Thursday, September 24, 2015
Vibration Dampers From Outer Space?
They're used on guy wires and long support cables. They prevent or reduce "aeolian vibration," which is to say, they keep the wind from playin' 'em like a harp. Literally. It's lovely when it's an instrument designed for the purpose but it can destroy large structures.
So we damp it out. Modern computer analysis has replaced the traditional hollow bell-type damper with a kind of dumbbell, effective over a wider range of frequencies and without the water-retaining properties of the older type.
But they sure do look freaky.
Ash, Stockbridge dampers. We use them on power transmission lines for the same reason, when we can't use a low enough tension to be below the threshold where aeolian vibration starts. Low tension wouldn't be a very good solution for a guy wire though...
ReplyDeleteThe other type of conductor motion we design for is galloping--much lower frequency and much larger amplitude compared to aeolian vibration. Is galloping ever an issue on long tower guys?
Interesting, I never realized they'd upgraded from the old type. And the 'dumbbells' do look strange!
ReplyDeleteUseless trivia- Ben Burt used undamped guy wires as the source for the blaster sound in Star Wars. Just whacked it with a wrench and recorded the results.
ReplyDeleteya learn something new every day.
ReplyDeleteI always wondered what those silly little antenna were on the hi power lines, and NOW I know. Thank You Ms Ecks.
Rich in NC
Don't forget the red spheres you sometimes see on power lines, guy wires and the like -- they're there to help airplane pilots notice where the wires are. Most common in areas where there's crop dusting, and near helicopter landing pads.
ReplyDeleteBack in my childhood and maybe still, local radio stations out in the flat agricultural areas also used them as small billboards, with call letters and frequency painted on the eye-catching gadget in white, and posted at fence level.
When I was in 7th grade, a small group of us were fooling around with the guy wire on a power pole at the edge of the school yard. We found the resonant frequency of the thing and ended up breaking it off at the upper end.
ReplyDeleteAnother resonance incident I remember was on the Capitol Beltway in Maryland. I looked over into the traffic going the other way and briefly saw a truck with the front wheels bouncing up and down at an alarming amplitude. I surmised that at the speed it was going, the resonant frequency of the front suspension exactly matched the spacing of the joints in the pavement. At least very briefly, until the driver got the hell on the brakes.
Bruce: Look up "Jeep death wobble". It's one of the reasons I traded in a Rubicon (a Wrangler with the full off-roading treatment). It included an aftermarket modification, jacking the body up 2 or 3 inches above the frame rails to make room for larger tires, which I think made it more likely. At highway speeds, a series of bumps (even small bumps) that is spaced just right will put the front end into a violent side to side oscillation. It doesn't interfere with steering, if the driver doesn't let himself be distracted by the shaking, but it feels like it's going to shake even a Jeep apart, and it does shorten the life of the ball joints. The only way to end it is to slow down below 35 mph.
ReplyDelete