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Truss Design

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#1 gnev

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Posted 12 January 2019 - 12:47 PM

My mind was rambling while trying to get to sleep last night. Have a question on trusses. Let assume we have a 8 pole truss set up. If telescope is pointing to zenith and we are standing behind it let say front two tubes of truss are at 12 o'clock, rear at 6 and sides at 3 and 9. At zenith all trusses should be in compression. As scope is lowered to horizon some tubes will be coming out of compression and into tension. Can we assume when scope continues getting below 45 degrees the 6 o'clock truss and the rearward tubes on the 3 and 9 are starting to take most of the load. Is there a point when the 12 o'clock truss and the front tubes on 3 and 9 are not helping at all. Is this why it is important that tubes need to be chosen based on diameter and wall thickness so they do not deflect. It would seem this would get more critical with a 6 tube design. This came to mind from other current build threads. Try to dumb it down a little when you answer, been a loooong time since i was in high school. 



#2 MitchAlsup

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Posted 12 January 2019 - 01:53 PM

Consider just 2 tubes of the truss and the third leg (rocker box) connecting to make a proper triangle.

 

At zenith the 2 tubes angle inward toward each other.

 

As one slews towards the horizon, when one of the poles reaches straight up, then the other pole transitions from compression to tension.

 

With your typical F/4-F/5 scope this happens quite close to zenith (80º or so.)


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#3 Bob4BVM

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Posted 12 January 2019 - 02:13 PM

Consider just 2 tubes of the truss and the third leg (rocker box) connecting to make a proper triangle.

 

At zenith the 2 tubes angle inward toward each other.

 

As one slews towards the horizon, when one of the poles reaches straight up, then the other pole transitions from compression to tension.

 

With your typical F/4-F/5 scope this happens quite close to zenith (80º or so.)

Yes , consider that the transition happens in a given tube when that tube reaches a perfectly vertical position, then it is in pure compression. As it moves away from vertical the gravity force vector transitions from compression to tension as the scope goes lower. Then there is also the torque (bending moment) at any angle below vertical. That's tube vertical, not scope vertical.

A truss structure is a fascinating study of force transfer. Especially in telescopes and bridges.:)

CS

Bob


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#4 gnev

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Posted 12 January 2019 - 02:18 PM

Consider just 2 tubes of the truss and the third leg (rocker box) connecting to make a proper triangle.

 

At zenith the 2 tubes angle inward toward each other.

 

As one slews towards the horizon, when one of the poles reaches straight up, then the other pole transitions from compression to tension.

 

With your typical F/4-F/5 scope this happens quite close to zenith (80º or so.)

Little confused. If we take the truss at the 6'oclock position like in my example. We have a tube on the left and right and the horizontal surface where connected to the mirror box to form a triangle. At zenith the left and right tubes are in compression. As the scope travels down towards the  horizon they [the left and right tubes} go from compression to tension.They are becoming more horizontal. Is there a point when these two tubes have diminishing strength in tension that the rigidity of the tubes becomes most important?



#5 Starman47

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Posted 12 January 2019 - 02:18 PM

The book by Dave Kriege (The Dobsonian Telescope) has a complete discussion of the engineering involved in truss design. 


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#6 gnev

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Posted 12 January 2019 - 02:53 PM

The book by Dave Kriege (The Dobsonian Telescope) has a complete discussion of the engineering involved in truss design. 

Have that book. Very informative, but sometimes a little too technical. Most times I find answers here that take a technical subject and make it easier to understand. 



#7 Oberon

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Posted 12 January 2019 - 05:11 PM

My mind was rambling while trying to get to sleep last night. Have a question on trusses. Let assume we have a 8 pole truss set up. If telescope is pointing to zenith and we are standing behind it let say front two tubes of truss are at 12 o'clock, rear at 6 and sides at 3 and 9. At zenith all trusses should be in compression. As scope is lowered to horizon some tubes will be coming out of compression and into tension. Can we assume when scope continues getting below 45 degrees the 6 o'clock truss and the rearward tubes on the 3 and 9 are starting to take most of the load. Is there a point when the 12 o'clock truss and the front tubes on 3 and 9 are not helping at all. Is this why it is important that tubes need to be chosen based on diameter and wall thickness so they do not deflect. It would seem this would get more critical with a 6 tube design. This came to mind from other current build threads. Try to dumb it down a little when you answer, been a loooong time since i was in high school. 

 

Yes.


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#8 gnev

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Posted 12 January 2019 - 06:09 PM

Yes.

Thank you. My rambling was not in vain.


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#9 MitchAlsup

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Posted 13 January 2019 - 12:12 PM

Little confused. If we take the truss at the 6'oclock position like in my example. We have a tube on the left and right and the horizontal surface where connected to the mirror box to form a triangle. At zenith the left and right tubes are in compression. As the scope travels down towards the  horizon they [the left and right tubes} go from compression to tension.They are becoming more horizontal. Is there a point when these two tubes have diminishing strength in tension that the rigidity of the tubes becomes most important?

One of the tubes remains in compression while the other tube transitions to tension.

 

The tube in compression bends

The tube in tension does not bend

 

The tube in compression collapses when the compression forces equal the yield strength of the tube material

The tube in tension fails when the tensional forces equal the yield strength of the material.

 

The tube in compression will fail earlier due to the geometry of the bending moments in the tube.


