How to tame pier vibration?

[Apriliax]

So I've swappped scopes sat on my home made pier and mounted up my new 14" Netwonian for the first time. The pier is hollow steel bolted to the floor and filled with kiln dried sand. I had my RASA11 mounted before and it was reasonably solid but I noticed today that the newt shook a fair bit after I manually moved it and the vibrations took a second or two to fully die down. I don't know if it's always been present and the extra mass (about another 15Kg) or the length of the scope that makes this more evident. Anyway with the RASA on I had noticed that even though the scope seemed solid the guiding detected if I was clomping around the observatory but since I rarely did I just put up with it, after all the scope was imaging unattended 95% of the time so no worries. That said I'd like to tame things a little. Any practical ideas? In order of ease (and therefore preference) I've heard that adding oil to the sand helps; other things I'm considering are casting a concrete sleeve around it, building up brickwork around it, or having some stabiliszing fins welded on. Has anyone experiencd similar and how did you cure it? Many thanks

[Peter Drew]

I think my first course of action would be to add some gussets to the base plate to pier position.  Difficult to tell from a photo but the base plate looks rather thin relative to the dimensions of the pier.      🙂

[Craney]

1 hour ago, Apriliax said:

The pier is hollow steel bolted to the floor 

Is it a concrete floor , as in a  foundation pad for the whole observatory and the pier sits on top of that ??

... or are the pier and the floor isolated from each other  and you have done a neat job with the flooring to hide any gaps ??

If the former, then it does not surprise me that tiny vibrations will be transmitted.

If the latter....hmmmm??....something is bridging the gap between the two.

 

 

[Apriliax]

It's a solid floor. There was an existing shed I converted to roll off. I've put cushioned flooring down and not too worried about the vibrations when I'm moving around, as I say that's infrequent enough not to bother me. But I want to damp out any vibrations that may occur. The whole system seems quite resonant at the moment and I'd like to damp it

 

[Kimboman]

Hi I  I agree with sub giant stiffening ribs are a must .

Mild steel box section is prone to vibration it would be beat filled with concrete.

The beat material for a metal pier is cast iron this absorbs vibrations better than  mild steel.

 Cast iron is hard to come by scrapyards may have suitable pieces and it is difficult to weld but it can be done

[kiwi_outdoors]

3 hours ago, Peter Drew said:

I think my first course of action would be to add some gussets to the base plate to pier position.  Difficult to tell from a photo but the base plate looks rather thin relative to the dimensions of the pier.      🙂

Yup (says me, a retired structural engineer)

[Davey-T]

When you remove the pier to get some gussets fitted if the base plate is too thin double it up with some 10mm steel plate then you could use a 6" diamond core drill to drill out where the bolts are, insulate the sides and drop some steel studding in and concrete them in, you'll need some large washers to lift the base clear of the solid floor.

Dave

[Rusted]

You have built an inverted pendulum with a low resonant frequency and high Q, much like a metronome.
The joint at the thin base plate to column is totally inadequate to control these vibrations.

As others have said: You need gussets. Tall steel triangles welded to the base plate and up the column.
The stiffness needs to be highest at the junction but can be reduced as the column rises.
This gives the column a better damped stiffness. With a lower Q across a much wider range of frequencies.
The gussets will make up for the thinness of the base plate if you gusset generously.
Uses a wide base to the triangles at the base plate.

I built a tall refractor pier of 4" thick wall, steel tube. Even empty it weighed "a ton!"
Later, I filled it with concrete after sand filling failed abysmally to cure the wobbles.
You could still see the vibrations at the top of the pier when the telescope tube was lightly tapped.
I had to dig the pipe out of the lawn again. The concrete filling made it so heavy I could not longer lift it!
I ended up using a very long steel pipe, a length of tree trunk as a fulcrum and Prusik loops to lever it out of its hole!

My present pier is nearly 4m or 13' tall. It is a huge pyramid built from timber 4x4s.
With 3/4" plywood cladding above observatory floor level.
Over 2.5m square at the base and tapering only slowly.
It stands on buried concrete anchors and is bolted to heavy steel brackets at ground level.
It works by avoiding flexure and is [of course] isolated from the 2 storey building.
The load at the top is probably a couple of  hundred kilos but the resonant frequency is satisfyingly high.

 

[pete_l]

Let's put the "clomping around" issue to the side, as you say it's not that important.
Have you positively identified the pier as being the source of vibrations after the mount stops moving the OTA?

Most driven mounts have a feature of slowing down and stopping gradually after they have moved most of the way to the target. Does your's do that - I don't recognise the make. If the mount stops suddenly, then I would expect the OTA to "whip" a bit. An effect that would increase significantly with the larger / heavier / longer tube of your Newt.

The problem may not be the pillar at all.

[inFINNity Deck]

I can only agree with previous suggestions that you need gussets. I once made a very similar 1 metre tall pier like yours to hold a 120kg collimator and that swung around happily like an inverted pendulum, even though it sat on top of a 15 metres (yup, 15m) concrete foundation pole. The whole problem is the connection of the square profile with the baseplate. I used a 10mm steel baseplate under a 140mm square sectioned pier, but that was not stiff enough to avoid flexing. In the end I more or less solved it by adding three pressure points around the central bolt that held the pier in place and putting as much pressure on them as possible (it was a temporary set-up). That reduced the wobble, but not fully eliminate it, while it was far from ideal solution.

When I created my observatory I needed a 3.4 metres pier to hold my GM3000 + four telescopes (total weight well over 150kg). That pier has 8 gussets: 4 tall ones going up to 2.5m and 4 small ones in between at the 0.8m diameter base (see below images). This pier weighs 270kg and shows almost no vibration (only when you hit it hard it trembles for a few seconds).

Nicolàs

Edited January 2, 2021 by inFINNity Deck

[fifeskies]

38 minutes ago, inFINNity Deck said:

(only when you hit it hard it trembles for a few seconds).

If you were to hit me hard I would probably tremble for a few seconds too 😜

 

That is one awesome pier, looks like it would fly if you filled it with fuel and lit it at the bottom.

[inFINNity Deck]

Perhaps it is good to mention that the pier is bolted down to the concrete floor using 16 M12 threaded ends with nuts and washers below and above the base. So instead of trying to pull the threaded ends out of the concrete by bolting down the pier, the base of the pier is clamped between the nuts and washers to avoid stress building up in the chemical anchors that hold the threaded ends in the concrete (so the base hovers a few centimetres above the concrete). The holes in the base are as close as possible to and on either side of the gussets to avoid further flexure in the base.

Nicolàs

[Apriliax]

Thanks guys, you've all confirmed my thoughts that I need to get it gusseted, I was hoping there'd be an quick fix but like everything in life there are no shortcuts only getting it done right! I'll get that it welded up and let you know how it goes. 

[pete_l]

2 hours ago, Apriliax said:

I was hoping there'd be an quick fix

Since you only experience vibration when the mount brings your new, longer, heavier, OTA to a stop the quick fix might be to tune the deceleration parameters in your mount controller. So that it slows the slewing speed down before reaching the target. It probably already does that effectively for your SCT. But the greater inertia of the bigger tube would require different accleration / deceleration characteristics.

After all, you don't experience any vibrration when the telescope is tracking.

[Rusted]

I was going to suggest steel cable "guy lines" with tensioning turnbuckles but this is like deliberately setting up hurdles in the dark.

While vibration may not be a problem during tracking, one only has to manually adjust the focus for the wobbles to return with a vengeance.

[Hals]

On 02/01/2021 at 14:17, inFINNity Deck said:

Perhaps it is good to mention that the pier is bolted down to the concrete floor using 16 M12 threaded ends with nuts and washers below and above the base. So instead of trying to pull the threaded ends out of the concrete by bolting down the pier, the base of the pier is clamped between the nuts and washers to avoid stress building up in the chemical anchors that hold the threaded ends in the concrete (so the base hovers a few centimetres above the concrete). The holes in the base are as close as possible to and on either side of the gussets to avoid further flexure in the base.

Nicolàs

That's not a pier...it's the first stage of a Saturn V ! 😀