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Size of bearing for dob

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#1 pete pumpkin

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Posted 17 February 2019 - 09:02 AM

Hi, I'm looking at rebuilding the base for my saxon (skywatcher) solid tube 12" dob (36cm OTA). What size alt bearing would you recommend and what degree of spacing would you use for the teflon pads. Also how would aluminium against teflon perform. Thank, Pete



#2 Myk Rian

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Posted 17 February 2019 - 09:43 AM

Well, my 13.1" Coulter has a bearing about 6" diameter, as does my 8" Orion. Aluminum may work, but wear to the bearing material might be a problem. 90 - 100 deg is fine.

Myk

#3 pete pumpkin

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Posted 17 February 2019 - 10:04 AM

Thanks Myk. The original plastic bearing on my 12"dob is also around 6 inches. As is the bearing on my 8" collapsible skywatcher also 6 inch. From what I've heard is that these aren't ideal and made to a price - which seems obvious seeing as the 12 and 8" have the same size bearing and rely on the tension/friction mechanism to work. I've been told that I should make them larger 8-10inch and have the teflon spaced out a lot further ie up to 160 degrees. I've seen info regarding large truss style dob designs quoting bearing sizes same as mirror size if not larger and teflon spacing of 70 degrees - but I can't really go by this as the dynamics are different.  I know it is a bit of an art form, and unless I have a good understanding of physics (which I don't) there is no other way to get it right besides trial and error or looking at what's been done before. Really I'm just fishing for opinions. Thanks


Edited by pete pumpkin, 17 February 2019 - 10:07 AM.

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#4 Arjan

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Posted 17 February 2019 - 10:33 AM

Hi, I'm looking at rebuilding the base for my saxon (skywatcher) solid tube 12" dob (36cm OTA). What size alt bearing would you recommend and what degree of spacing would you use for the teflon pads. Also how would aluminium against teflon perform. Thank, Pete

If you don't mind doing some calculations: Dobson Slide Bearings.pdf



#5 pete pumpkin

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Posted 17 February 2019 - 10:57 AM

Wow, looks like useful info a little over my head though. I guess I could find the friction coefficient of teflon and aluminium for a start. The main thing I took away from that was that the further spaced the teflon pads are, the more force required. I'm unsure if I could use the same formulas to work out the correct bearing proportions as the article looks to be more catered to truss style setups ie bearing at the bottom of the unit as apposed to in the middle of a solid tube - my math skills are bad..



#6 stargazer193857

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Posted 17 February 2019 - 11:14 AM

Hi, I'm looking at rebuilding the base for my saxon (skywatcher) solid tube 12" dob (36cm OTA). What size alt bearing would you recommend and what degree of spacing would you use for the teflon pads. Also how would aluminium against teflon perform. Thank, Pete


Most experts here will tell you to make bearings the size of your aperture and pad spacings 90 degrees apart. They applaud explore scientific for this reason. They says the random lumps on Formica can be felt with smaller bearings, and that pinches can happen when bearings are spaced apart further.




...

I respectfully disagree, on sizes under 12" and maybe even 12". though I've not tested my idea yet.

Big bearings work well on scopes whose beatings attach at the mirror plate along with the bottom of the one section truss. They have to be big to move the center of rotation up to the center of mass.

The course Formica has been replaced with some fine stuff called crystal.

The rear of the sides of the base, where the rear alt pad is, lacks lateral support. Using large bearings places lateral load on this area. All bearings keep the load tangent to the side, the direction the supports are strongest.

There are ways to avoid pinch. Don't make the pad anhles extremely steep, use delrin instead of Teflon, curve the surface, make the pads wider. With my light weight OTA, pinch won't be an issue.



...

We agree that alt friction should be equal to the typical az friction.

Edited by stargazer193857, 17 February 2019 - 11:25 AM.


#7 stargazer193857

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Posted 17 February 2019 - 11:33 AM

Thanks Myk. The original plastic bearing on my 12"dob is also around 6 inches. As is the bearing on my 8" collapsible skywatcher also 6 inch. From what I've heard is that these aren't ideal and made to a price - which seems obvious seeing as the 12 and 8" have the same size bearing and rely on the tension/friction mechanism to work. I've been told that I should make them larger 8-10inch and have the teflon spaced out a lot further ie up to 160 degrees. I've seen info regarding large truss style dob designs quoting bearing sizes same as mirror size if not larger and teflon spacing of 70 degrees - but I can't really go by this as the dynamics are different. I know it is a bit of an art form, and unless I have a good understanding of physics (which I don't) there is no other way to get it right besides trial and error or looking at what's been done before. Really I'm just fishing for opinions. Thanks



Good job realizing you can't compare one style dob to another.

