Design a shaft considering torsional failure

In summary, the designer needs to consider the torque and displacement surge that the shaft will experience when the load is attached to it securely. The designer also needs to consider the material and geometry of the shaft and the load in order to find an appropriate failure theory and design the shaft accordingly.
  • #1
ajayravishankar14
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[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?
 
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  • #2
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.
 
  • #3
trurle said:
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!)
 
  • #5
trurle said:
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.
 
  • #6
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.
 
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1. What is torsional failure?

Torsional failure is a type of mechanical failure that occurs when a shaft or other rotating component is subjected to excessive torque, causing it to twist or deform beyond its elastic limit.

2. How do you design a shaft to prevent torsional failure?

To prevent torsional failure, a shaft must be designed with sufficient diameter and material strength to withstand the expected torque and avoid reaching its elastic limit. Additionally, the shaft must be properly supported and aligned to prevent excessive bending or misalignment, which can also contribute to torsional failure.

3. What factors should be considered when designing a shaft to resist torsional failure?

When designing a shaft to resist torsional failure, factors such as the expected torque, speed, and operating conditions must be taken into account. The material properties of the shaft, such as its yield strength and modulus of elasticity, are also important considerations.

4. How does the shape of a shaft affect its resistance to torsional failure?

The shape of a shaft can have a significant impact on its resistance to torsional failure. A shaft with a circular cross-section is generally stronger and more resistant to torsional forces than a shaft with a non-circular cross-section, such as a square or rectangular shape. Additionally, a hollow shaft is typically weaker than a solid shaft of the same outer diameter.

5. Can torsional failure be prevented entirely through design?

While proper design can greatly reduce the risk of torsional failure, it cannot be completely eliminated. Factors such as material defects, manufacturing errors, and unexpected operating conditions can still lead to torsional failure even in well-designed shafts. Regular maintenance and monitoring of shafts can help identify potential issues before they result in failure.

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