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Old 10-02-2011, 11:22 AM
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worm and gear specifications

Hi,


i'm looking at buying worm and worm gears from either smallparts or pic design for a mount, but needed help with specifications.
else if someone bought one, could you'll share specifications or sources.

i was looking at the pic catalogue of anti backlash worm wheels,
http://www.pic-design.com/Images/pdfcat/section_12.pdf

they've got the following parameters listed

pitch
face width
pressure angle
no of teeth
O.D
P.D
Bore D

what are the common pitch and pressure angles used in most scopes, how do you specify the number of teeth that have to be meshed?
do the springs in the antibacklash gears reduce backlash noticeably? anyone tried these?

i'm looking at a reduction ratio of around 300:1.

Last edited by alistairsam; 12-02-2011 at 10:16 AM.
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Old 13-02-2011, 09:21 AM
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No need to reply - this doesn't answer your question directly. Alternatively, I went for a planetary gearbox or spur gear reduction, and final reduction of 6:1 (friction or timing gear) for the RA axis - as I understand it, the final reduction is where periodic error manifests the most. More moving parts bearings etc - Depends on the application I guess.
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Old 13-02-2011, 08:44 PM
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hi

would you have any pics or specs of the planetary gearbox and spur gears
i found that planetary gearboxes for steppers were ridiculously expensive, more than some mounts.
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Old 14-02-2011, 08:21 AM
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Alistair,
It is very unusual to use anti-backlash gears in those transmissions.. they are not really needed. Much better and cheaper approach is the use of timing belts and pulleys.
http://www.iceinspace.com.au/forum/s...ht=timing+belt
Planetary gearboxes are used where the high torques are involved (power tools, for example).. such conditions do not exist in telescope reduction train (unless you want extremely fast slewing, if you want to jump from object to object in a matter of seconds - I am guessing this requirement would be highly unlikely).

As for general design directions, why don't you have a look at how EQ6 is made? There are plenty of websites that discuss dismantling, cleaning and hyper-tuning of this mount, there you can find a lot of interesting design details...For example:
http://www.astro-baby.com/EQ6%20rebu...rip%20down.htm
http://ejcruz.smugmug.com/Photograph...85975881_wfgb7
http://www.bloomingstars.com/vega/ar...tmare/english/

Last edited by bojan; 14-02-2011 at 10:28 AM.
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Old 14-02-2011, 11:56 AM
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Thanks Bojan,

i have seen the eq6 threads, what I was after was the specs for a worm and gear like the one used in the eq6.
if I'm to order a worm and wheel for small parts, what pitch would i need to look for.

I will be using timing belt and pulley but the reduction with that alone wouldnt suffice. hence the thought of using a planetary gearbox.

I'm using 1/32 microstepping, so reduction ratio does not need to be too high to get the tracking rate.
for slewing, I just set the mcu to change to 1/8 or 1/4 microstepping to get high speed slews.
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Old 14-02-2011, 01:10 PM
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If you want a REALLY good worm gear reduction, go for Byers worm gears:
http://www.deep-sky.co.uk/telescopemaking/tm14.htm

Otherwise, I believe any pitch will do: What you need is a decent number of teeth (180 ~ 240, wheel diameter is derived from this and it should be as large as could possibly be found, to reduce torque.. but 100mm in diameter is still OK).
My dobson used 240 teeth worm gear and 1:12 friction wheel reductor... quite adequate for 32 microsteps.

Small parts has only one adequate option anyway:
http://www.smallparts.com.au/store/p...ection/wide/1/


Have a look at what Steve (Kinetic) is doing, here: http://www.iceinspace.com.au/forum/s...t=43091&page=7

Standard EQ6 (Synscan) has 200s/rev stepper, and overall reduction (from motor shaft) is 705.
(180-teeth worm wheel, and 12/47 spur gear reduction. This is done by use of 12 teeth gear on motor axis, a transfer gear (30 teeth or so? not relevant) and 47-teeth gear on worm shaft).
With 32 microstepping, this gives you enough resolution (0.14 arcsec per microstep), while still allowing relatively high slew rate (at 400steps/sec you have 0.5 per second slew.. decent).

A friend of mine is using only 1:4 timing belt reduction (on his EQ-6), which is directly driving the worm from motor shaft .
I was a bit more conservative, using 1:4 spur gears and 1:5 timing belt reductor to drive worm (I will remove 1:4 spur gears when Bartelising my EQ, when I am ready for permanent setup under the Sirius dome, one fine day (night)...).

It is worth considering alternative approach - friction roller drive: http://www.scopecraft.com/Technical%...r%20Drives.htm

And, finally, Bert (Avandonk) has EQ-6 with only one belt reduction, 1:6 or 1:8 (I think), directly from stepper to worm.
http://www.iceinspace.com.au/forum/s...ght=belt+drive

Last edited by bojan; 14-02-2011 at 03:25 PM.
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Old 14-02-2011, 05:26 PM
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Thanks for the info Bojan. those are good articles.

friction drive is an attractive option, but maintaining pressure on the rollers and concentricity of the rollers would be key factors.
if i can fabricate an assembly that can adjust pressure on the rollers, it might be practical.

my existing stepper has got a 20 tooth T2 timing pulley.
i also have a 130 tooth timing pulley.
so the first stage would give a ratio of 6.5:1

if I get a 100:1 reduction with a worm/worm wheel stage, i should get a reduction of 650:1 which is ample.

will also look at driving the worm directly with the motor with low freq pulses and a high torque motor.


