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Beefing up 13.1" cell

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#1 TayM57

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Posted 01 January 2022 - 03:24 PM

I have some shift on my 13.1" cell. Some of it is the springs not being strong enough, which I'll fix pretty soon by putting in stiffer springs, perhaps with extra springs on the sides of the bolts like in my 10". Steve did tell me that he was fighting shift on this cell over the summer. He planned to beef it up but never had the opportunity to do so. I pulled the mirror all the way in using the collimination bolts, and that got rid of some of the shift. So stiffer springs are indeed called for.

 

However, some of the shift comes from the way the cell is mounted, with the long bolts (3) holding the cell in place. When I tap the bolts, I can see the shift on the tublug thats facing me. One idea I had to stop the lateral movement of the bolts was installing large fender washers on the inside tube with large OD nuts. However, getting the fender washers to sit flush for optimal stability is a challenge.

 

What are some other ideas or thoughts that everyone here has, to cut down on the lateral movement of the mounting bolts? This is a Swayze designed cell, so it's a good cell. I'd rather keep this cell in, and not replace it with one similar to my 10". I do plan to mount a fan on this cell, once I get the lateral movement problem addressed.

 

Thoughts? Ideas?

 

 

 

IMG-20220101-102111.jpg


Edited by TayM57, 01 January 2022 - 03:26 PM.


#2 Bob4BVM

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Posted 01 January 2022 - 03:33 PM

replace the 3 springy all-threads with some flat steel bars (1x1/4") that bolt to the red ring and attach to the tube?    Like this roughly, but trim bar ends to fit tube ID and add 90* tabs at bar ends for bolts thru tube...

 

cell mnt.png


Edited by Bob4BVM, 01 January 2022 - 03:41 PM.


#3 Beeham

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Posted 01 January 2022 - 03:52 PM

I think instead of three separate elements (threaded rod) hooking the red ring to the outer tube, I'd try to do it with a single element.  I'd try 1" steel tube, welded into a "Y" shape.  The Y-shape could bolt to the red ring with simple bolts, and then you could put press-in female threaded inserts into the square tubes to allow for bolting to the outer tube.  PM me if this doesn't make sense and I'll send you a sketch.

 

Good luck!



#4 Oberon

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Posted 01 January 2022 - 04:30 PM

Fabricate an equilateral triangle from steel RHS tube with mounting points to suit existing telescope tube penetrations (currently used for threaded rod). Mount the old cell on top complete, or better still, discard the lower part of existing cell and utilise the triangle instead. The 2nd option has benefit of easier fabrication to maintain focal plane.



#5 Lee D

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Posted 01 January 2022 - 06:44 PM

Fabricate an equilateral triangle from steel RHS tube with mounting points to suit existing telescope tube penetrations (currently used for threaded rod). Mount the old cell on top complete, or better still, discard the lower part of existing cell and utilise the triangle instead. The 2nd option has benefit of easier fabrication to maintain focal plane.

Thinking of connecting square tubing of the cell to the OTA, I see at least one seller of star-type tube inserts that I associate with round truss tubes claims the inserts work with square tube as well. Might be a simple solution if 1" square tube works.



#6 MitchAlsup

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Posted 01 January 2022 - 07:29 PM

My 13" F/3 cell

 

assembly36.JPG

 

a) stiffer

b) more airflow

c) lighter

d) more support from 6-point cell



#7 TayM57

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Posted 02 January 2022 - 01:56 AM

Lots of good ideas here. Fabricating a steel Y shaped support to replace the lower part of the cell did cross my mind. It would be replacing Swayze's design, which I would only do as a last resort.

 

One idea I had was stacking belleville washers against the inner wall surface of the tube to provide extra tension without pulling the tube out of round. That, I think, could help reduce, or hopefully stop, the sag of the mirror when it comes back to zenith, or the forward sag when the tube is pointed towards the horizon. It's possible that a series of bellevue washers, or a parallel series of belleville washers will stop that sag. I think it's worth giving that a try before I have to modify the cell.

 

See attached. I think the series-parallel will work best. Probably 8 belleville washers in series-parallel for each bolt is what I will try. I want to try my best to keep the original Swayze cell design.

 

3-s2-0-B9780124713703500061-f05-40-97801



#8 Bob4BVM

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Posted 02 January 2022 - 04:28 PM

Lots of good ideas here. Fabricating a steel Y shaped support to replace the lower part of the cell did cross my mind. It would be replacing Swayze's design, which I would only do as a last resort.

 

One idea I had was stacking belleville washers against the inner wall surface of the tube to provide extra tension without pulling the tube out of round. That, I think, could help reduce, or hopefully stop, the sag of the mirror when it comes back to zenith, or the forward sag when the tube is pointed towards the horizon. It's possible that a series of bellevue washers, or a parallel series of belleville washers will stop that sag. I think it's worth giving that a try before I have to modify the cell.

 

See attached. I think the series-parallel will work best. Probably 8 belleville washers in series-parallel for each bolt is what I will try. I want to try my best to keep the original Swayze cell design.

 

3-s2-0-B9780124713703500061-f05-40-97801

Still pulling on tube...

I'd just hang the original cell on steel bars as in post#2. Inherent stiffness with no chance of deforming the tube. Would actually act as reinforcement for the lower tube.



#9 TayM57

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Posted 02 January 2022 - 04:49 PM

Still pulling on tube...

I'd just hang the original cell on steel bars as in post#2. Inherent stiffness with no chance of deforming the tube. Would actually act as reinforcement for the lower tube.

It would actually be pushing against the inner surface of the tube, not pulling on it. See pic.

