Tec~nical Report Fred M. Wolff, editor Pneumatic Jacks BY ALLAN FANJOY hat do you do when a structure must be moved and casters do not provide adequate stability? If money is no object, air casters can do the job. Another possibility is to rig a mechanicallift jack to lever casters into place. If you want to strike a middle ground with respect to elegance and expense, perhaps pneumatic jacks are the answer. I used them successfully to change the configuration of the audience seating in a makeshift theatre space. Each seating section was about 12' x 40' and each weighed about 1200 pounds. They were the right shape but in the wrong place for a particular show. Considerable time and effort could be saved if they could be moved without disassembling them by lifting them and setting them on dollies. My solution was to reverse that processfirst set the dollies, then lift the unit. Two to four pounds of air pressure per square inch is not much, but over an area of several square feet it can quickly add up to more than a ton. How many scene shops are equipped to build an adequate airtight cylinder and piston? All of them. The airtight part is a used truck inner tube. 1 paid $5.00 for each of them. The "cylinder" is a 2' x 4' platform framed with 2 x 4's and decked with %" ply. The piston is a plate of IN ply which fits inside that frame with W' clearance on all sides. Frame the platfonn cylinder first. Let the short dimension follow through the long one for maximum strength since the tube will exert more pressure on the sides than the ends. As hard as nails are to pullout, they are harder to shear off. Attach the deck and cut a W' ply plate to slip inside the frame. Bolt at least three casters to this "piston" with the nuts W Piston. tube. and cylinder ready for final assembly. Shape of tube accommodating to cylinder. Assembled pneumatic jeck. (not the bolt heads) on the castered face. Lay the inner tube onto this plate and drill a hole so that the valve stem can pass through near the center. Lay the tube into the inverted cylinder and set the piston on top with the valve stem through the center hole. Screw a couple of battens to the underside of the frame to trap the piston inside. Thus, the jack will not come apart when you carry it about, nor will an edge of the tube protrude if you should TD&T * SUMMER 1988 33