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ChrisEll

AVX Motor disassembly

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After discovering a periodic signal (once every 11 seconds) in my guiding in the RA axis I did a bit of digging and found the motor pinion rotates 1 rev/11 sec at sidereal rate.

PPEC_20mins.thumb.png.8d14c4162f3a1a83764fc7a4ee6b3578.png

By motor pinion I don't mean the big brass spur gear visible with you take the cover off... I mean the output from the motor before the reduction gearbox.

motor_wo_gbox_1.JPG.811e6b8906d40cf5a19fe4324bb18a82.JPG

When I spin the motor at a fast speed you can easily see that the pinion wobbles about which is not a good thing. The pinion is simply hammered onto the somewhat thin drive shaft rather than held in place by a grub screw. Dec motor wobbles the same. On the Celestron website there are photos of the CGX and CGEM motors and they look exactly the same... this may suggest the wobbly pinion is not an issue :/

I then opened up the encoder. Nothing odd there but the optical wheel was worryingly grubby so I cleaned it with a damp cotton bud. There were markings on the PCB: "E8300 Rev.A RoHS".

encoder_2.JPG.7bf046e465bc825e1250f17f0ad5614b.JPG

Time to open up the gearbox. Inside are 4 pairs of cogs.

gearbox1.JPG.07b24830dda17967ee418c337c08a542.JPG

gearbox_parts.JPG.9f508ecb4c30ebf3399096f995659367.JPG

The gear ratios are:

Motor pinion (9 teeth) -> 2nd gear (27 teeth) ==> 1:3
3rd gear (11 teeth) -> 4th gear (33 teeth) ==> 1:3
5th gear (11 teeth) -> 6th gear (33 teeth) ==> 1:3
7th gear (16 teeth) -> 8th gear (32 teeth) ==> 1:2

Overall reduction 54:1

Motor output speed 1 rev/ 11 sec
Worm period: 11*54 = 594 seconds

Note 2nd, 3rd and 4th cogs are plastic, possibly ok due to the low torque at that stage although someone here on SGL has experienced a split gear.

I found that cleaning off all the excess grease results in a much smoother gearbox.

I was tempted to open the motor casing itself but thought better of it ;)

So I was left with what to do about my RA high freguency periodic error... in the end it seemed a good idea to swap the Dec and RA motors over to see if that would help, especially as I had everything in pieces. Also my RA motor has had a hard life so far... my RA axis had been very, very tight - so tight I could not turn the worm by hand. This was due the tension ring being ridiculously overtightened and also the worm mounting bottom bolts being completely loose so the worm was not seated against on the ring gear properly. I only recently fixed this. I'm surprised the motor had not burnt out or a plastic gear split. I'm hoping it may have developed a slight problem and that the swap will help, though I admit it's a long shot.

So will test shortly and see what the outcome is :)

 

Edited by ChrisEll
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Very nice.....but scary to some.

I have had my AVX in parts several times and need to get it apart again this weekend as when I removed my OTA the other day to be able to  mod my DIY motor focuser, noticed there was a rather large amount of wobble.  One part is due to the re-assembly of the mount around the axis where you can alter elevation and the other is due to the worm housing.  I thought I had that nailed down, but it seems to not be the case.

As I will have to get the motor off to be able to adjust and tighten correctly, think I will have a look in the gear box and also see if I have this 11 sec wobble that you mention too.

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The gearbox is certainly interesting to poke in :) Also see if your motor's pinion spins a bit eccentrically looking from end-on... both mine do :/ I can't believe this is a good thing.

Anyway I swapped my 2 motors over and tested last night and the RA guiding wobble is still there :(

Fast running out of ideas now... it's not PHD2 cos the wobble is still there when guiding is off, so it's probably the mount, maybe RA bearings? (another long shot)

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Will do

Shame that the wobble is still there.  Did you swap the worm gears when you had it apart?  That was one thing that I have done on mine.  The old ones did not look to great when they came off as you can see in the image below, but my mount lives outdoors, so this will have contributed to it.  Plus I think that the worst is on the face and not the race.  Can't remember which bearings I went for to replace of the top of my head.  Will have a dig through my emails and see if I can find the invoice for the new ones.  Are very easy to swap out too.

 

bearings.thumb.jpg.808493b8f5585772192ea89bc526820d.jpg

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Had a nosey poke and the worm bearings are 686z bearings.

Believe I got the replacements from Wychbearings (have a paypal receipt from them, but it does not have a huge amount of info on there other than it cost me £17.11), and they have following specs

id = 6mm

od = 13mm

width = 5mm

 

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Thanks for the bearings info. Did they fit perfectly or did you have to drill an access hole for the RA tension ring grub screws? No, not swapped the worms.

