# Twist of two diameter shaft

I have this example:

• $M_k$ = 80 Nm
• $d_1$ = 30 mm
• $d_2$ = 20 mm
• $a$ = 600 mm
• $b$ = 500 mm
• $G$ = 8,5e4 MPa

I want to know the twist at free end of this shaft. I am using the formula:

$$\phi = \dfrac{M_k \cdot a}{G \cdot \dfrac{\pi d_1^4}{32}} + \dfrac{M_k \cdot b}{G \cdot \dfrac{\pi d_2^4}{32}}$$

But the result in the book is 0,013 rad. Where is the error?

• Whenever I end up with answers that contradict what my textbook states is the correct answer, I always double check that I am using the correct equation for the question.
– user16
Apr 8 '15 at 18:57
• Also 'working backwards' to try an determine where the errors may be my help
– user1184
Apr 8 '15 at 21:46
• I also get 0.037... Apr 8 '15 at 22:59
• Like series/parallel connections do they add up do their reciprocals add up? Compare contribution of each shaft whether it increases or decreases. Apr 13 '15 at 21:04