Pier Rat Cages

[bmitchell82 (Brendan)]

Tapered gussets do three things.

Geometrically changing your section
Increase the stiffness of your base plate
Changes inherit natural frequency

1. Geometrically changing your section.

I'll quote square sections as i know the formulas off the top of my head

Second moment of inertia of a Square section BD^2/12*(A*h^2)
B= Bredth
D= Depth
A = Area
h = Hight from the neutral axis

For SHS sections the second term is negated as it is doubly symetrical.

by adding gussets you are in term adding depth in turn that relates to a higher Ixx value which is the measure of Stiffness.

2. The gusset goes into tension and compression hence stopping the base plate from bending (often used in structural steel connections to increase the capacity of the connection).

3. Is a byproduct of the previous two points. the stiffer the section the higher the natrual frequency. it can be simplified and calculated by the use of matrix manipulation in the form of eigen values. This can shoot you in the foot as if your natural frequency matches the induced frequency (motor mounts fans etc etc) the sinusoidal waves are coincidental and hence additive (much how noise cancelling works it throws a inverse wave to make the -ive appear as the + appears 1-1=0. 1+1=2. thats what the whole meaning of resonance. the only way you can kill resonance is dampening. You see the little rubber doova on a tennis racket..... Dampening of the strings.

[bmitchell82 (Brendan)]

I never said i was good at spelling numbers are my game!

[Poita (Peter)]

Originally Posted by bmitchell82

I never said i was good at spelling numbers are my game!

Most browsers can have an in-line spell-check now, I must turn mine back on so I look less like a doofus!

[Poita (Peter)]

Brendan, can you also combat resonance by making the pier less symmetrical?

[koputai (Jason)]

Originally Posted by bmitchell82

You see the little rubber doova on a tennis racket..... Dampening of the strings.

So it's full of water?

[bmitchell82 (Brendan)]

Jason its a method of damping have the pier completely submerged in viscous fluid

Your a funny bugger

[bmitchell82 (Brendan)]

Answer to your question Peter, It would possibly help in the axis with less stiffness, but would still be present in the stiff axis..

[Poita (Peter)]

Speaking of viscous fluids, what about the practice of filling mounts with sand or oil or other 'non rigid' materials in an attempt at damping?

[allan gould]

I find this thread slightly be-musing. When I put my pier in for the observatory I realised that since it was located on ash residue from mining and clay it was unlikely to remain level either after it set or with the passing of time. So I buit it as shown in the thread
http://www.iceinspace.com.au/forum/a...se.php?a=72246
It has proven its worth over time for ease of adjustment in its initial setup and subsequently as it appears the whole shebang is slowly sliding down the hill due to the soil constitution.
I dont experience any resonance with my G11 or heavy scopes imaging at 2500mm for 15-30 min.
The maths may be there to say it should flex, resonate or whatever but my experience says this is not the case.
Its a bit like bumble bees mathematically proven not to be able to fly.
Just my 2c from experience.

[bmitchell82 (Brendan)]

Oil or viscous fluid would help with high frequency vibrations and as such could combat the high frequency experienced in Rally's situation.

[Poita (Peter)]

Originally Posted by allan gould

I find this thread slightly be-musing. When I put my pier in for the observatory I realised that since it was located on ash residue from mining and clay it was unlikely to remain level either after it set or with the passing of time. So I buit it as shown in the thread
http://www.iceinspace.com.au/forum/a...se.php?a=72246
It has proven its worth over time for ease of adjustment in its initial setup and subsequently as it appears the whole shebang is slowly sliding down the hill due to the soil constitution.
I dont experience any resonance with my G11 or heavy scopes imaging at 2500mm for 15-30 min.
The maths may be there to say it should flex, resonate or whatever but my experience says this is not the case.
Its a bit like bumble bees mathematically proven not to be able to fly.
Just my 2c from experience.

This was actually why I started the thread, my own experience is similar to yours. I had the same thing when using Vixen rails as well, everyone told me they were spaghetti and I should have losmandy ones, but I never had an issue, and suspected the vixen rails were more than stiff enough for the loads experienced with most amatuer gear.

My gut feel is that when it comes to piers, rails etc. that they tend to be capable of well in excess of the strength required, but there is so much opinion around about these topics and people spend big money sometimes on them, that I thought I'd like to know some actual factuals.

[bmitchell82 (Brendan)]

I think the dyanmic resonance we are talking about are in the catagory of 40-50kg telescopes + all their equipment.

