# Choosing a right motor



## major (Apr 4, 2008)

Nouman shaikh said:


> Hey. My project involves a motor which is driving a load of 20 kg through a pair of gears. A small gear of probably 100 mm diameter on the motor drives a big gear of 300 mm diameter. The load will be over the big gear. The motor provides azimuth movement to the load. I am a bit confused about the sizing of motor specifically the "torque"of motor. I have visited may pages about the torque calculation but they all calculate the torque with gravitational force keeping in the mind. In my case motor is not supposed to work against gravity instead it is supposed to provide enough torque to overcome the inertia and friction force. How to calculate the torque of motor in this case?
> Please help . Thanks


It does not sound like an electric car to me, but anyway: The motor torque is typically a specification given by the motor supplier, often as a torque constant (to be multiplied time the current) or as a speed/torque curve. Perhaps this link can provide some help: http://lancet.mit.edu/motors/motors3.html


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## tomofreno (Mar 3, 2009)

Torque is the cross product of a force and the radius through which it is acting about some axis. For a geared system the force is normal to the axis, so torque is simply the product of force and radius. The gears are a "torque multiplier", with 3:1 ratio in your case. You need the force > the frictional force, f, so T > r*f/3. How much greater will depend on the slew rate you require (how fast you want to accelerate the load).


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