# Safety factor assumed in exam?



## Capt Worley PE (Sep 4, 2007)

I was working the NCEES Mechanical sample questions, machine design depth number 528. In the solution, they used a safety factor of 2, but no safety factor was specified in the problem.

In the exam, are we to assume a safety factor of 2 when asked the minimum thickness of shafts, etc.?

Also, can anyone confirm the cross sectional area of a 1/2"-13 UNC bolt is .142 in^2? They used that value foe problem 524, and I can't find that anywhere.


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## EngRanger (Sep 4, 2007)

Captain Worley said:


> Also, can anyone confirm the cross sectional area of a 1/2"-13 UNC bolt is .142 in^2? They used that value foe problem 524, and I can't find that anywhere.


Machinery's Handbook, Table of Coarse-Thread Series, gives .1419 in^2

A quick and dirty method for calc. tensile dia. is Major Dia - 1/pitch. For example, for 1/2 -13, dia = .5 - 1/13 = .423. The area would then be pi * .423^2 / 4 = .141 in^2


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## MA_PE (Sep 4, 2007)

Mark's Standard Handbook for Mechanical Engineers. Coarse Thread Series - UNC and NC

1/2 - 13 Stress Area = 0.1416 in^2

FWIW: If you don't already have one, I found a Mark's Handbook to be a great reference book on a pretty broad number of subjects. Kind of like the Civil Engineering Reference Manual (CERM) is for civil.


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## Capt Worley PE (Sep 4, 2007)

EngRanger said:


> Machinery's Handbook, Table of Coarse-Thread Series, gives .1419 in^2
> A quick and dirty method for calc. tensile dia. is Major Dia - 1/pitch. For example, for 1/2 -13, dia = .5 - 1/13 = .423. The area would then be pi * .423^2 / 4 = .141 in^2


Thanks for Q&amp;D calc. Never heard of that, but I'll use it.

MA PE, no, I haven't gotten one, but I have heard there is a lot of good info in there.

Thanks for the info....anyone have any idea on the assumed safety factors? Maybe one of the NCEES guys?


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## maryannette (Sep 8, 2007)

For safety factors, I don't know how you could be expected to make an assumption if your answer depended on it. It could possibly be a distractor--the answers are so far apart that any reasonable SF would give the right "best answer". 2 is a reasonable SF if no other info is given. I've used SF as low as 1.5 and I know an engineer who designed elevators. Their SF was 7 or higher. If you are required to assume a SF, use "sound engineering judgement". Is safety or life at risk or is it a mere inconvenience if something breaks? Sorry I can't give a more definite answer.


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## IlPadrino (Sep 8, 2007)

Captain Worley said:


> Thanks for the info....anyone have any idea on the assumed safety factors? Maybe one of the NCEES guys?


I don't know of a single question on the exam that required you to assume on your own to apply a safety factor. My advice is to ignore safety factors unless specifically required by the question statement.

Edit: I can't speak for Mechanical, so please ignore my response.


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## Capt Worley PE (Sep 10, 2007)

IlPadrino said:


> I don't know of a single question on the exam that required you to assume on your own to apply a safety factor. My advice is to ignore safety factors unless specifically required by the question statement.


But at least one, and I think two problems in the NCEES Mechanical sample problems assumed a safety factor of two when asking something like "What is the minimum length of the key." I've noticed that in Lindeburg's sample problems and solutions as well.


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## IlPadrino (Sep 10, 2007)

Captain Worley said:


> But at least one, and I think two problems in the NCEES Mechanical sample problems assumed a safety factor of two when asking something like "What is the minimum length of the key." I've noticed that in Lindeburg's sample problems and solutions as well.


I'm sorry... I was thinking Civil not Mechanical and I don't have any experience in that. I'll change my previous response!


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