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KBBS Archive 10,000
Was that your troop in Boy's Life?
Posted By: Paul G. Jacobson In Response To: Hollow crank shaft thoughts (Ed Valley)
Date: Thursday, 11 November 1999, at 6:01 a.m.
> I have been thinking of a way to make a hollow core _crank_ shafted
> paddle. Your comment about the pvc reminded me of a project my Boy scout
> troop undertook one fall. . .I saw the article on this in Boys Life and was really impressed with the troop doing thise as a project. Optometrists use a warming pan filled with hot sand to soften eyeglass frames so they can be bent.
> I see no reason this could
> not be adapted to the paddle shaft project. But how to remove it?Why bother removing it? The crank sections are merely bent areas between long straight ones. Remove the straight sections, and leave behind short, bent sections as reinforcement inside the paddle. Depending on how short you can keep them, they will add very little to the weight.
Consider starting with a 6 foot section of pvc. Use a dowel rod to shove in a wad of paper towel, about 18 inches. Pour in the hot sand. The wadded paper towel should keep most of the sand in the end 18 inches -- and not soften the entire length of pvc. Shape the crank end in your mold to whatever angles you wish. When it cools, pour out the sand, and poke out the paper towel. Do the same at the other end.
Carefully clamp this PVC pipe to a 2x4. Put a scrap of wood under each end as you do so. This way the center of the paddle form should be slightly above the 2x4, and not touching it.
Now cut out the middle of the paddle form. Do this carefully so the ends don't move. You want to leave a straight stub a few inches long at the ends of each of those bent areas, but not too much. Using the piece of PVC that you have removed: wrap it in 4 to 6 layers of newsprint. Use the largest, flattest pieces you can and stagger the seams. Use a few drops of elmers school glue or a light application of library paste (you can even make a paste from flour and salt, or use wallpaper paste.) If you use th paste sparingly, and only put paste on the outer layer or two then you should be able to gently remove the plastic tube, and end up with a rigid paper one. Trim this paper tube to be 2 to 3 inches longer than the original pipe section, if you can.
slide the paper tube over the straight stub you left on the clamped down crank, then line it up so you can get the other end on the second bent crank end. wet out a piece of 3 inch fiberglass tape, or a cut scrap of fabric that is 3 inches longer than the paper tub you made, and cover the tube and 1 1/2 inches of each crank end. When this sets up you should be able to unclamp the ends, and they will stay in perfect alignment while you add additional layers of glass or carbon or whatever you plan to use. Sand down any bumpy seam that you might have from an overlap of the first layer of glass tape or cloth befoer adding more layers.
> Reheating to the softening point would probably make it less durable under
> tension to draw it out of the cured tube.If you get this hot enough to soften the PVC you are probably getting it hot enough to soften the epoxy, too! Maybe this principle sould be used to your advantage? Consider this experiment: Make up a few tubes of a single layer of fiberglass cloth over a paper core. Dump in hot sand to warm them, and if they behave like the PVC, they should get soft enough to bend into crank shapes, or to soften into an oval cross section. The sand inside them will keep them from collapsing as you ovalize (is that a word?) them. Once you have the shape, you can lay up additional layers of glass and resin to achieve the desired strength and diameter.
If you still have equipment for heating the sand try this with a few scraps and let us know if it works.
If you are going to make fiberglass tubes by forming them over paper ones, consider shoving a 8 to 12 inch piece of PVC pipe an inch into your paper tube, and then laying glass and resin up to the middle of this piece of PVC. A matching tube constructed on a paper tube of the same diameter as this pvc pipe would slide over the unglassed half, allowing the length of PVC to serve as an internal connector (a ferrule is an external connector, but this would do the same thing.)
Just a few ideas, late at night.
Paul G. Jacobson
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