Deep dive into addressing stiction in Synta 5 and 6 mounts

[Applal]

I have had my Atlas EQ-G since 2010. It's not a bad mount, and has the basics of good solid build. It was only since becoming involved in astrophotography that I began to really want to improve its tracking characteristics. First time through I did a thorough Hypertune, even bought a DVD on how to do it. It seemed ok and quieter. As my demands for better tracking increased, I felt I needed to go back in, and this time, following Astro-Baby's help, I did it again. Then added a Rowan Belt mod. 

Ok. Many of you are with me thus far. I've been in and out of my mount enough I don't even need to look at any guidance anymore, and I am still not where I want to be, and here's where I begin to need someone who may have also taken a deep dive into these mounts. 

My Dec guiding is superb, so no going there. This is all about the RA axis.

The rather large, ring gear that is driven by the worm gear, is integrated to be part of a "drum" that the RA clutch engages. This drum seems to be a precision piece that fits with little clearance into the mount casting. And I suspect that this fit is the cause of odd and unpredictable errors in guiding. The observed evidence I have for this is that if I back off my worm gear completely, the RA spins only in the large support bearings beautifully and without any stiction at all. None. However, engage the work gear to normal near zero backlash and now the rotation of the axis is sticky and irregular in how it is sticky. This HAS to be due to the brass drum fit to the casting.

So I have disassembled, multiple times, degreasing, using different qualities of grease, from lithium to Superlube, and even some high end fishing reel grease. No significant diminishment in this sluggish movement.

Last time through I degreased once again, solvent cleaned the barrel of the mount casting that the brass drum fits into, then wet sanded the entire interior with 600 grit and solvent to (hopefully) remove any high spots. Then, as a final experiment I also wiped the casting interior with graphited lock lube, rubbed it in and wiped it off.  The casting seems to be less than perfectly smooth, and I hoped the graphite might lay in the low areas... just guessing. 

That's the backstory to my question.  Which is this. Should I have a machinist measure and perhaps improve/perfect the casting hole that the brass ring drum fits into to allow for enough space for lubrication to bear the drum instead of what seems to be direct contact? Have any of you looked into this, as I interpret it, problem? 

[Lonestar70]

Hi Applal,

I don't think it will be the drum/gear touching the sides of the casting unless you have some slack (sideways or tilt) in the RA spindle bearings which should not happen with tapered bearing correctly adjusted.

You state that the RA spins easily, with no stiction  when you have the worm disengaged but you do when the worm is engaged and adjusted for minimal backlash.

What you are describing is generally termed Periodic error and is caused by tiny amounts of eccentricity and/or tiny variations in the tooth form at different locations on both the worm gear and the worm... which cause tightening or relaxation in the spacing during rotation.

These worms and worm wheels are mass produced by automatic machines and, whilst of very good quality, they are not high precision parts... for that you would be paying almost the cost of the entire mount on just the worm wheel alone.

If you are running EQMOD and ASCOM then you can run a PER sequence on the mount to collect error factors to a data file for these variations and use the results to automatically correct the tracking.... Periodic error correction... whilst running the mount with auto tracking.

I hope this helps.

 

 

 

Edited June 23, 2021 by Lonestar70

[Applal]

Thank you for your reply, and I understand what you are describing. 
Perhaps I should have been clearer about the resistance when the the worm gear is meshed normally.

Without the worm engaged, the RA axis runs beautifully on the bearings. With the worm properly meshed, AND the clutch disengaged, it should spin just as freely but it doesn’t. There is considerable drag that I feel is happening between the casing and the brass ring gear drum. 
either the casting is machined too tight or imperfectly, or the ring gear drum is not round/true. 
I’m going to have it apart again and mic the drum and check it for true. If it’s out of round I will have it turned true. If it is true, then I will have the casing honed. I am certain that the drum to casing clearance should be greater than zero, allowing for only the bearings to support all movement.

Outside of this one issue, the mount is a trooper. 
again, thank you for your time and answer.

[pete_l]

3 hours ago, Applal said:

I am certain that the drum to casing clearance should be greater than zero, allowing for only the bearings to support all movement.

Have you run a feeler gauge around the drum casing, with the gear in place (and the worm, too)?

If there is friction at some points with some thicknesses of gauge, that would point to something (maybe the casing, maybe the gear) not being perfectly circular. You would need to do this at multiple points of case / gear alignment to find any high or low spots. Or if the centres of the gear and casing are not at the same point

Edited June 24, 2021 by pete_l

[CraigT82]

You could try polishing the brass of the drum all around, just enough so what when you refit it and rotate it again, any binding spots will rough up the polished finish so you can remove it again and see where it is binding.

The binding isn't a major issue though as in use and tracking the clutches are locked and so there is no movement between housing and brass drum. If you can get it to freely rotate it might make balancing the OTA easier though. 

 

Edited June 24, 2021 by CraigT82

[Peter Drew]

You need a tube of "engineers blue".  Lightly smear some on to one of the mating surfaces and turn the other against it, the blue will transfer on to and reveal the high spots.  Be careful doing this to avoid the surfaces binding.  I think the amount of material to be removed will be less than can be achieved by re-machining, scraping as used to fit plain bearings is probably the best way forward, just removing the blue high spots.     🙂

[Lonestar70]

Could it be the RA clutch pressure pad rubbing on the drum, rather than lifting fully clear of it when released.

This could be caused by a small amount of old hardened grease in the clutch lever housing.

[Applal]

11 hours ago, CraigT82 said:

The binding isn't a major issue though as in use and tracking the clutches are locked and so there is no movement between housing and brass drum. If you can get it to freely rotate it might make balancing the OTA easier though. 

This. 
OMG! Apparently I was overthinking and underthinking at the same time.

comments about how to identify high/low/interference spots are all gratefully seen. 
I will proceed with care, while realizing guiding was not being impacted by the drum/casting interface. Addressing this to make balancing easier is my new goal. 
 

Thank you all! 

[newbie alert]

When you engage the clutch, it can push the ring gear to one side,  the clutch doesn't have to be fully locked, in fact many have the clutch barely holding, just enough to hold

 

[Starflyer]

I've owned / tuned a couple of EQ6 mounts and poor quality main bearings, the three on each axis, can allow quite a bit of lateral movement when the clutches are nipped up.  Better bearings and only just engaging the clutches helps, over tightening clutches can definitely adversely affect guiding.

 In my experience, the main cause of stiction in these mounts are the thrust bearings, again use good quality bearings, only a small smear of a light grease and don't over tighten them.