I have seen this somewhere in the wild, perhaps a space-efficient storage system in lieu of two shelving units. I'm trying to reproduce, and failing.

Imagine two stacks of trays which hold things that must not fall out, so the trays must remain horizontal. The trays are all connected to the same track, which loops around, so that the first stack is on "the way down" for the track, and the second stack on "the way up".

Q: how are the trays connected to the track, so that they can move along the track eternally without ever tipping over?

I sort of remember that there is a wheel on each tray that counteracts the rotation coming from the track, but I don't remember how.

  • 1
    $\begingroup$ One possibility among many: Let each tray swivel freely and rigidly attach a weight to the bottom to keep it upright. This only works if the items on each tray will be significantly lighter than the weight. $\endgroup$ Aug 25, 2015 at 0:44
  • $\begingroup$ Are you looking for a specific "mechanism" or a design/custom mechanism? Can you provide a sketch to accompany your description? Is this track circular, can it be? Or must be rectangular? Or any type of arbitrary path? Maybe a parallel four bar linkage: parallel four bar linkage $\endgroup$
    – GisMofx
    Aug 25, 2015 at 1:37
  • $\begingroup$ @ChrisMueller: I don't want the swaying back and forth I'd inevitably get. $\endgroup$ Aug 25, 2015 at 2:35
  • $\begingroup$ How level does level need to be? Does it need to be fixed level that at any point during the course of the path it is within 1-2mm of horizontal or does it just need to not reach an angle high enough that flat objects would tip over? $\endgroup$ Aug 26, 2015 at 13:33

1 Answer 1


Image in my mind: The track is on a wall, with the trays being perpendicular from the wall. The track is rectangular (with rounded corners obviously).

Solution 1:
If the things that the trays are holding are relatively light (to the tray/shelf), or if its feasible to perfectly balance the load on each tray, then you could just use gravity. You could attach the tray to the track carriage (for lack of a better word) via a bearing, and then just ensure that the CoG of the tray+contents is well below the pivot point (where the bearing is).

Solution 2:
Have an additional track/slot that controls the angular position of the tray (by it's position, so it would not just 'follow' the other tracks). It would probably need to be a slot, and the 'track' would probably need to be redesigned as a slotted/indented track as well that was of greater depth than your angular-controlling slot so that the angular-control-slot could pass over the normal slots/track.

crude hand-drawing

It would be an idea to offset the arm by something other than the depicted offset (let's call it 90 degrees) as then the two tracks would overlap less.

  • $\begingroup$ Appreciate the drawing! Question: If the tray(s) moved clockwise, I'd be concerned the whole thing stopped when it got to the bottom-right corner as perhaps the (green) track follower went backwards. Is there a way of avoiding this? $\endgroup$ Aug 25, 2015 at 2:42
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    $\begingroup$ I don't think there's an issue with the follower going backwards, however if you wanted to avoid it you could change the angle of the arm. I.e presently it's at 270 degrees say, changing it to 315 or 225 (45 degrees up or down) would mean that it's never going to be going directly "in" the arms direction. But again: I don't think it going backwards will be a problem. $\endgroup$
    – m4p85r
    Aug 25, 2015 at 3:57

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