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pier vibration

Started by mintakax, 12/03/2002 08:34PM
Posted 12/03/2002 08:34PM Opening Post
Hi-- I have a C14 on a Losmandy GM200 mount which sits on top of a Le Sueur 7" pier. The pier is attached to three large bolts suck into an 18" concrete cylinder in the ground. The botttom of the pier does not actually touch the concrete, but is bolted slightly above it using a pair of nuts...this is used for final leveling. I am noticing some vibration from my drive which was not present when I used a tripod. I'm trying to figure out if the vibration is coming from the 7" pier itself or the way it is bolted above the concrete. If its the pier, I may be able to damp the vibrations by filling the pier with sand or concrete without compromising the bolts and leveling nuts. Anybody else have any experience with this kind of thing ??

Thanks,
Dan
Posted 12/04/2002 12:28AM #1
Dan,
Since you didn't state the bolt circle diameter or the pier height, I'll have to make some assumptions that the height is probably 48" or more and that the bolt circle diameter is less than 16" and probably closer to 12". With the size of scope you have, the pier setting on nuts and the small bolt circle diameter are giving you an excellent spring. You also didn't say what you have for gussets between the base plate and the column. Any deflection you get in this area is multiplied by your height, millionths of inches easily become arc second vibrations. For starters you should try to bolt the base down tight to the concrete. When structural steel is used for buildings, setting plates are used which the building columns set on. These are leveled and set to the desired elevation and then grouted in place. The columns are set on the piers, usually several days later, and the hold down bolts are then tightened. The whole assembly is then in tension. Your current configuration is isolating the scope pier from the concrete base,allowing the scope to vibrate easily and the base to have little effect in dampening these vibrations. If after you remedy this situation and you still have a vibration problem, then you need to consider adding gussets to the pier base. A larger bolt circle would also help a lot but you will probably have to live with that now. Remember, to dampen vibration, you need to make the whole system appear as a single mass, any joint or connection that is added, in effect reduces this dampeneing effect. Filling your pier as it is with concrete will have only a minimal effect on the vibration problem and adding sand will make the pier sound dead but still will not
change the dampening characteristics by much.
Posted 12/05/2002 05:20PM #2
Thank you all very much for your suggestions. I was concerned about this problem before I purchased the pier, but my research did not turn up any problems. Unfortunantly, the 7" diameter is dictated by the GM200 mount base. I suppose that it might have been possible to use a larger diameter pier and try to rig some custom mounting plate.
I dont believe the problem is the motor, unless it has recently failed. Stuart--my dec motor grinds a lot also at 32x !! My gut feeling is that the problem is due to the way that the pier "floats" above the concrete. Also, what defines hf vs lf vibrations ? I would assume that if the drive itself is exciting the vibrations, that they would be hf ???
Since I have no wedge, I have no way of leveling the system other than the pier. I did pour the concrete very,very level and it is possible that just bolting the pier base to the concrete might be level enough, so I'll probably try that first. If not, I'll go to the wedges or some way of making that gap more ridgid.

Thanks again,
Dan
Posted 12/09/2002 03:11AM #3
Dan:
I had a similar problem with my pier. John and Marge Menke of Technical Innovations (Homedome) suggested 2-part industrial expoxy like that used to mount large machinery to concrete. After using the adjustments nuts to ensure that the pier was straight, I mixed up the epoxy and troweled it between the base plate and the concrete. Now it's solid as a rock. I also filled the pier with fine dry sand and it's very "dead". I've been using it for three years now...no problem.

Bruce Inscoe

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