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Just a quick question about piers


Philip Benson
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I have a Skyshed Pod pier from Altair Astro.  Having recently added a second OTA to my mount I have now detected some vibrations in my system when slewing.  So my plan now is to fill the pier tube in order to increase the damping effect.   I have some sharp sand and cement left over from when I located the pier which I intend to use for this.  Not sure whether to simply pour in some of the sand or mix up some cement and pour that in.

Thanks in advance.

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Yes the system (https://www.flickr.com/photos/30649972@N05/15890271124/) is not fully balanced yet but that is being addressed over the weekend when a friend will be helping me.  I have been sent links to a couple of very good videos on Youtube that will help with this.

I have a couple of bags of sand left over from when I built the base and since the pier is a hollow steel tube it occurred to me that I might as well fill the tube with that sand.  Can't harm.

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I wouldn't recommend filling a pier with sand.

It will add mass to the top of the pier which will increase the amplitude of any vibration and do nothing to make the pier more rigid. 

It may sound 'better' when you tap the side of the pier tube  - but all that's happened is the resonant frequency of the pier is lower.

( Think of a metronome - move the weight upwards and the rate is lower but the amplitude is the same.  

If a pier is vibrating you need to stiffen it, usually by adding bracing fillets between the pier tube and the base plate or using a larger diameter tube.

Often piers are bolted directly to an un-even croncrete surface.  Lay a thin skim of wet mortar where the pier sits and place the pier on to the soft mortar and gently tap to bed down.

Bolt down once the mortar has set and make sure the bolts go through the base plate as close to the braces as you can, not  in the gaps in between.

Dave.

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