Sand filled tripod - worse vibration!

[jeff65]

After going to the trouble of sand filling all leg sections of a clone of the Vixen aluminium tripod, I'm disappointed to find it significantly worse in both magnitude and settling time of the vibration.

Has anyone else experienced this? Maybe I've overdone it?

Cheers,

Jeff

[casstony]

I had vibration problems in a heavily loaded Losmandy LW(GM8) tripod. I tried inserting hardwood dowel in the lower legs which gave no improvement. I then tried fitting rubber stoppers for chair legs which completely cured the vibration in exchange for a slight feeling of being in a boat (due to the heavy load) as I push the Discmount around, but the end result is quite good.

So I'd recommend rubber stoppers or some other vibration dampening pad below each leg, in lieu of replacing the legs with wood.

[sheeny]

Jeff, I have to say I'm not surprised.

Filling with sand is not the best idea. I'll try to explain the physics so you (and hopefully others) may understand:

Vibration amplitude (the size of the vibration) is proportional to the exciting force (a bump, or periodic force) and inversely proportional to the frequency of vibration.

The frequency of vibration increases with stiffness and decreases with mass. So when you add mass (by filling with sand) you decrease the natural frequency of vibration or the tripod/pier. This means that any given size exciting force ( a bump, etc) will result in greater amplitude of vibration.

Get rid of the sand!
:)

Al.

[jeff65]

Thanks for the replies.

Yes, the physics jibes with what I remember from school, but I thought sand filling was recommended because it gave a damping effect. If I'm getting any damping benefit it must be more than negated by the mass increase.

The sand has to go! I wonder why it is so commonly recommended?

Thanks again.

[sheeny]

I think you'll find the term "old wives tale" applies Jeff. If there was enough movement in the flexure of any pier/tripod for the sand to have any damping effect through relative motion of the sand grains, then any astronomy would be impossible. My guess is someone has done it without understanding the physics, but they've been convincing when explaining why they did it to someone else, and its been perpetuated.

:)

Al.

[asimov]

I don't remember anyone ever suggesting just normal everyday ol' sand as a pier or tripod leg dampener :lol: :rolleyes:

> Casting sand <

[dannat]

Have you tried spray filler, light weight but expands to take up space

[jeff65]

I used fine, washed play sand. Didn't work.

Lot's of recommendations to do this though, as a quick Google demonstrates. There are varying reports on the efficacy but none indicating it made things worse.

[Barrykgerdes]

The answer Casstony gave is usually the solution. Rubber pads make the best tripod vibration dampeners. I think someone even markets them.

Barry

[Troy]

You could eliminate the vibration at the source (mount)

[jeff65]

No doubt. Thinking about replacing the telescoping aluminium legs with DIY fixed length wood legs.

[mercedes_sl1970]

Hi

I had an EQ5 mount once with aluminium legs which would just wobble incessantly. I did consider using sand but remembered high school physics... As a last resort, I tried using expanding gap filler and messed it up by trying to do it too quickly, with the result that it did not cure properly (sigh). I think if you use this approach, you need to do small sections at a time, let them dry then add more.

As I did not have much success with the spray filler, I moved to option 2 and tossed the legs away and made a replacement set using some hardwood and the components from the aluminium legs. The result was a very stable tripod/mount. The 5-10+ seconds of wobble dropped to about 1-2 seconds.

Just my 2 cents...

Andrew

[Troy]

Yep, hypertune first and if your still not happy new legs :)

[Bassnut]

Add cement, wet it, and turn it into stiff concrete :).

[avandonk]

The easiest way to see that sand filling is a furphy is to think of your system as a rigid rod with a weight on the end. This system will have a resonant frequency of oscillation. Adding more passive weight into the 'rod' only changes the resonant frequency generally lower and does nothing for the damping.

Rigidity of the 'rod' is the best way to stop unwanted oscillations. Rigidity raises the resonant frequency which is easier to damp. Foam in the aluminium legs will do this but if the legs are flimsy to start with the effect will be be minimal.

There is no substitute for rigidity. A light tripod will only hold a light weight.

Hope this helps. Bert

[jenchris]

Sand will take out the high frequency harmonics from a steel pier - but not the oscillation from a torsional mass.
My pier was a bit on the skinny side but very rigid so sand and a backstay helped enormously. I'm going to resite it anyway so it was a good experiment

[Bucky1379]

I had very good success with the thin-wall alumiium tube extrusion tripod on my Celestron Powerseeker 127 by putting a steel rod (maybe 4mm from memory) down the centre of each aluminium section and filling the rest of the tube with wet cement and waiting for it to set. I had to allow room for the plastic end blocks. Keep the bottom one in the tube, cut the rod to the correct length that the top end block will still go in correctly and fill the leg with cement to the top of the rod. I also had to seal the joints between the botton of the leg and the end block with sticky tape. Just keep the leg section upright while wating for it to set.

I guess that one reason for the use of aluminium extrusion is light weight and the tripod is certainly not light-weight anymore but it is much more rigid and reasonable vibrations stop within less than a second.

A minor issue I had was that I had also made a much more rigid triangular leg brace which incorporated bubble levels and a compass. The compass didn't work after that due to the steel I had put in the legs. :shrug: