We used a similar design as Petek described. 1x2 80-20 with some linear brackets and a chain drive. As for the chain drive on our bot, we needed the space between the uprights to be clear so we moved the chain to the outsides. A van door motor drives a chain that is fixed at one point to the lower extension. (see
our gallery) A shaft spans the towers to allow an even force to be exerted on each side. A window motor is mounted in a similar fashion to the smaller, 1x1 80-20 upper extension, except it drives a sprocket that travels up and down a fixed chain, rather than the chain moving around a stationary sprocket. The result is an extremely stable, relatively quick forklift mechanism that can go from as low as 16" to as high as 11-12'. It's not as cool as 173's awsomazing teloscoping arm, but we still won the GM Industrial design award in large part due to this mechanism.