[billbo911]

Originally Posted by M. Krass

For simple, two segment telescoping arms, I like using roller chain.

Our 2005 robot had two such arms. You can see them here: http://www.chiefdelphi.com/media/photos/20233

The chain was a complete loop that ran up the outside of the arm and around a sprocket at the end of the first segment. It then passed inside the tubing, between the first and second segments, and was bolted through the links to the inner piece of tubing. We used delrin blocks as linear bearings to make it all slide easily and to space the inside segment from the outside segment properly.

This is a very simple yet elegant design. It works well and is easy to implement and build. We used a similar design in 2005 with the exception of using cable instead of chain.
To add a third (fourth, fifth,sixth....) section, run a cable/chain loop that is attached to the bottom of the added section over a pulley/sprocket on the top of the next outer intermediate section and secure it to the section next further out. A pulley/sprocket on the bottom of the intermediate section(s) is highly advisable as well.