Torque and Shear stress this time

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Discussion Overview

The discussion revolves around calculating the external diameter of a tube required to transmit a specific torque while adhering to a maximum shear stress limit. The context includes mechanical engineering principles, specifically related to torque, shear stress, and the geometry of hollow tubes.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant expresses confusion regarding mechanical concepts and seeks assistance in determining the external diameter of a tube given specific torque and shear stress constraints.
  • Another participant provides relevant formulas, including the polar moment of inertia (J) and the relationship between torque, shear stress, and radius.
  • A third participant asserts that all necessary information is available to solve the problem, emphasizing that J is a function of the external diameter (D) only.
  • A later reply presents a formula derived from the provided information, indicating how to solve for the external diameter (D) and suggesting a specific numerical solution.

Areas of Agreement / Disagreement

Participants generally agree on the formulas and approach to solving the problem, but there is no explicit consensus on the final solution as it is presented by one participant without confirmation from others.

Contextual Notes

The discussion does not clarify certain assumptions, such as the material properties or specific conditions under which the shear stress limit applies. Additionally, the derivation of the final diameter solution is not fully explored or verified by all participants.

Who May Find This Useful

This discussion may be useful for students or professionals in mechanical engineering, particularly those studying topics related to torque, shear stress, and the design of tubular structures.

chrisking2021
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I'm :confused: I'm OK at electrical theory but mechanical just doesn't compute.

I have the following question if anyone can help.

Determine the external diameter of a tube needed to transmit a torque of 30KNm if it has an external diameter twice that of its internal diameter. The sheer stress is not to exceed 80 MPa.


Any help would again be gratefully received.

Also could anyone tell me how to put formula into this thread as i don't really know how.:smile:
 
Last edited:
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Useful formula

J = (pi/32)*(D^4-d^4)

T/J = Sheer Stress/r

T = Torque
r = Radius of external diameter
D = External diameter
d = Internal diameter

These are some formula I've tried working through but i need J or D to work it through.
 
You have everything you need. J is a function of D only. D and D/2.
 
Chrisking2021,
As Cyrus mentioned. All the info was given.
To finish off the question, for those like me found the thread.
The formula should look like:

T=τ/r*(J)
D= external diameter in meters

30*10^3 [Nm] = 80*10^6 [Pa=N/m^2] / (D/2) * (pi/32)*(D^4-(D/2)^4)

When solved
D=.126781154m

Thank you for the comments. I used this thread to help study for the FE exam.
 

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