Shaft radius from shear modulus

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Discussion Overview

The discussion revolves around determining the appropriate shaft radius for a motor design, focusing on the relationship between shear modulus, shear stress, and angle of twist in steel materials. Participants explore the challenges of calculating shear stress and the implications of selecting an angle of twist.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant is trying to find the shaft size for a motor and is uncertain about how to derive shear stress from shear modulus without knowing the angle of twist.
  • There is a suggestion to set a low angle of twist, with one participant proposing 0.5 degrees as a reasonable value.
  • Another participant questions whether the issue lies in performing calculations or in obtaining material properties for steel.
  • A participant expresses difficulty in finding maximum shear stress values for various types of steel and seeks guidance on acceptable margins for angle of twist.
  • One suggestion is to look for "Shear Strength" as a potential resource for finding shear stress values.
  • It is noted that for low/medium carbon steels, shear yield stress can be approximated as 0.6 times the tensile yield stress, prompting a question about the specific grade of steel being used.

Areas of Agreement / Disagreement

Participants express varying levels of uncertainty regarding the calculation of shear stress and the selection of an appropriate angle of twist. There is no consensus on how to determine these parameters or on the acceptable values for the angle of twist.

Contextual Notes

Participants mention the need for specific material properties and the relationship between shear modulus and angle of twist, indicating that assumptions about these values may affect calculations.

Who May Find This Useful

Engineers and designers working on motor shaft specifications, materials scientists interested in shear properties of steel, and students studying mechanics of materials may find this discussion relevant.

fahraynk
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Hi. I am trying to determine the size of a shaft for a motor I am designing. The thing is, I can't seem to find shear stress alone for steel. I can find shear modulus easy though... But I don't really know the angle of twist. I know torque and I am trying to figure out radius.

Should I just set angle of twist really low and get it that way? What is a reasonable angle of twist for a motor shaft... 0.5 degrees? I was just going to find similar torque motors and plug their shaft radius into the torsion equation but I would like to know how they determine it from the Shear modulus from scratch. I am guessing guess the angle...
 
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Just to be clear - is your problem with doing the calculations or with finding material properties for the steel that you are using ?
 
Nidum said:
Just to be clear - is your problem with doing the calculations or with finding material properties for the steel that you are using ?

It is with finding material properties. I never thought about angle of twist for a rotor shaft. I don't really know what acceptable margins of this are. I was trying to find a maximum shear stress of various types of steel but could only find a shear modulus. To use the shear modulus you need to know the angle of twist (I think).
What metric would tell me the maximum angle of twist before the shaft reaches a plastic zone? That way I could plug that angle into the shear modulus to get the maximum shear stress.
 
fahraynk said:
I can't seem to find shear stress alone for steel.
Try "Shear Strength".
 
For ordinary low/medium carbon steels and approximate calculations you can assume shear yield stress to be 0.6 times tensile yield stress .

What grade of steel are you actually using ?
 

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