# How to keep a tray horizontal when moving around a closed, vertical track?

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.

• 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. Aug 25, 2015 at 0:44
• 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 Aug 25, 2015 at 1:37
• @ChrisMueller: I don't want the swaying back and forth I'd inevitably get. Aug 25, 2015 at 2:35
• 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? Aug 26, 2015 at 13:33