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Equatorial Platform troubles

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#1 tag1260

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Posted 25 April 2013 - 05:54 PM

HELP!!!!! I just built my equatorial platform. It's a VNS type.

Why will it only work with the scope facing North or South but if I turn it East or West, the shaft just spins and it doesn't move?

#2 Almagne

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Posted 27 April 2013 - 11:31 AM

Pictures?

#3 tag1260

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Posted 29 April 2013 - 05:01 PM

OK, an update here. I balanced my scope and now it works when pointed West but still doesn't work pointed East. I can add weight to the base and get it to work but then it doesn't work again pointed West. I'm not real sure where to go next but I'm very close to giving up on this thing.

#4 Ed Jones

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Posted 29 April 2013 - 09:10 PM

Pictures?



#5 tag1260

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Posted 29 April 2013 - 09:47 PM

Here's a pic. It's rough, but I am hoping to take it a part and do some trimming and finishing if I can get it to work.The six pounds of weight on the NW corner makes it work in both directions. I just can't figure out what is going on with it.

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#6 John Carruthers

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Posted 30 April 2013 - 03:54 AM

not enough friction on the drive roller?

#7 Ed Jones

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Posted 30 April 2013 - 07:06 AM

Can you show your calculation of the COG? If that's off it would only get work on one side. I use 2 motors on my platforms so you can put any scope on it. With one motor you need to get the COG right.

#8 Nils Olof Carlin

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Posted 30 April 2013 - 08:48 AM

One simple thing to try is move the telescope ground board as far "north" as possible and see what happens. If it is top heavy in the picture (it looks pssible), this would lower the COG relative to the polar axis. You may get a better idea of what could be done.
Apart from that, I get a feeling that the N supports are too close for comfort - ideally they should be as far apart as the support points for the telescope (groundboard feet if used, platform teflon patches if not), to minimise flexure.

Nils Olof

#9 Bill Kocken

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Posted 30 April 2013 - 08:51 AM

With the scope tilted left like that the COG is much more on the left support and probably not enough over the motor. So it slips. Adding the weight on the right hand corner compensates. Even if the COG is on the virtual axis, to minimize the motors workload, there isn't enough weight over the motor for friction.
I see that the scope's feet are wider than the north bearings. Another solution would be to build a wider platform, but that is pretty much like starting over. :(

#10 tag1260

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Posted 30 April 2013 - 12:00 PM

Hi Ed,
I can't find the spread sheet that I used but my OTA is 50 Lb And my Rocker box is 26 Lb. My bearing center is 25.5" when in the box. My balance point of my box is 8". The spread sheet said my balance point would be 18". I'm not sure if I measured something wrong but I just re-measured and they all come up the same.

Thanks

#11 tag1260

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Posted 30 April 2013 - 12:04 PM

Bill
The feet aren't wider than the NB's. The bearings are 24" and the feet are 19" wide. Are you meaning the NB support bearing and drive?
Thanks

#12 tag1260

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Posted 30 April 2013 - 12:06 PM

Nils
I centered them on the North Bearing segments. That puts the "about 12" wide where the scope support feet are 19' wide.
I have moved the whole thing as far North as possible but it was no help.

Thanks

#13 Ed Jones

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Posted 30 April 2013 - 12:43 PM

Assuming the top of your platform weight roughly 10 lbs. the I get a COG of only 12.75 inches. That's a bit different.

#14 tag1260

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Posted 30 April 2013 - 01:00 PM

Darn it ED!!! I didn't figure that weight into the equation!!!!I figure the biggest change will be my bearing radius' so starting over isn't out of the question.

Your CG can be only about half of your total height to the bearing center of the OTA?

I Thank You for finding this out for me. Maybe now I'll get this thing to work.
Is your support bearing placement critical?


Thanks again
Tag

#15 Ed Jones

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Posted 30 April 2013 - 02:31 PM

Your CG can be only about half of your total height to the bearing center of the OTA?


It all depends on weights and distances and you must include everything that moves. Did you read my web site?
The motor shafts should go under the rocker board Teflon pads and the motor shafts need to be over the ground board pads (as close as possible) for greatest stability.

#16 tag1260

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Posted 06 May 2013 - 05:53 PM

Changed out the North and South bearings today but still no joy. I've got the scope tail heavy but it still won't travel when pointed East. I'm going to try shimming my drive shaft bearings up a little to see what will happen if I put a little more pressure on them. I just don't know!!!!

#17 Ed Jones

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Posted 06 May 2013 - 10:02 PM

Did you move your motor and idler bearing farther out? They were much too close.

#18 tag1260

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Posted 07 May 2013 - 09:27 AM

Yes Ed I tried that. What I am trying right now is some really fine emery cloth glued to the North bearings to give some traction. This "SEEMS" to be working. I just don't know how this is going to work with a reset. I don't think I will sand flats in my shaft but who knows. If I do, I'll just turn another shaft.

I don't want to knurl the shaft as that will just dig into the wooden bearings if it gets stopped.
Thanks

#19 tag1260

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Posted 12 May 2013 - 02:32 PM

Well, I had it out last night. Seemed to work fine. I do need to keep an eye on the shaft to see if I end up sanding flats on the drive shaft but it is working finally. My shaft and and wooden bearings must just be too smooth and hard to grab. If anyone has any other ideas instead of the emery on the bearing runner, let me know.
Thanks again for all the help.

#20 tag1260

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Posted 04 June 2013 - 10:40 AM

Finally got to try my platform again last night. Nothing but troubles. Now I find that something must be wrong about my calculations because it just doesn't track properly. I have built many projects over the years and have never had this many problems. I GIVE UP on this thing. I may pick back up on it at a later date but my time and frustrations just have not been worth it.

#21 Ed Jones

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Posted 04 June 2013 - 08:00 PM

tag,
Sometimes failure just happens but you can learn from it too. The things I see wrong are the COG was wrong especially for one motor. The motor and roller should be directly under the ground board feet, you have them too far in. I didn't see a pic of the south bearing if any issues are there. I would advise anyone to make curved bearing on a table saw like I do here. It's easier than trying to make VNS sections.






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