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OK, some results from my first run with the ball thrust bearing in DEC and no thrust bearing under the RA ring gear.? See the analysis dated 2023-06-28.? Note that these are PHD2 logs with guiding removed (ie., raw).? Not conclusive, but.....

Due to smoke and clouds, this was a very short run (about 15 minutes) but like Peter found, it looks like my raw PE is worse without the RA thrust bearing so I think I'll clean it, re-grease with 50/50 Mobilgrease and MP-50 and re-install.? I'm hoping the MP-50 will help the rollers slide as they run around the race.

I've included a couple other frequency analyses for comparison. Yes, my mount is messy.... I think most of it is the Oldham since it is predominantly harmonics of the worm?

DEC seems like it is at least no worse and probably better with the ball thrust bearings.? PHD2 backlash correction was at 90mS and TVC was at about 80 (4X worm resolution).? With the ball thrust bearings at both ends of the DEC shaft, I can now easily observe and feel the right side worm bearing pistoning as I gently torque the DEC shaft. As soon as I release, it springs back to the rest position as expected.? This was difficult to observe with the roller thrust bearings so for now I will keep the ball thrust bearings installed. However, I do not feel this is a viable solution for all mounts.? To make it a drop in replacement for the roller thrust bearing, the balls are only 2mm in diameter (to attempt to match the rollers). Since the balls run on the same flat races as the roller thrust bearing, I am concerned that the very small contact area (infinitely small in a perfect world) is insufficient to carry the full rated load of the mount.? Possibly OK for me since my load is maybe 25 pounds including the 11 pound counterweight.

I will keep the ball thrust bearings for the short term and at some future date will disassemble and look for wear on the races.? In the mean time, I tried printing a test piece with various hole sizes in the first and last layers.? I think I have a combination that will (marginally) retain the 2mm balls but let them roll freely.? The balls are not held as securely as I hoped but I am stretching the capabilities of my printer so it will have to do.? I know someone with a liquid resin printer.... that might be the best way to print the ball thrust bearing guides.

I note that with Peter's spring worm bearing preload mod, reducing DEC and RA friction is crucial.? If the thrust bearing friction causes the worm to overcome the Belleville spring and move the worm bearing, this will cause tracking issues.? Remember that on RA, normal tracking causes the worm to want to move to the right, against the preload spring.

Lastly, if anyone wants the STL files for the DEC shaft ball thrust bearing guides, let me know and I'll send them to you.? The 2mm balls are silicon nitride ceramic balls from Amazon.? The overall stack is a couple thou thicker than the stock roller bearing.

Paul G.

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Paul Goelz
Rochester Hills, MI USA
pgoelz@...
www.pgoelz.com

Very nice work, Paul, in testing out your ball bearing vs roller bearing
idea.

I agree with your thinking that the ball contact area is tiny and might
wear out the race surface vs the rollers larger contact area. So a light
scope and counterweights might be ok, but not Peter's 12 inch Newtonian.

The rollers can work fine with the right polished surface hardened races,
but not at a high clutch knob force, because our data shows the "flat
parallel" metal faces are not able to stay parallel under huge clutch
force. You need a high friction clutch disk in each axis to allow you to
keep the clutch firm at a lower ckutch knob force to prevent the metal
distortion and save the roller bearing races from premature wearout.

Looking at your PemPro output PE frequency data, I see these peaks:

The 239.4 peak is of course the G11 RA sidereal rate (worm) period. But it
can be caused by the Oldham coupler also, (and I think half that at 120
sec, when the coupler turns 180 degrees may be half that at 60 sec too.)
To get rid of the Oldham coupler period contribution, you must carefully
align the worm shaft to the drive shaft (might be the gearbox shaft on your
unit). But the gearbox mounting holes usually don't allow you to line it
up. So I use a "rat tail" file to enlarge or maybe slot the gearbox
mounting holes, to then allow you to shift the drive shaft lined up with
the wirm axis. You know you are done when the plastic white center of the
Oldham coupler only rotates, but does not shift in X or Y directions.

The 32 sec peak is there too, from the stock plastic gearbox having the
well known 1/7.5 gear ratio error. (Mark Crossley describes this error in
his nice website from nearly 25 years ago. www.wilmslowastro.com ). You
can retrofit a metal McLennan gearbox, about $35, and solve that problem.
( Contact me if you need the sleeves to complete the retrofit. See attached
PDF on how to do that retrofit. )

The 76 sec error is there too. That's well known from a ball in the RA
worm bearings getting pinched. The solution much of the time is to allow
the far worm bearing to both slide in and out of its block, but also rotate
its OD slightly. Like the design of this "rod end" bearing...not the size
of course! This product has the concept in the ball shaped pivoting
bearing:

uxcell PHSB6 Rod End Bearings 3/8-inch Bore Pre-Lubricated Bearings 3/8-24
Female Thread Right Hand 2pcs

This way, as the worm moves in and out to follow the ring gear movement,
the end worm bearing will just pivoting to keep its ball bearings in the
race and not get pinched. To do that you must remove the end bearing from
the block and polish its OD (I use a few grits of wet or dry sandpaper).
Just polish it to be slightly rounded, and reduce the OD so it can slide
(with some lube) in the end bearing block. Add in one R4 Belleville spring
under the far sliding bearing, to accompdate thermal expansion and
contraction. This normally gets rid of the 76 sec error.

All the best,
Michael

On 6/29/2023 2:35 PM, Michael Herman wrote:

The rollers can work fine with the right polished surface hardened races,
but not at a high clutch knob force, because our data shows the "flat
parallel" metal faces are not able to stay parallel under huge clutch
force. You need a high friction clutch disk in each axis to allow you to
keep the clutch firm at a lower ckutch knob force to prevent the metal
distortion and save the roller bearing races from premature wearout.

I run my clutches VERY loose these days.? I see no need to tighten them beyond about 1/8 to 1/4 past first contact.? In fact, I suspect it will track OK at most pointing angles with the clutches completely loose.

I currently plan on keeping the DEC ball thrust bearings but if I put the stock roller bearings back in the RA axis I also plan on using the 50/50 mix of Mobil grease and MP-50 since the rollers need to slide against the races.? Seems like a good use for it.

The 239.4 peak is of course the G11 RA sidereal rate (worm) period. But it
can be caused by the Oldham coupler also, (and I think half that at 120
sec, when the coupler turns 180 degrees may be half that at 60 sec too.)
To get rid of the Oldham coupler period contribution, you must carefully
align the worm shaft to the drive shaft (might be the gearbox shaft on your
unit).

Last I checked the plastic center of the Oldham does not move radially but I should check it again.? Back when I adjusted it, there was enough slop in the gearbox plate holes to line it up.? It is not exactly round, though, so it is hard to judge when it is perfect.

The 76 sec error is there too. That's well known from a ball in the RA
worm bearings getting pinched. The solution much of the time is to allow
the far worm bearing to both slide in and out of its block, but also rotate
its OD slightly.

My bearing blocks were reworked by Peter to make the bearings a slip fit and also to add a brass screw plug that tensions the bearings against a Belleville washer.? That works fine.... I suspect the 76s signal is because the bearings may be damaged from me having to tap them out of the blocks as well as tap the worm shaft out of the left bearing after the Oldham grub screws marked the shaft.? The shaft damage is much reduced after I rounded and smoothed the ends of the grub screws.? I will eventually replace the bearings and see if the 76 second signal goes away.

Note that the worm bearings in my mount do not have to swivel at all.? The two bearing blocks are rigidly fixed to the worm cover when assembled and the whole assembly pivots on the left hand worm block pivot post.? Note that the worm assembly does not move in or out at all while tracking.... the stock worm spring is far too strong for that.? I have tried a weaker worm spring and no backoff screw but found that if the spring is weak enough to avoid excessive worm drag, the worm assembly gets pushed outwards when the worm turns.? This if course shows up in the guiding.

With the stock worm spring, the worm position is fixed by the backoff screw and that backoff screw never lifts off the edge of the baseplate.? The shaft of course moves relative the transfer gear shaft if the worm assembly moves but the Oldham takes up any shaft misalignment and is (I hope) aligned when the worm is in position against the worm wheel.

Paul

--
Paul Goelz
Rochester Hills, MI USA
pgoelz@...
www.pgoelz.com