If you took five crack checked ground the same early cranks and supported them off one and four mains, then hung a fifty pound weight on the center main and checked the amount of deflection of each.
Then four out of five checked about the same but the fifth had noticeably more deflection.
Would that be because it suffered to many stress cycles and should not be re used?
(I think you meant to say #1 & #3 mains).
I would guess that a varience of only a few thousandts of an inch in the thickness of the forging could make a great deal of difference.
If your crank was work-hardened by too many stress cycles, the work-hardening should actually make it more resistant to flexing.
By the same token, if it was cracked, the cracks may make it flex more.
Sometimes when you hire a machine shop to do crack detection, they look for major cracks in the "usual areas" and don't do a very careful job. Thats why I decided to invest in my own Magnaflux setup. (been there - done that - had that problem)
Heat treat could be a factor.
Adam, please contact me off line as I too have been thinking of setting something up for blocks and such. Thanks.email@example.com
Thanks Adam it would be nice to find a way
The modulus of elasticity is the same for all steel- heat treated, not heat treated, work hardened etc. The only factor that determines the deflection of a steel shaft for a given load is the section properties of the shaft- the moment of inertia of the shaft. So if the cross section of a crank is different, or the cross section had been modified by, say a crack, the shafts will deflect different amounts. I hope this helps.
Would you mind telling us what it cost to get your crack checking setup going?
I am not sure what you are saying. In simple terms if you tested a set of cranks as described at the start of the thread, would my thinking be correct or wrong? My simple experience with metal is if you start bending back an forth it slowly becomes weaker until it brakes
Paul. I'm wondering if you are a professional bendologist, or if you are suffering from flexaphobia and need an intervention.
Have you tried welding a brace onto the crank to stiffen it up?
Paul- all steel (and many other metals) regardless of alloy or heat treating has the same
"stiffness" (deflection vs load for a given cross section) as long as you stay within the elastic range of the steel. When you exceed the elastic limit (the yield strength) the material will take a permanent set- you have bent it. All heat treating or work hardening does is increase the point (elastic limit) at where permanent deformation occurs. Take the suspect crank and load it with the crank in several rotated positions and check the deflection at those positions and see if the amount of deflection changes at each. Since you say the crank did mag ok, (no cracks that would make the effective cross section less than the other cranks, I have no idea what is going on, unless that crank is noticeably different from the others in cross section.
What I cant get through my thick head is I have straightened several T cranks, it takes over 0.300" to start bending one so the bend will "take".
On cars with say leaf springs over a period of time they sag with metal fatigue I guess.
Logically if you were to test the deflection of one of the leaf springs at a certain push it would test less then a new one.