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608 bearings for CGEM worm

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#1 orlyandico

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Posted 14 September 2012 - 01:41 PM

So I snagged an Aeroquest worm off the other site for a low price, I thought, what the heck.

Major problem, my periodic error (which was bad enough at +/- 25") has gotten worse! at +/- 34"

Maybe I shouldn't have put those allegedly "ABEC-5" skate bearings in there... (that I removed from a pair of skates)

Anyway I don't feel like putting back the stock CGEM bearings - which are lousy anyway.

My question is, what bearings do I use to replace them? I see the SKF part number 608-2RSH. But how different is this from say a 608-2ZJEM? do I need those exact bearings?

Also there are these pricey "full ceramic" bearings - will these do me any good?

#2 brokenwave

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Posted 14 September 2012 - 03:27 PM

Ed Thomas at Deep Space Products could help you out,
He has a DYI Hyper Tune kit available, if that is something you were interested in.

#3 DaveJ

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Posted 15 September 2012 - 07:54 AM

Ed Thomas at Deep Space Products could help you out,
He has a DYI Hyper Tune kit available, if that is something you were interested in.


"DYI" = "Do Yourself In" :grin:

#4 orlyandico

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Posted 15 September 2012 - 10:40 AM

Yeah looks like I've done that.

Found some NKF "Japan" bearings and put them in. The periodic error is just as bad, maybe even a bit worse (probably within uncertainty limit).

Looks like it's lapping time. Toothpaste will probably do...

#5 Mert

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Posted 15 September 2012 - 10:58 AM

Do you have some sort of log-file to analyze the
mount's basic periods??
Typically the bearings influence but not in the amount
you have written!
Maybe you have put too much pre-load on the bearings??

I don't know the CGEM but likely mecanically it is not
too different from an EQ6.
Just my 2 :penny: worth :gramps:

#6 orlyandico

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Posted 15 September 2012 - 11:17 AM

I have a bunch of logfiles.

Looks like it's not the bearings and it's not the Aeroquest worm.

It's the wretched 8/3!!!

On my original worm, the fundamental was 29" according to PECPrep (agrees with my old PEMPro run) and the 8/3 was 19" for a total PE of 42"

With the Aeroquest worm, the fundamental is 16" (much better!) but for some reason the 8/3 has ballooned to 24" (now making it larger than the fundamental) for a total PE of.... 40"

So much for that.

The Aeroquest worm does have a quantifiable improvement, but the 8/3 can't be helped. I don't even know if lapping the worm would do any good...

#7 Mert

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Posted 15 September 2012 - 01:07 PM

Do you have the layout of the CGEM gearing?

That is from the stepper-gear - X - X - Worm - Wormwheel.
How many teeth on the wormwheel? ( 180 ?? )

When you state 8/3 what do you mean?

This could be helpful analyzing with PECPrep.
Also some logfile of 5 cycles would help!

Right now I'm off to do some imaging tonight ( I hope )

#8 orlyandico

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Posted 15 September 2012 - 01:12 PM

Mert, the CGEM has a very well known 8/3 harmonic (2.667X or a period of roughly 179 seconds = 479 / 2.667) due to the layout of the gears in the servo -

http://tech.dir.grou...EM/message/7441

Some CGEM's have a small 8/3 term (8") and some have a large term like mine. This cannot be corrected by PEC because it's out-of-phase with the fundamental (and the CGEM only has 88 PEC cells).

#9 Alph

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Posted 15 September 2012 - 01:54 PM

I have a bunch of logfiles.


Where can we find them?

#10 orlyandico

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Posted 16 September 2012 - 01:17 AM

here ya go Alph...

stock gear, Meade DSI (9.6x7.5 um pixels), Vega, 432mm FL

https://dl.dropbox.c...ega_DSI_Z70.txt

Last night, also Vega, also 432mm FL, with the Aeroquest worm

https://dl.dropbox.c...0_Aeroquest.txt

Last night, Altair, with the Aeroquest worm

https://dl.dropbox.c...0_Aeroquest.txt






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