As for the physics and art, it is just sines and cosines to calculate the tangent and normal forces. I can do the math fo you if you tell me you base pad diameter, ota weight, total weight minus base plate, and bearing sizes.

I plan to buy 4" plywood circles stacked 1"-1.5". I will put my pads about 120-140 deg, once I know the final weights. My base oad diameter will be 17" on my 11" f4.9. I will try to match the 45 deg az force.

#8 pete pumpkin

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Posted 17 February 2019 - 11:34 AM

Those explore scientific dobs do look nice. I've looked at the dob page on this forum, so many photos to compare. I have a feeling it's going to be a stab in the dark at the end of the day when it comes to dimensions. Lots of variables. I'll try and work out the spacing of the pads on the fly. As for the size of the trunnions I guess a bit bigger than the stock is better seeing as I won't be using the original 'friction' mechanism and this in combination with pad spacing should allow for more adjustment.


Edited by pete pumpkin, 17 February 2019 - 11:35 AM.


#9 Pinbout

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Posted 17 February 2019 - 12:54 PM

I start at 1.8x mirror dia 

 

12.5x1.8=22.5” dia 



#10 stargazer193857

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Posted 20 February 2019 - 01:34 PM

I start at 1.8x mirror dia

12.5x1.8=22.5” dia


People who try to save weight or height by using bigger bearings attached lower will benefit from the 1.8x. But at that point we are looking at center of gravity not just friction. Good luck fabricating those smoothly and cheaply. I can order 4" anywhere.

#11 Pinbout

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Posted 20 February 2019 - 06:42 PM

People who try to save weight or height by using bigger bearings attached lower will benefit from the 1.8x. But at that point we are looking at center of gravity not just friction. Good luck fabricating those smoothly and cheaply. I can order 4" anywhere.

Wow it’s not even your thread and you sarcasm is astoundingly clear  tongue2.gif


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#12 MitchAlsup

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Posted 20 February 2019 - 09:29 PM

People who try to save weight or height by using bigger bearings attached lower will benefit from the 1.8x. But at that point we are looking at center of gravity not just friction. Good luck fabricating those smoothly and cheaply. 

There is thing called a router. You put a 1/8" 3/32" or 1/4" carbide bit in the router.

Next you take 2 pieces of all-thread and 4 nuts, and a block of wood drilled at the same width as the holes in the router.

Adjust jig so the router will route the intended circle (or arc)--that is the proper radius.

Next, drill a hole in the wood to be cut so that a post on the wood block with the all-thread attachment can hold itself in the center of the circle.

Make light passes--the first about 1/16" deep--this will give a nice clean edge. Then increase at 1/8" per adjustment until the circle is 1/2 way through.

Flip the wood over find the hole again, and repeat routes.

 

Done correctly, this will give you a circle (or arc) within a few thousandths of being perfectly round.


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#13 stargazer193857

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Posted 20 February 2019 - 09:59 PM

There is thing called a router. You put a 1/8" 3/32" or 1/4" carbide bit in the router.

Next you take 2 pieces of all-thread and 4 nuts, and a block of wood drilled at the same width as the holes in the router.

Adjust jig so the router will route the intended circle (or arc)--that is the proper radius.

Next, drill a hole in the wood to be cut so that a post on the wood block with the all-thread attachment can hold itself in the center of the circle.

Make light passes--the first about 1/16" deep--this will give a nice clean edge. Then increase at 1/8" per adjustment until the circle is 1/2 way through.

Flip the wood over find the hole again, and repeat routes.

 

Done correctly, this will give you a circle (or arc) within a few thousandths of being perfectly round.

Thanks. I thought that was out of my fabrication ability. Now it sounds doable.

 

I read that dobs that have clutches to tighten their small bearings have stiction issues. So the big bearings may have a point.



#14 stargazer193857

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Posted 20 February 2019 - 10:07 PM

Wow it’s not even your thread and you sarcasm is astoundingly clear  tongue2.gif

?????

I read my post again and still have no clue what that means.

 

Do you think a jigsaw on a pivoting arm and sled would do nearly as well as a router? I'm guessing the router won't splinter.



