# Thread: how can I calculate the Torque requirement?

1. ## how can I calculate the Torque requirement?

Hello, I am new here but I need some help. I've been commanded to choose a Stepper motor to an application but I've never worked in this field (not even with motors) so I am completely lost and I really am in a rush...
Iwould like to receive some help with the calculations and to know if what i am doing is ok!

My problem is the following:

The stepper has to rotate a mechanism through 4 different possitions (each at 30 º from the previous one, the speed requirements say that the mechanism has to change from each possition to the next one in 1 second, which translates in a speed of 30 º/second (0.53 rads/secnd). If the step size is 1.8 º, We need a 17 steps per second speed. (not a big deal right?).

The only way i know to calculate the stepper requirements in terms of torque and power is to calculate the Intertia torque and the friction torque that the motor have to provide to make that movements. (There won't be any external torque when the stepper is stopped so i think holding torque is not a problem here).

SO if the mechanism has a 0.005 kg m2 of intertia and the stepper 0.03 Nm friction torque, the motor torque needed is

Tmotor = Tinertia + Tfriction= a*Izz + 0.03 (without Safety margins)

The only thing I have to calculate is the maximum angular acceleration (a) that will be the critical parameter right?. how would you calculate that angular acceleration? the catalog doesn't tell anything about the maximum angular acceleration of the stepper so i don't know the limitations.
what's the typical way to accelerate it? for example, can it reach its top speed in just one step (that would need a 9rad/s2 acceleration) or it needs a more progressive increase of the speed? I just need someone to give me some orientation with this.

using a = 9rad/sec2 -> Tmotor = 0.08 Nm (withough safety margins)

The only Torque data provided in the catalog (SAGEM) is the Holding Torque. But i am not interesting in that torque, i need to know the torque provided by the motor in movement! right? but i don't have that data!!!

The highest holding torque provided in the catalog is 0.5Nm, would it be enough?

2. Trying to calculate the torque from the load moment leaves out a lot of vital info. Especially since it seems you just want to connect the load device on the end of the stepper motor shaft?

Exactly what is the load? What does it do? Is it sensitive to resonance? What are the mounts/bearings etc? Do you have a photo? What precision/accuracy of final angular position is needed?

3. Originally Posted by RomanLini
Trying to calculate the torque from the load moment leaves out a lot of vital info. Especially since it seems you just want to connect the load device on the end of the stepper motor shaft?

Exactly what is the load? What does it do? Is it sensitive to resonance? What are the mounts/bearings etc? Do you have a photo? What precision/accuracy of final angular position is needed?
the load is something like a filter wheel with 4 positions, i don't have a picture but it is something like this:
The speed we have chosen is 30º/secnd

http://img.directindustry.es/images_...ico-282911.jpg

and yes, our idea is to directly connect the "filter wheel" to the motor shaft, but we haven't decided yet, we are considering using a frameless configuration and add some bearings...

the motors we are looking at have a 1.8º step size but we would like a precision lower than 1º so we are thinking in using the motor in half step mode or even considering a reducer (we don'tlike this idea because the mechanism have toooo many cycles and a reducer may reduce the live of the mechanism)... anyway before taking those desitions we want to know the torque we are going to need

what about the holding torque? can I assume that the "working torque" is more or less 60%of the holding torque?

4. There is a very good rule of thumb for this and a calculator at techref.massmind.org/techref/io/steppers.htm#Estimating which will help figure it all out for you.