One-way turn of the screw

One-way screw

One-way screws, such as the above (image from Designing Against Vandalism, ed. Jane Sykes, The Design Council, London, 1979) are an interesting alternative to the usual array of tamper-proof ‘security fasteners’ (which usually require a special tool to fit and remove). There’s a very interesting illustrated listing of different systems here.

A fastener requiring a special tool is effectively addressing the “Access, use or occupation based on user characteristics” target behaviour – and is functionally equivalent to a ‘what you have’ security system such as a padlock, except that anyone can look at almost any engineering catalogue and buy whatever special tools are needed to undo most security fasteners, pretty cheaply and easily, whereas it’s still a bit more difficult to obtain padlock master keys.

However, this kind of one-way clutch head screw, which can be tightened with a normal flat screwdriver, but is very difficult to undo using any tool (without destroying it) can be thought of as addressing a slightly different target behaviour: this is “No access, use or occupation, in a specific manner, by any user”. Even if the original installer wants to undo the screw, he or she can’t do it without destroying it (e.g. drilling it out). A few of the other systems illustrated on the Security Fasteners website also have this property:

Image from Securityfasteners.netImage from Securityfasteners.netImage from Securityfasteners.netImage from

I’m particularly intrigued by the Shear Nuts and No Go enclosures (last two images above) – these two types effectively self-destruct/render themselves permanent as they are fixed into place. Something about this step-change in affordance fascinates me, but I’m not sure why exactly; it’s a similar idea to a computer program deleting itself, or Claude Shannon’s ‘Beautiful Machine’ existing only to switch itself off.

A step further would be a fastener or other device which (intentionally) destroys itself if the wrong tool (by implication an unauthorised user) tries to open/undo it, but which will undo perfectly well if the correct tool is used – along the lines of the cryptex in the Da Vinci Code, just as an ATM will retain a card if the wrong PIN is entered three times: it’s both tamper-evident and limits access. What other cryptex-style measures are there designed into products and systems?


  1. Some bolts (posidrive, philips, hex) can also be drilled to remove the drive pattern. And then they can have a groove dremelled back in (for screwdriver).

    There are probably some fasteners based on the fish hook mechanism, i.e. easy to get in, but not easy to get out. Cactus thorns, bee stings, arrows, rivets, cavity wall anchors, etc.

    One-way fasteners can also be incorporated into the design of larger structures. There are plenty of snap shut plastic cases of non-user serviceable electronic equipment. These either require special tools to prise apart, pressure in precisely the right places, or are not expected to be opened without irreversible damage to the case (either cheaply replaced – or not).

    The simple cable tie is one of the cheapest instances of irreversible fastening. There are also the metal customs fasteners with malleable seals – you can break them, but then you can’t refasten them unless you can reproduce the seal. Some food containers also need to be opened with significant force but not closed without it being obvious.

    Don’t forget the sand released one-time doors of pyramids, etc.

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  3. Moz

    Many manufactured objects simply practice security through obscurity – it’s not at all obvious how to open them up without damaging them.

    One you haven’t mentioned is glue. Quite a few things have a cover glued on that must be removed before the fasteners can be removed. At times this is as simple as putting holes under the stick-on rubber feet under something, but some cameras also do this with their rubber-like grips. Evn if it’s easy enough to remove the glue, generally it’s impossible to glue it back in place because it deforms when removed.

    A surprising number of bigger things are welded into place. At one extreme are things like railway lines and many bridges – you can remove them, but only by cutting them up. But on a more “only done to make removal harder” level it’s not uncommon to see guard rails and so on welded in place simply to stop people unbolting them and removing the guarded object. Or an expensive CNC machine with a nice solid protective frame around it to stop people driving forklifts into it, that also “just happens” to stop the machine being easily dismantled and loaded onto a truck without first cutting the frame away.

  4. moz

    CNC = computer numerical control. Broadly, any computer controlled cutting machine (lathes, laser cutters etc). Not uncommonly costing more than $100k/tonne to buy.

  5. As a supplier of security fixings, one thing we have noticed in recent times is the move away from clutch heads to the sentinel screws. Like clutch heads, this type of one way screw can be tighted with a conventional screwdriver (in this case a Phillips). Arguably its key advantage is that it has a higher torque. As such, it can be tightened without the same limitations as the clutch head. For the full range of clutch heads and sentinel security screws see

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