14 replies to this topic

[Edward E]

I'm working on converting my 20" Push-To Dob to the OnStep Go-To system.  I need to reduce the torque to move the Altitude bearing.  The existing 4 teflon pads require too much torque for the NEAM 17 stepper motor to handle based on my measurements made with a digital scale.  I am looking into using  two of these liner glide bearings made out of nylon.  The bearings are the right width and with 1/16" shims on the sides, the correct height to match the remaining teflon pads heights.  They would need to be cut to the proper length (come 75mm long).  Do you think the nylon has the strength not to deform under the weight of the fully assembled Dob?

Edited by Edward E, 19 November 2019 - 06:22 PM.

[nstiesi]

Are you planning to have some sort of clutch system to disengage the motor and still be able to push-to?  If so, I would think you need more torque out of the motor, not less friction on the altitude.  Are you using a reducing gearhead on the stepper motor?

 

I am actually struggling with some similar issues at the moment.

[Edward E]

Yes the NEAM 17 stepper motor has a precision planetary gear system with a 50:1 ratio; plenty of torque to do the job once I get the Alt bearing friction down.  Yes there will be a clutch/brake installed.

[nstiesi]

Got it.  I was trying to use no gearhead, but all readuction via pulley and belt to minimize backlash.  Do you experience much blacklash with the planetary gears on your stepper?

 

As for friction, some people here swear by a little bit of dry bar soap to lubricate the teflon pads, but I've seen that debate get a little contentious.  Can you reduce the size (area) off your teflon pads?

 

The link you provided for your needle rollers links to this thread so I cant comment on them.  Good luck!

[Edward E]

There is no backlash with this geared down stepper.  Thanks for letting me know that the link was not set correctly.  I have fixed that issue.

 

Let me know how your OnStep conversion goes.

[m. allan noah]

Only the cage on those bearings is plastic, the needles will be hardened steel. Unless you have a steel surface for this to roll on, it will indent the surface and move poorly. I cannot tell from the picture if the needles protrude from the bottom of the cage as well. If they do, they cannot be fixed in place, but need to slide in the joint.

 

allan

[Bob4BVM]

Try the bar soap before you go to all that trouble with the bearings. You might be surprised

[gregj888]

On my little 8" SCT I milled out the Dec and replaced the plastic sleeves with real needle bearings. 

 

Your motor and clutch would provide the friction for the push to, you'll probably want to replace the Alt bearings with ball or needle bearings. 

 

If you go to the Observatory forum, you'll find 3-4m domes being turned by steppers, so you could do that with some of the larger ones (Nema 34 (?)), but your power requirements goes up a lot too.

 

I'ld look at come AC ball bearings at least for discussion.

[jpcannavo]

Have you checked the angular separation of the Teflon altitude bearings? Decreasing the angle will decrease the needed torque. Of course when they are too close to each other stability suffers. Worth checking though, as if they are further apart than needed an easy fix might be at hand.

Joe

[Edward E]

Thank you for the suggestions.  I tried the soap technique; it did reduce the torque need to start moving the alt bearing but not enough to get in the specs of the stepper motor.  Moving the alt pads closer also helped but not enough and the motion started to suffer. 

 

I will go with needle bearings and see how that works out.  At $5 and free shipping, not much to loose to try.  I will keep you posted on how things turn out.

 

Cheers

[Pinbout]

id Use sleeve bearings over a shoulder bolt

[Edward E]

I thought of that but found that there is not enough room between the outside edge of the mirror box and the inside edge of the alt support structure for a bolt and mounting.  The alt bearing structure must fit on the upper surface where the alt pads currently reside.  This is the reason I am looking at the needle bearings.  The needle bearings I have ordered fit the width of the alt support structure and the bearings are slightly wider than the alt bearings.  Plus with 1/16" shims, the top of these needle bearings will match the height of the remaining Teflon pad.

[Edward E]

UPDATE:

The needle  rollers did not work, as many of you thought would be the case.  I have tried the soap on the "Teflon" pads trick, moved the soaped pad closer together, no improvement; still takes ~ 22 lbs/foot to to overcome inertia and keep the Alt bearing moving with the scope balanced.  Next tried replacing the "Teflon" pads with steel rollers; the Alt bearing slides over the steel rollers, sometime the rollers turn and sometimes not, none of the 4 rollers turn at the same time.  Still takes ~ 25 to 30 lbs/foot to overcome inertia and keep the Alt bearing moving.

 

Next idea; replace the Formica material on the Alt bearings with "thin" aluminium sheet(s) and keep the soaped "Teflon" pads in place, just moved close together but not so close as to compromise stability.  What are your thoughts?

 

The Alt bearings are ABS plastic, width is 1.5 inches and  ~ 15 inch radius.  The Dob is a 1990s 20" f 3.9 StarSplitter II scope.

 

Goal is to have a 1.25 inch O.D. Neoprene roller drive the Alt bearing.  The Neo roller will be connected to an intermediate, 100 tooth (~ 4 inch O.D.) timing belt pulley which is connected to a 40 tooth timing belt pulley by a 11mm wide "toothed" belt.  The 40 tooth pulley is connected to the stepper motor with a 50:1 precision planetary gearbox.

[Macguyver909]

Have you looked at getting a larger #17 motor. They come in different lengths and tourque ratings, also higher quality motors have higher values. 92 oz/in are the strongest I see with a quick search. I have had great results from OMC stepper motors.

 

Clear and dark sky's

Edited by Macguyver909, 10 March 2020 - 05:54 PM.

[Edward E]

Found an older Sky & Telescope article that plotted the friction coefficient curve of combinations of material used as bearing/pad for Dob scopes.  The friction coefficient for Teflon/Aluminum was high.

 

Solved my Alt bearing motion problem.  With a suggestion from an article by StellarCat, I used fine steel wool to "polish" the Teflon pads, then cleaned the pads well with 99% alcohol; after re-mounting the pads on the scope, spacing the pads closer together, I rubbed some "Armor All" onto the pads.  The Armor All was a suggestion from Dean at Starzonia, here in Tucson, AZ.  The results, it now takes 11 pounds to overcome inertia and 6 lbs to keep the Alt bearing in motion; it is the same moving the Alt bearing N or S from a full stop.  Now to get the 20 & 80 tooth timing belt pulleys with a 8mm bore for a 10mm belt mounted.

Edited by Edward E, 16 March 2020 - 01:39 PM.