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stupid question - bushing on bolts connecting OTA to dob base

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#1 quercuslobata

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Posted 06 December 2019 - 08:06 AM

Is there a reason why the bushings around the bolts that connect the OTA to a Dobsonian base are loose, and what do folks do to make sure they don't get lost when moving stuff around?

 

This is probably one of those things that is obvious to everybody but me... After many cloudy nights I went to set up the scope and had the bolts and bushing loose in the base and of course, on of the bushings fell out and rolled away. I found it, but only after searching the flowerbeds with a flashlight for an hour (not the nighttime exploration I had in mind).

 

One solution I could think of would be to glue the bushing in the hole, but maybe this is a bad idea? Would it be better to store both the bolts and bushing on the OTA? The third thing that I can think of is to secure the bolt and bushing to the base with a nut when not in use, but I'm not sure that type of threads to use.

 

What do folks do to keep from staring at the ground instead of the sky?

 

Thanks.



#2 TOMDEY

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Posted 06 December 2019 - 10:47 AM

Make and model... is it your Orion SkyQuest XT8" F6 Dobsonian that you have listed there? Many/most Dobsonians don't suffer that flawed design/build problem. I guess I would epoxy them in. For some reason, many mass-produced commercial concoctions have terrible adjusters and fasteners; user-friendly not considered... as if the designer never actually uses his creations.   Tom



#3 quercuslobata

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Posted 06 December 2019 - 11:06 AM

Make and model... is it your Orion SkyQuest XT8" F6 Dobsonian that you have listed there? Many/most Dobsonians don't suffer that flawed design/build problem. I guess I would epoxy them in. For some reason, many mass-produced commercial concoctions have terrible adjusters and fasteners; user-friendly not considered... as if the designer never actually uses his creations.   Tom

Yes, the Orion SkyQuest XT8. Actually the plus model, but doubt that matters for this. The manual did advise me not to loose them, so there's that, but I can't figure out why it would be designed in such a way.

 

So, on many Dob bases these bushings are permanently fixed to the wooden base?

 

Epoxy was my first thought, but thought I'd ask before I did anything I would later regret.

 

Thanks



#4 TOMDEY

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Posted 06 December 2019 - 12:01 PM

On the minus side, once you glue something in, may be near-impossible to get out --- but, honestly... the liklihood of later wanting to extract something is (statistically, realistically) quite remote. Things usually wind up in the garage corner or town dump long before that happens.    Tom



#5 SteveG

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Posted 06 December 2019 - 02:00 PM

Is there a reason why the bushings around the bolts that connect the OTA to a Dobsonian base are loose, and what do folks do to make sure they don't get lost when moving stuff around?

 

This is probably one of those things that is obvious to everybody but me... After many cloudy nights I went to set up the scope and had the bolts and bushing loose in the base and of course, on of the bushings fell out and rolled away. I found it, but only after searching the flowerbeds with a flashlight for an hour (not the nighttime exploration I had in mind).

 

One solution I could think of would be to glue the bushing in the hole, but maybe this is a bad idea? Would it be better to store both the bolts and bushing on the OTA? The third thing that I can think of is to secure the bolt and bushing to the base with a nut when not in use, but I'm not sure that type of threads to use.

 

What do folks do to keep from staring at the ground instead of the sky?

 

Thanks.

Are you talking about the center bolt in the dob base? I don’t know of a bolt that holds the tube into the base. Most tubes just rest in their altitude bearings, via gravity.



#6 vtornado

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Posted 06 December 2019 - 03:08 PM

You might try silicone adhesive,  enough to keep the bushings in, but not forever strong, if you ever had

to take them out for any reason, I could see if something happened to your mount like warping due

to humidiyy, you might want to replace the rocker box. 

 

Steve G, he has the plus version a completely different mount for the OTA than the classic.

The atlitude bolts can be tightened to force the sides of the  the rocker box against the hubs on the ota to add the

right amount of friction

 

https://www.amazon.c...739927098&psc=1


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#7 Roger Corbett

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Posted 06 December 2019 - 05:41 PM

If you epoxy it in, won't that later be a problem for adding DSCs?

 

In assembling the 8XT Plus, one uses an Allen wrench and makes the center bolt tight, but not overly tight.  If the azimuth spinning while observing becomes too easy, tighten the bolt again.



