What stress Concentration Factor to use for Crankshaft Bending Calcs

[JCReynolds79]

Hi all,

Trying to automate Fatigue Factor of Safety calcs in a pro/e model and I found myself questioning what a sensible K factor (stress concentration factor) to use when determining the effect of the fillet where the pin meets the web.

Standard diagrams I have used before in books has the usual handful of 'turned - stepped' looking shafts with graphs showing K factors for bending and torsion, but none really look suited to the situation of a diameter (the pin) meeting a flat face perpendicularly, as with the layout of a crank bay.

I'm sure there must be some universally accepted standard to use so my question is where should I be looking to get sensible figures for typical crank layouts for the K factor in the fillet?

Many thanks in advance.

 

[Bribyk]

Sounds like FEA is needed here. You could find out the K-value by a simple Mechanica (or other software) analysis. Mechanica will also do the fatigue analysis outright.

 

[R1100S]

Finding the stress concentration factor for a crankshaft in bending is a complicated problem. Fortunately it is also a well studied one. Unfortunately most of the research was done in the 1930's and the books detailing it are long out of print. If you spend some time looking around you local engineering school's library you can most likely find a book on the subject.

Outside of the older texts, it seems that FEA is the preferred method these days.

From experience, the stress concentration factor is higher than you think, most certainly higher than for a stepped shaft. SCFs of 5 to 6 and higher are not uncommon for crankshafts.

 

[JCReynolds79]

Hi both,

Thanks for your answers. I have been looking through a Peterson book on Stress Concentration Factors that I found in the library at work and there was a section on crank bending!

Although some of my figures were slightly out of the range of his diagrams, the SCF certainly is larger than expected, ~4 for my design.

Cheers.