Torsion on a shaft calculated by hand vs FEA

In summary, the conversation discusses the confusion surrounding where to apply the moment for torsion calculations in a BLDC motor with FEA. One person suggests applying the moment on the sides of the shaft while the other suggests applying it on the end face. They also mention the difference in results between applying the torque to different faces and the balance between driving and reaction torque.
  • #1
fahraynk
186
6
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).
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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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  • #2
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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  • #3
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?
 

1. What is torsion on a shaft?

Torsion on a shaft refers to the twisting force that is applied to a rotating shaft. This force causes the shaft to bend and deform, which can lead to structural failure if the force is too great.

2. How is torsion on a shaft calculated by hand?

To calculate torsion on a shaft by hand, you will need to know the applied force, the length of the shaft, and the material properties of the shaft. You can then use equations such as the torsion equation or the maximum shear stress equation to determine the amount of torsion on the shaft.

3. What is FEA?

FEA stands for Finite Element Analysis. It is a computerized method used by engineers and scientists to model and analyze complex structures and systems. It can be used to simulate the behavior of a shaft under torsion, providing more accurate results than hand calculations.

4. What are the advantages of using FEA for calculating torsion on a shaft?

The main advantage of using FEA for calculating torsion on a shaft is that it can provide more accurate results. FEA takes into account the non-linear behavior of materials and the complex geometry of the shaft, which may be difficult to account for in hand calculations. Additionally, FEA can also provide visual representations of the results, making it easier to understand and interpret.

5. Are there any limitations to using FEA for calculating torsion on a shaft?

While FEA is a powerful tool for analyzing complex systems, it does have limitations. One limitation is that the accuracy of the results depends on the accuracy of the input parameters and assumptions made in the model. Additionally, FEA can be computationally expensive and time-consuming, making it less practical for quick calculations or simple systems.

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