Question about torsional shear stress

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Discussion Overview

The discussion revolves around the calculation of maximum torsional shear stress in a circular rod subjected to opposing moments. Participants explore the implications of a freely rotating rod and how applied moments affect the resulting shear stress, focusing on the equation (T*c)/J.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions what the maximum moment in the rod is for calculating maximum torsional shear stress.
  • Another participant explains that the unbalanced moment will cause the rod to accelerate, suggesting that the effective moment acting on the rod is M, balanced by M/3 and the moment causing acceleration.
  • A later reply indicates confusion about the distribution of moments, proposing that M should be partially used to resist M/3, leading to a belief that the moment causing torsion might be M/3.
  • Another participant emphasizes the need for clarity, suggesting that a diagram or original wording is necessary to resolve multiple interpretations of the situation.
  • One participant proposes that if M/3 is applied to both ends, the stress would be calculated as M/3 times c/J, concluding that the maximum twist moment is M/3 due to the lack of equilibrium.

Areas of Agreement / Disagreement

Participants express differing interpretations of how the moments interact and their implications for torsional shear stress. There is no consensus on the correct approach or calculation method, indicating ongoing debate.

Contextual Notes

Participants highlight the need for additional information, such as diagrams or original problem wording, to clarify the situation and assumptions involved in the calculations.

rcw2004
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If a circular rod is allowed to rotate freely and a moment (M) is applied to one end of the rod. A moment acting in the opposite direction, with magnitude (M/3), is applied to the other end of the rod. What is the maximum moment in the rod that would be used to calculate the maximum torsional shear stress in the equation (T*c)/J?
 
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Welcome to PF.

Because the rod is free to rotate, the unbalanced moment will result in an acceleration of the rotation of the rod. The rod will be subjected only to the moment (M), which will be balanced by (M/3) plus the moment causing acceleration.
 
Welcome to PF.

rcw2004 said:
If a circular rod is allowed to rotate freely and a moment (M) is applied to one end of the rod. A moment acting in the opposite direction, with magnitude (M/3), is applied to the other end of the rod. What is the maximum moment in the rod that would be used to calculate the maximum torsional shear stress in the equation (T*c)/J?
Please show your work based on the hints given by Baluncore. Thank you.
 
Baluncore said:
Welcome to PF.

Because the rod is free to rotate, the unbalanced moment will result in an acceleration of the rotation of the rod. The rod will be subjected only to the moment (M), which will be balanced by (M/3) plus the moment causing acceleration.
Thank you for the answer. I am still confused, is M not the moment that is causing the acceleration? My initial thought was that one third of M would go towards resisting the M/3 moment, and the other two thirds of M would accelerate the rod. Then the Moment that is actually twisting the rod and causing torsion would be M/3. Is this incorrect? and if so why?
 
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rcw2004 said:
Is this incorrect? and if so why?
The moment, M, is applied to one end of the rod.
The rod must be able to transmit that M, along the rod, to where it will be countered by the 2M/3 that accelerates the rod, and the far end M/3.

There are multiple interpretations of your question. We need a diagram, or the original wording to determine the situation.
 
Let's say you apply M/3 to both ends - then the stress is M/3 times c/J. Since one end applies M then there is no equilibrium and rod accelerates since free to rotate. So max twist moment is M/3
 

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