Glass Insulators and Earthquakes
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S.F. Bay: 3 of 6
|Peninsulators HQ was mid-San Francisco peninsula, only few miles from the
San Andreas fault. Yes, that San Andreas fault. Visualize an angry
giant picking up your house and shaking it. HARD. That's a big
earthquake, and they will happen here, the only question is when.
Lots of little ones are preferred, but sometimes they save up and you get
the big kahuna. But big or little, you must plan for them all.
At PHQ, we live by the motto "You can't fall off the floor." If it's not mounted securely on a pin, it sits on the carpeted floor-- or is safely packed in a box. Museum Wax et al are not positive enough.
|Crossarms with wood/steel pins are a good period choice-- every collection
should have a pole-top. But with original spacing, they don't hold many
insulators. Plus, the solid pins block the light, and they're not suitable
for windows or display cases. So: shelves.|
But without a threaded pin, the glass is just sitting there, not held in place. Waiting for the inevitable shaking. In a display case with front doors the carnage is contained, but open shelving is disaster. How then to safely display on shelves in a way that's unobtrusive?
|This is one solution-- can you see it? Hopefully not. Each insulator
is sitting on an acrylic pin made of a round ball on 5/16"
round rod, which friction fits into through holes in softwood shelves. The
rods are made in various lengths (on ½" increments) and adjusted up
and down in the hole to achieve a good fit so there is no lateral motion: the
insulators can't move around.
Pro: nearly invisible, positive holding;
|Here's a close look at a typical insulator on an acrylic pin. The acrylic is crystal clear, and since both rod and ball are rounded, the lack of sharp edges means you can barely see it. Often it's entirely invisible; sometimes, a bit more obvious-- but it's always unobtrusive enough not to stand out. Visitors often ask what keeps the insulators on the shelves, not seeing the pins at all.||Here's the same picture with the insulator removed-- the crystal clear acrylic is barely visible. The ball shape on top was chosen because it fits snugly in the pinhole and doesn't catch, so if an insulator were to ride up on it (shaken by the giant), it would tend to slide right back down again to its original position. However, the balls are expensive, retail about $0.25 each in quantity.|
|Balls are available in various sizes to fit most pinholes, with 1" obviously the most used. Each has a hole drilled to accept the rod, chamfered to ease insertion. Rod is cheap, a penny or two per pin, but it's oval in cross-section, not round, and the diameter varies enough from piece to piece that you can't just select any rod-- you must bring calipers or a micrometer and measure them: the ideal large diameter is .3100" ± .0005. Too small and it's a sloppy fit in the hole that's difficult to set perpendicularly, and too loose in the shelf to stay put by friction alone. Too large and it's difficult to fit to the ball and will be too tight to adjust in the shelf (wax can be used as a lubricant). The right fit snaps into place with just a little force, and forms a mechanical joint which is then bonded with acrylic cement. There's no trick to making the rods, just saw to length and finish the ends: they should be rounded or chamfered to aid insertion into the ball and shelf. They're made in various lengths in ½" increments, since each insulator's pinhole is different. Most common length: 2½".|
|Center-drilling the balls is more difficult, but there is a drill press trick: clamp a softwood scrap to the table and drill a hole 1/8" smaller than the ball size and about ½ a ball deep. For 1" balls, use a 7/8" bit (shown is a Forstner) and go down about ½". Drill the same size hole all the way through another piece of thin scrap. Now without moving the table or clamped board, remove the first bit and replace with a 5/16" twist bit.||Drop a ball onto the hole-- it should rest on the hole edges, not the bottom, and be firmly centered. Use the second scrap to hold the ball down while still allowing it to be drilled. Depth: about 3/16". Drill quickly, acrylic has a low melting point! Chamfer the holes by hand or by replacing the twist bit with a chamfer bit and re-using the same set-up.|
|To assemble, take a ball and put one or two drops of acrylic cement in the hole. Roll the ball around to distribute the cement and wet all the hole surfaces. Now, take a rod and snap it into the hole! It should pop in place and have a nice tight mechanical fit. Add one or two more drops at the interface of the two parts and make sure capillary action draws the cement all the way around.||Stand the pins with the rods straight up in an egg carton or similar that will hold their positions. Let dry overnight and the joint will be stronger than the acrylic. To use, pick correct rough size for insulator's pinhole (to ½"), wax bottom if needed, then insert into wood shelf and twist down until the insulator just rests on its base, with minimal side-to-side play at the ball.|