Back yard pier

[Bart]

http://www.iceinspace.com.au/forum/a...e.php?a=223455

Check out this remote observatory. I wonder how bad the images are from these as the mounts are a long way from the bolts so the wobble must be accentuated?

[AndrewJ]

Gday Peter

Not sure where you are going with this...but surely not to suggest 3x stainless steel bolts at the top is the best to preserve the stiffness of the lower section of Al’s beaut pier.

Of course it wont be as stiff as a solid pier, but the question is will it be stiff enough to do the job.
As long as he does his nuts up tight :-), i would suggest it is probably more than adequate for what is required.

Andrew

[Bart]

What I would do here is reduce the length of the adjusting bolts so they are as short as you can get them while still providing level. Your polar alignment will be easier if you are reasonably level. If you need to pull the centre bolt out you can undo the top nuts and pull the plate off.

[axle01 (Alan)]

Originally Posted by Bart

What I would do here is reduce the length of the adjusting bolts so they are as short as you can get them while still providing level.

Arrr yes but because I’m using 3 different mounts I need to get my hand between the two plates to do up and undo the center bolt.

Al

[AstralTraveller (David)]

Originally Posted by Bart

What I would do here is reduce the length of the adjusting bolts so they are as short as you can get them while still providing level. Your polar alignment will be easier if you are reasonably level. If you need to pull the centre bolt out you can undo the top nuts and pull the plate off.

+1 You should be able to keep the amount of visible thread down to 10-12mm. That will move far less than the stilts you have now.

[AstralTraveller (David)]

Originally Posted by axle01

arrr yes but because i’m using 3 different mounts i need to get my hand between the two plates to do up and undo the center bolt.

Al

I see.

[Peter Ward]

Originally Posted by Bart

http://www.iceinspace.com.au/forum/a...e.php?a=223455

Check out this remote observatory. I wonder how bad the images are from these as the mounts are a long way from the bolts so the wobble must be accentuated?

Very droll

Just because it works, does not mean the solution was optimal.

Also begs the question about the false need to level an EQ head.

Sure, if not level, you will get a little “crosstalk” in altitude/azimuth adjustments, but this is hardly fatal, perfect polar alignment is possible even if the EQ head is on lying its side.....but I’d suggest a degree or two of a lean is way better than reducing the rigidity of a massive mounting system down to that of a spindly (i.e weakest link) and unrequired component.

‘nuff said

[troypiggo (Troy)]

Originally Posted by Peter Ward

For circular sections the moment of inertia is proportinal to the diameter to the 4th power. If my arithemetic is correct you have reduced the stiffness of the pier by about 6000x by using 10mm bolt(s) instead of the full diameter of the pier (150mm and say 4mm wall).....hence my comment.

I use a 10” steel pier with 1/2”walls. doesn’t move much

Originally Posted by Peter Ward

Indeed, ideally you want low mass and high moment of inertia, so any perturbation will have a low amplitude (eg immune to small breeze) and decay quickly. That said even an EQ6 probably deserves better than a 8-10mm diameter bolt...the attached diagram from my old engineering text don’t lie.

The formulae are obviously correct, but they are only considering a circular section in isolation, not a group of 3 or 4 where they are some distance from the neutral axes. A single bolt will have fraction of the stiffness of a larger CHS section as you say, but a bolt group can have similar stiffness with sufficient spacing and/or diameter.

I do agree that pier heads don't need to be 100% level, although it does make polar alignment easier the more level they are, less iterative.
I do also agree that it seems a bit silly to put a "soft storey" if not needed when what we're after is minimising risk of deflections. However the calcs do show that stiffness can be maintained by carefully selection of bolt diameter and spacing.

[Peter Ward]

Originally Posted by troypiggo

The formulae are obviously correct, but they are only considering a circular section in isolation, not a group of 3 or 4 where they are some distance from the neutral axes. A single bolt will have fraction of the stiffness of a larger CHS section as you say, but a bolt group can have similar stiffness with sufficient spacing and/or diameter.

I do agree that pier heads need to be 100% level, although it does make polar alignment easier the more level they are, less iterative.
I do also agree that it seems a bit silly to put a "soft storey" if not needed when what we're after is minimising risk of deflections. However the calcs do show that stiffness can be maintained by carefully selection of bolt diameter and spacing.

Did you really mean that: heads “don’t” need to be 100% level?

Lord knows I make enough typo’s (But I suspect there is a virus on my PC that randomly remaps the keyboard)

Assuming that this is the case, I guess we are agreed.

[troypiggo (Troy)]

Oh yeah, I meant "don't". Off to bed for me...

[Kunama]

Nice pier Al, never mind the mathematicians, just remember not to hit the pier with a hammer while viewing or imaging

Great idea to have it so you can use the three different mounts on the one pier.

[axle01 (Alan)]

Originally Posted by Kunama

Nice pier Al, never mind the mathematicians, just remember not to hit the pier with a hammer while viewing or imaging

Great idea to have it so you can use the three different mounts on the one pier.

Yeah it’s all above my my pay scale.
Ok I must remember not to do that.
Thanks appreciated.

Al