EQM35 Pro Belt Mod - Skywatcher

Found on a forum with no responses to share - want to share with the world. EQM35 Belt Mod.

I’m using the standard EQM35 motor controller with no changes made to EQMOD settings.

The aim of the exercise was to achieve the same ratios as the hard coded ones because I recognised at the start that there was nothing I could do to change them. On the RA axis I did it with 48 & 12 tooth pulleys (ratio 4:1, as in the original gear train). On the Dec axis I used  66 & 12 tooth pulleys, but that ratio is 5.5:1 and would run at twice the required speed because the original ratio is 11:1, so I halved the speed with a 0.9 degree motor in place of the 1.8 degree motor. I chose 66 & 12 (i.e. 5.5:1) specifically because I could halve the motor speed. I’ve also got a 55 and a 10 on the way from China (and a 66, to go with the 12, just in case…)

Parts list for the whole job:

RA Axis

NEMA 17 1.8 degree motor. You could re-use the existing motor though (I replaced mine because it had failed) 

12 tooth GT2 timing pulley – 5mm bore (for a 6mm wide belt)

48 tooth GT2 timing pulley – 6mm bore (for a 6mm wide belt)

158mm GT2 timing belt (6mm wide)

Custom mounting bracket and socket housing

2x M5 (5mm x 15mm) thumbscrews (standard M5 bolts would do if you're not fussed) + 2x M5 nuts + the existing bolts used to attach the motor (add some washers)

Dec Axis

NEMA 17 0.9 degree motor (a NEMA 14 might do if you can get a 0.9 degree version, but  the stock motor needs to be replaced here. Keep it as a spare in case your RA motor ever fails)

12 tooth GT2 timing pulley with 5mm bore (alternatively, 10 tooth - but not yet tried)

66 GT2 tooth timing pulley with 6mm bore (or 55 tooth if using 10 above)

200mm GT2 belt 6mm wide (or 196mm)

custom motor and socket housing 

1x M5 nut + the existing bolts used to attach the motor (add some washers). 

The RA gear train is as follows:

• 12 teeth on the drive gear (attached to the motor) - call it gear A
• 48 teeth on the first driven gear - gear B
• 35 teeth on the co-axial gear rigidly attached to that - gear C
• 35 teeth on the last driven gear (attached to the worm) - gear D

Gear A drives gear B. Gear C rotates with gear B because they are attached to each other co-axially. Gear C drives gear D. So:

• 1 rotation of gear D requires 1 rotation of gear C (both with 35 teeth).
• 1 rotation of gear C requires 1 rotation of gear B, since B and C are co-axial
• 1 rotation of gear B (48 teeth) requires 4 rotations of gear A (12 teeth)

From all of that, 1 rotation of the worm (gear D) requires 4 rotations of gear A, hence 1:4

For dec the gear train is:

• 12 teeth on gear A (attached to the motor)
• 66 teeth on gear B
• 35 teeth on gear C (co-axial with, and attached to, gear B.)
• 70 teeth on gear D (attached to the worm)

As for RA, Gear A drives gear B. Gear C rotates with gear B because they are attached to each other co-axially. Gear C drives gear D.  

• 1 rotation of gear D requires 2 rotations of gear C (70:35)
• 2 rotations of gear C requires 2 rotations of gear B (co-axial)
• 2 rotations of gear B requires 11 rotations of gear A (66:12)

So, 1 rotation of the worm (gear D) requires 11 rotations of gear A, hence 1:11