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For the RA axis big bearing I have had to drill another access hole to get to the grub screw as the replacement is a wee bit larger. Is silly east to do as the casting is quite soft

Was a straight swap for the main RA thrust bearing and also the worm bearings too

 

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Ok I was getting a bit muddled with worm and RA bearings... got it now :) It seems tricky to get the exact size RA bearings, I heard that some people's RA axis stick out a bit more at the top and holes need to be drilled, like yourself.

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You will struggle to get an exact width replacement as the one Celestron put I there is frankly pants.

I have a thread from last year with some images in ref the bearing here.

You can see that there is a size difference, but this is easily overcome with a wee bit of fettling.

Think if people saw my mount now they would wince...has had a fair few tweaks and adjustments to it.  The last was an inspection hole drilled into the front by the Az adjusters.  I have these swapped out for some micro-adjusters and was not sure they were hitting the Az post....so one 12mm hole later I was able to get my inspection camera into there and confirm they were hitting.

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Just read your thread, very informative! Looks like your AVX has been through the wars :) I need to be careful how much I tinker with mine... it's got to remain in a newish, resellable condition!

Edited by ChrisEll

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Aye..it has been dropped a few times and did get a wee bit of a knocking when I was removing the RA axis as was not aware of the last grub screw........is well out of warranty, so am not that concerned about the looks of it.  Just need it to function to the best that it can and all will be good

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A post re PE at different frequencies:

http://ccdware.infopop.cc/eve/forums/a/tpc/f/401101098/m/3097074586

"Looking at the frequency spectrum tab, it looks like there are integral peaks at 1x, 2x, 6x, 18x, and 54x. I think you could correct all except for the 54x peak, as it's frequency (11 seconds, may be two fast to correct)."

Not what I want to hear...

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UPDATED: A bit more info on the motor pinion... it's a 9 tooth, 0.4 module gear. The motor shaft is 2.3mm in diameter and the pinion is simply hammered on to the end. The motor shaft is not splined or keyed... the pinion is held in place by friction. It's a nasty cheap looking cog, no proper bore and no grub screw which accounts for the off-centre motion. I would be disappointed to see such a gear in a Hornby locomotive never mind a £800 mount. So my plan now is to find a suitable replacement if possible. Radio controlled car/planes/helicopter parts seem to be the best bet.

 

motor_pinion_end_on.jpgmotor_pinion_side_on.jpg.6bb9a6a09457e1bd3f4faeeaa977fb43.jpg

 

Edited by ChrisEll

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I did find with mine that the housing that is screwed to the motor was slightly misaligned...that is the motor shaft was not centred in the hole.  I slacked off the screws, adjusted and then re-tightened.

Have rebuild the gearbox now and used some white lithium WD40 to lube up in there.

Will be finishing the rebuild of the mount over the next week or so.  Am in no rush as the weather is up to it's usual and being wet and miserable.

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I envy you guys knowledge of how to take the AVX apart - I've no idea where to start.

Both RA and DEC are tight to the point of not even completing a half revolution when taken to horizontal and left to drop

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15 hours ago, iapa said:

I envy you guys knowledge of how to take the AVX apart - I've no idea where to start.

Both RA and DEC are tight to the point of not even completing a half revolution when taken to horizontal and left to drop

Adjusting the RA and Dec axes tightness is really quite straightforward, no need to take anything apart (well, not too much!). I'd be happy to create a post on the subject if you're interested.

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40 minutes ago, ChrisEll said:

Adjusting the RA and Dec axes tightness is really quite straightforward, no need to take anything apart (well, not too much!). I'd be happy to create a post on the subject if you're interested.

Please !

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Am sure the post Chris will put up will cover this, but to loosen the axis off you need to slacken off the grub screws that hold the tensioning rings on the end of each axis and then wind them slightly clockwise.  This will reduce the pressure on the axis and so make it easier to rotate.  I have a couple of threads (here and here) which shows the AVX in several pieces as had a few issues with both of my axis freezing.  This has a fair few pictures of the steps that I took and shows these retaining rings.

Is not a hugely difficult job, but you only need to loosen off very slightly other wise you will introduce a huge amount of play in the axis......which is bad......very very bad.

also...the tension ring for the RA axis has three grub screws holding in place, where as the Dec one only has two.  Don't miss the third one or you are in a whole world of not moving ring, hammer, screwdriver, messed up thread, anger.  Have the t-shirt for that one.

Edited by Shelster1973
typos
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