 

1.png

 

Only by loosening the nut on the outer rim of the cell, and tightening the nut on the inner rim of the cell can you pull on the tube. I tried that, but could saw that I could start pulling on the tube if I overtightened.

 

Adding the belleville washers in the locations shown in the picture, would create tension without pulling on the tube, and add surface area for the bolts to grab against the inner surface of the sono, to cut down on the lateral movement.

 

Don't know if it will work but it's worth a try. I'll post later this week, if it does.


Edited by TayM57, 02 January 2022 - 04:50 PM.


#10 Lakec

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Posted 03 January 2022 - 08:35 AM

If you want to keep that design your better off with a diagonal brace giving an alternate tube support to the rear. Perhaps even with a flat stock running along the tube to reinforce both points. It would be almost trivial to 3d print a bracket that goes between the red ring and outer nut and sweeps to the rear of the tube to create a triangle, and fairly easy out of a flat steel piece.



#11 TayM57

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Posted 14 January 2022 - 11:24 PM

Well I got the belleville washers from McMaster-Carr ordered, and got them in today. I took the 13.1" mirror out, cleaned it, and then installed the belleville washers on the 5/16" threaded rods in series order along with a 5/16" x 1 1/2" fender washer for the belleville washers to sit flush against the sonotube. The fender washers also provide a large contact area against the sonotube so they don't dig into the sono as much as a smaller one would. Tightened everything up, especially the belleville washers. They are pretty tightly torqued against the sonotube.

 

And it holds collimation now! No drift on the tublug when moving the scope from zenith to horizon and back.

 

I'm very pleased, as I can keep the Swayze cell design which was the goal of this project. Now, I can disassemble everything, get the mirror coated by Jeff Decker, and work on re-doing the paint on the sonotube.

 

 

IMG-20220114-195758.jpg


Edited by TayM57, 15 January 2022 - 12:56 AM.

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#12 psionik

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Posted 15 January 2022 - 10:45 AM

I think that since you're using small structures (the bolts) to hold the ring, some instability is baked-in.  You can try to improve the stability by increasing tension, but the tube strength will probably be the limiting factor.

 

The problem is that you have no "triangle" or tetrahedron to secure the 3-D location in space.  (Well... you do actually but it's very small and located inside the bolt itself)

 

You need to spread apart the "feet" of the structural supports.   I might suggest cutting a triangular piece of wood or metal to go between the ring and the tube, and mount the wide part of the triangle against the tube.  Do this for all 3 supports and you'll be in business.

 

Attached Thumbnails

  • 9156358937834785345.jpg

Edited by psionik, 15 January 2022 - 10:47 AM.

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#13 Stardust Dave

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Posted 15 January 2022 - 06:15 PM

I do understand the spring washers , but what if

 

If the "ring" of the cell were aluminum ( looks to be wood) the threaded bolts would make the assembly very stiff.

IOW the complete cell, mirror & threaded rod assembly stiff enough itself acts as a tube brace. 

 

The triangle would be the three points inside the tube where the cell contacts the wall of tube.

My cell using 5/16" rod and fender washer .  No other mod needed IMHO     



#14 Pinbout

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Posted 15 January 2022 - 06:55 PM

this is how I'd redo your cell. not beef it up, but remake it.

 

with two plates top or inside plate has the support points [6]

 

the bottom or back plate which supports the springs for the collimation bolts and attaches to the tube.

 

i'd take the ends of the bottom plate all the way out. I glue tnuts into the edge of the 3/4 ply.

 

https://www.youtube....hqaMXFT&index=8



#15 Pinbout

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Posted 15 January 2022 - 06:56 PM

here's a 6 point cell... and if you don't like the tnuts in the end of the ply do this

 

https://www.youtube....qaMXFT&index=11



#16 Waynosworld

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Posted 15 January 2022 - 07:26 PM

If I understand correctly you want to keep everything the way it is but stiffen the tube up, I would make a metal ring that fits the inside of the OTA snugly, how thick the ring should be is the question, the thicker the better.

This would put all the forces on the ring rather than the OTA, but it will change the balance depending on how heavy the ring is, but it will make no new holes and it will not distort the OTA.

 

You could also put tangs on the ring to hold a fan in position.

 

What you have done in post 11 will likely work but it also will likely make a circle the size of the washers in the OTA.


Edited by Waynosworld, 15 January 2022 - 07:27 PM.


#17 Pinbout

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Posted 15 January 2022 - 11:41 PM

I’d just rebuild the rear plate at the least, to get rid of the long bolts that was a poor design to begin with from the dark ages…


Edited by Pinbout, 15 January 2022 - 11:41 PM.


#18 TayM57

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Posted 16 January 2022 - 01:31 AM

I’d just rebuild the rear plate at the least, to get rid of the long bolts that was a poor design to begin with from the dark ages…

Which is exactly what I may do, if it proves to be a PITA to clean the mirror when it needs it. The long term goal is to build a truss for the 13.1" but that's after I finish the 17.5". So for now, the 13.1" will stay in the coulter tube, so, rebuilding the rear plate may be what I'll end up doing in the short term.


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#19 Pinbout

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Posted 16 January 2022 - 11:49 AM

....rebuilding the rear plate may be what I'll end up doing in the short term.

pretty much all jigsaw work... a couple drilled holes. easy peasy. but i always draw it in cad and print 1:1 and spray mount it to the plywood

 

similar to how i'm welding up this cell frame. I used this drawing/wood for the moving plate of the same cell... a 2 for 1.

 

https://www.youtube....qaMXFT&index=12




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