[jenchris (Jennifer)]

I use a plastic/concrete/steel combination pier with 2x 15mm steel plates forming a ratcage on top for my alt/az on a wedge.
The only vibration I get is across the forks on the top - nothing is apparent in live view x10 on my laptop.
Maybe the thee different materials of the pier damp the effect of each other in the presence of a vibration. I've not noticed resonance or any resonant frequency of the pier at all - it should be very low and very quickly dispersed by the mass of the differing materials, but I have only empirical evidence of this and no way of measuring.

[Poita (Peter)]

Anecdotally I find the wooden tripods recover more quickly from a bump than the metal ones do.

[rally]

Brendan,

It was a real case ! and I think that it does happen with other large OTA's

The P-ME was "built", rather than really engineered and designed at the sort of levels mooted here !

We can only assume it was a resonance, not necessarily high frequency, but that depends on what you mean by high.

Whilst it could be the pier or the pier as part of that particular system, I believe this is unlikely, it occurred in different situations and appeared to be unaffected by any dampening applied to the pier or the piers ground connection.

It seemed to be an OTA/Mount system related problem, that was able to be remedied by changing the counterweight mass around.
That was a solution but not necessarily a fix of the source.
It wasn't specifically an overloading mass related problem because the total mass was ultimately increased to solve the problem !

I didn't solve it, just supplied some of the pieces.

In any event the purpose of this example was to illustrate that vibration in telescope systems does exist and does cause real problems in all sorts of unexpected and complex ways.

Rally

Originally Posted by bmitchell82

Thankyou for your reply Rally,

So your issue was high frequency vibrations forming a dynamic resonance. Is this a issue of the pier or is this a issue of the mount?

To me it would be more a case of in the pier, with adjustments and tolorances. Possibly due to the telescope being sufficiently big enough coming upto the PME's rated capacity and hence any dynamic vibrations start to affect.

If this where a real case and it happend all the time wouldn't mount design take this phenomena into account?

I also think that placing the CW further out would make the issue worse not better. Yes the leaver arm is bigger hence less weight but the ability for a weight further out where your natrual frequency decreases and hence has more chance of coming into resonance.

It is a tricky one high frequencies because they are hard to measure unless you have some really expensive tools.

[Nico13 (Ken)]

Not directly related to vibration but what if you lovely new peer has been installed with an East/West cant??
Surely that can't be adjusted out without the adjustable top plate although ok if you can shim the bottom, just another area for flex?

[koputai (Jason)]

Originally Posted by Nico13

what if you lovely new peer has been installed with an East/West cant??
Surely that can't be adjusted out without the adjustable top plate

That's the whole point, yes it can. The base of EQ mounts can be adjusted in altitude and azimuth.

So, imagine you have set up your mount perfectly and have your polar axis absolutely perpendicular to the axis of the Earth.

Now, keeping the axis parallel to that of the Earth, you can undo the alt and az locks and whiz that mount 100 miles away, and the base will adjust itself to the different plane of the Earth, but your polar axis is still parallel to that of the Earth, and you still have perfect polar alignment. Do up the locks and you're done.

That's the thing. The mount can be used anywhere on Earth, so of course the base does not have to be flat, you could have it at 30 degrees tilt if you wanted to, because that is exactly what someone 30 degrees away from you does.

Cheers,
Jason.

[Screwdriverone (Chris)]

Another thought provoking and interesting thread Peter (Brendan & others....)

I've often thought myself, that it was a little strange there is SOOOO much concrete and stiffening in the pier with Xmm thickness steel and gussets etc etc and then there are 4 piddly bolts holding up a steel plate with 20-40Kgs of spread load (especially at Zenith when say a newt reflector is horizontal and the counterweights and tube are at opposite ends of what essentially looks like a see saw at rest)?

It seemed to defeat the purpose of all that mass and rigidity to then introduce a point of (my initial thoughts) weakness simply for what I thought was ease of mounting and perhaps clearance for the base of the EQ mount.....

I know it is more complicated than this, but as a few have said and Jason has mentioned recently, as the EQ base is inherently adjustable, I didnt see the point of having so much air gap and "unsupported" load simply resting on the compression properties and rigidity of some bolts sticking out of some concrete or steel?

I suppose its all relative, based on how accurate and tolerant the system is to vibrations induced by walking, motors, resonant frequencies, torque introduced by the optics and mount etc etc. so I suspect for most amateurs who simply want the convenience of a mount that can be left set up and is more stable than a tripod (ME) and dont need the accuracy to image above 1000-1500mm FL, a simple pier without an adjustable top plate/rat cage, would probably do?

