Max shearing stress problem

[Pepsi24chevy]

Hey guys, I got a problem that goes like: A shaft with a circular cross section is subjected to a torque of 120ft-lb. Its shaft's diameter is 0.750 in and its length is 15 in., determine the maximum shearing stress.

I did the following: I tried to use shearing stress = Tr/J in which i plugged in as followed (120*.750/2)/(pi/32)*.750^4 and i get 1448, but the answer should be17.39ksi. I assume i am not getting the right answer because I am not factoring in the length but I am not sure. Any help is appreciated, thanks

 

[Pepsi24chevy]

nm, i figured it out, Stupid me forgot to convert the ft-lb to inch-lb.

 

[quicknote]

Hello,

Based on the information provided, it seems like you are on the right track but may have made a few calculation errors. The formula you are using, shearing stress = Tr/J, is correct for determining the maximum shearing stress in a circular shaft. However, there are a few things to keep in mind when plugging in the values.

Firstly, make sure you are using consistent units throughout the calculation. It seems like you have used feet for torque (120ft-lb) and inches for diameter (0.750 in) and length (15 in). It would be best to convert all units to either feet or inches before plugging them into the formula.

Secondly, when calculating the polar moment of inertia (J), make sure to use the correct formula for a circular cross section, which is J = πd^4/32, where d is the diameter of the shaft. In your calculation, it seems like you have used the formula for a solid cylinder, which is πd^4/64.

Once you have corrected these errors and plugged in the correct values, you should get the correct answer of 17.39 ksi for the maximum shearing stress. Remember to always double check your units and formulas when solving engineering problems like this.

I hope this helps. Good luck with your calculations!