Our scissor problem pt1

[Dick Linn]

Pop off the acme nut, lift the mechanism manually and see if you can tell where it's binding.

[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

[Al Skierkiewicz]

Jacob,
I go along with Chris on his assesment and the others. You have a tremendous amount of force that you are trying to overcome. As you start to raise the scissors, you are encountering some rather exceptional friction in the horizontal direction and well as trying to raise the weight of the assembly. That in of itself is bad enough, but it also appears that you trying to lift a motor, gearbox and shaft as well. That throws all the forces off balance and multiplies the work that the lead screw and motor must perform. When you try this again, take the motor and other parts off the top section and see if balancing helps. As pointed out, try lifting a few inches by hand and see if the motor can take over from there.
Lastly, the joints in a scisor lift must stay in position and have low friction. As you can see in one of your stress photos, the bottom of the scissor has started to move but the top layers have not yet moved at all. This is the free play in your joints adding up. When the joints have little free play, all layers should move about the same. It is hard to tell from here, but a washer between bars helps reduce the friction of the joint. I would also recommend you use a lock nut (nylon insert) instead of the dual nuts. That will give you some additional control over the friction of each joint.