swordfishy

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.

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.

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".

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

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!

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?