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Torque needed to drive EQ3-2


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I have a Helios 200p (identical to the Skywatcher) mounted on an  EQ3-2.
I know it is overloaded but I find it OK for visual provided I give it a couple of seconds to settle down at high powers.

I intend to motorise the mount using stepper motors & an Arduino. It will be tracking for visual only with a possible upgrade path to GoTo.

Since I dont need fantastic precision and having read the relevant threads on this forum, I propose to drive the RA worm directly & the DEC with a simple belt drive. By avoiding a step-down gear set I will still be able to use the manual slo-mo controls without disconnecting the motors. I think that centering the target with the slo-mo's & then switching on the tracking will be nicer than using fast-forward & fast-reverse controls.

But no step-down means no torque amplification. So how much torque do you need to drive an overloaded EQ3-2. I decided  to do some quick & dirty tests.

Apparatus
200p with 25mm Plossl & 9 X 50 Finder, payload about 9kg.
EQ3-2 with 10kg of counterweights
Improvised wooden pulley, effective radius 0.375 inch
2 X 1kg barbell weights
piece of string

Method
For each axis ,with the clutches locked, a 1kg & then 2kg load was hung from the pulley & released. This was repeated for the  opposite direction of rotation.

Results
DEC    1kg    Torque    13.5 oz.inch    Rotated smoothly at about 90 degree/minute
DEC    2kg    Torque    27   oz.inch     Rotated quickly at about 300 degree/minute +
RA      1kg    Torque    13.5 oz.inch    Did not rotate
RA      2kg    Torque    27   oz.inch     Rotated smoothly at about 90 degree/minute

Conclusion
The required torque for RA  would be on the limit for a (cheap, ungeared) NEMA 14 stepper but a NEMA 17 motor with a torque of 84 oz.inch should be up to it.

So now all I have to do is  order the bits & learn how to use an Arduino.

 

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