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#10 jtsenghas

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Posted 13 January 2019 - 12:33 PM

The tube in compression will fail earlier due to the geometry of the bending moments in the tube.

I've thought about this a few times in the context of making truss scopes as ultralight as possible. The rear tubes only see a trivial compressive force of a fraction of the UTA when the scope is pointed at zenith.  The opposing tubes see a compressive force of at least several times the weight of the UTA, except for really fast geometries such as Mel's 25" scope featured in the February 2019 Sky and Telescope article. The first failure mode if things are pushed to an extreme would be buckling of those front, lower tubes.

 

My thought is this: for making an ultralight as light as possible, there is no reason all six or eight trusses need to be made of the same material or of the same dimensions.  The front lower tubes can be beefier, or at least stiffer. Even extremely slender tubes stretch very little under tension.  I did the math on this when comparing truss scopes and string scope designs. 


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#11 kb58

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Posted 13 January 2019 - 01:21 PM

Even extremely slender tubes stretch very little under tension. 

Right, as long as the tubes are under tension, they can be replaced with wire/cables.



#12 CrazyPanda

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Posted 13 January 2019 - 01:22 PM

gnev, I just want to second picking up The Dobsonian Telescope by Kriege and Berry. 

 

The effect you're describe is partially alleviated by creating a compound truss design, whereby the radius of the connection points at the mirror box is larger or smaller (very preferably larger!) than the radius of the connection points at the upper tube assembly. 

 

This means that all 8 poles will always be in some degree of compression and tension regardless of the direction force is applied.

 

It's more complex to fabricate because now you have to consider compound angles, and some will be very slight, but it produces a much stronger truss system than 8 poles where only 4 of them are doing any real work at any given time.

 

Example:

 

truss.jpg

 

The design on the right will do a better job of loading all 8 trusses regardless of orientation and regardless of which direction force is applied. This will be true of 6 truss designs as well.


Edited by CrazyPanda, 13 January 2019 - 01:55 PM.

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#13 gnev

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Posted 13 January 2019 - 01:48 PM

This is all extremely interesting, and I actually understand it!

Thanks  Glenn



#14 gnev

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Posted 13 January 2019 - 01:55 PM

By dumb luck my 10 inch will actually come out like the right hand  picture. I made the attachment point on mirror box using captured plastic balls. Solved one angle problem.

 

Attached Thumbnails

  • IMG_0144.jpg

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#15 ckh

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Posted 13 January 2019 - 02:04 PM

I like those. I'm making something like that now with Delrin balls and two pieces of aluminum.  It's an alternative to Heim joints for a hexapod truss.  More work to make but a bit cheaper and lighter.


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#16 dddileo

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Posted 13 January 2019 - 08:02 PM

  I have used this FEA program to aid some past builds. It includes a material library with properties data. Way back in the early years of truss structure development, design was heavily debated on the old ATM list, it shows that a six element structure has advantages over the 8 element standard.

 Not a difficult program to master:

 

http://www.grapesoftware.mb.ca/

 

 

Dom DiLeo



#17 Pierre Lemay

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Posted 13 January 2019 - 09:02 PM

The book by Dave Kriege (The Dobsonian Telescope) has a complete discussion of the engineering involved in truss design. 

Albert Highe's book "Engineering, Design and Construction of Portable Newtonian Telescopes" not only discusses truss tube designs but he has actually demonstrated empirical formulae through experimental data for all natures of telescope trusses. Highe's book is, by far, the most complete work on this subject and it will answer any questions one may have on telescope tube trusses.


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#18 Starman47

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Posted 25 January 2019 - 10:23 AM

Just received the Albert Highe book. I like the fact that there is a disk included with spreadsheets that will help with calculations in the design process. 


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#19 kb58

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Posted 25 January 2019 - 12:36 PM

Was on the fence about ordering this book, but this thread pushed me off and it's been ordered.


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#20 CrazyPanda

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Posted 25 January 2019 - 01:02 PM

Was on the fence about ordering this book, but this thread pushed me off and it's been ordered.

Same here. Just ordered it myself. Still lots to learn about making telescopes!


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#21 mark cowan

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Posted 25 January 2019 - 04:42 PM

It's a very good book, as is the sequel.  Recommend both highly.



#22 CrazyPanda

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Posted 25 January 2019 - 05:10 PM

It's a very good book, as is the sequel.  Recommend both highly.

Ooooh. Which one is the sequel?



#23 kb58

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Posted 25 January 2019 - 08:47 PM

Sequel?! Noooo...

#24 mark cowan

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Posted 25 January 2019 - 09:34 PM

The one that came second, and it's not quite as general but it's pretty interesting.  Let me use the magic internet to find it:

 

http://www.willbell....Telescopes.html



#25 Jeff B1

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Posted 26 January 2019 - 07:45 AM

Try reading these articles:  

 

Albrecht, Richard E., “The Design of Telescope Structures - I,” Sky and Telescope, Vol. 77, No. 1, pp. 97-101, January 1989
https://archive.org/...1-pdf/page/n117

 

Albrecht, Richard E., “The Design of Telescope Structures - II,” S&T, Vol. 77, No. 2, pp. 210-214, February 1989
https://archive.org/...2-pdf/page/n101

 

Brooks, John J., “Mechanical Considerations of Telescope Makers,” S&T, Vol. 51, No. 6, pp. 423-428, June 1976.
https://archive.org/...06-pdf/page/n53


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