I'm hoping someone can help me with a problem I am having. I have a telescope mounted on a 54 inch tall, 3/16 inch wall, 8x8 square aluminum tube with 1.25 inch plates welded to the top and bottom and bolted to 2/3 of a yard of concrete. The problem is I am getting a little too much movement out of the tube and I need to increase the stiffness of the tube. The ideal situation would be to replace the tube with something larger but that's just not possible at this point in time. I don't need a radical increase in stiffness either so I think there is something I could do to for a slight increase in stiffness. I need to kill low frequency vibrations so please don't suggest sand or concrete, beside concrete and alumimun don't get along with each other. Any ideas ? Would putting the tube under tension help ?
Adding tension or compression won't help unless your have a joint that is giving a bit someplace in the system. Tension or compression will only take up slop, it won't make the underlying structure stiffer unless the tensioning members them selves start to be as stiff as the original structure.
In what direction is the vibration you are trying to reduce?
How much stiffer does it need to be? It needs to be 4 times as stiff to double the frequency of vibration.
Can you isolate the movement to the 8x8 tube, or is it possibly where the 1.25 plat fastens to the concrete? You need to find where the movement is coming from that allows the vibration before you fix the vibration. A couple of dial indicators on a stand to measure coulumn movement when you force vibration in the problem mode can help you see what's going on.
I wouldn't rule out sand. It would lower the frequency :( but would add a lot of damping. You could make some holes in the top and bottom sides of the column and try the sand. If it doesn't work, take it out. That would be my first effort if I were doing it and it were a column bending type of vibration and not a column torsion problem.
It it's in bending you could also make a significant improvement by adding some struts from the top plate down to anchor points a foot or two out from the base. Make it like a pyramid. If you can't stand the larger foot print, you could make some gains by securely fastening (idealy welding) some significant plates to the sides of the column. Perhaps
3/8 to 1/2 inch thick plates as long as you are getting more aluminum and setting up to weld. No sense in underdoing it and not being satisfied.
I should have been clearer the flex is defintely coming from the 8x8 tubing and not the base or the concrete. The tube does not appear to be twisting and flex is does not seem to occur diagonally. I need about a 50% improvement to get where I want it.
Maybe wrap the whole thing in fiber reinforced plastic (fiberglass, carbon fiber, kevlar)? Wicks Aircraft Supply
sells graphite carbon fiber "tape" that's 5" wide and compatible with all major resin types:
They also have kevlar cloth (which is difficult to cut) and epoxy tapes (cheap). As I believe someone else already mentioned, you would want to determine whether the tube is bending or twisting before you decided how to apply a uni-directional tape, or use a bi-directional cloth. Possibly wrap the uni-directional carbon fiber tape in a "barber-pole" pattern from top to bottom... once in each direction? If you scrub the thing down with a scotch-brite pad soaked in an acetone/resin mixture... it will "prime" the aluminum and prevent oxides from forming on the surface, so you get a better bond. David
"qedude" wrote in message news:bo8qt7$e1$ firstname.lastname@example.org...
I'm trying to kill a low frequency vibration (think a slow sway back and forth). I agree that sand will kill the high frequency stuff but it's not going to help in this situation. I'm having a hard time determining exactly what is causing this vibration. It's not the mount as I can kill the power and there is still a slow sway to the mount and it doesn't seem to die out. Either I have created a really good pendulum or I may be getting a low frequency harmonic from a highway about a 1/3 of a mile away.
I was thinking about running a tensioning rod from each corner down to the edge of the concrete pier to triangulate the top and help add a little resistance to the mount bending in tension.
If I can't solve the problem simply in the next couple of weeks I'll probably just replace everything with steel. I have the steel but the aluminum came available at the right price (scrap price) and I didn't have to spend the time to sandblast and paint it like I will the steel. The only reason I am spending time with it is that it is so close to working. If I can cut the size or the error or move the frequency lower so that I can guide it out I'll be happy.
If you can live with a big footprint at the bottom, then triangulating it will make a big difference. Something like same overall pyramid width as column height. The tensioning rods will need to be tight enough so that they are always under tension.
Why not fill it with sand, as others have suggested, but in order to add stiffness to the sand's damping qualities, mix it with a resin, so that as well as having added mass, you have added considerable stiffness/rigidity to it at the same time. The increased mass will damp the high frequency vibration while the rigidity of it will damp the lower frequencies.
good suggestions in the other posts to this question. How "low" is the vibration you are seeing and how close is the highway? It may be that the only way to fix this problem is to decouple the telescope from the ground movement. For example, you could pull the support shaft loose, & cast a large weight of concrete around it (say...300 lbs). Be sure to put four lifting rings on the upper surface of the concrete block, near the corners. Then...build a heavy frame mounted on the original pad. Make it a few inches larger than the block on the shaft...say..maybe 3-5" per side. USe some heavy springs to support the block base for the telescope inside the frame, so it is free-floating. If necessary, you can get some cheap Lawn Tractor inner tubes from Northern Tool, or some other place, and, put them between the block and the frame, to add some additional damping. It is best that they only be inflated to a couple of PSI, though. If your problem is low frequency vibrations from the highway, this should do it. Oh...it occurs to me it should be pretty easy to see if this is the problem, though, as it should vary depending on traffic... Regards Dave Mundt
You might try clamping some weight on the top of the column. That will lower the column resonate frequency and may help. If it does, filling the column with sand will help.
Do you stand on the same concrete as the tube is mounted on?
A rod or cable from the corners of the top plate to the corners of the base will help a lot if it is the tube flexing. Compression or tension on the aluminum won't make a difference, but adding to the structure will.
It would be good to understand what is causing the vibration before changing to steel. The problem may not be in the column flexing. Maybe you could clamp a magnet to the column top and figure out a way to support a coil so you can measure the vibration. We used a geophone from a surplus store on the BPAA telescope.
I think Dave is on the right track here with local vibrations creeping in thru the foundation.
What kind of soil is your 2/3 of a yard of concrete poured into? Sand, muck, clay, rocky?
The other issue also to consider is how are you determining movement? Are you doing a long exposure camera shot and catching a star squiggle or are you visually seeing a movement with your eye at the viewing piece?
Consider building a deck around the base of your scope to isolate you the viewer from the foundation that the scope is connected to.
Had to give up looking up due to increased local light pollution.
If the problem IS torsional, when does it occur? Assuming that the undesirable motion is torsional, does it occur during traverse, or when the telescope is "touched". If it occurs during traverse at the start of swing and then again during stopping of swing, is it possible to put a reverse force collar under the existing, so that the head is "counter-balanced"? IE an equal mass counter-rotating?
Brian Laws>I'm hoping someone can help me with a problem I am having. I have a telescope
Or how about adding the suggested rods/cables, but instead of taking them to their respective corners in triangulation, take them to the "next" corner and force the highest torquing action you can? Takes the aluminum out of it's normal harmonic range and close to it's elastic limit.
Brian Laws>You might try clamping some weight on the top of the column. That
If it's really not dying out, then it is VERY likely to be extremely frequency-dependant. Adding sand might just do it. Test whether changing the frequency will help by clamping a large weight near the upper part of the column with "lossy" pads between weight, column, and clamps. If the vibration changes in frequency or amplitude, you may be on track to cure it.