Torsion in Shafts: Conceptual Doubt Explained

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Discussion Overview

The discussion revolves around the conceptual understanding of torsion in shafts, particularly in the context of strength of materials. Participants explore the implications of treating one end of a rotating shaft as fixed while the other end receives torque, and the effects of this assumption on stress and strain calculations.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant expresses confusion about how a rotating shaft can be considered fixed at one end, questioning the application of equilibrium equations in this scenario.
  • Another participant explains that the torque in the shaft depends on the relative angle between the ends rather than their absolute angles, suggesting that treating one end as fixed is a matter of convenience rather than necessity.
  • A similar point is reiterated, emphasizing that the induced torque is based on relative rotation, and the concept of a fixed end is a simplification.
  • One participant raises a scenario involving a shaft supported by bearings and questions why it would twist if it is free to rotate, prompting a discussion about the relationship between torque and shear stress.
  • Another participant asserts that if the shaft is truly free to rotate, there would be no torque and thus no shear stress, questioning the purpose of applying torque in practical scenarios.
  • There is a consensus among some participants that treating one end as fixed is a simplification, acknowledging that it is not truly fixed in an absolute sense.

Areas of Agreement / Disagreement

Participants generally agree that treating one end of the shaft as fixed is a simplification for analysis purposes. However, there remains some disagreement regarding the implications of this assumption, particularly in relation to torque and shear stress in practical applications.

Contextual Notes

The discussion highlights the dependence on definitions of fixed and free conditions, as well as the assumptions made in analyzing torsion in shafts. The nuances of torque transmission and the role of bearings in supporting shafts are also noted but remain unresolved.

ramzerimar
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I have some conceptual doubts about shafts subjected to torsion. When studying Strenght of Materials, to find stress and strain in power transmission shafts, we consider that one of the ends of the shaft is fixed, with all degrees of freedom restricted, and the other one is receiving torque. I'm having trouble to understand this fixed end. If it's a rotating shaft, how can it be fixed? But if it's not fixed, we can't apply equilibrium equations to solve the problem.
 
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Suppose that the shaft is strain free (no twist) to begin. Make a mark on each end at the same angular position. Now, twist the shaft so that the angle between the marks is phi. There is now shear stress = T*c/J in the outer fibre, OK?

Now, suppose you roll both ends through an angle theta, maintaining the angular difference phi. The torque in the shaft is the same as it was before because it depends on the relative angle between the ends, not the absolute angle.

Where you choose the reference mark, at theta = 0, theta = pi/27, or something else, it is only the relative rotation, the twist, that matters as far as the induced torque. Treating one end as fixed may be convenient, but it is certainly not necessary.
 
Dr.D said:
Suppose that the shaft is strain free (no twist) to begin. Make a mark on each end at the same angular position. Now, twist the shaft so that the angle between the marks is phi. There is now shear stress = T*c/J in the outer fibre, OK?

Now, suppose you roll both ends through an angle theta, maintaining the angular difference phi. The torque in the shaft is the same as it was before because it depends on the relative angle between the ends, not the absolute angle.

Where you choose the reference mark, at theta = 0, theta = pi/27, or something else, it is only the relative rotation, the twist, that matters as far as the induced torque. Treating one end as fixed may be convenient, but it is certainly not necessary.

Suppose I have a shaft supported by a pair of bearings, one at each end, and there's a motor connected to the shaft. When we start the motor, the shaft will rotate with some angular velocity. Why would the shaft twist then, if it is free to rotate because of the bearings?
 
If it is truly free to rotate, there will be no torque in the shaft and no shear stress. Why would you do this?

The usual reason for turning a shaft with a motor is to enable the shaft to drive some machine, to do work, and that takes a torque in the shaft to transmit power to the driven machine.
 
Dr.D said:
Where you choose the reference mark, at theta = 0, theta = pi/27, or something else, it is only the relative rotation, the twist, that matters as far as the induced torque. Treating one end as fixed may be convenient, but it is certainly not necessary.

Okay. So when I treat one end as fixed is just a way of simplifying things. Actually, it's not really fixed, only in relation to the other end.
 
That's about the size of it.
 

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