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Quick Question about 3 strut systems

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#1 calibos

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Posted 02 February 2012 - 02:48 PM

In all the 3 strut scopes I've seen two of the struts are on the underside of the OTA so to speak (OTA pointed horizontal) and the third strut is on top of the OTA so to speak. Is this arrangement necessary for strut flexure or compression reasons or could I have the single strut on the underside of the OTA and the other two on top (Ota pointed horizontal)

#2 tcmzodiac

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Posted 02 February 2012 - 03:19 PM

The 3 pole that Im building is a 10" f6. I wanted 2 of the poles to be aligned verically ala the wonderful Ron Ravnenburg...I added a 3rd pole.

The poles occupy 3 of the 4 corners of a square, and so form an equilateral triangle. The 3rd pole occupies the "top" corner, so I have 2 on top and two on one side...so to speak.

Im using 2" OD aluminum with a .063" wall thickness. That may be total overkill, however I am not building for uber lightweight rather, for stiffness.

I could also configure it as a 2 pole ala Ron R. if I want..will be interesting to see how well that works.

In addition, Ill have plenty of aluminum for finish fun! :)

#3 wirenut

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Posted 03 February 2012 - 06:04 PM

I'm building a 10" F4.5 3strut scope to I going with the traditional 1 on top 2 on bottom. I not doing this for mechanical reason I did it for viewing reasons. when a scope is near horizontal I find it easiest to view with the EP stick up around 45°.

#4 tcmzodiac

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Posted 03 February 2012 - 07:26 PM

I'd love to see any comments by those who have built 2-and-3 pole scopes.

What do you seasoned ATM's predict will be the outcome of my layout as described above...also of course your answer to the OP...threadjack over!

wirenut....I assume that you are using a flat focuser board and thus the 45 degree spec? That means that your UTA will be quite lighter than mine, as I am using an undrilled 12" birch drumshell for my UTA. You may well have less balance challenges than do I! :)

#5 m. allan noah

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Posted 03 February 2012 - 08:33 PM

Most materials are stronger in tension than compression. In general, this means you need more material on the bottom than the top of the tube. When pointing at the horizon, you could replace the top tube with a string. Now, all that theory is a bit wasted as soon as you point upward.

allan

#6 killdabuddha

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Posted 03 February 2012 - 11:42 PM

Hi calibos,

I know that I brought this up elsewhere and mentioned that the reference was to the traditional 8 tube truss design, but I did manage to find the sites I remembered that spoke to the greatest load bearing parts of the traditional OTA

http://www.cruxis.co..._trusstubes.htm

and he's referencing this site

http://bossanova9.org/astro/ATM/

Specifically, this page

http://bossanova9.or...bes&Trusses.pdf

Dunno whether or how the traditional forces would translate in your 3-tube design, but I thought I should at least provide this. I also remember the advice to determine yer design first, whether it's according to aesthetics, viewing comfort, etc., and after this to find the engineering that'll get you there. For instance, here's someone maximizing portability and the single pole must support the focuser, so the rest had to be engineered accordingly

http://www.litescope...e products.html



#7 Sean Cunneen

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Posted 04 February 2012 - 09:24 AM

I built my Newt as a 3 pole scope with the single tube at the "top". My altitude bearings clamp to the side-poles to help stiffen the structure. In matters of tension/compression, the stresses on our scopes are no where near large enough for that kind of stress to be an issue.

Building a structure that doesn't twist or flex is another matter and designing the geometry of your components so they reinforce each other allows you to have a light and stiff structure.

I've built more than a few scopes and I like for each build to explore a unique direction. That being said for me personally I am finding the more visually interesting the scope is to look at, the less exciting it is to look through. Bear in mind I have never followed my own advice, so once built, my scopes have gone through more than a few periods of redesign. As I use them more, they tend to move towards the more conventional design aesthetic.

So if you are contemplating an asymmetrical pole layout with a big heavy mirror on the bottom end of the scope and a focuser that is all the way at the top, hangs off the side and you have a penchant for heavy widefield eyepieces, you can get to know flexture and be a whiz at collimation-on-the-fly just like me!

#8 tcmzodiac

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Posted 04 February 2012 - 10:35 AM

A prayer...please o please do not allow my scope to adopt "Twisting The Night Away" as it's anthem! :)

#9 killdabuddha

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Posted 04 February 2012 - 11:51 AM



A 20" 3-strut string job with the poles "on top," from a fellow CN-er

http://nemoworld.com...ghtsthread.html

A 3-strut with poles at 3 corners

http://www.cloudynig...hp?item_id=1123

#10 Jon Isaacs

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Posted 04 February 2012 - 03:50 PM

Most materials are stronger in tension than compression. In general, this means you need more material on the bottom than the top of the tube. When pointing at the horizon, you could replace the top tube with a string. Now, all that theory is a bit wasted as soon as you point upward.

allan


Allan

If you do the math, I think one finds that neither the compression strength nor axial stiffness of the tubes are important, it's the bending stiffness that is important and at first guess, it doesn't matter the orientation in terms of whether the two struts are below or above.

Equal spacing of the tubes probably optimizes the stiffness of the upper ring which ties the tubes together and increases the stiffness of the entire structure in bending. A single pole with one fixed end is a cantilever, if the upper end is "guided, which means it can move laterally but it is prevented from rotation at what would be a free end free end, then the pole is 4 times stiffer. This is one purpose of a stiff upper ring.

Jon

#11 killdabuddha

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Posted 04 February 2012 - 04:54 PM

Most materials are stronger in tension than compression. In general, this means you need more material on the bottom than the top of the tube. When pointing at the horizon, you could replace the top tube with a string. Now, all that theory is a bit wasted as soon as you point upward.

allan


Allan

If you do the math, I think one finds that neither the compression strength nor axial stiffness of the tubes are important, it's the bending stiffness that is important and at first guess, it doesn't matter the orientation in terms of whether the two struts are below or above.

Equal spacing of the tubes probably optimizes the stiffness of the upper ring which ties the tubes together and increases the stiffness of the entire structure in bending. A single pole with one fixed end is a cantilever, if the upper end is "guided, which means it can move laterally but it is prevented from rotation at what would be a free end free end, then the pole is 4 times stiffer. This is one purpose of a stiff upper ring.

Jon


Jon,

I think you raise an interesting point. Look at the 3-strut Portaballs. That said, the focuser and eyepiece ARE introducing a torsional component that must be compensated for, either by strut placement, solid mounting, cage/ring rigidity, or maybe all of the above? If he wanted to go there, he could always run some FEM (Finite Element Method) calculations. In our case, with our focusers/diagonals/EPs/Barlows hangin off the inside bottoms, we're gonna beef up the "top wings" (top corner tube sections) that are indicated (above links) for the greatest load-bearing stresses. But for a 3-strut design, it seems he has full freedom to put them anywhere he wants, especially inasmuch as the Portaballs are always doin exactly this? We need an engineer. Or maybe Portaball users can share whether they've noticed any differences in bending or collimation according to the orientation of their scopes.

#12 calibos

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Posted 05 February 2012 - 07:09 PM

Thanks for all the contributions so far. I thought another mod I had in mind would necessitate the aformentioned strut arrangement but for other reasons I realised the other mod was not feasible so there is now no reason for me not to go with the conventional strut arrangement.

Some of you already know this from other threads but for the other guys, I should point out that this was not atm but a strut mod to a 16" Lightbridge to give it the functionality of those Skywatcher Flextubes with their collapsible UTA's.

I'd be using moonlite truss hardware and stiffening the structure with a string mod of metal cable and turnbuckle.

In another thread I had been discussing using a lazy susan ring to make the UTA rotatable also but a Homer Simpson DOH! realisation has put paid to that flight of fancy. The lazy susan ring would get in the way of my Moonlite filterslide and I love my Moonlite filterslide! :D So instead what I will do for the rotatable UTA is to simply fit a second trio of moonlite truss hardware to the UTA. ie Plan my sessions better and when viewing low down remove and refit the UTA to the second set of clamps and then recollimate.

I'll also follow Jons recommendation from another thread to use 1.25" clamps and poles instead of 1".

I have some questions about the string scope mod though.

My plan is to use one length of metal cable with a turnbuckle. My Lower OTA endring with the old Lightbridge truss clamp hardware would be rotated 120º. ie. the 3 old clamps would be equidistant between the 3 struts. The cable would loop under the smooth shaft of the old truss clamp bolts up over a bolt through the top of the next strut, down to the next old truss clamp bolt, up...... etc etc If needed I could fit pulley type bearings to the bolts so that the looping cable doesn't snag and that when I turn the turnbuckle it applies equal pressure on all the struts.

Jon you mentioned previously that I couldn't use moonlite ball and socket clamps for the UTA attachment to the struts and that I should use UTA clamps instead. Can you explain why the moonlite ball and sockets don't work in letting the string tension the struts and prevent flexure?

#13 careysub

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Posted 06 February 2012 - 11:19 AM

...
I have some questions about the string scope mod though.

My plan is to use one length of metal cable with a turnbuckle. My Lower OTA endring with the old Lightbridge truss clamp hardware would be rotated 120º. ie. the 3 old clamps would be equidistant between the 3 struts. The cable would loop under the smooth shaft of the old truss clamp bolts up over a bolt through the top of the next strut, down to the next old truss clamp bolt, up...... etc etc If needed I could fit pulley type bearings to the bolts so that the looping cable doesn't snag and that when I turn the turnbuckle it applies equal pressure on all the struts.

Jon you mentioned previously that I couldn't use moonlite ball and socket clamps for the UTA attachment to the struts and that I should use UTA clamps instead. Can you explain why the moonlite ball and sockets don't work in letting the string tension the struts and prevent flexure?


This scheme is not going to work (I thought of a similar one myself awhile back, before I understood the mechanics correctly).

Despite what you read on some websites, string trusses do not work by applying compression on the struts. If you do this you make the struts bend more, not less (this is a classic effect of column loading) - it makes them less stiff. What string trusses do is use the string's resistance to elongation to prevent the scope from bending, adding a structural element that would only be loaded in tension. Putting some initial tension on the string is necessary to take up all slack, but more than this is undesirable. The looping scheme will allow the string to slip (which is actually your design to create even compression) and prevent the string from taking the tension load as the tube shifts in elevation.

What you want is for the string to be rigidly attached on its ends, with a tensioner to take out the slack. When the scope attempts to bend it tries to stretch the string, and the string's area-elastic modulus product resists that and thus reduces the bending. If doing one string, it should be on top. If used on the sides they should be at a downward angle.

Steel has a very good modulus (30,000,000 psi), but it is heavy. Consider using Dyneema sailing line instead. Dyneema (also called Spectra) has almost the same modulus as steel (24,700,000) but is little more than 1/8 the weight giving it the highest specific modulus of any material known (except perhaps for theoretical carbon nanotube cable that does not yet exist). Quarter inch Amsteel Samson sailing line at westmarine.com costs 88 cents per foot, having about the highest modulus/price ratio I have seen. Sailing boats use Dyneema line for exactly the same benefits you are seeking - a light weight line that resists elongation.

#14 Jon Isaacs

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Posted 07 February 2012 - 12:59 AM

Jon you mentioned previously that I couldn't use moonlite ball and socket clamps for the UTA attachment to the struts and that I should use UTA clamps instead. Can you explain why the moonlite ball and sockets don't work in letting the string tension the struts and prevent flexure?



I can't say they won't work. But one important factor in a strut design is that if the tube is attached to the upper cage so that it cannot rotate (I am not talking about twist/torsion), that is the ends of the struts remain parallel, then it is much stiffer in bending. A ball and socket is generally not designed to keep the end parallel... If the end can rotate, we are talking very small rotations, then the bending stiffness is decreased by a factor of 4 because the tube is now essentially a cantilever.

As far as using one length of cable. I think the of each run of cable need to be firmly fixed. Otherwise, small deflections can be accommodated by slight motion of the cable.

If the cable has pulleys then the length would not be fixed and as one section shortened, the next one would lengthen. If you don't have pulleys, friction would probably hold it but the tension would not be equilibrated and the cable would reposition itself and the scope would go out of collimation. I can tweak the secondary of my scope simply by twisting the turnbuckles of the tension rods. In my view, both ends of each section of cable need to be fixed.

As far as using finite element codes to analyze a scope, I doubt very much telescopes are designed using FEM, they are designed using some simple calculations like I have been doing, some engineering intuition and just experience.

A simple calculation says that for tubes of equal wall thickness, a 2 inch tube is 4 times stiffer than a 1.25 inch tube but only weighs 1.6 times as much. A simple calculation says a single ring that is 1.125 inches thick is 4 times stiffer than a ring that is 0.70 inches thick but weighs 1.6 times as much. So, calculating allowable weights, there is a lot of stiffness to be had and stiffness is rarely a bad thing to stockpile.

Jon

#15 killdabuddha

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Posted 07 February 2012 - 12:41 PM

careysub, is Dyneema/Spectra better than the archery string everybody uses?

Jon, I agree that FEA may be overkill for our scales, though I like to find those who've used it for builds similar to mine. But for anyone interested, there are two shareware programs available:

PCSTRAN
http://www.chet-aero...nload/other.php

Grape 3D
http://www.grapesoftware.mb.ca/

Any opinion whether square aluminum tubing would be better/just as good for our truss tubes? There's this:

"The round tube is more efficient in compression and the square tube is more efficient in bending if the wall thickness and mass are the same for both. For compression, the minimum diameter matters most. For bending, the average diameter in the plane of bending matters most. But these apply only to simple materials on simple structures."

Otherwise, we're about to order 1" OD/.065" walled tubes and begin the OTA (75" focal length with a light secondary cage). But square would definitely be easier to mount. Maybe what I'm really askin is, given the mounting advantages of square, why are all the scopes made with round? Have I answered my own question? Thanks

#16 careysub

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Posted 07 February 2012 - 02:04 PM

careysub, is Dyneema/Spectra better than the archery string everybody uses?


The archery string that is commonly used is BCY-452X (or similar), a 67% Spectra/33% Vectran blend.

There are two very high modulus fibers commercially available, Dyneema/Spectra and Vectran. High modulus Dyneema/Spectra (there are lower modulus versions, but the high modulus version dominates the market) has a modulus of 24,700,000 psi and a density of 1.0; Vectran has a modulus 10,200,000 psi and density of 1.4. As you see Vectran has a much lower specific modulus (but still quite high by comparison with most anything else) - it is used because it has near zero creep (gradual elongation under load). This means that a bow, once strung, will not have the tension drop over time.

This zero-creep property seems of little importance to a string scope, where a tension adjusting element is incorporated anyway (unlike most bows), and the exact tension itself is of little importance (other than it being balanced for multiple strings, if used).

Some bowstrings use 100% Dyneema anyway. 8190 Universal Bowstring, sold as the highest performance bowstring available, is 100% Dyneema.

There are actually a range of Dyneema/Spectra (and Vectran blend or pure Vectran) line products available, sold for archery, rock climbing, arborist climbing, and sail boats. Any of these can be used.

It appears to me the most cost effective, and well described, are the sailboat lines. I shopped around at REI for Spectra climbing line, but found a lot of climbing line is not well-labeled about what it is actually made of. Bowstring seems to take more effort than necessary since it has to be doubled a number of times and tied. Sailing line is available ready-made in many thicknesses at reasonable cost.

Note that it is an advantage to get line that is seemingly ridiculously over-spec'ed. The quarter inch line I mentioned has a breaking strength of 7400 lb. The reason for getting such amazingly strong material is that you want to have a very high modulus-area product so that the load placed on it produces a very small elongation. A 50 Nt (11.25 lb, the weight of a fully loaded UTA plus some truss pole mass perhaps) load placed on a 7400 lb cable will stretch it 27 microns, an amount too small to affect collimation in even a very fast scope. This is probably overkill, but the cost and mass of the line is not unreasonable (of several dozen products I gathered data on it had the best modulus-product/cost ratio).

Going with 2.2 mm arborist line with a strength of "only" 300 lb allows a stretch of 660 microns, enough to adversely affect collimation in a very fast scope.

#17 calibos

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Posted 07 February 2012 - 09:27 PM

Steel cable and Tensioning

Walking past a shop a window today made me think of cable display systems which I then looked up on the net. In terms of cable tension their system of inline cable tensioning is much more compact and elegant than carbineers or turnbuckles.

Link - Half way down page, item CASS-T

With the little fasteners to creat loops on the ends of the cables that can loop over the old truss clamp bolts on one end and bolts on the struts on the other end I think I would have tension and ease of attachment/removal sorted. However is the 1.5mm steel cable of these systems enough. The sales blurb quotes a weight carrying capacity of each 1.5mm steel cable as 120kg.

However is the final word that each cable run has to be a discrete cable. ie. I'd need 6 tensionable cables? I mean, OK lets forget about the single cable that loops all the way round the scope on one tensioner/turnbuckle, but is 3 cables doable. ie. say each of the three cables hooks onto the top of adjacent struts but is just looped under the bolt on the old lightbridge truss clamp on the endring between those struts.

I'm trying to see can I minimise the setup time and setup hassle here.

......Or......

Do I go for a KISS option and just make a removable plywood Tensioning ring like this:

Posted Image


Ball and Sockets

The ball and socket on the tops of the struts idea was to ease replacement of the UTA when setting up after a full scope breakdown for transport. I accept Jons reasoning for it not necessarily being a good idea in terms of Cantilever etc. However, what about ordinary Moonlite truss clamps for two of the struts and a ball and socket on the third strut. Would I then be able to have my cake and eat it too. ie. Ball and socket on one strut allows me to firmly anchor/take the weight of the UTA on one strut leaving both arms free to guide the UTA clamps onto the other struts. While the Truss clamp attachment to the other two struts prevents the problems assosciated with using all ball and sockets???

(PLease note. I was only ever talking about ball and socket clamps on the top of the struts. Bottom of the struts always attached with ordinary truss clamps)

#18 derangedhermit

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Posted 08 February 2012 - 02:19 AM

\ Ball and Sockets

The ball and socket on the tops of the struts idea was to ease replacement of the UTA when setting up after a full scope breakdown for transport. I accept Jons reasoning for it not necessarily being a good idea in terms of Cantilever etc. However, what about ordinary Moonlite truss clamps for two of the struts and a ball and socket on the third strut. Would I then be able to have my cake and eat it too. ie. Ball and socket on one strut allows me to firmly anchor/take the weight of the UTA on one strut leaving both arms free to guide the UTA clamps onto the other struts. While the Truss clamp attachment to the other two struts prevents the problems assosciated with using all ball and sockets???

No, you only get the desired stiffness in the direction between the two struts using clamps. You lose it in the direction toward the strut with the B&S joint.

That is, (and I believe true for the original discussion above as well) if the B&S connectors allow rotation once clamped. I think they resist rotation, meaning that while the arrangement won't be as stiff as, say, 1.25" long cylindrical clamps, it will be much stiffer than if you mounted the ring on the B&S connectors and didn't tighten them down (the factor of four Jon mentioned). Most flex may come from the neck of the ball, not from rotation.

I'll be interested to hear if Jon has some comment on this last part.

#19 Jon Isaacs

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Posted 08 February 2012 - 07:55 AM

That is, (and I believe true for the original discussion above as well) if the B&S connectors allow rotation once clamped. I think they resist rotation, meaning that while the arrangement won't be as stiff as, say, 1.25" long cylindrical clamps, it will be much stiffer than if you mounted the ring on the B&S connectors and didn't tighten them down (the factor of four Jon mentioned). Most flex may come from the neck of the ball, not from rotation.

I'll be interested to hear if Jon has some comment on this last part.



Without having seen the joints and played with them, I just don't know, it may be that they can be tightened sufficiently so that there is no rotation. It may be all academic at these low loads.

Jon

#20 calibos

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Posted 08 February 2012 - 11:35 AM

Ah, sorry guys, I thought you were familiar with the Moonlite Ball and socket truss hardware. I actually have examples of both already. I used some 1" Moonlite Truss clamps for attaching my Wheelbarrow handles and wheel chuck to my rockerbox and I actually have one of the 1.25" ball and sockets that I purchased a few years ago for another project. I'll post a pic later but with it in my hand right now and the tightening knob tightened down, with great force I can get the UTA mounting block(socket) to tilt very slightly in relation to the ball, however I cannot get it to twist.

link to photo on Moonlites site I am talking about the single socket version of this rather than the double ball/socket version shown. Theres is a picture of the single verion down the page but the top photo of the double gives a better idea of the constituant parts.

Given the fact that the bottom of the strut won't be able to twist at all because its locked in a conventional truss clamp in use then hopefully I am good to go.

I specifically said 'in use' there because I think I have an idea that removes the need for 6 discrete turnbuckles(of any type) on the 6 discrete cables. Examples of string scopes with extendable struts got me thinking. With some tube threaded inserts, a small section of tube and an extra set of Lower OTA clamps, I have a way of putting the strings under tension by merely rotating the 3 struts.

Heres is a simple exploded drawing of what I mean.

Posted Image

The clamps will be mounted stacked right against each other but I have them separated in the drawing for clarity.

So imagine the scope in its collapsed state like a Skywatcher flextube. I wheel it out on its built in wheels.

I attach the 6 cables to the UTA and Lower OTA attachment points.
I loosen the knobs on the UTA clamps so the poles can be twisted.
I extend the UTA/Struts till the bottom small tube sections are lined up in the bottom LOTA clamps.
I tighten those clamp knobs.
Scope is now 99% extended. Cables not quite tensioned yet.
I then twist each strut which rides up the threaded insert bolt and puts the strings under tension. I pluck each string and when the tone is the same I know I have even tension.
Now I can tighten the top LOTA clamps which grip the main tube section.
Finally I retighten the UTA clamp knobs.

#21 derangedhermit

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Posted 08 February 2012 - 02:13 PM

Joint looks bendy to me.

#22 careysub

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Posted 08 February 2012 - 02:39 PM

A good presentation of the principles of a string truss telescope is section VII of this document:
http://bossanova9.or...bes&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.

#23 killdabuddha

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Posted 08 February 2012 - 03:24 PM

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...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.

#24 calibos

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Posted 08 February 2012 - 05:56 PM

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.

#25 careysub

careysub

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Posted 08 February 2012 - 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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