Popular Woodworking 2002-08 № 129, страница 83
ROTOR, CAM AND 'NEEDLE' BEARINGS Most whirligigs employ some sort of multi-vaned propeller mounted to a horizontal drive shaft with a tail at the end to bring the whole assembly into the wind.Through the use of a crankshaft or gear box, the axis of rotation is transformed from horizontal to vertical. This complex bit of mechanical engineering was simplified in the early 1920s by the Finnish designer Sigurd J. Savonius. The Savonius-type rotor that I used for the octopus whirligig has the advantages of always facing the wind and providing rotation directly about the vertical axis. For the top and bottom of the rotor, I planed some cedar to V4" thick. The wind-catching sides are made using aluminum-roll flashing available at any hardware store.To join the elements of the rotor, I used epoxy and sheet-metal screws. The cam is made from a smooth hardwood to reduce friction -in this case maple. The edge is beveled 5° so that only a sharp edge is contacting the pivoting arm extensions. The "needle" bearings at the ends of the drive shaft are, in fact, clipped sewing-machine needles. Because it would have been extremely difficult to make plumb bores in the ends of the shaft for mounting the needles, I came up with an intermediate transition piece. With my adjustable hole cutter in my drill press, I made two 3/4"-thick x 1"-diameter disks. I bored a V4" stop hole on one face for the drive shaft and a V16" hole on the other for the needle. Again using my hole cutter, I cut a 1V2"-diameter gluing washer from the V4" cedar stock. Mounted at the intersection of the rotor top plate and the V4" x 13" dowel drive shaft, it adds stability to the assembly by spreading the shear forces on the glue joint. The rotor is then mounted in the whirligig with the needles resting in the cross-grooves of the smallest brass Phillips-head screws I could find. Half-scale rotor 82 Popular Woodworking August 2002 |
