Rocky Plains Observatory
- Wheel Beam Fabrication
The heart of the roof design are the wheel beam assemblies. These beams are made up of 3x3 steel tube (1/8" wall), and were machined to house 2 wheels each. The 6" roller bearing wheels are commercially available, and have a central machined V-groove wheel that is designed to ride on inverted angle iron. A 3/4" hardened steel shoulder bolt serves as a wheel axle. The end wheels are mounted inboard from the end of the beam to enable the roof to overhang the cantilevered track beams, to minimize the length needed for the roof to completely roll off the building.
Four angle cross pieces are welded to the top of each beam, to provide attachment to the roof trusses. The trusses are bolted to these angles through slotted holes, to enable fine tuning (adjustment) when building the roof in place on the rails.
Though I had learned to weld in college (too many years ago), I was able to pick it up well enough to get the job done - but my welds weren't too pretty. The structural requirements for all these welds are very minimal. I used a simple AC-DC arc welder (Lincoln). Honey, I Need a welder... :-)
- Track Fabrication
Four steel tracks are mounted to the top track beams (each 10 feet long). The steel tracks are a fabricated from 5" wide strip steel and steel angle. Tack welds along the length of the angle are sufficient to attach the angle to the strip. You need to limit the weld lengths, clamp the angle, and skip around to prevent bowing of tracks from the welds (even so, I got a little bowing). Mounting holes in the strip are used to bolt the strip to the top of the 4x4 wooden track beams. Holes are drilled oversized to allow some adjustment - you want the separation distance between tracks to be very consistent (parallel). Some float in the wheel mounting allows the wheels to handle some track to track variation.
- Truss Fabrication
The roof truss pieces were very carefully cut to be very consistent one to another, then the trusses accurately clamped and fastened with glue and ring nails. The trusses came out very good, and very consistent (dimensionally) from one to another. Getting good, straight lumber is a real challenge these days. I ended up purchasing my wood over 2 months in advance, then storing it in my garage. As the wood dried, some pieces curled unacceptably. Even though I had hand picked the wood at the lumber yard, I still ended up returning almost half the wood. For the main truss pieces, I spent the extra and purchased some truss grade wood - it tends to be drier and straighter, and is selected to be free of defects and knots along the outer edges (and it meets a minimum strength level). I still had to be a little particular with that wood as well.
- Roof Construction
The first task was to attach the tracks to the already mounted 4x4 wood beams. These beams nailed and glued to the top of the east and west walls. Though cantilevered out from the building, angled gussets are bolted into place to support most of the roof load when rolled off. The steel tracks were carefully aligned (parallel, and at the correct spacing), clamped in place, and lag bolts used to attach them.
Next, the wheel beams were placed and the trusses attached (a two man job). The trusses were adjusted to be in line with each other (remember the slots in the angle cross ties?). Once bolted down, the roof was adjusted to be square (important) and clamped in place during the sheathing operation. The second roof was clamped to the first and sheathed. This results in a close fit when the roofs close together.
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