We've built 3 different lifts in 4 years. Here's some of the things we've learned from each:
2005
In 2005 we used a cable (3/32" core, 1/8 OD) driven lift. Each side of the lift had a cable, this was to prevent racking and for redundancy. The cables were tensioned with two turn buckles. It was power up, gravity down using an old van door motor. We used no encoders for position sensing.
Pros:
The cable never broke. The turn buckles provided a quick and off the shelf method for cable tensioning.
Cons:
When the motor stalled we would lose the spool of cable and be out for the rest of the match.
2007
In 2007 we used a chain driven lift. A single 25 chain gave us the ability to power up and down. We used a single fisher price motor -> AM Planetary -> BaneBots Planetary to drive the lift. We added an encoder to simplify the control on the lift. We also used 80/20 extruded aluminum and 80/20 slides for the structure of the lift. The motor assembly was on a slide so tensioning the chain was as easy as loosening a couple screws and sliding the motor our.
Pros:
We were power up and power down. The single fisher price motor was plenty of power to move the lift up and down. We were able to go from ground to the top row of the rack in just a couple seconds. The encoder allowed us to maintain our height without a lot of driver finesse.
Cons:
The chain was very heavy, however once it was on we never had to mess with it again. I think we calculated something between 20-25 ft. of chain being used. Between our competition and practice robot we used about 75ft. of 25 chain at $162/ 50ft. spool. Obviously not a cheap solution.
2008
This year we used a 968/254 style lift. The lift was driven by two cables (1/16, no coating) and two fisher price motors running into a custom gearbox. We used to cables for redundancy and to prevent racking. Once again an encoder was used for position sensing. The cables were looped into two eye bolts. By tightening the nut on the eyebolt we could tension the cable.
Pros:
Power up and down, again. Fairly inexpensive to build. Lighter than the 2007 lift. The cables never broke, however we did lose a crimp because someone over tensioned the cables. The encoder allowed us to easily set and maintain positions.
Cons:
In our first competition we had some minor racking problems due to spacing and alignment, which is critical for a lift of this type. By Atlanta we had fixed all of our racking problems.
Overall lessons to be learned:
- Do not power up/gravity down. This can become a major headache at competitions.
- 1/16 steel wire rope is plenty strong enough.
- Tensioning should not be over looked on any lift system. Not mater what you're using it's going to stretch over time and stretching can end up killing your lift if you don't have a tensioner.