#15 TxStars

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Posted 20 February 2019 - 10:51 PM

I can do up to 48" circles on my cnc mill without moving anything in the work room.

If I move one of my benches further away I could go up to 62" and still maintain 0.001"

Perhaps Pinbout will cnc cut the rings?



#16 pete pumpkin

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Posted 21 February 2019 - 12:21 AM

I'll be using a jig i made up for my jigsaw for the circles. Will then use a length of thin aluminium (~2mm thick) flat bar wrapped around the circumference which will ride on 4mm thick teflon pads (waiting on this to arrive). I know laminate is the ideal thing to use but I don't know where to get hold of the good stuff and the aluminium strip is readily available. Anyhow still undecided on size of circles, pads and placement. I'll probably gently nail the pads in place and see how they work until I find the ideal location - then I'll screw into place.


Edited by pete pumpkin, 21 February 2019 - 12:22 AM.


#17 TxStars

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Posted 21 February 2019 - 01:11 AM

Make sure that you countersink the nail or screw heads.



#18 pete pumpkin

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Posted 21 February 2019 - 01:19 AM

Make sure that you countersink the nail or screw heads.

Thanks, yeah I'll do that. countersink + small hole for nail, then larger whole for screw once I work out positioning. I hope the 4mm will be thick enough. Just waiting on parts and final decision on size of bearing. I'll probably go 250-300mm or there abouts.



#19 pete pumpkin

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Posted 21 February 2019 - 01:27 AM

I'm rebuilding the base because the original one had started to swell from moisture and was sticking - particle board. I though instead of just copying it, I may be able to improve on the design and rebuild using a good plywood - I bought some  17mm 'formply' which is a described as 'manufactured using a high density (HDO) of phenolic resin impregnated paper bonded to an exterior grade plywood core. The overlay provides a smooth surface suitable for applications such as concrete formwork as well as decorative purposed'. It's black in color so all I'll have to do is seal and paint the edges.

 

 



#20 Oberon

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Posted 21 February 2019 - 03:18 AM

?????

I read my post again and still have no clue what that means.

 

Do you think a jigsaw on a pivoting arm and sled would do nearly as well as a router? I'm guessing the router won't splinter.

A jig saw is a crude low speed recipricating blunt floppy tool compared to a router. That said, because the next step is sanding followed by laminating a bearing surface, then it will do. You just need to do lots more sanding, and that involves spinning the job against a fixed place machine sander. 



#21 a__l

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Posted 21 February 2019 - 04:51 AM

If you have open arcs of large diameter and you plan to install the encoder. Think about how you will accurately and rigidly position it. Encoder on long rod is not the best option for your GoTo accuracy.



#22 Arjan

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Posted 21 February 2019 - 07:19 AM

Pete,

 

Indeed a jigsaw blade will go anywhere when you cut a curve. You have to move it slowly, and go up and down several times. The blade has a tendency to bend outward.

 

I think that Aluminum on PTFE is not an ideal combination in general, CoF is 0.19 (sliding) to 0.24 (static), but I think there is variability.

The published CoF on steel usually assumes the steel is polished.

IMHO any hard plastic or formica will give better results. You should either choose a variant with small scale structure (bumps) or make the contact area small.

 

Good luck,

Arjan



#23 Jon Isaacs

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Posted 21 February 2019 - 07:22 AM

Hi, I'm looking at rebuilding the base for my saxon (skywatcher) solid tube 12" dob (36cm OTA). What size alt bearing would you recommend and what degree of spacing would you use for the teflon pads. Also how would aluminium against teflon perform. Thank, Pete

 

Pete:

 

My J-Star 12.5 inch uses Teflon against Aluminum for the altitude bearings.  It works reasonably well.  With all my Dobs I use bar soap to lube the bearings but I found the Aluminum-Teflon worked as well as anything without lube too.  

 

5102862-Jstar Balance Cloudy Nights.jpg

 

Something to consider:

 

You could attach the new bearings to the old bearings by using a hole saw to drill out the center of the new bearing and just slipping it on and then clamping it over the old bearing. 

 

Jon


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#24 Brianfromsyr

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Posted 21 February 2019 - 01:14 PM

I thought New Moon telescope bearings were cast aluminum and they ride on teflon type material.

#25 Pinbout

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Posted 21 February 2019 - 02:18 PM

a lot are doing textured powder coating


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