There's two ways to do it:

1. Continuous (preferred): You line up vertically stacked 8020 in front of each other (we'll say 2 for simplifying the example). A cable runs from the base of the back 8020 to the top of the back 8020 (through a pulley) and then down to the bottom of the front 8020 (another pulley) and is then tied to the top of the front 8020. You put a motor with a winch near the base of the back 8020 (which is bolted to the robot's frame) and when driven, it will lift the front 8020. The front 8020 will slide up and down, and you can mount any type of forks on it. This isn't true "continuous" because you're relying on gravity to pull it back down, so it's much better to have it continuously looped and powered.

2. Cascade: Four 8020 stacked vertically next to each other. Cable starts at bottom of back 8020 on winch with motor. Cable then runs to top of back 8020 (through a pulley), over, and then under second-to-back 8020 (another pulley) and then tied to bottom of second-to-front 8020. Another cable is tied on top of second-to-back 8020, looped under second-to-front 8020, and then tied to bottom of front 8020. Et cetera! The cables become taught and push up the 8020 in front of them.

These are both very simple methods, and I know I made it sound very complicated. If I had pics I would show you, but I don't.

Good luck,
Joe