We built a telescoping arm last year and it worked pretty well.
http://www.issaquahrobotics.org/photos.shtml
The mechanism was pretty simple: three lengths of 80/20 extruded aluminum linked together with nylon sliders. Pulleys guided spectra cable (from Small Parts) which raised and lowered the arm (one set of cables for up and another for down). We used 50lb test cable for up and 300 lb for down, since that had to lift the weight of the robot.
Our only mistake was using two different motors for the two directions. We thought we could pick motor speeds that would keep the cables in synch, with tensioning loops to give us some slack to play with. That didn't work well at all and resulted in broken cables, including one disaster that broke the 300 lb cable in the middle of a match. It made quite a load SNAP that we clearly heard from the nose-bleed seats.
We didn't produce engineer documents, but I'm happy to answer any questions and/or take more pictures to show the mechanism.
Go with a Big Block. The torque is awesome and you cant beat the sound of a built Big Block with a high lift cam and some open headers. A blower would be a nice addition as well.
Good luck getting past the inspector. - GBK [more] 
17-01-2005, 02:24
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