#8 quercuslobata

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Posted 06 December 2019 - 09:33 PM

Probably should have started with a picture. The bushing (B) goes in the hole (A) and the bolt © goes through that and screws into the OTA. It adjusts the tension on moving the ota up and down. The other weird thing is that the bushing on the other side is twice as long, but I’m not sure why. 
 

I think I might try the less permanent adhesive route.

 

 

 

 

IMG_0717.JPG



#9 Ulmer Spatz

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Posted 07 December 2019 - 05:55 AM

The other weird thing is that the bushing on the other side is twice as long, but I’m not sure why. 

The two side bearings ("disks") attached to the optical tube are not identical. On these mounts with a tensioning feature, often only one side does the tensioning. The side bearing on that side usually has some compression material attached to it. The other, non-tensioning side lacks that material and gets the slightly longer bushing to compensate. It's a bit strange to have one bushing twice as long as the other. Are you sure you have the right-size bushing in there?

 

Not sure it's wise to fix the bushings. They might need to move in their holes to accommodate manufacturing tolerances. Also, the longer bushing is slightly longer than the mount's side plate is thick, so one end sticks out. Fixing that bushing could make assembly or disassembly difficult if not impossible.

 

Keep in mind that these two knobs do not hold up the optical tube, so a bit of play is not a problem and might even be needed. The weight-bearing connection is made with the disk-shaped side bearings attached to the optical tube resting on four bearing cylinders.


Edited by Ulmer Spatz, 07 December 2019 - 07:01 AM.


#10 quercuslobata

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Posted 07 December 2019 - 07:08 AM

The two side bearings ("disks") attached to the optical tube are not identical. On these mounts with a tensioning feature, often only one side does the tensioning. The side bearing on that side usually has some compression material attached to it. The other, non-tensioning side lacks that material and gets the slightly longer bushing to compensate. It's a bit strange to have one bushing twice as long as the other. Are you sure you have the right-size bushing in there?

 

Not sure it's wise to fix the bushings. They might need to move in their holes to accommodate manufacturing tolerances. Also, the longer bushing is slightly longer than the mount's side plate is thick, so one end sticks out. Fixing that bushing could make assembly or disassembly difficult if not impossible.

I guess twice as long was a rough approximation. The short one is the width of the wood on the rocker box, the other one is about 1/4 inch longer. I think I have the shorter one on the correct side with the friction bolt that does the tensioning. I had not considered that the side bearings on the ota were different, and that the longer bushing probably fits into the plate, so as you say glueing will prevent the ota from going in properly.

 

Does anyone know if there are special threads on these bolts? Putting on a nut temporarily seems like the safest way to keep the bolt and bushing from slipping off during transport, but maybe screwing both to the ota would work too if it doesn’t stick out too much?

 

thanks


Edited by quercuslobata, 07 December 2019 - 07:09 AM.


#11 Ulmer Spatz

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Posted 07 December 2019 - 08:07 AM

That 1/4" longer for the right-hand side bushing sounds correct. The threads are bound to be standard metric. Hardware stores have thread gauges which will tell you the exact thread size and pitch. I wouldn't keep the tensioning bolts on the optical tube during transport--they stick out too much. Any kind of unintentional force on their ends could bend them or even damage the centers of the optical tube's side bearings.

 

For retention, I've bent some paper clips into retention clips. If the bend is tight enough, it'll settle into the threads of the bolt and the clip can't slide off. Punching a slightly undersize hole into a small piece of cardboard or plastic will also work. It's good for many more push-ons and pull-offs than you might think. Looks ugly, though. A nut will work well, of course. I'd get two white nylon nuts so you can find them in the dark, and they're more pleasant to the touch on a cold night. :-)

 

cli.jpg


Edited by Ulmer Spatz, 07 December 2019 - 09:36 AM.

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#12 vtornado

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Posted 07 December 2019 - 09:17 AM

I am building my own dob mount for a skywatcher 300p dob with the same altitude system.

The threaded holes in the hub  are M10 on mine,  So good news you can get replacement bolts, and nuts.

I got my bolts from Menards

 

My tube has friction material on both hubs.  I don't know why one bushing would be longer than the other.

Some xt8's are xt8i's, with the intellescope feature, could the longer bushing be to keep the ota away

from the intellescope electronics, Which is a small printed circuit board on the inside of the rocker box?

 

I think the bolt idea is a great one.  If the scope bolts/knobs stick out too far you can buy a short stubby bolt and a nut.


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