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new tensioner
Saturday, March 6 2010
Almost since the beginning of the laboratory, I've had a disco ball hanging from its ceiling, and that disco ball has been attached to some sort of homebrewed rotator mechanism. Unfortunately, I've never been able to build a rotation system that could be counted on to work for more than a few days. It turns out that it's fairly difficult to build robust mechanical systems. The main problem with the various rotator designs has been with the drivebelt, which has had to be elastic in order to cope vibration, wide tolerances, temperature, humidity, and material deformation over time. The most convenient elastic drive belt is a rubber band, and while these work great for a day or so, exposure to either oxygen or ultraviolet light eventually destroys them, and begin to stiffen and crack. Interestingly, I can keep a rubberband for six years in a drawer, but if I take it out and attach it to a mechanism that runs continuously, it will quickly transform into a material not unlike dried pasta. The difficulty of getting the rotator mechanism to work reliably over the long term has meant that I'd largely abandoned the disco ball and the system that rotates it. I don't think I've operated it in four and half years, since the solar deck building inspection scare of September, 2005.
But in the past few days I've been trying out some alternative materials for use as drivebelts. The other day I used a piece 23 gauge insulated copper wire (the kind found as a conductor in CAT-5 cable). I made a loop and twisted the ends together, and it ran for most of a day before metal fatigue broke the wire off at the junction. Next I tried a piece of narrow hemp rope, its ends knotted together. That worked great until either the humiditiy increased or the material stretched, at which point it fell off one or the other focus of its circuit. To use such inelastic drivebelts, I'd need a mechanism to take up slack.
Those of you who have threaded film through an film projector know about the little spring-loaded lever that takes up slack in the film as it is pulled through. I decided to build a similar device for my disco ball rotator. So I went through my can of pulleys and knobs until I found a perfect little aluminum spool, which I attached to the end of a 3.5 inch long piece of mild steel, using a bolt as an axle so the spool could rotate. I attached the other end of the steel lever to the wooden chassis of the rotator system with another pivot bolt, and then attached a spring so the lever would feel happiest applying pressure to the rotator drivebelt.
With this new system, though, I couldn't develop enough friction on a piece of hemp rope to force its bulky knot past the tensioner. So I made a drivebelt out of polyester cord, the ends of which I melted together using my soldering torch (resulting in a much smaller junction than a knot). Unfortunately, though, the polyester cord had even lower friction that the hemp rope. So I gathered pine sap from nearby White Pines to use as rosin (serving the same function as the substance applied to a violin's bow). It's usually possible to gather a teaspoon of dried sap from various places around the base of a moderate-sized pine, and that's far more than one would ever need for rosining up a string. After melting the rosin in a spoon in a manner that looked identical to the preparation of heroin, I applied it to the cord. This worked perfectly, and I found I could stop and start the rotation of the discoball very reliably. If the polyester cord proves durable, I expect this new system to last for years.
The new disco ball rotator (you cannot see the large discoball this rotates in this picture). Note the new tensioner arm in the upper middle.
This picture is from a few days ago, on a rare laundry day when we made the bed. The critters love a made bed, but they don't want to crowd one another.
Ray had his first day at work at New World Home Cooking today, and showed up tonight as Gretchen and I were watching the recent version of Battlestar Galactica. He'd be spending the night at our place because we're his Upstate residence until he gets his own place. We have a big house, so there's no reason he can't live in our Gunther room indefinitely.
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