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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: Quick Question about 3 strut systems new [Re: calibos]
      #5062325 - 02/08/12 02:13 PM

Joint looks bendy to me.

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careysub
Carpal Tunnel
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Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Quick Question about 3 strut systems new [Re: calibos]
      #5062369 - 02/08/12 02:39 PM

A good presentation of the principles of a string truss telescope is section VII of this document:
http://bossanova9.org/astro/ATM/5-Tubes&Trusses.pdf

A true string truss does require a minimum of 3 pairs of cables, all six of which are separate tension elements.

The key difference between a strut telescope and truss telescope is that the struts bend, one side of each strut is under tension and the other under compression, while in a truss the elements are each either under pure compression or pure tension - no bending. A string truss is thus a true truss - it relies on pure tension with rigid poles under pure compression serving only to maintain the tension.

If you adopt a true string truss design then the requirements on the poles are greatly relaxed compared with the strut telescope. You could reduce the number of poles to two, make them much lighter, and if you spring-load the poles then none of the strings need a separate tensioner (the poles themselves provide this).

You can also make a hybrid "string assisted" strut telescope. This uses struts to provide the basic structural rigidity, with strings to take up additional load in certain directions. To handle the deflection caused by shifting in elevation, only one string on top is needed, though you could use one on either side instead. You can provide load handling in all directions with 3 strings, you don't need six.


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killdabuddha
Pooh-Bah
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Reged: 08/26/11

Re: Quick Question about 3 strut systems new [Re: careysub]
      #5062452 - 02/08/12 03:24 PM

Quote:


and if you spring-load the poles then none of the strings need a separate tensioner (the poles themselves provide this.





I can't remember who it was but they did this on a string job. They eventually removed the springs because they couldn't suppress their play, opting eventually for a more rigid mount.

Here it is, version #1 "Springy Things"

http://nemoworld.com/astro/8_inch_string.htm

He says that it was the "father" of strings, Dan Gray, who pointed out to him the weakness of his particular spring design.

Edited by killdabuddha (02/08/12 03:38 PM)


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calibos
Carpal Tunnel
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Reged: 11/18/07

Loc: Ireland
Re: Quick Question about 3 strut systems new [Re: derangedhermit]
      #5062703 - 02/08/12 05:56 PM

Quote:

Joint looks bendy to me.




Thats supposed to be an exploded view of the clamps/pole sections with threaded inserts and bolt.

In reality the clamps will be butted up against each other, the bolt wont be as long, there will be a small section of tube that fits inside the larger ID tubes.

Perhaps I need to clarify again, that this is not me building a new scope around my Meade LB16 optics, this is a strut conversion of a standard Meade Lightbridge. The reason I am seemingly taking this structural rigidity step backwards from a Truss system is that I want the OTA to be collapsible like a Skywatcher Flextube dob such that the scope is less obtrusive in the room it is stored in while at the same time being quick and easy to setup.

To maintain this 'Telescopic' collapsibilty of the OTA the strut poles need to be able to slide up and down inside the clamps. This requirement complicated what would have been a simple mod with a single clamp, single threaded insert and single bolt for each strut.

Remember both clamps in reality will be butted up against each other. The main strut tube section and the small tube section with the bolt pictured below it are permantly bound together and would have an inner tube sleeve not pictured such that outwardly it would almost look like a continuous length of pole. This means the pole can slide up and down through the clamps when collapsing and extending.

The UTA is extended and I tighten the lower clamps. Now the bottom section of the strut tubes are anchored/locked. Now the main section of tube has something to push against when I start twisting the struts. They ride up the threaded bolt and put the string under tension. Remember the bottom of the main section of strut is still within the boundaries of the top clamp. So when I tighten the knob on the top clamp it firmly grips the main section of strut tube. I am not relying on the bendy bolt as the connection between the struts and the Lower OTA. The 'bendy' bolt is just there for the strut to ride up a few millimetres on to put the strings under tension. Once I tighten the top clamp which grips the strut then its the strut and clamp that are taking the strain.


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careysub
Carpal Tunnel
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Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Quick Question about 3 strut systems [Re: killdabuddha]
      #5062742 - 02/08/12 06:25 PM

Thanks - note that they did work in his 8 inch scope though. Some sort of non-springy screw-adjustment pole, a version of which he used for his "Mark 2" model is probably the best all-around approach.

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calibos
Carpal Tunnel
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Reged: 11/18/07

Loc: Ireland
Re: Quick Question about 3 strut systems new [Re: careysub]
      #5062748 - 02/08/12 06:34 PM

Quote:

A good presentation of the principles of a string truss telescope is section VII of this document:
http://bossanova9.org/astro/ATM/5-Tubes&Trusses.pdf

A true string truss does require a minimum of 3 pairs of cables, all six of which are separate tension elements.

The key difference between a strut telescope and truss telescope is that the struts bend, one side of each strut is under tension and the other under compression, while in a truss the elements are each either under pure compression or pure tension - no bending. A string truss is thus a true truss - it relies on pure tension with rigid poles under pure compression serving only to maintain the tension.

If you adopt a true string truss design then the requirements on the poles are greatly relaxed compared with the strut telescope. You could reduce the number of poles to two, make them much lighter, and if you spring-load the poles then none of the strings need a separate tensioner (the poles themselves provide this).

You can also make a hybrid "string assisted" strut telescope. This uses struts to provide the basic structural rigidity, with strings to take up additional load in certain directions. To handle the deflection caused by shifting in elevation, only one string on top is needed, though you could use one on either side instead. You can provide load handling in all directions with 3 strings, you don't need six.




I think some of the clarification in my post above concerning the Telescopic collapsibility like a skywatcher flextube that I am aiming for will explain why I can't spring load the poles and Killdabudda highlighted the downsides of a spring loaded pole as well I think.

When you talk about 3 pairs of strings, do you still mean 6 discrete cables because you have used 'pair' and 'discrete' in the same sentance so I am still confused.

I had actually realised how I might be able to have 3 physical pairs. A loop on each end and in the middle of the cable. I've seen the metal cable doodads for this. Perhaps a new drawing will show what I mean



I presume this new idea eliminates the problem of cable creep that was talked about when I asked if I could just loop a single run of cable over and under bolts. While at the same time meaning I don't have to deal with 6 individual cable runs but instead 3 pairs so to speak.

So if we have arrived at a workable cable/string system and a workable tensioning system given what you said in your last paragraph, can I switch back to using 1" OD poles and clamps instead of the 1.25" versions. In the grand scheme of things given the weight of the Lightbridge UTA, Moonlite focuser, Paracorr, 21mm Ethos, 80mm finder etc etc the weight savings in the poles is not a consideration. What is, however, is that I already have 4 spare 1" Moonlite Truss clamps and would save myself $72 by not having to buy all 10 I would be using on this project. After all the Lightbridge truss poles are 1" OD so with a 'Virtual Truss' with the strings could I get away with my 3 struts also being 1" OD.


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calibos
Carpal Tunnel
*****

Reged: 11/18/07

Loc: Ireland
Re: Quick Question about 3 strut systems new [Re: careysub]
      #5062764 - 02/08/12 06:44 PM

Quote:

Thanks - note that they did work in his 8 inch scope though. Some sort of non-springy screw-adjustment pole, a version of which he used for his "Mark 2" model is probably the best all-around approach.




The Mark 2 approach is basically what I came up with on the last page I obviously just didn't explain it well enough though. The difference with my version is that unlike Nemo where the threaded bolt hardware is permentently fixed to the top of his mirror box, my hardware is permently fixed on the end of the main section of strut because the strut needs to be able to telescope up and down through the clamps. I need the 2 clamps per strut because the bottom clamp locks my "Mark 2" hardware which then gives the main section of strut with the threaded insert something to push against. After twisting the strut has done its job of putting the cables under tension then I can lock the top clamp to secure the strut in that position and to the OTA.


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tcmzodiac
professor emeritus


Reged: 11/11/11

Re: Quick Question about 3 strut systems new [Re: calibos]
      #5062810 - 02/08/12 07:22 PM

Playing Devil's Advocate...

Could you "bag" the string idea and instead order a set of 2" OD poles from Speedy Metals, etc with ID's that would allow making a trio of telescoping poles...that would index, ala crutches, by means of the telescoping pole spring clips available from McMaster Carr?

You may want to flare one end of the "inside" pole so as to eliminate wiggle.

Make the pole clamps at either end uber-sturdy...the poles are a straight-shot from mirror box to UTA..and there you go!


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Gordon Rayner
Carpal Tunnel
*****

Reged: 03/24/07

Re: Quick Question about 3 strut systems new [Re: killdabuddha]
      #5063098 - 02/08/12 10:42 PM

I have about 30 each of USMC command post tent poles , magnesium alloy, anodized, olive drab. 12 feet long, x 1.625 square, with 3/16 inch wall thickness . In southern California.

Would these make a good 1, 2, or 3 pole scope?

Magnesium is 2/3 the mass of aluminum, and damps vibration better.


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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Quick Question about 3 strut systems new [Re: calibos]
      #5063812 - 02/09/12 01:13 PM

Quote:

...
When you talk about 3 pairs of strings, do you still mean 6 discrete cables because you have used 'pair' and 'discrete' in the same sentance so I am still confused.





Umm... actually none of my posts use the word "discrete" (as far as I can tell).

I did say "3 pairs of cables, all six of which are separate tension elements" - in other words, each cable in the pair is a separate run of cable rigidly anchored on each end. A cable pair could share the same anchor device at the point of the vee, but they are secured separately.

Quote:


I had actually realised how I might be able to have 3 physical pairs. A loop on each end and in the middle of the cable. I've seen the metal cable doodads for this. Perhaps a new drawing will show what I mean







Yes, that is exactly what I mean by a cable pair (assuming the clamp doodad forming the loop mechanically isolates both sides, say by clamping). I think tying two cables to a metal ring in the middle would work very well (I have mind to try this in a travel scope build).

Quote:


I presume this new idea eliminates the problem of cable creep that was talked about when I asked if I could just loop a single run of cable over and under bolts. While at the same time meaning I don't have to deal with 6 individual cable runs but instead 3 pairs so to speak.





Right.

Quote:

So if we have arrived at a workable cable/string system and a workable tensioning system given what you said in your last paragraph, can I switch back to using 1" OD poles and clamps instead of the 1.25" versions. In the grand scheme of things given the weight of the Lightbridge UTA, Moonlite focuser, Paracorr, 21mm Ethos, 80mm finder etc etc the weight savings in the poles is not a consideration. What is, however, is that I already have 4 spare 1" Moonlite Truss clamps and would save myself $72 by not having to buy all 10 I would be using on this project. After all the Lightbridge truss poles are 1" OD so with a 'Virtual Truss' with the strings could I get away with my 3 struts also being 1" OD.




Yep, again.


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calibos
Carpal Tunnel
*****

Reged: 11/18/07

Loc: Ireland
Re: Quick Question about 3 strut systems new [Re: careysub]
      #5064013 - 02/09/12 03:27 PM

Ah yes, It was the word 'Pairs' and 'six separate tension elements' that had me a little confused. Thanks for the clarification.

Now, can I throw a spanner in the works again

Now that I have struts that run parallel to the OTA I am wondering can I move my Stellarvue F80 Finderscope (2.5Lbs) from my overcrowded OTA to the uppermost strut. I would mount it to another one of those moonlite clamps on the strut. ie nothing to do with those same types of clamps I use to affix the struts to the OTA but using yet another one of those clamps a few inches down the strut from the TA as a movable mounting along the length of that uppermost strut. Why a Moonlite clamp instead of just bolting a Dovetail shoe to the strut?? Twisting the strut for tension might put the shoe at the wrong angle. With the shoe bolted to the moonlite clamp I just loosen the clamp and rotate it.

Declutters my UTA and helps with counterweight issues.

So the question is would this affect the rigidity of the strut? Final position of the shoe on top of the strut or final position pointing sideways towards me standing at the side of the scope. This would mean I wouldn't have to lean in as far to the Finderscope eyepiece. ie. Striaght up and the weight would be in the direction of the strut, whereas mounting off the side of the strut would put the lever arm alongside the strut. Would this torquing of the strut cause issues?

If it did cause issues to a 1" OD strut, Could I just make the Finderscope strut a 1.25 incher but leave the other 2 struts as 1 inchers. Thus still being able to use the 1 inch clamps I already own.

Thanks again for everyones help. I applaud your patience with me.


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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Quick Question about 3 strut systems new [Re: calibos]
      #5064865 - 02/10/12 02:18 AM

Quote:

...
Now that I have struts that run parallel to the OTA I am wondering can I move my Stellarvue F80 Finderscope (2.5Lbs) from my overcrowded OTA to the uppermost strut. I would mount it to another one of those moonlite clamps on the strut. ie nothing to do with those same types of clamps I use to affix the struts to the OTA but using yet another one of those clamps a few inches down the strut from the TA as a movable mounting along the length of that uppermost strut. Why a Moonlite clamp instead of just bolting a Dovetail shoe to the strut?? Twisting the strut for tension might put the shoe at the wrong angle. With the shoe bolted to the moonlite clamp I just loosen the clamp and rotate it.




Definitely clamp it. You do not want to put a hole in the strut and compromise its integrity (you get away with using lightweight poles by ensuring that they are sound). I would use a clamp of some kind myself for that.

Quote:

... So the question is would this affect the rigidity of the strut? Final position of the shoe on top of the strut or final position pointing sideways towards me standing at the side of the scope. This would mean I wouldn't have to lean in as far to the Finderscope eyepiece. ie. Striaght up and the weight would be in the direction of the strut, whereas mounting off the side of the strut would put the lever arm alongside the strut. Would this torquing of the strut cause issues?




I doubt the pole's function as a string tensioner would be compromised, but side mounting would increase the possibility of the mount twisting on the pole, or the pole twisting in its mount - but these may be non-problems too depending on implementation.

Quote:


If it did cause issues to a 1" OD strut, Could I just make the Finderscope strut a 1.25 incher but leave the other 2 struts as 1 inchers. Thus still being able to use the 1 inch clamps I already own.




Yes, 1 inch struts should be fine. After all, in truss pole systems 1 inch poles are already successfully operating loaded in compression, exactly the same as you are doing.


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calibos
Carpal Tunnel
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Reged: 11/18/07

Loc: Ireland
Re: Quick Question about 3 strut systems new [Re: careysub]
      #5066050 - 02/10/12 07:18 PM

It takes a day or so for the knowledge you guys impart to me to be absorbed

Its just clicked for me what you were saying about the relationship of the strings and struts. ie. That when you add strings to a strut system the struts cease to be the main structural member so to speak, its the strings doing all the hard work and the strut just becomes the means of keeping the strings under tension as it were.

So would I be correct in saying that I can have my threaded joint that enables tensioning by means of twisting at any point along the strut. You see I thought I had to have a contiguous length of strut between the Lower OTA clamp and UTA clamp because the tiniest amount of play between telescopic strut pieces would mean collimation shift. However I think I now realise that a tiny amount of lateral play doesn't matter in the strut if the strut is only there to keep tension on the strings which are the real structural elements and maintainers of collimation.

So I can do away with my Double Lower OTA clamps idea and have the short twistable section of strut up near the top of the struts with an outer sheath of tube over the split threaded sections. Or inner sheath I guess. Which ever I find first that fits outside a 1" OD pole or inside a 1" OD .050" wall thickness pole. Hopefully this inner or outer sheath tube keeps the sections in alignment with little or no play but that a small amount of play is not a big deal anyway.

I can use an outer sheath if need be because the pole length and height of OTA means even in the collapsed state the UTA can telescope down no closer than about 6 inches to the Lower OTA before the poles bottom out on the floor of the rockerbox. Still a huge difference to the height of about 3 feet which makes the scope much less obtrusive in the room. Thus I don't have to worry about an outer sheath meeting the lower OTA clamps and preventing the scope collapsing further. The threaded joint between the two strut sections and the sheath will be positioned within that 6" top section of pole that never goes through the LOTA clamps anyway.

If you guys can confirm that this is OK then I can put the research for this part of my modding project to bed till I am ready to proceed......and start my next thread about what part of my rockerbox sides I can cut off/out to reduce weight without affecting structural rigidity or causing more rockerbox flexure and also ways to increase rigidity and remove already present flexure

Thanks again for all your help. Its been invaluable.


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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Quick Question about 3 strut systems new [Re: calibos]
      #5068603 - 02/12/12 10:49 AM

Quote:

...
Its just clicked for me what you were saying about the relationship of the strings and struts. ie. That when you add strings to a strut system the struts cease to be the main structural member so to speak, its the strings doing all the hard work and the strut just becomes the means of keeping the strings under tension as it were.





If it has been rejiggered into a true string truss system, then this is true. But remember there also string-assisted strut hybrids where that isn't true - the string(s) are simply adding additional resistance, generally to oppose the load caused by tilting the tube from vertical to horizontal. Jon Isaacs who has posted on this thread has one of there - a DobStuff truss dob with an after-market cable. He posted excellent pictures of it recently on the Reflector forum.*

BTW - I was thinking about whether string loaded poles in a true spring truss were ever a good idea or not, and realized that the problem with spring loading is that the poles can jiggle and jitter up and down with varying load (the wind), since the strings don't oppose movement down toward the mirror(shortening them all at once), unless the spring is very stiff. This would make the UTA prone to a mode of vibration. A rigid pole is needed to prevent this and stabilize he system fully.

Quote:

So would I be correct in saying that I can have my threaded joint that enables tensioning by means of twisting at any point along the strut. You see I thought I had to have a contiguous length of strut between the Lower OTA clamp and UTA clamp because the tiniest amount of play between telescopic strut pieces would mean collimation shift. However I think I now realise that a tiny amount of lateral play doesn't matter in the strut if the strut is only there to keep tension on the strings which are the real structural elements and maintainers of collimation.




I believe so. Lateral play is fine, the strings maintain that rigidly, the tube (once tensioned) is of fixed length and so won't play vertically. You need a rigid element along the optical axis to handle the vertical load component.

BTW - try Googling "tensegrity structures" in Google Images. You see many perfectly rigid structures made entirely of cables and rigid poles (which don't even touch each other). A string truss scope is a type of tensegrity structure. The absence of spring loading in these structures suggest this is not a good idea.

Quote:

So I can do away with my Double Lower OTA clamps idea and have the short twistable section of strut up near the top of the struts with an outer sheath of tube over the split threaded sections. Or inner sheath I guess. Which ever I find first that fits outside a 1" OD pole or inside a 1" OD .050" wall thickness pole. Hopefully this inner or outer sheath tube keeps the sections in alignment with little or no play but that a small amount of play is not a big deal anyway.




Another scheme I was considering is a 1/2" screw thread mounted one the base, and a wood socket with a hold bored from top to bottom for the 1/2" thread, and a threaded inserted so that it screws up and down on the thread, with a hole bored for a snug pole fit, the excess 1/2" thread fits into the hollow pole. Seating the pole in the socket, then turning it to raise it locks the pole in position. In this scheme the pole remains a plain pole, with no special mods or treatment at all, though I see you are looking for a collapsible pole as an essential feature.

*This can be considered to be half an X-brace frame. In earthquake (or high wind load) building designs vertical columns can have X-braces between the columns (usually rigid, but essentially cables as they only work in tension) to resist lateral loads. In any one direction of lateral load only one half of the X brace takes any load at all. Since the braces are very strong in tension (and take no vertical load) they are much lighter than the columns, yet give nearly all the lateral load stiffness. Since Dobsonian OTAs tilt in only one direction we only need half of the X.


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scopemankit
sage


Reged: 01/27/09

Loc: south africa
Re: Quick Question about 3 strut systems new [Re: careysub]
      #5068900 - 02/12/12 01:35 PM Attachment (20 downloads)

I have made a couple of 10", 3 truss scopes. Very successful although I did have two stiffening wires to stop vibrations.

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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Quick Question about 3 strut systems new [Re: scopemankit]
      #5069422 - 02/12/12 06:59 PM

Quote:

I have made a couple of 10", 3 truss scopes. Very successful although I did have two stiffening wires to stop vibrations.




That's interesting, the more examples I see the more I learn. Since you have a cross brace wire on the bottom, were you experiencing lateral vibration?


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scopemankit
sage


Reged: 01/27/09

Loc: south africa
Re: Quick Question about 3 strut systems new [Re: careysub]
      #5072728 - 02/14/12 08:26 PM

Re the cross brace wires, one is on the bottom but there is also one down the right hand side. The wires damp down low frequency vibrations a lot.

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