I want to make a robot arm where a joint is powerful enough to lift 8 kg up at a distance of 1 meter.
This requires torque $tau = r*F = r*m*g$ = about 80 nm.
So now I am trying to find the requisite parts to put this together, i.e. motor + gears and then controller.
It seems that I will require a very high gear ratio to make this happen. For example this motor: http://www.robotshop.com/ca/en/banebots-rs-550-motor-12v-19300rpm.html has stats:
stall torque = 70.55 oz-in = 0.498 nm no load roation speed = 19300 rpm
To get my required torque, I need a gear ratio of 80/0.498 = about 161:1 (and the max speed drops to about 120 rpm).
1) Do my calculations so far seem correct? It seems a bit suspect to me that an $8 motor with some gears can cause a 17.5lbs dumbbell to rotate about a circle of radius 1m twice a second (I'm barely that strong). This type of torque would be awesome for my application, but perhaps I'm missing something in the calculations and being too optimistic (e.g. efficiency).
2) Is it safe to operate a motor at such a high gear ratio? Gears are small, and I'm worried they'll easily crack/break/wear down quickly over time. Does anyone know of example gears that I should consider for this?
Thank you very much for any help, cheers.