which stepper and gearbox?

[danielsun]

Been doing a bit of reading but a little unsure!
I have a dob driver set up wich i'm looking at fitting and i am chasing stepper motors for it.I know that i need a 1.8deg and 12v but looking at rs aust web site i dont know about the mNm torque.
There is 70nNm, 260mNm 500mNm and higher torques but i figure it would be one of these three.
Does anyone know which one ? Or do i only need a stronger torque just to drive the az.

Also would i need a 50:1 gearbox, or would a 100:1 be better considering i dont half step the motor?
Any help would be much apreciated.

[Vermin]

Does this help?

http://zone.ni.com/devzone/conceptd.nsf/webmain/FC873A0E31F13E928625683F006D5803

[Starkler]

When I was researching equatorial platforms, the accepted wisdom was that discrete steps should be 1 arcsecond or less at the telescope to avoid vibrations and actually seeing them.

1.8 degrees = 6480 arcseconds. So if you have a 100:1 gearbox, you will still need a further 64.8:1 reduction or better in the final drive.

[davidgate]

Rather than guess, try an experiment - measure the starting torque required to get the scope moving. This could be done by applying a force with a spring balance at a known radius (this would be a small number so an appropriately sensitive spring balance would be required).

For example say you applied the spring balance at 100mm radius (0.1m) and you get a starting force of 0.1kg.

For a steadily applied load a Newton (N) = kg x g (gravitational constant 9.81m/s2)

Torque = force x distance it is applied at

Therefore the measured starting torque = 0.1 x 9.81 x 0.1 = 0.0981 Nm

Multiply by 1000 to get mNm (milli newton meters) = 98.1mNm.

Remeber also, a reduction gearbox will multiply the torque by the same ratio e.g a 100:1 ratio will give approx 100x more torque!

The spring balance is not too accurate but it should get you in the right ball park. I hope this helps.

Dave G :D

[danielsun]

Thanks for the help guys it's much apreciated ;)
I found some stepper motors out of some old 5 1/4" floppy drive systems after reading some tips from old posts from David P's dob driver thread. :thumbsup: