Torque Calculations

[AndyB]

I am working on designing a 4-bar linkage arm and was attempting to do some calculations on which motors and gear ratios I should be using. I calculated it at 60in with 20lbs of force. Below are my calculations with a dual-globe motor setup. I came away with an absurd amount of supplied stall torque. If I have 20000 oz-in of required torque, how much actual torque should I be shooting for? I understand that one needs to supply more than just the stall torque, but I don't understand how much of a difference there needs to be between the required and supplied torque. Maybe you guys can help me.


Arm Length: 60 in
Arm Weight: 20lbs
Required Torque = Length * Weight = 19200 oz-in

Motor: Globe
Quantity: 2
Stall Torque: 3194 oz-in
Free Speed: 80 RPM
Gear Ratio: 48:1
Supplied Stall Torque: 306675 oz-in

Motor Specs: http://www2.usfirst.org/2005comp/Specs/Globe_Motor.pdf
I can't find any torque curves.


Thanks for the help