There a few neat mechanisms that allows only one way motion without the use of a pawl or ratchet. This is one of them:


What I'm looking for is a mechanism that can't be undone once it has gone from one position to another (without taking it apart; i.e., removing a pin) after it is rotated. This could be useful in certain applications, but my search turns up nothing like I remember.

I can think there might be a four or five-bar mechanism that has a critical point where the mechanism can't undone, but can't remember (or where to search for it)!

I do remember seeing one many years ago at one of my first engineering jobs, but can't recall the details other than "that was neat!" The mechanism I saw had I believe 3 to 4 moving parts.

There are some other mechanisms that don't use springs or latches; a search for this turned up something also nifty, but not quite what I was looking for:


Any thoughts on this?

No magnets, no springs; this was a strictly mechanical thing (nothing hidden; I could take it apart, and the mechanical lock was fascinating). The closest analogy I can think of is to those Sokoban games (and perhaps this is a way to think about it) where if you perform a certain order of operations, you lock up the puzzle.

This is one solution I came up with this morning, but I'm wondering if there is a list of them, a search term I'm missing, or other examples.

In this example, a plunger enters from the left; I'm not too sure of the geometry, but I think this is an example of how it could work. When the plunger brings the two locking elements behind it, it can't go back, because of a stop at the end of the plunger, prevents the linkage from reversing.

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Even cooler - I believe the mechanism I saw had no linkages - just a series of Sokoban-like moves where the system went from one state to another, but couldn't be reversed without taking a part out of the 2D plane it was in.

  • $\begingroup$ Would something that involves a magnet meet your requirements? There are diagonal dovetail locking puzzles that have a bearing to block movement, held in locked position by a magnet. A sharp knock moves the ball to free the dovetail. I suspect one could modify such a puzzle to make the lock permanent, apart from disassembly. $\endgroup$ – fred_dot_u Mar 30 '19 at 9:44
  • $\begingroup$ Nikola Tesla invented a one way valve for water/air that has no moving parts, but I don't think this helps you :-( The existence of one way functions is also a MAJOR open problem in computer science/mathematics. $\endgroup$ – William Hird Mar 30 '19 at 13:57
  • $\begingroup$ I belive that what you ask is possible. for example you can replace the linkage with gravity in your example. Or by having a prong and let the pushing part puch down the plunger, itsel or via a 3rd part $\endgroup$ – joojaa Aug 27 '19 at 19:40

In order for a mechanical movement to be "one way", something has to change state as the elements in the mechanism move. The expansion and contraction of a spring and the appearance or disappearance of a magnetic field constitute such a change of state. I don't think what you are looking for exists , I would be amazed if there was a device that was "one way" without having any springs or electro-magnetic phenomena. You claim you saw such a device early in your career, but did you actually inspect it to see if it had any hidden springs or magnets?


A simple weighted ratchet fulfills your specification. Gravity is the "symmetry breaker" in that case. So what else do you implicitly assume that excludes a ratchet?

And: Your plunger thing does not work, I think: Unless the "connection wires" are elastic, you either cannot shove through the plunger; or it can move back - unless you assume that gravity keeps one of the two pallets in the slots - but then, you would not need the "wires".



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