# Torsion on a shaft calculated by hand vs FEA

#### fahraynk

Hi, I am a bit confused. So I have a shaft with a step on both sides so it gets really large in the middle. I am trying to model torsion on the rotor of a BLDC motor with FEA (or by hand calculations).

If I apply a moment on the sides and fix the faces at the end of the shaft I get 1 result. If I apply a moment on the end face and fix the other end face I get another result.

If I run a motor on max power and hold the end of the shaft so it does not move... Where should I apply the moment in my calculation, on the end of the shaft I am holding or on the side face of the shaft where the motor is creating the power? How do you express this difference in a torsion calculation? In a torsion calculation there is nowhere to put the face the torque is applied to, but in a finite element analysis the results come out differently if I apply the torque to different faces of the object.

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#### Nidum

Gold Member
The driving torque produced by the armature is balanced by a reaction torque coming from the load .

So where does the driving torque act on the motor shaft ?

Where does the reaction torque from the load act on the motor shaft ?

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#### fahraynk

Driving from thicker part of shaft on sides, rection from sides of thinner part of shaft. Why does the FEA analysis come out different though? Shouldn't torque be the same at different parts with different radius regardless of the torque coming from driving end or reaction end?

"Torsion on a shaft calculated by hand vs FEA"

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