# Choosing Motor (Servo or Step motor)

Hello everyone. I wonder if someone could help me choose the proper motor for a project I am working with.

So the idea is to move a lever of 10cm (which rotates from one end) using a servo ou step motor. My doubt is about selecting the right amount of torque necessary from the motor.

Let's say the lever has to be pushed with same force as to lifting 10kg. So, my reasoning is as follows:

10Kg needs around 98.0665 ~ 100 Newtons of force.

It is known that Torque = radius x Force, thus Torque = 0.1m * 100N = 10Nm (Assuming no loss of force due to vector directions).

Thus, this tells me I need a motor of at least 10Nm. But I should double it to be safe.

Conclusion,
A servo/step motor with Torque of 20Nm or ~200kg-cm, and a 10cm arm, should be enough to lift 10kg.

Is my reasoning ok? Or am I omitting something?

Thanks for the help!
Fernando

ferlnm:
am I omitting something?

The speed at which you want it to move. Servos are spec'd at some number of seconds / 60 degrees*

Then you need to calculate the mechanical power (P=Tw, Watts= Nm x rad/sec*) and then make a stab at the electrical power required allowing for some losses in the conversion process, and then arrive at the electrical current from P=VI.

• since a radian is 57degrees, seconds/60 is not a million miles off seconds/rad, so the inverse of that is close to rad/sec.

Neither, this sounds like a match to a linear actuator. That's a lot of force, better to apply the force directly to the load.

If you definitely need to use rotary motion, you need a fair amount of reduction gearing on whatever motor you choose.

You haven't said anything about how fast you want the motion. Without both torque and speed there's no way to size a motor. torque x speed = power.