Telescope Reviews: 608 bearings for CGEM worm

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Equipment Discussions >> Mounts

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orlyandico
Carpal Tunnel

Reged: 08/10/09

Loc: Singapore

608 bearings for CGEM worm new

#5421067 - 09/14/12 02:41 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?

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brokenwave
super member

Reged: 05/10/11

Loc: Scottsdale, AZ

Re: 608 bearings for CGEM worm new

[Re: orlyandico]
#5421233 - 09/14/12 04:27 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.

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DaveJ
Pooh-Bah

Reged: 01/07/05

Loc: NE Ohio

Re: 608 bearings for CGEM worm new

[Re: brokenwave]
#5422016 - 09/15/12 08:54 AM

Quote:

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"

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orlyandico
Carpal Tunnel

Reged: 08/10/09

Loc: Singapore

Re: 608 bearings for CGEM worm new

[Re: DaveJ]
#5422226 - 09/15/12 11:40 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...

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Mert
Post Laureate

Reged: 08/31/05

Loc: Spain, Pamplona

Re: 608 bearings for CGEM worm new

[Re: orlyandico]
#5422243 - 09/15/12 11:58 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

worth

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orlyandico
Carpal Tunnel

Reged: 08/10/09

Loc: Singapore

Re: 608 bearings for CGEM worm new

[Re: Mert]
#5422270 - 09/15/12 12:17 PM

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...

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Mert
Post Laureate

Reged: 08/31/05

Loc: Spain, Pamplona

Re: 608 bearings for CGEM worm new

[Re: orlyandico]
#5422416 - 09/15/12 02:07 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 )

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orlyandico
Carpal Tunnel

Reged: 08/10/09

Loc: Singapore

Re: 608 bearings for CGEM worm new

[Re: Mert]
#5422420 - 09/15/12 02:12 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.groups.yahoo.com/group/CelestronCGEM/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).

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