# How do I choose the stepper motor torque for my robotic arm

I'm a communication engineer so my knowledge about robotics is very little.

I know the basic formula of torque T=mgL

in my project I have 2 arms & 2 motors

i used a torque calculator online and it came out with a very high torque a s a result which I didn't even found in the market!

here is a photo of the calculations So the higher torque motor should have a torque of 580 Kg cm which is equal to about 60 Nm

I never found a stepper motor with such a great torque.

i read somewhere else in Quora that to lift a weight of 50 KG you need only 1NM so how come my 10 KG load requires that high torque?

so is there is something wrong with my calculations? and what is the holding torque? is it the same one I'm calculating or it has another relation.

Note: I'm not concerned much about torque resulted from angular acceleration as I want a constant rotational speed.

• You seem to be missing any consideration of including any type of gearing ratio that might benefit you enormously such as a gearbox, worm gear, threaded rod, etc. To calculate the force delivered to the load you also need to consider any mechanical advantage which you will gain from gearing. The torque rating on a stepper expresses the force it can produce at the far tip of an arm of length L attached to the motor shaft. But the motor will likely go faster than you require, and that excess velocity can be sacrificed and exchanged/traded for force through gear reduction or the like. Nov 2, 2017 at 1:47
• I did a similar simulation using SolidWorks: youtube.com/watch?v=crJXUlzJ918 Feb 19, 2018 at 19:50
• There's no way to solve this completely without knowing the velocities and accelerations you want to support. The system equation is M(q) * q'' + C(q, q') * q' + g(q) = τ, where M is the mass matrix, C is the Coriolis/centrifugal term, g is the gravity term, τ is the vector of joint torques, and q' and q'' are the first and second derivatives of the joint angle vector, q. Apr 17 at 15:58