Calculating Max Torque on a Ø140mm x 936mm Shaft

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Discussion Overview

The discussion revolves around calculating the maximum torque that can be applied to a stainless steel shaft with specified dimensions. Participants explore the necessary formulas and calculations related to torsional stress, polar moment of inertia, and section modulus, focusing on theoretical and practical aspects of mechanical engineering.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant seeks clarification on the formula needed to calculate maximum torque for a shaft of Ø140mm x 936mm.
  • Another participant emphasizes the need for the maximum torsional stress of the material and the calculation of the polar moment of inertia, J, and section modulus, Z.
  • Participants discuss the allowable stress figure of 77,000 N/mm² for the shaft material.
  • There is a request for the specific equations needed for torque (T) and polar moment of inertia (J).
  • One participant provides the formula T = Z * (allowable stress) and J = π/2 * (r^4) for a solid shaft, questioning if the shaft is indeed solid.
  • Another participant calculates the torque and expresses confusion over a seemingly low result of 3.5E-5 Nmm, suspecting a miscalculation.
  • Discrepancies arise as one participant claims a torque of 4.15x10^7 N-m, suggesting a significant difference in calculations.
  • Participants verify the calculation of J and discuss unit correctness, with one participant recalculating J and Z, leading to a torque result of 2043146490 Nmm.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct torque value, with differing calculations and interpretations of the formulas leading to significant discrepancies. There is uncertainty regarding the allowable stress figure and its implications on the calculations.

Contextual Notes

Participants express confusion over unit conversions and the application of formulas, indicating potential limitations in their understanding of the strength of materials and mechanical principles. The discussion reflects varying levels of familiarity with the topic.

pegghead
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I have a stainless steel shaft that is Ø140mm x 936mm long.

I need to calculate what the maximum torque is that I can apply to this shaft. I have looked around for formulas and have just got myself confused. Could someone tell me what the formula is that i need to work this out.
 
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What is the maximum torsional stress allowed for the shaft material? You will need this for the equation. Also, you need to calculate the polar moment of inertia, J, of the shaft and calculate the shafts section modulus, Z. The equation for Z is Z=J/c and c=outer diameter/2.
 
Last edited:
The figure i have for the material is 77,000N/mm^2
 
Ok now you can run the equation. Do you know how to calculate J?
 
Sorry no i don't know how to calculate J.
 
Do you have a book on strenght of materials?
 
I don't have a strength of material book.

Could yoou tell me the equations that i need.

Like T = ? ? ? ? ?

and

J = ? ? ? ? ?
 
Well,

T = Z*(allowable stress) where Z = J/c

J = PI/2(r^4) if the shaft is solid. Is the shaft solid?

r = c = Outside Diameter/2

Think you can handle the rest?
 
Last edited:
Lastly, be sure to check and make sure all of the units are correct. You should end up with N-mm

Thanks
 
  • #10
I have calculated the torque using the formulas given and i get 3.5E-5 Nmm of torque that a Ø140mm shaft can transmit when material allowable stress is 77,000N/mm^2.

I think somewhere i have miscaculated, i have checked a couple of times and got the same result.

O/D = 140mm
R = 70mm
Allowable stress = 77,000N/mm^2

The amount of Torque transmittable seems very low.
 
  • #11
Check your math.

I have 4.15x10^7 N-m

That is a lot of torque.
 
  • #12
the figure i am getting for J is around 6.5E-8
 
  • #13
Check the allowable stress. It's about 2 orders of magnitude higher than a strong grade of steel.
 
  • #14
The allowable stress figure that i have is 77KN/mm^2.
 
  • #15
For J,

J = PI/2(r^4) = 3.141592/2*(70^4) = 1.570796*(24010000)=37714811.96 mm^4
 
  • #16
Are you using all the correct units?
 
  • #17
I know where i was having problems it was in the formula, i was dividing Pi by 2x(r^4) and not Pi/2 x (r^4).

I get a result of 26534.37mm^3 for Z.

However when i now multiply this by the 77,000N/mm^2 i get 2043146490Nmm. Not the 4.15x10^7 that you got.
 

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