Here's a link to a previous post that I wrote about our experiences with constant force spring motors (in a thread that is ripe with tons of other goodies). Our elevator in 2011 was without a doubt the best part of the robot; it was speedy, going to full extension in a second, and was only run off a tiny little RS550; all made possible by counterbalancing the lift so that it would be able to stay in whatever position it was left in when unpowered.
http://www.chiefdelphi.com/forums/sh...1&postcount=12
If you want any more information on our lift and how it works, I'd be more than happy to help you out, just message me. Some of the finesses on that lift are things that I still find really cool, 3 years down the road. Also, do some searches of Chief Delphi; you'll find that there are a lot of threads on this subject already, and they contain information that you might find extremely useful.