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ChrisEll

AVX Motor disassembly

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Quick update on the AVX motor pinion situation... I am waiting for some pinions to arrive. In the meantime I've been experimenting with different ways to fix the pinion to the motor shaft... the new pinions do not have a boss and grub screw so will need to be glued on with something like Loctite or JB Weld. Hopefully I'll have something interesting to post in a few weeks.

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hi

just been looking at your avx  strip-down…

on the pics of the worm drives they don't show locknuts on the end of the backlash adjusters… are they supposed to be there?

great article btw.

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1 hour ago, Fspp369 said:

hi

just been looking at your avx  strip-down…

on the pics of the worm drives they don't show locknuts on the end of the backlash adjusters… are they supposed to be there?

great article btw.

Not sure which pics you're referring to... maybe to @Shelster1973 's posts?

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2 hours ago, Fspp369 said:

hi

just been looking at your avx  strip-down…

on the pics of the worm drives they don't show locknuts on the end of the backlash adjusters… are they supposed to be there?

great article btw.

From what I can remember, yes they do  have a lock-nut on them.  You have to remove this to get the worm out.  It is a snug fit in there and they do need a wee tap to get them out,  but come out clean and easy.  The one you can see in the pics was missing yes,  but was replaced later on...... I think :icon_biggrin:

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My AVX motor pinion replacement finally arrived. The only suitable gear I found were 5mm in height with no boss or grub screw. This meant bonding the new gear to the motor with glue. :unsure:

DSC00012.JPG.41d6ad7b3bc1bbc5e18babd5796d4492.JPG

First job was to check it's the right diameter bore, so I removed the optical wheel off the back end of the motor and tried the gear there... perfect fit. I removed the RA motor, PEC sensor and control panel all as one piece, much easier than removing the motor from the control PCB. Also you can spin the motor using the handset. So the motor went in the vice and after many failed attempts to lever off the pinion with a screwdriver I eventually removed it by gripping the motor in my left hand rather than the vice. The pinion was hammered on very tight and was only held in place by friction... no keying or splines.

DSC00006.JPG.947d7f18df72dcd2c215de2564b036af.JPG

Next to prep the bonding surfaces... I scored some lines inside the gear hole to give the glue something to grip onto, ditto the motor shaft. Quick clean then I put a small piece of masking tape at the base of the motor shaft (make sure the gearbox base can sit back on the motor without the masking tape interfering). This prevents any wayward glue entering the motor and also I can use it to help position the new pinion at the correct height. I put the gearbox base back on with the 2nd gear to get the correct height - I jiggled the masking tape accordingly. The new pinion is 5mm in height so only 3mm of it will actually bond to the motor shaft, not ideal but hopefully enough.

DSC00010.JPG.d65e9ad381c29d53e181822a8339e51e.JPG

Now to glue it on. I used Loctite 638, a "high strength retaining compound". I smeared some onto the motor shaft and inside the pinion using a tiny screwdriver, then a small blob into the hole and fitted the gear to the required height. I expected to have a bit of time to jiggle it and double check the height but the glue set very quickly and I was not able to move it. Luckily it was at the right height. I spun the motor using the handset to check the gear ran true which it did, much better than the original pinion. I left it for an hour and then scooped out the small pool of residual glue from the top of the pinion... this is not going to set and it's good to get rid of it now to help the curing process (Loctite only cures in small spaces in the absence of air). Left overnight.

Next day I reassembled everything and switched on the mount. I could see the RA gears start to turn very slowly. I manually slewed the mount in ever increasing speeds, finally reaching 9... it worked fine. More slewing and then I left it on for the day at sidereal speed. I also loaded the mount with scope, camera, etc. (this is only a 5kg payload, the AVX can handle up to 16kg... I should test with a max payload sometime).

So after all that here are the results. The high frequency periodic error is gone and the RA RMS has dropped considerably. Success! :hello2::happy1:

phd2_before_and_after_mod.thumb.png.b62f32530897f5784a4daf26d407aac3.png


N.B. In case you need to remove the gear for any reason here's how: you need to apply heat to the Loctite, so hold the gear in a match flame for about 3 seconds then pull the gear off immediately.

N.B.B. No need to do this mod on the Dec motor.
 

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Great news 

Do you have a link to where you got the pinion from? With all the awful weather at the moment I think I will strip mine down and change it. 

I do need to look at it again as after my last rebuild the worm is not as it should be as you can see a wobble when flexing the counter weight bar. Is just a case of finding the sweet spot of tightening Alan bolts and not binding the worm. Is a pain having to have the motor off to do it too.....

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I bought mine here:

http://www.dancingwingshobby.com/free-shipping-rc-model-metal-pinion-gear-04m-23mmhole-diameter-8t-9t-10t-12t-13t-14t-15t-18t-20t-24t-height5mm-10mm-12mm_p0558.html

Takes about 2 weeks. They come in fives, so I have some spare, so PM me and I'll pop one in the post if you like :)

Anyway have a look first and see if your pinion does run off-centre... I thought you said it was all ok?

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Howdy. I have a similar issue with a CGEM mount. 2 motors with 2 plastic gears missing teeth. I cannot find a source for the plastic gears, willing to go brass if I must. Any idea where a good place to look may be?

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On 21/01/2018 at 10:23, ChrisEll said:

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 :)

 

Any idea where we can find the nylon gears? Mine is stripped. CGEM, and I cannot for the life of me find any information on it. Anything would be appreciated!

 

20190502_205824.jpg

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On 03/05/2019 at 03:21, swordfishy said:

Howdy. I have a similar issue with a CGEM mount. 2 motors with 2 plastic gears missing teeth. I cannot find a source for the plastic gears, willing to go brass if I must. Any idea where a good place to look may be?

You probably have to buy a whole motor replacement for each... i've not come across any gearboxes which are the same as the ones for the those motors.

The broken teeth seem to suggest the RA and Dec axes are far too tight... the axes are not well assembled and tend to be overly tight. They can be easily corrected.

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