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What's the advantage of belt drive?

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#1 Ralph Paonessa

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Posted 10 December 2014 - 05:23 PM

I've seen a lot about mounts with/without belt drive, but I don't really know what difference this makes, and why belts are supposedly better.

 

Thanks for any enlightenment.


Edited by Ralph Paonessa, 10 December 2014 - 08:21 PM.


#2 schmeah

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Posted 10 December 2014 - 10:54 PM

The major advantages are absence of backlash, and minimal maintenance requirement due to reduced friction, wear and no need for lubrication. They are also not susceptible to periodic error in the traditional sense of the term since there is no worm/gear period.



#3 blueman

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Posted 10 December 2014 - 11:13 PM

Aren't most belt drives just the motor to the worm? I don't know of any that drive the axis with a belt and no worm.
Blueman

#4 siriusarcher

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Posted 10 December 2014 - 11:16 PM

On some like mine (Sky Watcher AZEQ6) the belt drive connects the motors to the worm gear. There is backlash and periodic error, but probably less of it because there isn't a reduction gear train. One big difference is quieter operation. Another is less need for lubrication. You don't have to lubricate the belt drive, whereas a gear train you do.



#5 orlyandico

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Posted 10 December 2014 - 11:46 PM

Less backlash is the biggie. A spur gear engages it's partner on only one or two gears, while a belt engages it's pulley over a large number of teeth. So much less backlash.

Second win is they are quieter.

#6 schmeah

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Posted 10 December 2014 - 11:55 PM

Aren't most belt drives just the motor to the worm? I don't know of any that drive the axis with a belt and no worm.
Blueman

 

The Avalons use toothed belt - pulley drives, no worm. There is a diagram down the middle of the page in this thread.

http://stargazerslou...belt-mod/page-4



#7 OzAndrewJ

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Posted 11 December 2014 - 01:21 AM

Gday schmeah

 

The Avalons use toothed belt - pulley drives, no worm.

 

They are also not susceptible to periodic error in the traditional sense of the term since there is no worm/gear period.

Actually, they can suffer large ( tho slowly changing ) PE, that is periodic relative to pulley ratios used and the teeth pitch on the belts.

Unfortunately, the true period for the error is 24 hours :-)

In fact, quite large PE can actually be seen in the Linear mount due to ( what appears to be ) the teeth on the final output gear meshing and unmeshing.

ie Even with a belt drive, you can get PE from eccentricity in the pulleys, and also from the irregular action of the teeth on the belt meshing with the pulley, esp if a large pitch belt is used.

Backlash reduction and quietness are the big advantages, as well as much more tolerance on misalignment.

 

Andrew Johansen Melbourne Australia



#8 Ralph Paonessa

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Posted 11 December 2014 - 01:49 AM

Thanks for all the replies. I guess I'm not clear on how many gears there are in the gear train on my Orion Atlas. I've seen the belt mod kit by Rowan Astronomy http://www.rowanastronomy.co.uk/ that replaces the gear that turns the worm. Are there also a lot of other gears in the motor?

 

There would still be a worm gear turning the worm wheel. I thought that most of the periodic error came from the worm gear? How much does the worm gear contribute to backlash? I always assumed that was the main source. Or is it divided among all the other gears?



#9 dotnet

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Posted 11 December 2014 - 01:58 AM

Being a purely visual observer I couldn't care less about PE. For me the main advantage of belt drives is the reduced noise. My HEQ5 Pro used to make screeching sounds when the transfer gear went into resonance, typically at the start and end of fast slews. It was loud enough to wake up the neighbours in the middle of the night. Since I've installed the Rowan belt mod kit I can observe quietly all night without a worry. I can still hear the stepper motors whining of course, but the sound level has dropped substantially overall.

 

Backlash in my mount had previously been caused by poor engagement between the reduction gears, between worm and worm gear, and also by axial play in the worm. The belt drive modification has removed the first of those causes, but I still have to adjust the worm properly to minimise backlash.

 

Cheers

Steffen.



#10 DuncanM

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Posted 11 December 2014 - 03:39 AM

Thanks for all the replies. I guess I'm not clear on how many gears there are in the gear train on my Orion Atlas. I've seen the belt mod kit by Rowan Astronomy http://www.rowanastronomy.co.uk/ that replaces the gear that turns the worm. Are there also a lot of other gears in the motor?

 

There would still be a worm gear turning the worm wheel. I thought that most of the periodic error came from the worm gear? How much does the worm gear contribute to backlash? I always assumed that was the main source. Or is it divided among all the other gears?

One of the advantages of a microstepped stepper motor is that it doesn't required much in the way of reduction gearing, so the only gears are the spur gears that connect the motor to the worm and then the worm gear itself.

 

The key advantage of a belt drive is that it tends to have a smoother periodic error, and smooth errors are easy to guide out and easier to correct via PEC.


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#11 schmeah

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Posted 11 December 2014 - 09:57 AM

Gday schmeah

 

The Avalons use toothed belt - pulley drives, no worm.

 

They are also not susceptible to periodic error in the traditional sense of the term since there is no worm/gear period.

Actually, they can suffer large ( tho slowly changing ) PE, that is periodic relative to pulley ratios used and the teeth pitch on the belts.

Unfortunately, the true period for the error is 24 hours :-)

In fact, quite large PE can actually be seen in the Linear mount due to ( what appears to be ) the teeth on the final output gear meshing and unmeshing.

ie Even with a belt drive, you can get PE from eccentricity in the pulleys, and also from the irregular action of the teeth on the belt meshing with the pulley, esp if a large pitch belt is used.

Backlash reduction and quietness are the big advantages, as well as much more tolerance on misalignment.

 

Andrew Johansen Melbourne Australia

 

Hi Andrew. I think it depends on how we are defining periodic error vs tracking error which has been quoted for Avalon mounts. Hopefully you guys can explain this to me a little better. My understanding is that these drives by definition cannot have "periodic" error which is repeatable and related specifically to the gear/worm cycle. This error can be improved with PEC because of the consistently repeating cyclical measurable error. Now, obviously all mounts have "tracking error" of which periodic error is just one component. For the Avalons, tracking error could be related to slight belt elasticity, misalignment of the pulleys, etc but this is not periodic, and therefore cannot be improved with PEC. So all periodic error is tracking error, but not all tracking error is periodic. Correct? So how does one measure unguided error for an Avalon mount, and over what period of measurement?

 

Derek

 

Edit: I see you did explain how it could be "periodic" related to pulley ratios and belt tooth pitch. I wonder how this is measured, however. It seems that periodic error has become the standard measurement with which all mounts are compared, which I think is not applicable in this instance. 


Edited by schmeah, 11 December 2014 - 10:47 AM.


#12 hjd1964

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Posted 11 December 2014 - 10:30 AM

 

Thanks for all the replies. I guess I'm not clear on how many gears there are in the gear train on my Orion Atlas. I've seen the belt mod kit by Rowan Astronomy http://www.rowanastronomy.co.uk/ that replaces the gear that turns the worm. Are there also a lot of other gears in the motor?

 

There would still be a worm gear turning the worm wheel. I thought that most of the periodic error came from the worm gear? How much does the worm gear contribute to backlash? I always assumed that was the main source. Or is it divided among all the other gears?

One of the advantages of a microstepped stepper motor is that it doesn't required much in the way of reduction gearing, so the only gears are the spur gears that connect the motor to the worm and then the worm gear itself.

 

The key advantage of a belt drive is that it tends to have a smoother periodic error, and smooth errors are easy to guide out and easier to correct via PEC.

 

And sometimes you can even connect the stepper directly to the worm and omit the spur gears entirely, as I did on my G11.



#13 Whichwayisnorth

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Posted 11 December 2014 - 12:01 PM

Astro Physics 1600 http://youtu.be/n1T_2wbgajY

 

Avalon M-Zero http://youtu.be/1AXSN3iLL-E

 

I'm sold on belts or direct drive.  



#14 Per Frejvall

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Posted 11 December 2014 - 03:21 PM

Ouch! They are noisy, aren't they?

 

/p



#15 Ralph Paonessa

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Posted 11 December 2014 - 03:46 PM

Being a purely visual observer I couldn't care less about PE. For me the main advantage of belt drives is the reduced noise. My HEQ5 Pro used to make screeching sounds when the transfer gear went into resonance, typically at the start and end of fast slews. It was loud enough to wake up the neighbours in the middle of the night. Since I've installed the Rowan belt mod kit I can observe quietly all night without a worry. I can still hear the stepper motors whining of course, but the sound level has dropped substantially overall.

 

Cheers

Steffen.

 

My Orion Atlas EQ-G has been very quiet out of the box.


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#16 OzAndrewJ

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Posted 11 December 2014 - 05:09 PM

Gday schmeah

 

 

Edit: I see you did explain how it could be "periodic" related to pulley ratios and belt tooth pitch. I wonder how this is measured,

 

You do long duration unguided data grabs in different locations and see what drops out.

FFT analysis clearly shows the timings for the teeth.

With a worm drive, the worm spins once per rev of the worm, and ( if you are lucky ), the worm PE is very large "relative to" the gearbox/drivetrain.

In that case the error is truly periodic AND consistent. Ie PEC can fix most it .

If the gearbox error is large relative to the worm, and doesnt repeat every turn of the worm ( like the 8/3 error mentioned for some Celestrons ), it cant be fixed by PEC, as the true period of the error is 3 turns of the worm.

Meade LX200s also have this 8/3 quirk, but Meade got around it ( partly ) by making their PEC model three turns of the worm, not one.

 

With the linear fast reverse, it has a 66 "tooth" final output drive.

Errors due to the teeth "meshing" are still periodic, but each tooth is different in shape and spacing and hence its not consistent in the shape/size of the error.

