Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Tuned Vibration Damper

  • Please log in to reply
8 replies to this topic

#1 Dick Jacobson

Dick Jacobson

    Viking 1

  • -----
  • topic starter
  • Posts: 982
  • Joined: 22 Dec 2006
  • Loc: Cottage Grove, Minnesota, USA

Posted 09 December 2012 - 01:31 AM

The strange looking device pictured below is a vibration damper that cuts the vibration of my telescope about in half. When the tube is bumped, the damper absorbs much of the vibration and damps it out.

The need for this device arose several months ago when I added a seven-focuser rotating secondary cage to my 14” Newtonian (see Seven focusers, one telescope). This about doubled the weight of the tube. Since I did nothing to strengthen my lightweight equatorial mount, it was no surprise that the telescope developed a severe vibration problem. It took eight seconds for the vibrations to die out after a “thump” on the tube. With a pair of dampers, one on each side of the tube, the vibration time was reduced to 3-4 seconds.

The damper is very simple to construct. It’s just a weight at the end of a wooden dowel. The dowel is firmly attached to the balance point of the telescope. I used a pair of identical dampers on opposite sides of the tube to maintain balance, also adjusting counterweight behind the mirror so the tube is balanced about the declination axis. A split-block clamp at the left of the photo attaches the dowel to the aluminum frame that is at the balance point of the tube.

I call this a “tuned” damper because the vibration frequency has to be adjusted so it is about equal to the natural frequency of the telescope’s vibration. There are three ways to adjust the frequency: change the position or amount of the weight, or the strength of the shaft. With the shaft pointing upward, it was easy to make adjustments by adding or removing large fender washers to/from the weight. I did this until I found the amount of weight that made the damper vibrate most vigorously after a thump on the tube. Later I replaced most of the fender washers with a steel cylinder. The weight is held in position by a pair of shaft collars.

The device is based on the physics of coupled harmonic oscillators. When two objects that vibrate at nearly the same frequency are coupled together, vibrational energy is passed back and forth periodically. In this case, vibration from the telescope is passed to the damper, then back to the telescope and so on. This isn’t exactly what I wanted. The goal is to damp out the vibration before it can be fed back into the telescope. This goal is achieved by adjusting the size and material of the shaft. I experimented a little with a threaded steel rod, but it worked poorly since the damping action was very slow. A wooden dowel has a lot more damping power. My limited experiments indicate that the total weight of the damper(s) needs to be about 5 percent of the weight of the tube in order for it to work effectively. When I used a smaller damper, it vibrated a lot but did little to reduce tube vibration.

Of course it’s best if the telescope doesn’t vibrate in the first place, and I have plans to strengthen the mount. But if you have a vibration problem, this simple and cheap device might be a better alternative to building or buying a heavier mount.

Attached Files


  • Rusted and Lightning like this

#2 ckh

ckh

    Viking 1

  • -----
  • Posts: 595
  • Joined: 21 Feb 2015
  • Loc: Arizona

Posted 26 February 2016 - 09:48 AM

Nice solution.  Some plastics may have good damping as well.

 

Carl



#3 ChristianG

ChristianG

    Apollo

  • -----
  • Posts: 1386
  • Joined: 18 Oct 2012
  • Loc: Ottawa, Canada

Posted 26 February 2016 - 09:12 PM

Hi Dick.

 

This is a very smart idea: remove energy from a stiff, non-dissipating system (the telescope) and send it to a dissipating system (the rod). You used a polyethylene rod, I guess from your picture?

 

--Christian



#4 Dick Jacobson

Dick Jacobson

    Viking 1

  • -----
  • topic starter
  • Posts: 982
  • Joined: 22 Dec 2006
  • Loc: Cottage Grove, Minnesota, USA

Posted 27 February 2016 - 07:54 AM

Hi Dick.

 

This is a very smart idea: remove energy from a stiff, non-dissipating system (the telescope) and send it to a dissipating system (the rod). You used a polyethylene rod, I guess from your picture?

 

--Christian

I used a wooden dowel. A plastic rod might have been a better idea since the dowel eventually broke because of clumsy handling.



#5 Ed D

Ed D

    Soyuz

  • *****
  • Posts: 3665
  • Joined: 30 Apr 2010
  • Loc: Sunny South Florida

Posted 27 February 2016 - 09:00 PM

I like your idea.  It's not off beat at all.  Archers have been using similar devices for decades, especially in the Olympic class,  and competitive rifle shooters have also employed adjustable tuning weights on the muzzle end of the barrels to regulate harmonics.  I like that you can fine tune it by sliding the rod/weight back and forth in the clamp, as well as regulating the weight at the end of the rod.   :waytogo:

 

Ed D 


Edited by Ed D, 27 February 2016 - 09:03 PM.


#6 don clement

don clement

    Vendor (Clement Focuser)

  • -----
  • Vendors
  • Posts: 1038
  • Joined: 02 Feb 2011
  • Loc: Running Springs, California

Posted 28 February 2016 - 02:07 PM

Good idea. Did you adjust the the dampener for critical dampening? An electrical analog to your mechanical dampener is the X10 Oscope probe where using a square wave input to the probe the probe is adjusted to be critically damped. See: https://www.youtube....h?v=uOAzRlhrnYE



#7 Bill Schneider

Bill Schneider

    Vostok 1

  • *****
  • Posts: 155
  • Joined: 25 Jun 2015
  • Loc: Athens, OH

Posted 28 February 2016 - 03:01 PM

I recognize this kind of harmonic damper!

 

I've done a similar thing to smooth out my drill press. Out of the box, vibration from the motor "walked" small vises across the table unless they were secured. That's not handy for a quick-n-dirty hole.

 

I put this damper on several years back. It is made of a couple of long steel flats bolted between motor and press, and the counterweight is a number of cross pieces of the same stock. The cross pieces are clamped across the steel flats some distance away from the motor mount. This stack of steel plates (counterweight) can be adjusted up and down to find an optimum location. The number of cross pieces (counterweight mass) and the distance from motor was determined through trial and error.

 

When I was done, I could stand up a machine screw on its threaded end and not have it topple when the motor was running.

 

drill_press_damper-11tsml.jpg


Edited by Bill Schneider, 28 February 2016 - 03:14 PM.

  • Lightning likes this

#8 Hallzers

Hallzers

    Vostok 1

  • *****
  • Posts: 106
  • Joined: 04 Jan 2016
  • Loc: Tumbleweed, USA

Posted 28 February 2016 - 03:21 PM

Nice.

 

If you use a material for the mounting block with a different resonant modulus, it will help to diffuse the vibrations slightly faster. 

 

So instead of the first system passing the vibrations on to the second system, some are scattered in the transfer.  So if your frame is aluminum, you could use a corian block for mounting, with brass screws, a wooden dowel, and a rubber weight. Small wavelength diffusors do diffuse large waves a little... just not very well.

 

I saw a picture on the web where a guy used a tension-set wire between the end of his scope and his counterweight, to effectively accoplish something similar. But in his system, he was transferring vibrational energy to ground.

 

I've been thinking about doing the tension-set wire. 



#9 Dick Jacobson

Dick Jacobson

    Viking 1

  • -----
  • topic starter
  • Posts: 982
  • Joined: 22 Dec 2006
  • Loc: Cottage Grove, Minnesota, USA

Posted 29 February 2016 - 09:38 AM

Good idea. Did you adjust the the dampener for critical dampening? An electrical analog to your mechanical dampener is the X10 Oscope probe where using a square wave input to the probe the probe is adjusted to be critically damped. See: https://www.youtube....h?v=uOAzRlhrnYE

Yes, I wanted the damper to be as long as possible without extending below the bottom of the tube, so I made it a fixed length and adjusted the weight by trial and error until the damper vibration was maximized. I did this by adding or removing fender washers, then replaced them with a steel cylinder of the same mass.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.







Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics