Jump to content


Photo

How many trusses?

  • Please log in to reply
22 replies to this topic

#1 RonnyKelly

RonnyKelly

    Explorer 1

  • -----
  • Posts: 65
  • Joined: 15 Aug 2012
  • Loc: Portrush, NI.

Posted 10 December 2012 - 07:56 PM

My brother and I are building a dob. It's 2500mm with a focal length of 1250mm. It will have trusses connecting the primary section to the secondary. Question is do we use 3 or 4 trusses? My brother seems to remember from geometry at school that 3 trusses are stronger than 4. I want to use 4. Any thoughts?

Thanks, Ron.

#2 careysub

careysub

    Surveyor 1

  • *****
  • Posts: 1895
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 10 December 2012 - 08:08 PM

I don't have my copy handy, but the definitive treatment of this issue, and really everything having to do with truss design is Albert Highe's new book "Portable Newtonian Design" on Willman-Bell Press. Once I have a chance to consult it, I can post what he has to say.

#3 jgraham

jgraham

    Voyager 1

  • *****
  • Posts: 13692
  • Joined: 02 Dec 2004
  • Loc: Miami Valley Astronomical Society

Posted 10 December 2012 - 08:15 PM

As long as the trusses form triangular structures they should both be strong. A triangle converts the bending loads into compression and tensile loads. Having said that, I've built two trusses, both had four struts arranged as a right prism. (Right angles were within the grasp of my meager wood working tools.) In contrast, my LightBridge 16 uses 3 pairs of truss struts arranged in a triangular form and it is surprisingly light and sturdy. Excellent design.

#4 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7597
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 10 December 2012 - 11:09 PM

with 4 triangular trusses you can get the ota closer to the mirror diameter. 3 triangular trusses will have a larger dia to keep the trusses out of the light path.

:grin:

Attached Files



#5 RonnyKelly

RonnyKelly

    Explorer 1

  • -----
  • Posts: 65
  • Joined: 15 Aug 2012
  • Loc: Portrush, NI.

Posted 11 December 2012 - 07:11 AM

Thanks for that guys.

Ron.

#6 m. allan noah

m. allan noah

    Viking 1

  • -----
  • Posts: 752
  • Joined: 14 Aug 2009
  • Loc: Virginia, USA

Posted 11 December 2012 - 08:32 AM

Is there an extra 0 in your primary diameter?

allan

#7 Dick Jacobson

Dick Jacobson

    Viking 1

  • -----
  • Posts: 898
  • Joined: 22 Dec 2006
  • Loc: Plymouth, Minnesota, USA

Posted 11 December 2012 - 09:28 AM

For maximum compactness, the end-on view at the right above should be an octagon, which is what an 8-pole (4-pair) truss looks like from the end. However, most Dobs with a wooden mirror box use a square 8-pole pattern. On my 20-inch with a square mirror box I put the bottom of the poles inside the corners and angled them outward so the end-on pattern was an octagon. I've also used 6-pole trusses. Either style works well. 8-pole octagonal is a little more compact, 6-pole a little quicker to assemble.

#8 rboe

rboe

    ISS

  • *****
  • Posts: 67078
  • Joined: 16 Mar 2002
  • Loc: Phx, AZ

Posted 11 December 2012 - 09:43 AM

Strength is relative. Any of these designs; if well executed will be plenty strong for a scope. If building a bridge or building then we could get down to brass tacks and debate this in ernest.

What you really want to worry about is stiffness so vibrations are kept to a minimum. In fact, you should not see any at all.

Since that can be achieved with single pole, two pole and three pole designs (which are not truss designs at all) you can see that a good design is a good design (if well built) no matter how many poles are used. Or not used. :)

#9 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7597
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 11 December 2012 - 09:57 AM

the end-on view at the right above should be an octagon, which is what an 8-pole (4-pair) truss looks like from the end.



hey Mr. Jacobson,

what you see above is obviously a schematic with the trusses coming together at the same point so it would be a square.

Attached Files



#10 killdabuddha

killdabuddha

    Apollo

  • *****
  • Posts: 1137
  • Joined: 26 Aug 2011

Posted 11 December 2012 - 02:31 PM

We were originally gonna use a string design and resorted to this page a lot

http://sd2cx1.webrin...l=http://dbp...

but when we decided on a conventional truss we used Robert's site a lot

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

to understand where the load-bearing was moving. (Besides his engineering background and finite element analysis, he originally included links to other studies showing the same.)

#11 RonnyKelly

RonnyKelly

    Explorer 1

  • -----
  • Posts: 65
  • Joined: 15 Aug 2012
  • Loc: Portrush, NI.

Posted 11 December 2012 - 03:49 PM

Is there an extra 0 in your primary diameter?

allan


Yes, doh!

#12 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7597
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 11 December 2012 - 04:03 PM

For maximum compactness,



with the octa-thingy don't you think you compromise the strength of the truss by lower the angle?

also by putting your poles outside doesn't everything tilt inward so the poles are straight also strengthing your trusses.

Attached Files



#13 Dick Jacobson

Dick Jacobson

    Viking 1

  • -----
  • Posts: 898
  • Joined: 22 Dec 2006
  • Loc: Plymouth, Minnesota, USA

Posted 11 December 2012 - 04:13 PM

the end-on view at the right above should be an octagon, which is what an 8-pole (4-pair) truss looks like from the end.



hey Mr. Jacobson,

what you see above is obviously a schematic with the trusses coming together at the same point so it would be a square.

Sorry, I didn't make myself clear. Your schematic shows the way 8-pole trusses are usually built, with the pole ends forming a large square at the bottom and a smaller square at the top. My point is that it is possible to build a more compact 8-pole truss if the squares at both ends are the same size, so the poles look like an octagon end-on. With a wooden mirror box, this is done by angling the poles outward. That way you can have a compact Obsession-style mirror box, with the poles inside instead of outside the corners, yet not interfering with the light path.

If I was familiar with CAD software I could make a picture that would make this idea clearer.

#14 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7597
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 11 December 2012 - 04:24 PM

like? but i have it upside down cause the bottom should be the wider.

Attached Files



#15 Dick Jacobson

Dick Jacobson

    Viking 1

  • -----
  • Posts: 898
  • Joined: 22 Dec 2006
  • Loc: Plymouth, Minnesota, USA

Posted 11 December 2012 - 04:25 PM

Pinbout, to illustrate my idea you could take your purple schematic (above) and push the top ends of the poles outward so that the distance across a pair of poles at the top equals the diagonal distance at the bottom. Then it would look like an octagon end-on, though still using a square mirror box. Pairs of poles still meet at the corners both at the top and bottom. This is the way I built my 20-inch. I don't think it causes any significant loss of stiffness, but gets rid of the awkward Obsession-style pole clamps on the outside of the box.

(Added later) Yes! You're one step ahead of me.

#16 derangedhermit

derangedhermit

    Apollo

  • -----
  • Posts: 1147
  • Joined: 07 Oct 2009
  • Loc: USA

Posted 11 December 2012 - 08:05 PM

Albert Highe goes into the question from an engineering view, um, rather thoroughly, in his book "Portable Newtonian Telescopes". I found his analysis of the importance of truss connector structural detail and location to be very enlightening, and worth the purchase price alone. I'm not sure his treatment of (and enthusiasm for) the use of drum shells and bass rings is as convincing - I have to go through it again and give it more thought.

As far as connectors on the outside of the box vs on the inside (or top), I have read the outside connector enthusiasts say it results in a smaller, therefore stiffer (and lighter) box for a given truss base distance. IMHO it comes down to personal preference - either works.

#17 careysub

careysub

    Surveyor 1

  • *****
  • Posts: 1895
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 12 December 2012 - 01:25 AM

Albert Highe goes into the question from an engineering view, um, rather thoroughly, in his book "Portable Newtonian Telescopes". I found his analysis of the importance of truss connector structural detail and location to be very enlightening, and worth the purchase price alone.


Yes, this really is the definitive treatment on truss design and construction - and he exposes a number of practices that are commonplace, but poorly conceived.

Anyone building a truss (especially on a larger project) should read the appropriate sections of this book first.

I'm not sure his treatment of (and enthusiasm for) the use of drum shells and bass rings is as convincing - I have to go through it again and give it more thought.


I was unfamiliar with drum shells and bass rings, so it was interesting to be introduced to them. But when, on pg. 232, he drills 160 (!) recesses partially through the OTA hoops to lighten it, the wisdom of taking this path does seem less than convincing.

