Torsion: General (Comprehensive) - Strength of Materials

[@chrispaul9194]

Hey, great vid!! I had one question, for the last question of this problem, it asks if we had the same amount of material...If the question said same cross-sectional area would it be the same thing? Thanks in advance.

[@StructuralEng]

Thank you for the comment! For that question, the issue is the crosssection area is not consistent throughout the shaft. So, if we have two different types of crosssection, we can basically use the same process as you see in the video. In other words, we are giving weights based on length to various types of crosssections that exist through the whole shaft. The bottom line is the process will necessarily be the same, and you can use the exact same process I did in the video.

[@chrispaul9194]

@@StructuralEng Thanks for your response! I had another quick question, which way should we start our torque diagram from? Suppose in shaft CDEFG we started from the right, then the torque would start negative and all the torques in the shaft would be negative. Also suppose there was a gear to the left of gear C, Lets say gear L, starting our torque diagram from the left or right would both give 2 different diagrams. Starting from left we would have negative torque on that gear L but starting from right, it would end up as positive torque there. Thank you in advance for your reply!!

[@StructuralEng]

@@chrispaul9194 It is best to start torque diagrams from the left. But in reality, you could start from the right, but remember that you're going in reverse, and your positive convention is the opposite of what you had if you started from the left. So it can be confusing. In general, you don't need to start from the right in any scenario. Even in this question, as you see, although the given info is about shaft FG (right side) and we need the torque value in portion FG, intuitively, we can see the magnitude of torque in gear G and portion FG are the same.

[@karimabouelela3665]

Hey sir I didn't quite understand what will be the procedure if say that CDEFG is transformed into rectangular shaft (depth(a) = 3 times of width(b)) having the same cross-sectional area. will depth(a) and width(b) be the same along CDEFG. Which one is right ? procedure 1: Volume(cdefg)=V(cde)+V(efg) then V(cdefg)=V(new rec shaft) procedure 2: Area(cdefg)=A(cde)+A(efg) then Area(cdefg)=A(new rec shaft) procedure 3: Area(cde)=Area (new rec shaft) will get a & b for cde only the Area(efg)=Area (new rec shaft) will get different a & b for efg only ?

[@StructuralEng]

Hey Karim, thank you for your comment! Procedure 1 is just about right! Please notice that this question asks about the "same amount of material," which translates to "same volume." We will automatically go with the same process if the question asks about "the same cross-sectional area" (same volume). The reason is we have two types of areas and as a result, formulating the question as "having the same cross-sectional area" can be misleading. Also, the relationship between width and depth is given for us to derive dimensions from the volume. Please let me know if that answers your question!

[@karimabouelela3665]

@@StructuralEng yes it does thank you so much.

[@dilpreetrahal5780]

Hi Shayan, I believe the gear ratio only applies between gears b and c, as well as g and j. I don’t think you use the parentheses for all the twists occurring after c. /*Never mind, your solution is correct, as per Dr. Anwar*/

[@StructuralEng]

Hi Dilpreet, thank you for your comment and for pointing out the mistake! Yes, it will be exactly as you mentioned. The parentheses will apply only to the C/D part, in this case, regarding the rotation ratio between the gears. I will pin your comment and add some points in the explanations soon to address this. Thank you for watching vigilantly!

[@dilpreetrahal5780]

Thanks Shayan, If you don’t mind me asking, where did you obtain this example from?

[@StructuralEng]

@@dilpreetrahal5780 all of the questions solved in this channel are designed by me.