After seeing the pictures of the tetra grabbing 2 stage lift, I was wondering how to attach a structural piece to a chain so that as the chain moves horizontally, it provides linear motion. Obviously, limit switches would be neccesary to prevent the part from being sucked into the sprockets. So how do you experts do it?
we tried it with the tank treads and drilled a hole in the middle of one of the tracks and bolted into the slide
robohippo, can you link us to that picture? I’m not exactly clear on what you are describing.
I think he is refering to this pictures:
http://www.vexrobotics.com/index.php/user_profile?user_id=234
those are the profile with several pictures of their robot.
and regarding to your question I hope this link will help you with the way the system works:
http://www.vexlabs.com/conlift.shtml
thats basically how it works with a few expections
cheers
~gdo
Yup, that’s it. 
For a single stage elevator, you can simply make a loop of chain and secure your linear motion part to a single point along this chain. (Much like a garage door opener.) Here is how I attached the chain to the elevator slide when the Triple Play Robot was only a single stage elevator.
http://img471.imageshack.us/img471/1249/vexfasteningchain4jc.png
For the multistage elevators, things start getting more complicated. Here are a few of the pictures from this post:
http://img287.imageshack.us/img287/5713/s43000361hr.jpg
http://img287.imageshack.us/img287/6129/s43000223rn.jpg
The way the multistage elevators work, is that the chain loops through every stage of the elevator. They are also attached to a fixed position somewhere on the last (uppermost) stage of the elevator. I attached the chain to the sprocket itself by zip tying the chain in place. To make this even more efficient, try locking the sprocket from rotating as well. (The sprocket will not rotate much (maybe 10° or so) if the chain is zip tied to it, but it works better if this sole sprocket is locked into a single position.
(See this link from gdo’s post for a diagram of the Continuous lift concept.) In that diagram, if the motor in the base of that robot was to start pulling on the red chain/rope, the chain will shorten. Since it is attached at the other end, the elevator will have to find a way to make get shorter. It does this by beginning to pull up the uppermost stage first. (Because this stage is the lightest). It will then proceed to do this until all the available “slack” is used up, resulting in all the stages of the elevator being extended.
But there are a few very important things to keep in mind when developing a multistage elevator. In the pictures of the beta stage of my “Space Elevator” Vex Robot, I used elastic bands as tensioners, because I did not have the geometry of the elevator perfect at that point. When building the elevator, any of the segments of the chain that actually “shorten” or “lengthen”, such as the green ones in the picture below, must be in EXACTLY the same plane as the direction of the elevator. In other words, the chain in these green segments must be parallel to the direction of the elevator.
http://img221.imageshack.us/img221/3872/contlift5qy.gif
If these green segments are not perfectly parallel to the direction of the elevator travel, they will essentially form a triangle’s hypotenuse. As these green segments become “shorter” and “longer”, the change in triangulation of these green segments will make your chain become unbearably loose or so tight that it snaps. In the above pictures of the “Space Elevator”, all the “green segments” were all done correctly, except the last return section in the front. Hence the elastic bands. But after a quick modification, I fixed this issue (no more elastic bands required!) and the elevator worked perfectly. (You can kind of see the new modification in this picture. I’ll get a better picture online tomorrow, as all my Vex pictures are on my other computer right now.)
