# Homework Help: Determine angle of twist in shaft

1. Feb 16, 2012

### togo

1. The problem statement, all variables and given/known data
Compute the torsional shear stress and angle of twist in degrees in an aluminum tube, 600 mm long, having an inside diameter of 60 mm and outside diameter of 80 mm when subjected to a steady torque of 4500 N*m. Then specify a suitable aluminum alloy for tube.

2. Relevant equations

angle = TL/GJ
J = piD^4/32 (Do^4 - Di^4)

3. The attempt at a solution

angle = (4500 N*m)(0.6 m) / (2.8 x 10^-6)(80 x 10^9) = 0.012 rad

2. Feb 16, 2012

### LawrenceC

J = piD^4/32 (Do^4 - Di^4)

This is wrong. Check formula for polar moment of inertia for hollow shaft.

3. Feb 17, 2012

### togo

my book doesn't have it

two examples for hollow shaft use the same J formula

4. Feb 17, 2012

### LawrenceC

The way you have it written, you have the units of length to the 8th power. That is not correct.

The correct formula is

J = pi (Do^4 - Di^4)/32

5. Feb 17, 2012

### togo

Thanks for clearing that up

6. Feb 17, 2012

### togo

7. Feb 17, 2012

### LawrenceC

Did you use Young's Modulus instead of the shear modulus for aluminum?

8. Feb 17, 2012

### togo

Yes I did. Thanks problem solved