I was reading up on worm parameters, apparently lower pressure angle yields lower noise but lower power transfer. 20 degree is the industry norm.
also lead angle.
smallparts actually have a pretty extensive collection. this is why i started this thread as I wasnt sure of what to choose.
http://www.smallparts.com.au/store/categories/worms/

I found these options close to what i could use. only issue is that with a quadruple start, I think the ratio is teeth/4.

what is the significance of number of starts, does it affect efficiency?


100 tooth quadruple start
http://www.smallparts.com.au/store/i...ries5metallic/

100 tooth antibacklash worm wheel
http://www.smallparts.com.au/store/i...ries5metallic/

single start 120 tooth
http://www.smallparts.com.au/store/i...ries2metallic/
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Old 14-02-2011, 07:06 PM
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Yep, single start worm is the only option.
The problem with 120 teeth gear is relatively small diameter (63mm - OK for smaller loads, up to 2kg methinks), which will mean higher torque required for bigger telescope (and possibly higher rate of wearing).
I would go for 360 teeth (as suggested earlier).
Don't worry about antibacklash worm gears, they are not really that useful (especially here, where moment/torque may be pretty high, and that must be counteracted with spring), as backlash can be adjusted if pair is made with sufficient precision.
Also, there will be additional friction present (because of that spring), which may be a problem for lower torque motor.

Quote:
Originally Posted by alistairsam View Post
friction drive is an attractive option, but maintaining pressure on the rollers and concentricity of the rollers would be key factors.
if i can fabricate an assembly that can adjust pressure on the rollers, it might be practical.
Maintaining the constant and controlled pressure is not a problem at all (see picture below - my own "invention" )
To manufacture the truly concentric friction drive is actually easier than truly concentric gear - because the whole operation can be done in one go, without taking the part off the lathe before the job is completed. BTW, 200" Mt Palomar telescope uses friction reduction wheels (I read that a long time ago in a book "the design of big telescopes" .. or something similar, I don't remember the exact title any more).
Attached Thumbnails
Click for full-size image (Friction_drive.jpg)
139.1 KB42 views

Last edited by bojan; 15-02-2011 at 08:49 AM.
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Old 15-02-2011, 12:12 PM
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i was looking for suppliers of 360 or even 180 tooth worm wheels, didnt find too many that were reasonably priced, sacrificing quality of course.

but came across this suplier, fairly expensive.

http://www.aeroquest-machining.com/index.html

http://www.aeroquest-machining.com/page11.html

did you machine the brass wheels in your drive? i'm guessing you machined the aluminium wheel. where can we get such wheels?
I don't have access to a lathe so cant machine as such.

the overall gear reduction in your drive seems fairly large with the worm stage and the roller wheel stage. what is the actual reduction you've achieved?

for single start worm wheels, would you know if the sliding friction is higher or lower with high and low pressure angles and lead angles?
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Old 15-02-2011, 12:30 PM
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my colleague just gave me an idea of using a non vented solid brake disc as a possible roller wheel. wonder if they'll work.

http://www.valiantspares.com.au/inde...roducts_id=429


if the driving roller and motor assembly is spring mounted, that should allow for some tolerance to imperfections in the driven wheel edge.
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Old 15-02-2011, 01:21 PM
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Quote:
Originally Posted by alistairsam View Post
did you machine the brass wheels in your drive? i'm guessing you machined the aluminium wheel. where can we get such wheels?
I don't have access to a lathe so cant machine as such.

the overall gear reduction in your drive seems fairly large with the worm stage and the roller wheel stage. what is the actual reduction you've achieved?

for single start worm wheels, would you know if the sliding friction is higher or lower with high and low pressure angles and lead angles?
A friend of mine has a machine shop... so everything you saw on that image is machined as per my specs.
Reduction of the wheel set on the image above is around 1:12. You can't get those wheels from anywhere, this is my own design.
The worm has 240 teeth, reduction is 240 X 12 = 2840, this gives me 2.25" per step.

Sliding friction is the same for single and multiple start worms, and it depends on load (that means, imbalance of your telescope)

Last edited by bojan; 16-02-2011 at 02:41 PM.
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Old 15-02-2011, 01:42 PM
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Quote:
Originally Posted by bojan View Post
Sliding friction is the same for single and multiple start worms, and it depends on load (that means, imbalance of your telescope)
any advantages with multi start worms over single start?
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Old 15-02-2011, 02:23 PM
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Quote:
Originally Posted by alistairsam View Post
any advantages with multi start worms over single start?
None, from the ATM point of view.
Actually the influence of the load is slightly lower for single start worms.
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Old 15-02-2011, 03:00 PM
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Quote:
Originally Posted by alistairsam View Post
the overall gear reduction in your drive seems fairly large with the worm stage and the roller wheel stage. what is the actual reduction you've achieved?
Reduction of the wheel set on the image above is around 1:12. You can't get those wheels from anywhere, this is my own design and they have been made for me.
The worm has 240 teeth, reduction is 240 X 12 = 2840, this gives me the resolution of 2.25" per full step (200step/rev motor).

Last edited by bojan; 16-02-2011 at 02:42 PM.
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