Finding diameter of torsion shaft

1. Jul 30, 2013

Mechanical94

1. The problem statement, all variables and given/known data

The solid shaft is made from brass that has an allowable shear stress
= 100 MPa and a shear modulus of elasticity G = 39 GPa. The length of the shaft is L = 2 m, and over the length of the shaft the allowable angle of twist is θ = 0.10 rad.the shaft is subjected to a torque of T = 25 kNm, what is the required diameter of the shaft

2. Relevant equations

T/J=t/r=Gθ/L

Last edited: Jul 30, 2013
2. Jul 30, 2013

Staff: Mentor

Hi Mechanical94. Welcome to Physics Forums.

In this situation, what is the equation for the local shear strain as a function of how θ is varying with distance along the shaft, and as a function of radial distance r from the axis of the shaft? Does the shear strain vary with axial location? Does the shear strain vary with radial location? What is the equation for the local shear stress?

Chet

3. Jul 30, 2013

Mechanical94

this is all the information given to me i have attached a picture to this post hopefully it can answer your questions

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• question torsion bar.jpg
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4. Jul 30, 2013

SteamKing

Staff Emeritus
Those are questions which you, Mechanical94, have to answer. After all, it is your problem to solve. In case you missed it when you read the Terms & Conditions, PF is not a homework service.

5. Jul 30, 2013

Staff: Mentor

As SteamKing said, you already have enough information to answer all my questions. I already know the answers to these questions. Now, it's up to you.

Chet

6. Jul 30, 2013

Mechanical94

i understand about this not being a homework service however this is my workings out and i got a diameter of 7mm, apparently my answer is not correct can anyone spot my error or can anyone help me to get the correct answer??

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Last edited: Jul 30, 2013
7. Jul 30, 2013

Staff: Mentor

In the right hand side of the equation in picture 1, the algebra is incorrect. It should be:

$$J=\frac{TL}{Gθ}$$

Chet