The next gear down has belts and again, the teeth on that provide a "periodic" but non consistent error.

As such, it changes smoothly, but the swings are very large ( relative to other drives ), hence why they say PEC cannot be used to fix it, and it cant be run unguided.

Some data for the Linear shows near 100 arcsec of "PE" over an hour or so, but it appears smooth so should be guidable.

 

Andrew Johansen Melbourne Australia



#17 schmeah

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Posted 11 December 2014 - 07:05 PM

Thanks, Andrew for that detailed and clear explanation. So with that in mind, how does one objectively compare the potential tracking performance between all belt/pulley drives and gear/worm drives? 



#18 OzAndrewJ

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Posted 11 December 2014 - 08:29 PM

Gday schmeah

 

You need to look at the mount/scope as a full system.

Looking at the periodic/non periodic error is only part of the analysis.

Ie you cant just do a comparison on unguided pk-pk error etc, as it only gives you an indication of the underlying "mechanical" errors.

Large smooth errors are far easier to work with than short fast changing errors, soo the slow 100 arcsec error in the Linear may actually be better than a 3arcsec fast error.

At the end of the day, the only real comparison is how well it can be guided, and that comes down to lots more variables.

ie now the guiding software and its settings come into play, as well as how the mounts firmware responds to commands etc

Or in more simpler terms, can you get round stars with it.

 

Andrew Johansen Melbourne Australia



#19 orion61

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Posted 12 December 2014 - 07:39 PM

You get rid of that annoying rubber banding where the mount will surge then lag,

RIRRrirRIRRrir. and..the backlash IF the belt isn't loose..



#20 pianoman2

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Posted 13 December 2014 - 02:20 AM

Ralph,

 

Jon Rista has a belt mod on his Atlas going on now on the beginning astro forum you might find interesting, with pictures. 

 

victor



#21 mikefulb

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Posted 13 December 2014 - 11:44 AM

I used the Rowan Astronomy belt upgrade for the RA axis on my Orion Sirius (HEQ5P) mount and it has been an amazing change.  These (any I would assume other mounts using teethed gears) have a component of periodic error which occurs everytime the gear teeth release and mesh as the two gears turn.  On my mount this happens every 13.6 seconds.  The periodic error for my mount has a 13.6 second "sine wave" with a magnitude of around 2.7 arc-seconds.  Impossible to guide out or handle with PEC.  The belt mostly mitigates this problem and it seems to be around 0.5 arc-seconds now.  Stars are now round instead of elongated.

 

I have parts to do the dec axis as well but it guides well enough I'll probably hold off as the 13.6 second period on RA was my main issue and the belt drive solved it.

 

EQ6/Atlas mounts have a similar 10.8 second period as I understand it.  There is a belt mod for that was well.



#22 Ralph Paonessa

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Posted 15 December 2014 - 01:55 AM

Thanks to all for the advice and info. I now better understand belts.

 

Ralph,

 

Jon Rista has a belt mod on his Atlas going on now on the beginning astro forum you might find interesting, with pictures. 

 

victor

 

Thanks, Victor, I'm looking at that thread now, which is directly applicable to my Atlas mount. The belt mod seems worth trying.



#23 ccdmaker

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Posted 05 June 2015 - 02:39 PM

I just performed the same mod on an Orion Sirius, but I had to interpose 3 washers between the motors and the mounting plate to depress the motor pulleys low enough to allow the belts to ride centrally on the pulleys, even with the motor pulley seated as far as it will go. The suggested clearances in the instructions accompanying the Rowan kit did not quite work for me. There must be some dimensional variations in the HEQ5/Sirius depending on the production date. 

 

I must agree that the overall quality of the product is high.

 

Prior to the upgrade, on the average my RMS guiding errors as reported by MaximDL wre 0.8 arc secs in R.A. & 0.4 arc secs in Dec. I did generate a PEC graph with EQMOD and it was obvious that there were several high frequency, non repetitive spikes riding on top of the worm error. I am hoping that the belt mod would eliminate or reduce these.

 

Waiting for the clouds to part now....



#24 Peter in Reno

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Posted 05 June 2015 - 03:19 PM

Astro Physics 1600 http://youtu.be/n1T_2wbgajY

 

Avalon M-Zero http://youtu.be/1AXSN3iLL-E

 

I'm sold on belts or direct drive.  

 

I am not sure that's a fair sound test. The video is showing an A-P1600GTO WITHOUT counterweights or scope so it's grossly mis-balanced and I believe this setup would generate louder sound than a well balanced mount. The video showing Avalon mount with a well balanced setup and a very light load (tiny refractor and counterweight).

 

Peter


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#25 rmollise

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Posted 05 June 2015 - 04:38 PM

I've seen a lot about mounts with/without belt drive, but I don't really know what difference this makes, and why belts are supposedly better.

 

Thanks for any enlightenment.

 

It makes for quieter mounts. It does little for periodic error which is the reason most people state in its favor, alas.




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