Re: Notes on tuning the EXOS-2/PMC8 mount hardware V2 #EXOS2


You're most welcome!

When I assembled the doc, I had to decide how far down in the weeds to go.  I know that there are others who have had a  sticky RA.  But as to how many, I had no clue, and the only way to fix that problem is to do a full teardown.  We're well out of 'adjustments' territory and so many folks are leery of cracking the covers in the first place - I certainly was.  So I sawed off short of full teardown and tried to stick to what most would need.

I think the worm bearing retensioning idea may have come from Kent via Jim McKee, back in the day - it's quite remarkable  how much difference it makes and have a spanner... , how easy it is to make a real improvement.  The mount has good bones, but it can benefit from a little help in places.  :-)

I'm actually on my second mount head.  ES was quite good about replacing my first over a sticky RA, but the replacement had the same problem!   So that settled it, and I did the teardown and repair.  Here's a picture of me polishing trial-fit scuff marks out of the ring's ID.   

- Bob

On Mon, Aug 24, 2020, at 13:33, guitsboy@... wrote:

Robert, Thanks for writing up this document.   

I recently consulted it quite a bit while tracking down the cause of my RA sawtooth error.   Ultimately I found the bearing preload on the RA worm shaft was far too much.  The shaft felt gritty and notchy and hard to turn by hand.  Unfortunately the bearing retainer was so tight that it couldn't be removed even with a piece of plate steel, notched and held in a vice.  It simply sheared off the soft aluminum, leaving me no choice but to use a pair of pliers and sacrifice the threads.  I can no longer replace the lock nut, but luckily the fit is so tight, no lock nut seems necessary.  Anyway, reducing the preload allowed the worm shaft to spin smoothly, and has removed the vast majority of the RA error I was seeing.  From my understanding, the retainer should have been at or just over finger tight.  I would estimate mine to have been somewhere around 30 to 40 ft lbs from the factory.

My mount also had a high degree of friction on the RA axis with the clutch loose.  The fit between the worm drum and the RA shaft was tight.  I did wind up polishing the drum and RA shaft as mentioned by the myriad CG5 and LXD75 hypertuning threads online.  There was also a bit of deformation in the aluminum on the end face of the worm ring side of the drum, possibly cause by the worm screw itself (Shipping bumps and bruises?).  I took care to sand and polish down the material that stood proud.  I can't remember if this side interfaced with a bushing or not.  I suspect that side is supported by the RA bearings but I could be mistaken.  Anyway, the RA still does not move as freely as the DEC axis does with the clutches loosened, but it is much, much closer now.   I'ts considerably easier to get an accurate balance.  Previously, I had to move the OTA and judge by which direction slowed quicker.

Anyway, I mention these issues just in case you find them useful to caution others about in future revisions.



Mounts: ES PMC-8 EXOS2
Scopes: SV 102EDT, ZWO 60/280 Guide
Cameras:  Nikon D5300, Altair GPCAM2 290M
Software: ASCOM, CdC, AstroTortilla, BYN Pro, Sharpcap, PHD2
Computer:  Thinkpad x230, Win7Pro/64

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