PNEUMATIC LOCKING CASTERS PHOTO: BILL BROWNING by Chris Fretts Indiana Repertory Theatre The primary scenic element for a production of Huckleberry Finn, designed by Rob Koharchik, was a 10´x10´ raked wagon that could pivot around a fixed corner to serve as a variety of locations required by the play. The wagon needed to move easily-it was to be moved by the actors playing Huck and Jim, with poles, when they were on the raft in the Mississippi-but it also needed to be solidly anchored for the land-based scenes. The pivot point was the center of the stage with the audience in the round. It was decided that the actors could lock and unlock the wagon if the process was simple and quick. My first thought was an air-powered system so a discreet switch could trigger the entire locking mechanism. I could put an air supply tank onboard and not have to worry how I would deal with making the unit revolve more than one revolution and not foul or tangle the air lines. I first considered using air-lift casters we had in stock but had to eliminate that idea because of the clearance under the raked wagon. I also considered some kind of friction pads pushed to the floor that would hold the wagon in place. I finally decided on a simple friction brake that applied THEATRE 44 DESIGN & TECHNOLOGY SPRING 2007 pressure directly to several of the fixed casters themselves. It is a simplified version of a car's disc brakes. I figured I would not need to put a brake on all the casters to effectively hold the unit in place. Unlike the air-lift casters or the friction pads, this solution would not cause any visible movement in the unit as the brakes were applied. I also thought I had major parts necessary to build these units in stock. The design was based on a 3-1/2˝ rubber tired fixed caster and a 3/4˝ bore x 1˝ stroke pancake double-acting pneumatic cylinder. If I were not trying to use components I already had, I could have used a single-acting air cylinder and used the spring return for removing the pressure on the caster and simplified the plumbing. A piece of flat stock steel was used as a base and a piece of angle iron was used for a cylinder mounting. A wood block cut to the caster radius served as a brake pad bolted to the end of the cylinder rod. A simple U-shaped metal guide welded to the base kept the wood block in line with the caster. In testing we found that about 40psi air pressure kept the caster from rolling. One further refinement was the addition of several