Unibody/Monocoque Construction

[robotguy67]

The entire frame was chromoly and consists of two main pieces - a main "tower" and a "trolley" that rolls up and down the tower. These two pieces together are about 25lbs with the rollers and bearings. The lifting platforms (aka - wings) used monocoque construction (mostly .040" 6061 T6). These were made just like aircraft wings - skin over two main spars made from sandwiched aluminum. The actual wing pivots are made out of 0.1" thick chromoly plate - these plates ARE heavy, but very strong. The first set of wings were 17lbs each, I believe. I think the lightened wings were about 15lbs each (you may have seen us frantically finishing these in our pit in Atlanta). Other than these main parts, of course we had our control panel (monocoque box), monocoque battery box and pnuematics panel, two outriggers also made of chromoly (2-3lb each, not included in the 25lb), transmissions, wheels, etc... I'll have to let one of the other team members post some good pictures of the frame.

If we were doing a more standard base, we would have used monocoque. We used entirely monocoque the two previous years - frame and manipulator/shooter. It goes together much quicker and is plenty strong if you have a "normal" shape. To accomplish our design goals last year, we needed a large part of our robot to move 12" and it needed to support large internal forces (upwards of 1000lbs) from the wings and it needed to have a big hole through the middle of it. This could have been done with monocoque given enough analysis and design time, but it just wasn't practical to do in 6 weeks. The tube structure was much easier to design and is more durable than monocoque would be in this application. The chromoly tubing is tougher than mild steel, so thinner wall tubing tubing can be used most places which keeps the weight down.

Monocoque is ideal for distributed loads - like the areodynamic loading of an aircraft wing. The rolling trolley in this robot concentrates very large forces on just a few points (the rollers). Using monocoque, these points would require some fairly thick material along the roller tracks to distribute the point loads. We may have saved a few pounds using monocoque for the lift bot, but it would have taken a lot more design effort and for the same weight, the tube structure is a lot stiffer.

Mike