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MathsRetard09
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Homework Statement
Question
Assuming the torque and maximum shear stress values to be the same for both shafts, determine the size of a suitable soild shaft which could be used instead of the hollow shaft.
Information/Data Known
Firstly, a previous question - A ship's propellar shaft transmits 7.5MW at 440rev/min. The shaft has an external diameter of 230mm. Calculate the maximum permissable bore diameter if the shearing stress in the shaft is limeted to 150MN/m^2. The modulus of rigidity for the shaft material is 79GN/m^2
Ok, the following is known:
Bore Diameter = 108mm = 0.108m
DH = Hollow shaft External Diameter = 0.23m
touH = touS
TH=TS
Homework Equations
T/J = tou/r = Gθ/l
We're using T/J = tou/r
J=pi(D^4-d^4)/32 [Hollow Shaft]
J=pi(D^4)/32 [Soid Shaft]
The Attempt at a Solution
Ok so to show understanding, the question says that the torque and stress values are the same for both shafts therefore:
touH = touS
TH=TS
Which means we need to find JS, rS, JH and rH
JS/rS = JH/rH
We can find our H values but not our solid so:
JH = pi(DH^4 - dH^4)/32
DH = 0.23m
dH = 0.108m
JH = 2.6138x10^-4m^4
rH = D/2 = 0.115m
JS = pi/32 [because we can only assume DS atm is equal to 1]
JS = 0.0982m^4
rS = D/2 = 0.5Dm
Back to the equation: JS/rS = JH/rH
Muliply over:
JH(rS)/JS(rH)
Which looks like so:
(2.6138x10^4)x0.5 / 0.0982 x 0.115 = d^3 = 11.5727x10^-3
to get the diamter, we cube route the answer:
(cube root)11.5727x10^-3 = D
D = 0.226m
Therefore the diameter of the solid shaft use to replace the hollow shaft is: 226mm
What I'm asking for here is clarification that what I've just done here is correct. I've been told by my collegues that i may have gotten the bore diamter wrong, someone said they got 198mm instead of 108mm.
So from the second question i have typed out p above, could you find the time to check if i have the correct or incorrect bore diamter and the second request is simply to read the above (my workings out) to see if it looks / is valid.
Thankyou for your time, i really do appreciate your help even if it's a simple word.