What Material Is Best for a 2740 mm Shaft Transmitting 2100 kNm at 40 RPM?

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Discussion Overview

The discussion focuses on determining a suitable material for a shaft with specific dimensions and operating conditions, including allowable shear stress and the impact of various stresses on material selection. The context involves mechanical engineering principles related to torque transmission and shaft design, with considerations for bending moments and fatigue.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant emphasizes the need for a detailed analysis of stresses, noting that all loads and geometry must be considered to determine suitable materials.
  • Another participant argues that neglecting bending moments in the analysis is unwise, suggesting that torsional and bending stresses should be calculated to find the maximum stress induced.
  • A later reply highlights the importance of fatigue calculations in the design process, indicating that these may govern the final material choice.
  • Some participants point out that the support conditions of the shaft, including bearing locations, are critical to understanding whether neglecting bending moments is valid.
  • There is a suggestion that factors of safety should account for various stresses, including fatigue, in the material selection process.
  • One participant expresses skepticism about the original poster's level of experience, implying that the question may stem from a student perspective.

Areas of Agreement / Disagreement

Participants generally disagree on the validity of neglecting bending moments in the analysis, with some arguing it is crucial while others suggest it may not be necessary depending on support conditions. The discussion remains unresolved regarding the best approach to material selection and the importance of various stress factors.

Contextual Notes

Limitations include the lack of information about the shaft's support conditions and the specific application context, which may influence the relevance of bending moments and fatigue in the design considerations.

shams alam
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I want to find out suitable material with it physical properties, like allowible shear stress, which will be suitable for the shaft with following given data.
R.P.M = 40 rpm
Transmitting torque = 2100 kNm
Length of the shaft = 2740 mm
Smooth drive.
Neglecting the bending movement of the shaft.
 
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Selecting the material requires knowledge of the stresses, and no one can know those stresses without doing the analysis on the particular shaft geometry. You need all the loads (not just torque) in addition to all the geometry.
 
shams alam said:
I want to find out suitable material with it physical properties, like allowible shear stress, which will be suitable for the shaft with following given data.
R.P.M = 40 rpm
Transmitting torque = 2100 kNm
Length of the shaft = 2740 mm
Smooth drive.
Neglecting the bending movement of the shaft.

2100kNm & shaft length of over 2m, neglecting Bending Moment would be stupid.

Start finding all the stresses, eg, torsional stress & bending stress, calculate the maximum stress induced, assume an appropriate factor of safety depending upon the application, get the minimum strength required using FoS * Stress induced, select a material.
 
ank_gl said:
2100kNm & shaft length of over 2m, neglecting Bending Moment would be stupid.

Start finding all the stresses, eg, torsional stress & bending stress, calculate the maximum stress induced, assume an appropriate factor of safety depending upon the application, get the minimum strength required using FoS * Stress induced, select a material.
That's pretty good ank_gl, and it's not incorrect. But don't forget that a shaft is typically subject to fatigue, and it's typically the fatigue calculation that governs the final design.
 
ank_gl said:
2100kNm & shaft length of over 2m, neglecting Bending Moment would be stupid.
Not necessarily. The OP hasn't mentioned anything about how the shaft is supported, i.e. the locations/spacing of the bearing surfaces. That is going to be the biggest factor in the real design in that the rotordynamics will dictate whether the bending moment argument is valid or not.
 
That's pretty good ank_gl, and it's not incorrect. But don't forget that a shaft is typically subject to fatigue, and it's typically the fatigue calculation that governs the final design.

yes right, but I meant that FoS should take care of all those things.

Not necessarily. The OP hasn't mentioned anything about how the shaft is supported, i.e. the locations/spacing of the bearing surfaces. That is going to be the biggest factor in the real design in that the rotordynamics will dictate whether the bending moment argument is valid or not.

oops!:redface: forgot that part altogether about BM
 
ank_gl said:
2100kNm & shaft length of over 2m, neglecting Bending Moment would be stupid.

and so is neglecting friction and aero drag in dynamics problems, but when you're a student you have to start somewhere. basically, I am assuming the OP is a student, since the question has that school-ness ring to it
 

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