Based on my limited Engineering Science knowledge from High School, I would assume that the vertical bolts that hold up the pier plate, would benefit from some sort of structural truss design that "tied" them together, a bit like a bridge truss where the triangle is stronger than the individual girders and can handle more compression and bending stresses induced on them?

I wonder if this would produce any more rigidity in the system than simply four bolts pointing vertical? At least the plate could still be there for adjustment (or whatever its needed for) and the assumed weak points could be reinforced?

Or have I got this all wrong? Is the system as it stands with the EQ mount on top all centred and balanced about the vertical axis of the pier and therefore, any bending moment or stress induced into the base would be evenly distributed down into the structure and not transverse to the mount and pier?

I think I confused myself more....oh well, it may be a while before I get around to a pier, so I shall read on with interest in the meantime....

Cheers

Chris

[bmitchell82 (Brendan)]

Rally, im with you 100% and the first thing i said it would be a scope/rig that was on the borderline of the PME's capabilities... My point more assists to the Pier and Vibrations induced and that a simple static deflection will assist you in finding the maximum a pier can move due to a force. Harmonics or resonance applies a force generally according to the right hand rule or a reaction in the opposite direction of the rotation of the harmonics which the mount/piers ability to resist is affected by the youngs modulus of the material, geometry of the section and the length.

Higher Youngs modulus = higher natural frequency
Higher stiffness (second moment of inertia) = higher natural frequency
Longer length of pier = lower natural frequency

By high frequency im talking in the 30+ Hz probably more. at the end of the day what you where doing by moving the weight away was changing the length and hence the harmonic properties of the CW bar.

The stiffer the components the more chance you will induce vibration into a system as it can develop easier into a harmonic. Unlike G11's, EQ6s and the like that have to have back lash for gear meshing high frequency vibrations cannot set them self up in to the natural harmonics.

For this reason is why my EQ6 doesn't use a CW extension bar. I bunch the weights up as close to the mount as possible. Short CW bar = extremely high frequencies and hence the system cannot react fast enough and so the vibration doesn't pose enough movement to see or quantify either that or they cannot get to the natural harmonic.

for instance. get a slinky pull it out a bit. and then start oscillating one end up and down. slowly go faster and faster you will get to the point of the first harmonic, that is it looks like a pure sine wave. get faster and itll go nuts for a bit and then itll get into the second harmonic you will see two sine waves. on and on it will go. What i am trying to demonstrate is the system needs to hit one of those harmonics for you to see anything the higher the order of the harmonic the less amplitude it has aka the peaks are smaller. It makes it easier to track down your issue!

Possibly and this is more an idea than too much science behind it. to install between either the pier base or the adaptor (see logiberras pier that is one of my designs although Logan unscaled due to section availability and cost) high density industrial rubber only a few mm thick this may take a lot of the vibrations out. Once again this is more ideas to design than pure mathematics.

BM.

Originally Posted by rally

Brendan,

It was a real case ! and I think that it does happen with other large OTA's

The P-ME was "built", rather than really engineered and designed at the sort of levels mooted here !

We can only assume it was a resonance, not necessarily high frequency, but that depends on what you mean by high.

Whilst it could be the pier or the pier as part of that particular system, I believe this is unlikely, it occurred in different situations and appeared to be unaffected by any dampening applied to the pier or the piers ground connection.

It seemed to be an OTA/Mount system related problem, that was able to be remedied by changing the counterweight mass around.
That was a solution but not necessarily a fix of the source.
It wasn't specifically an overloading mass related problem because the total mass was ultimately increased to solve the problem !

I didn't solve it, just supplied some of the pieces.

In any event the purpose of this example was to illustrate that vibration in telescope systems does exist and does cause real problems in all sorts of unexpected and complex ways.

Rally

[bcoote (Brian Coote)]

Clearly the larger the diameter of the pier, the greater the stiffness and lower the vibration potential. For me however a thicker pier really gets in the way for imaging. I prefer to image all the way past meridian until the camera hits the pier if the target is far enough south so as usual you have two conflicting requirements. My first "decent" pier was an RTA traffic light pole which I mounted on four x 25mm threaded rods set into concrete floor slab. Vibration was never a problem I was aware of but the slab shifting with water content variations in the clay underneath remains a real problem especially with polar alignment.
Brian
www.bcoote-astro.com