Hi everyone, I am working on a shaft redesign for a gearbox project for school.
ID: 2074013 • Letter: H
Question
Hi everyone,
I am working on a shaft redesign for a gearbox project for school. This involves me taking a previously designed gearbox and redesigning such that it will meet requirements. I have performed the stress calculations on the shaft and ended up finding a factor of safety of 2.32 at the critical point, which is at the keyway in the center of the shaft. the diameter of the shaft at this point is 3in. In order to redesign this, i need to find the required shaft diameter in order to achieve a fact or safety of 3 at the same critical point. I performed this calculation and my result was a lesser value than my original diameter. Would it make sense for the factor of safety to go up when decreasing the shaft size? If so, why is this?
ps. i am using the DE-ASME Elliptic equation.
Thanks,
- Harry
Explanation / Answer
Hello,
I have gone through the procedure for calculating factor of safety using diameter = 3 inch. I have not checked calculation but the procedure is correct. Required factor of safety comes out to be 2.32.
Now, for making calculations for Factor of Safety 3, you are getting diameter 2.18 inch which is less than the diameter for which you had Factor of Safety 2.32.
Upon checking I found the mistake in calculation of Corrected Endurance Strength which has been over-estimated as you have taken Kb value of 0.9 as a guess. Since Kb value is inversely proportional to Diameter its value as a guess must be taken less than 0.7658 as the higher value is causing the corrected endurance strength to rise which in turn calls for a smaller diameter to satisfy the requirements as per the desired Factor of Safety 3.
If you solve again with Kb value around 0.7 you will get a higher value of diameter which is justifiable as well for Factor of Safety 3.
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