Regarding Factor of Safety 2

[BADVE]

What is the criteria for comparing Von Misses Stresses with Yield or endurance stress?

How much factor of safety should be selected?

Are there any guidelines to select FOS?

 

[freiFEM]

von misses stress can give u the idea the at which max stress conditions ur sturcture or model will fail..coming to the yield stress...after the max plastic deformation..it occurs..nendurance limit..u can hve a look at stress-strain curve..or hook`s law..
factor of safety..depends the structure..and the application..also on the rigidity of the structure..probably FS of 2 to 4 is recommended..

cheers,
tobias

 

[corus]

There is no FOS. If any safety measures are made then it is in the loading that you have chosen, ie. use the worst case or spefified design loads. The calculated Von Mises stress will be comprared against a limit speficied in the design code you are working to. This limite tends to be between 2/3 and twice yield depending on the stress classification. You really need to look at design standards to see how the type and class of stress is assessed. The endurance stress you refer to relates to the endurance limit, I presume. This is a fatigue limit for which you need to look at the principal stress range, location, and detail.

corus

 

[JStephen]

Depending on the application, the design code may allow localized yielding, which makes it hard to define a factor of safety. What is your application?

 

[BADVE]

Let me specify the project.

A compressor crank shaft is analysed for static loads. After doing FEM, I have set of values of Direct stresses both normal as well as shear and also a set of Von mises stresses for various location on the crank shaft.

Now I want to chech the margin of safety (how much the opearting stress is away from either UTS or Yield or Endurance.)

For von mises stresses, I learnt from one of my colleague that this should be compared with yield tresses and FOS of order of 2 is OK. I am not able to locate a book or a tech paper for that.

Also for direct stresses is endurance strength a corrrect criterion ?

Please comment.

Thanks and Regards

Badve

 

[GregLocock]

I very much doubt that a simple stress analysis of a crankshaft will tell you anything useful. It is much more likely to fail in fatigue. I suppose you could design it to the endurance limit, depending on material.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[prost]

Won't the stresses in the crankshaft as calculate by a simple or even a complex stress analysis be useful for the fatigue design? Where else would you get the stresses needed in fatigue analysis?

I think what most here are trying to say is that the answers to simply posed questions such as "how much force can I put on this structure and how do I determine that force?" are much more complex the simplicity of the question would indicate. Multiple failure modes are possible with most structures and each must be considered to yield something what I like to call a 'holistic analysis', though others might have a different phrase for the same idea of considering all failure modes. For instance, in designing typical airplane structures, many failure modes are possible--static strength, residual strength, and fatigue (fracture mechanics and damage tolerenace, that is). Whatever analysis gives you the lowest 'max. stress' (which itself is another decision if static strength limits the design--do I use Tresca criterion? How about von Mises?) is the failure mode that often is the design limiter. I personally don't design crankshafts, so couldn't tell you which failure modes must be examined, and how to handle the analysis for each failure mode calculation. I am frankly surprised no one on this forum designs crankshafts and would have an answer for you as to where to look for crankshaft design criteria.

 

[GregLocock]

I don't design crankshafts (I have in the past). I do work very closely with the people who design suspension components, which can fail in fatigue or in one-off events.

Typical automotive crankshafts run past their first resonance, and are expected to do that for 10s of millions of cycles. A static analysis is not going to cut it.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[gurmeet2003]

BADVE,

I read from your description that you are Electrical, but designing crankshafts? Anyhow I have designed a few crankshafts using FEA. Generally the loads are bending and torsional. In my application the bending loads are a small percentage of torsoinal.

You do not have to worry about yielding in crankshafts. The primary failure mode is fatigue and will occur at oil holes or a stress concentration (groove, fillet) on the drive end. Compare Von Mises stress with endurance limit of the material. Use correction factors for size and surface finish. Also use a correction for mean stress effect such as Goodman diagram etc.

Factor of safety will depend on how well you know the load. Will the crankshaft see some torsional vibrations (all crankshafts do)? How much margin should be included for torsional vibrations? I use 50%. In my application bearing support stiffness is high enough to be ignored (bearing supports are considered rigid). What will be the situation in your application? Does the bearing support flexibility need to be included? I guess for a simpler analysis they could be considered rigid.

Considering all of above a factor of safety of 1.5 or higher may be accpetable. But generally when deciding the factor of safety prior experience with the type of equipment if avialable should be considered. If you have a working equipment that is doing a satisfactory job, you could use that as a benchmark.

Gurmeet

 

[JStephen]

"But generally when deciding the factor of safety prior experience with the type of equipment if avialable should be considered."

I think that comment hits the nail on the head.