Dampeners

Somewhat like the huge mass that sits on top of the Citicorp bldg to help it counter sway, I am looking for two dampeners.

  1. Counter the sway of a mass hanging at the end of two ropes. There will be sway in the same direction as the lineup of the two suspending ropes, and
  2. Sway is a cross direction (like a garden swing).

The physical size in the direction of the two suspending ropes will be a few feet. The physical size in the cross direction will be a few inches.

In general terms, are there dampening devices, techniques that might meet this cryptic description? (Application is real!)

Best Regards, Wayne

Reply to
Wayne G. Dengel
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That block on Citicorp uses a simple principle - put a spring-mass system of the same natural frequency as the system to be damped.

In short, calc the natural frequency of the system, and attach a small spring-mass of the that same frequency to the system .

You will need one for each axis you don't constrain (think link-arms vs ropes to cut your axes to one).

(FWIW - That Citicorp damper two-axes mass-arm frequency-multiplier and mass-skirt-float was my design, from when I was with MTS. I still have the four pages of partial differential equations determining the significance of torsional vibration on that building on stilts, which the orginal equations had missed. I mocked that damper up in the lab for a single axis, and I as the ME/EE and our group's EE worked out and tested the feedback loop and controls, using a cheap-ass twenty foot steel bar and sheet-metal-plate-in-a-sidewise-half-barrel-of-oil mockup which I had cobbled together. A few snips here and there in the sheet metal petal that was swamping in the oil, and the damping factor curves on the scope were just the same as we had determined for the building. Cheap and dirty precision engineering - the best kind.)

fwiw

Reply to
hob

Hob: Please play that back again . . . . .about link-arms to cut dampening solution to one axis??? Are you saying that I can dampen in both axis with a single axis solution?? BTW, while living for years in the greater NY area, I would certainly have loved to see that system! Now in Florida, oh well . . . . .

Reply to
Wayne G. Dengel

Hob:

Another problem I have is that the length of the ropes will change, let's say from 8' to 10'. Maybe enough room in the design for a single solution to accommodate this spread??

Wayne

Reply to
Wayne G. Dengel

Reply to
insideman

solution to one >axis???

1) the link arms are used to lower the effective spring constant. To get a lower frequency with given mass and spring, mount the mass at the end of an arm, and mount the spring transverse to and part way down the arm. 2) each axis needs its own solution, i.e., its own spring-mass.

no. But you can turn a two axis problem into a one axis problem by using bars instead of ropes and not letting it oscillate across the bars' pivots.

BTW, while living for years in the greater NY

Not much to it - a 600,000 pound block of concrete (30' x30' as I remember it) floating on a polished bed, supported by oil under pressure. Amplifying arms on the four sides are connected to the building, and along the arms are pressurized-nitrogen bags connected on arm and on building, used as variable springs. A few sensors and a feedback loop, and some accounting for hysterisis, and it seemed to work (if it didn't, people would have gotten seasick)

I left MTS and didn't even get my name on the plaque in the Citicorp lobby.

Now in Florida, oh

Reply to
hob

There are ways, but now its getting complicated if you go with a same-frequency subsytem. As long as the system and the attached subsystem have the same natural frequency, it will be very heavily damped. So if you change the rope length, you will need to change the frequency of the subsytem used for damping to exactly match the change (its spring constant or mass.)

Can you use two bars instead of two ropes, part of the bars as pistons with

2 ft stroke, and thus limit one axis by using the pivot sleeve reactions - and then use a cammed friction or stepped fluid damper for the primary axis? In other words re the primary axis, as the primary axis mass begins to gain energy due ot natural frequency, it will travel more, and using the step or cams, there will only be damping when the travel forces the friction faces into contact at high travel or when the pin holding the fluid damper is contacted in the overswing.
Reply to
hob

In the general run of things, there are passive devices - magnetic; like an aluminum vane in a narrow gap with a big magnetic field, viscous, like a piston sitting loosely in a cylinder filled with water, oil, or air springs to provide a restoring force etc., etc. There are active devices, like air jets, counter-moving masses solenoids etc., etc. In other words, there is a wide spectrum of devices suited to damping sway/swing

Brian Whatcott Altus OK

Reply to
Brian Whatcott

Thank You. A stickler for spelling myself, I blew it here!

W

Reply to
Wayne G. Dengel

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