Fairing Model: Tool Bracing
Dateline: 04/28/99The tool is assembled with the foam and bulkheads in place, almost ready for finish work. Although we're impatient to start laying up the part, the tool sags too much.
The basic problem is that the central shaft is simply not stiff enough. Partly that's because we just discovered it was the wrong material, and partly because we should have gone with a larger diameter.
In any case, the smart solution would be to disassemble the tool and get a larger shaft. That seemed like an awful lot of work, so we looked hard for another solution.
I thought that installing braces between the bulkheads would give us enough stiffness. By bracing the outer edges of the bulkheads, making the tool more like a cylinder than a shaft, the sag should decrease significantly.
Brace Design
The tool has six bulkheads, forming five "chambers." We decided to brace in the three middle chambers. The center-most chamber hadn't been enclosed by foam yet (so we could still use the overhead crane to move the tool), but we didn't have access to the next chambers out.
The bulkheads had been designed to slide off the shaft. By breaking the Liquid Nails bond between the bulkheads and the foam, we were able to keep the foam in one piece and gain access to one chamber at a time.
Rebecca and several other students put together the braces from particle board and shelf brackets. The braces were designed to rest against the shaft, with corner cutouts to span the bulkhead blocks.
My intent had been to brace at the outer edges of the bulkheads, but Rebecca thought her design would work. In situations like this, I tend to be overbearing, so I make an extra effort not to dictate a solution. The trick, though, is to learn when it is necessary to make an executive decision. This was one of those cases, but I didn't realize it until later.
Brace Installation
Installing the braces required some contortion. The foam was still attached to the outer bulkheads, meaning we had to reach inside of a two or three foot cylinder. The released bulkheads couldn't be moved very far, leaving little room for access.
Even with the close quarters, we managed to get three people at a time working on the bulkheads.
This view also shows the brackets which attach to the moveable bulkhead. The brace is first attached to the stationary bulkhead. The other bulkhead is then moved into place, and we reach through the access holes to install the remaining screws.
On one of the outer chambers, the foam broke away from the stationary bulkhead. We were worried the foam cylinder would break apart without the bulkhead support, but it held together. It also had the advantage of giving us easier access to the stationary bulkhead.
With the braces installed in the three middle chambers (the foam has been replaced in all but the center-most chamber), the tool still sagged too much. In fact, the braces barely had a noticeable effect.
I believe there were two reasons for the remaining sag: 1) bracing near the shaft did little to increase the inertia, or "I", of the entire assembly; and 2) there was enough "slop" in the brace tolerance to allow movement of the tool.
Additional Bracing
At this point, we definitely had too much work invested in the tool to take it apart and install a bigger shaft. I decided to install stouter braces at the outer edges of the bulkheads, and to make them very tight.
We were now running into schedule pressure, so we didn't want to remove any foam. That meant only the center-most chamber could be braced. We first installed a temporary brace, and that seemed to remove most of the sag. Bracing the center chamber only, then, looked like it would be sufficient.
For braces, we used 4x4 lumber. To get a tight fit, we used a hydraulic jack to spread the bulkheads apart just a bit.
The braces were wedged in very tightly, and wouldn't move even if we pulled on them. To be safe, though, we attached a single shelf bracket to either end of each brace. (There were a total of four braces in the chamber.)
Ready for Finishing
The new braces brought the total sag down to about 1 inch in the center with the tool simply supported at either end (i.e. resting on ladder rungs). This compared to close to 9 inches without the braces.
We were now ready to move the tool to the winder, put the foam over the central chamber, and finish the surface. Unfortunately, Rebecca wouldn't be around for the rest of the project. It was time for her to go back to school, so we would have to finish without her. Her last task was to pose in front of the tool she had built.