#18 careysub

careysub

    Surveyor 1

  • *****
  • Posts: 1895
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 12 December 2012 - 10:00 AM

Question is do we use 3 or 4 trusses? My brother seems to remember from geometry at school that 3 trusses are stronger than 4. I want to use 4. Any thoughts?


To answer this question directly, based on the lengthy discussion in Highe's book:

Not stiffer in general, but could be equally stiff if certain conditions are met.
Either:
a) use heavier poles than the four pole comparison so the weight is the same, using the same base separation distance;
or
B) using the same poles, increase the base separation by 15% (in which case it will be 25% lighter)

He points out that the geometry of arranging 3 poles is a bit tricky, and spends a number of pages discussing how this can be done and the trade-offs involved.

#19 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 22716
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 12 December 2012 - 11:15 PM

One engineering detail I learned from designing bicycle frames:
if you use 8 poles, do NOT have pairs across the scope from one another be parallel. Either the poles should narrow at the top or at the bottom, but the structure will be stiffer and sag less when the scope points low if the pole pairs are not in the same plane.

This idea is used a lot on double truss or catadioptric scopes with truss tubes. I took a scope with parallel strut pairs and made them non-parallel, and the improvement in stiffness was noticeable.

#20 Dick Jacobson

Dick Jacobson

    Viking 1

  • -----
  • Posts: 898
  • Joined: 22 Dec 2006
  • Loc: Plymouth, Minnesota, USA

Posted 13 December 2012 - 08:50 AM

if you use 8 poles, do NOT have pairs across the scope from one another be parallel. Either the poles should narrow at the top or at the bottom, but the structure will be stiffer and sag less when the scope points low if the pole pairs are not in the same plane.

Don, are you saying that the end-on view should look like a trapezoid instead of a square? I'm having trouble seeing why this should improve stiffness. The triangles formed by the side pole pairs should be about equally stiff whether they are parallel or not.

#21 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7597
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 13 December 2012 - 09:11 AM

Don, are you saying that the end-on view should look like a trapezoid instead of a square?



Mr. Jacobson,

I believe his statement is similair to what you said about tipping the tops outwards/inwards.

I've read that before also but I can't remember where.

Attached Files



#22 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 22716
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 13 December 2012 - 10:41 AM

When 8 truss tubes are used with pole pairs parallel, the weight of the UTA is basically borne by the top two poles on the side. The bottom poles have little tension and they mostly sag.
The top and bottom pairs have almost no tension at all and only prevent the UTA from rotating around the attachment points of the side poles.
When the poles are non-parallel (yes, looks like a trapezoid from the side), now the upper and lower poles carry some of the weight from the UTA because the downward force vector now has some horizontal direction as well as vertical, applying tension and compression to the upper and lower pole pairs.
Having the weight of the UTA at least partially borne by the upper and lower pairs of poles results in less sag of the UTA, greater collimational stability, and less "wiggle" in a breeze.

It's no accident that professional observatory telescopes with truss structures (whether Serrurier or not) are widest at the top, or bottom, or middle but do not have pairs of poles parallel.
Here is an example of a commercial truss scope that takes non-parallelism of opposing truss pairs seriously:
Heavy Truss scope

#23 Dick Jacobson

Dick Jacobson

    Viking 1

  • -----
  • Posts: 898
  • Joined: 22 Dec 2006
  • Loc: Plymouth, Minnesota, USA

Posted 13 December 2012 - 01:47 PM

When 8 truss tubes are used with pole pairs parallel, the weight of the UTA is basically borne by the top two poles on the side. The bottom poles have little tension and they mostly sag.

In an 8-pole truss, when horizontal, the two upper side poles are purely in tension and the two lower side poles are purely in compression. This is a classical truss structure where all the members are either in tension or compression, with no bending force on any of them. The top and bottom pairs, as you say, do not bear any weight but merely prevent the UTA from rotating or moving sideways.

The beautiful Officina Stellare telescope that you link to has exactly the octagonal truss structure that I mentioned above. The Officina Stellare scope is a double truss (balance point at the middle, I assume) and my 20-inch was a single truss with a wooden mirror box, but the geometry is the same.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics