011
   

       what fun!
—po, Jul 07 2011
   

       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.   

       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.
—metarinka, Oct 27 2011
   

       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.
—Alterother, Oct 27 2011
   

       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...   

       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.   

       If the only proposed advantages to this idea are tamper proofing and security from loosening, then why not just use huck bolts?
—Custardguts, Oct 27 2011
   

       Did I mention how much I despise this idea?
—Voice, Oct 27 2011
   

       // If the only proposed advantages to this idea are tamper proofing and security from loosening, then why not just use huck bolts? //   

       Because hucks A) already exist and B) aren't nearly as problematic.
—Alterother, Oct 27 2011
   

       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.   

         

       Will double turn screws somehow be protected against being bent?
—ye_river_xiv, Oct 28 2011
   

       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.   

       [metarinka], how could you design the head so that // backing out the inner one wouldn't work // ?
—pertinax, Oct 30 2011
   


 

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