Torsion problem for strenght of materials help

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SUMMARY

The discussion centers on solving a torsion problem in strength of materials, specifically calculating the diameter of a shaft given a torque of 210,085 lb-in. The user initially miscalculated the angle conversion from degrees to radians, which led to incorrect results. After correcting the angle to approximately 0.069813 radians and using the correct torque value of 52,521.1312 lb-in, the user successfully derived the shaft diameter as 3.44 inches or a radius of 1.72 inches. The importance of maintaining significant digits throughout calculations was emphasized for accuracy.

PREREQUISITES
  • Understanding of torsion equations: τ = Tc/J and θ = TL/JG
  • Knowledge of torque calculations in mechanical systems
  • Familiarity with converting angles between degrees and radians
  • Proficiency in significant figures and their application in engineering calculations
NEXT STEPS
  • Study the application of torsion formulas in different materials and their properties
  • Learn about the significance of significant figures in engineering calculations
  • Explore advanced topics in strength of materials, such as shear stress and deflection
  • Investigate the impact of varying torque values on shaft design and material selection
USEFUL FOR

Mechanical engineers, students studying strength of materials, and professionals involved in shaft design and torque calculations will benefit from this discussion.

duffman1278
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Homework Statement


question2.jpg



Homework Equations


tao=Tc/J
theta=TL/JG


The Attempt at a Solution



I got the torque of the motor as 210,085lb-in
Then I converted the 4* to .698 rad

From there I plugged it into the equations. I solved for T from tao=Tc/J then plugged that T into the theta=TLJG and put everything else in there.

When I did that I got a diameter of 3.44" or 1.72" radius. I don't get what I'm doing wrong?
 
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duffman1278: Conversion mistake; 4 deg is not 0.698 rad. Also, generally always maintain four (or
five) significant digits throughout all your intermediate calculations, then round only the final answer
to three significant digits, unless the final answer begins with 1, in which case round the final answer
to four significant digits.

Initially compute T from the given data, at which point T is known, not an unknown. Do not solve for
T in the stress nor deflection equation; solve for d. The unknown is d. Try it again.
 
The 4 degree's I put on here was just a type-o this is what I got when I redid it.

\phi= .069813
T(shaft)= 52,521.1312 lb-in
G= 12x106
\tau= 12,000psi
L= 120"

I solved for T as you said in which I got

T=\stackrel{\tau*J}{c}

I then plugged that T into the \phi=\stackrel{TL}{JG}

That then gave me \phi=\stackrel{L\tau}{Gc}

I solved for "c" which would be the radius and it gave me 1.72 or 3.43in still.
 
The answer for T is already given in the third line of post 3. Don't solve
for T again after that; just use it in your other equations thereafter to
solve for d (or c).
 
I love you! omfg this stupid problem was so easy the entire time.
 

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