Design a shaft considering torsional failure

[ajayravishankar14]

[I've searched the site and did google. I couldn't find a satisfactory answer]
I have the motor characteristics and the moment of inertia of the object I'm supposed to spin with it. I have to design the shaft considering the torsional failure.
Now, if I consider the inertia of the body, there is a small difference in rotational displacements, for example, consider starting the motor. The motor is at spin but the body isn't spinning yet. This yields a torsion to the shaft. What is the maximum value of the "difference" that can be allowed? How do I state these mathematically and find the diameter of the shaft? What are the parameters?

 

[trurle]

You are likely mixing here the concerns of coupling design and shaft design.
Shaft must withstand the specified torque (torsion), while withstanding rotational (angular) displacement difference and resulting surge of torque is the task of coupling.

 

[Tom.G]

Shaft must withstand the specified torque (torsion), while withstanding rotational (angular) displacement difference and resulting surge of torque is the task of coupling.

I agree that could be valid if there is a separate coupling device or the load is not securely attached to the motor shaft.
If you assume the load is securely attached, then maximum shaft torque would be the peak motor starting torque. (don't forget safety factors!)

 

[CWatters]

Not really my field but have you looked at pages like this..

https://www.engineeringtoolbox.com/torsion-shafts-d_947.html

It has examples towards the bottom.

 

[Dr.D]

You are likely mixing here the concerns of coupling design and shaft design.
Shaft must withstand the specified torque (torsion), while withstanding rotational (angular) displacement difference and resulting surge of torque is the task of coupling.

Is it not true (for some odd reason) that the same torque passes through both the shaft and the coupling? I fail to see the distinction being made here.

 

[Dr.D]

Usually the design of shafts (and couplings too) is concerned with both peak torque and also torsional fatigue. For the fatigue considerations, you need to adopt an appropriate failure theory (suited to the material and geometry involved) and consider the driven load as a source of torsional excitation as well as the motor.