[Steve_C]

Hi Steve,

If you re-read my post earlier in this thread where I replied to your Q about motors, I stated that DC motors are difficult to gear down. It's because of their high native output speed.

Surely if I got a 4rpm o/p motor off ebay and used 4:1 gearing that would work as per Gary's article?

Let's say you build a tracker with a 20TPI 1/4" whitworth rod and a 10 inch arm radius, every turn of the nut advances the arm by

206265 ÷ 20 TPI ÷10 inch = 1031 arc sec per turn of the nut. But the sky only moves at 15.04 arc sec per sec. So you need to turn the nut at 1 turn every 68.6 seconds.

I need to understand the maths for this, long time since I studied math! Gary's article mentions a formula R= rpm/(.004375*tpi) to get the rod radius and rod position from the hinge, but I don't understand where the .004375 constant comes from.

If you were to order this 6rpm motor.

http://www.ebay.com/itm/12V-6RPM-Oup...item2a52ae43c2

You will need approximately 6:1 gear reduction in between the motor gearhead output and the gear that turns the shaft.

I understand that, and it is fairly easy to do. I plan to go with a stepper, either a28BYJ-48 or a Nema 17 format, using the driver / controller board I mentioned previously. Just need to get the arm radius and arm postion calculations correct. I'll look at the spreadsheet mentioned elsewhere. I haven't decided on motor voltage yet, I have some spare 11.1V li-pol batteries from some Rc gear I can use, I'll probably regulate them down to around 6V.