[dtengineering]

We made a scissors lift for our very first FIRST 'bot four years ago. I think you are experiencing something that we experienced on that design....

Judging from the photos the screw is able to lift the bottom stage of the lift almost perfectly... but the top stages stubbornly remain locked in place horizontally. When a member, pinned at both ends, approaches the horizontal, it can only convey horizontal force. You need the upper members of your structure to be able to convey vertical force... but since they are bascially horizontal they cannot.

To confirm this, try supporting the top of your lift (hold it up) so that the members are at least 30 degrees from the horizontal. Now try running your screw up and down with the motor. I suspect it will work just fine. Now try lowering the lift all the way down... is there a point where the upper stages "collapse" while the lower stages, supported by the screw, do not?

If this is the case then there is nothing wrong with the lift itself. (In fact, it looks like a very nicely designed scissors lift). You problem lies with gravity and physics.

Some things we discovered with our scissors lift: don't collapse it ALL the way down... this robs the members of any ability to convey vertical force. Secondly, to aid with the lift, try wrapping bungee cords around the "spacer" pieces that keep the two sides of the lift running parallel (I can NEVER spell parralell correctly...) so that the lift is pulling itself together.

Hmm... that doesn't make a lot of sense... try this lovely piece of ASCII art...
.....\/
...../\
.../....\
./........\
OBungeeO
.\......../
...\..../
.....\/
...../\

That will take a lot of the load off the motor, if nothing else.

Play around with it... scissors lifts are cool... and so are the mecanums.

Jason