This is the gas spring Z-axis lift I put together. The head is supported by a cable which is fixed at one end and looped over a roller on the end of a gas spring.
(Click on the pictures to bring up the full size image.)
Below you can see the upper and lower extremes of travel. A cool thing about the pulley design is that the spring only needs half the travel of the head so you don't need a very long spring. This also means that you need a spring with a force approximately double the weight of the head (~35lbs). This spring has roughly 60-80 lbs of force.
Another feature of this mod is that it helped to increase the Z travel to just over 10". I say "helped" because the other thing needed to increase the travel is a longer rack available from Littlemachineshop.com. Here is a picture of the old rack compared to the new rack.
The spring was actually made for use in an adjustable height chair. I found it at American Science and Surplus for about $8. The button at the bottom is what lets the spring adjust. When the button isn't pressed the spring is locked in place. For this application the button has to be pushed all the time so that there is always lift provided for the head. So basically I just cut a blind tapered hole in a block of aluminum to fit the taper at the button end of the spring. The hole is just the right size so that the button is pushed when the spring is sitting in it. Unfortunately I didn't take a picture of the block before I put it in the column so you'll just have to use your imagination!
The block is mounted in the column using 2 screws. The lowest rack screw just happened to be in exactly the right place so I only had to drill one hole in the column. Because of the longer rack I also had to make another countersunk hole (in the rack) just above the existing stock hole.
Here are some pictures of the roller.