I had a call laSt night inquiring about what is the bottom limit for grinding a crankshafts' mains to. I looked in a few books to no avail.
Most think 0.030" undersize is max for the bent hairpin style "T" crank, but there are examples running ground even more. If the babbitter is equipped to cut different sizes, then maybe only one main need to be cut deeper? And maybe it doesn't have to be an even number? The less grinding the better
I think it's just the luck of the Irish with a crank, like the saying go's, it's not if the crank breaks but when, now in saying that.
I re-built a club members T sedan engine that the mains had been ground .060" who knows when, high compression head. He has owned the T for several years, lives in a hilly area and his wife drives the car fully loaded with passengers more often than not as well. Still going strong.
My rod journals are at .040 and yes I do cross my fingers. Mains are close to Std. but not perfect.
I haven't found anything written that states .030" is the limit. It maybe just be a line drawn in the sand so to speak.
I know of a couple of engines that have .050" undersize cranks on the mains that have been in service for 30 - 40 years.
IMO I feel .030" under is OK but I would prefer finding a good crank that would clean up between .010" and .020" mains and or rod pins.
When having a crank ground just have it clean up to what ever size it will .. Rods and mains can be cut to size.
Don't forget to get the proper radius on the mains and throws.
Thanks everyone. I will pass the info on.
One way to get more life out of a crank shaft is by "rolling " the fillets. This method is used regularly by the car companies. By rolling the fillets under high pressure, they impose residual compressive stresses in the high stress areas of the fillets. This may take away a little bit of effective bearing area, but fatigue strength will significantly improve. I do not know if any machine shop has the equipment to perform this procedure. Khonke, do you know?
I'm with Mike Bender on this one. I have my machine shop grind the crank to the minimum required to clean it up. Mains are all the same size, throws are all the same size, but neither needs to be the same as the other. The bearings are machined to suit the shaft. That way the minimum is removed from an already puny shaft.
Allan from down under.
I am not familiar with that process, Roar.
Sorry about misspelling your name.
Try a search on "rolling crankshaft fillets", and a lot of information will come up.
Not sure if this is something easily accomplished for the average machine shop.
Here is an except from a test performed on a rolled radius crank shaft:
The residual stresses due to fillet rolling and the bending stresses near the fillets of crankshaft sections under bending fatigue tests are important driving forces to determine the bending fatigue limits of crankshafts. In this paper, the residual stresses and the bending stresses near the fillet of a crankshaft section under fillet rolling and subsequent bending fatigue tests are investigated by a two-dimensional plane strain finite element analysis based on the anisotropic hardening rule of Choi and Pan [Choi KS, Pan J. A generalized anisotropic hardening rule based on the Mroz multi-yield-surface model for pressure insensitive and sensitive materials (in preparation)]. The evolution equation for the active yield surface during the unloading/reloading process is first presented based on the anisotropic hardening rule of Choi and Pan (in preparation) and the Mises yield function. The tangent modulus procedure of Peirce et al. [Peirce D, Shih CF, Needleman A. A tangent modulus method for rate dependent solids. Comput Struct 1984;18:875–87] for rate-sensitive materials is adopted to derive the constitutive relation. A user material subroutine based on the anisotropic hardening rule and the constitutive relation was written and implemented into ABAQUS. Computations were first conducted for a simple plane strain finite element model under uniaxial monotonic and cyclic loading conditions based on the anisotropic hardening rule, the isotropic and nonlinear kinematic hardening rules of ABAQUS. The results indicate that the plastic response of the material follows the intended input stress–strain data for the anisotropic hardening rule whereas the plastic response depends upon the input strain ranges of the stress–strain data for the nonlinear kinematic hardening rule. Then, a two-dimensional plane-strain finite element analysis of a crankshaft section under fillet rolling and subsequent bending was conducted based on the anisotropic hardening rule of Choi and Pan (in preparation) and the nonlinear kinematic hardening rule of ABAQUS. In general, the trends of the stress distributions based on the two hardening rules are quite similar after the release of roller and under bending. However, the compressive hoop stress based on the anisotropic hardening rule is larger than that based on the nonlinear kinematic hardening rule within the depth of 2 mm from the fillet surface under bending with consideration of the residual stresses of fillet rolling. The critical locations for fatigue crack initiation according to the stress distributions based on the anisotropic hardening rule appear to agree with the experimental observations in bending fatigue tests of crankshaft sections.
Too academic for my retired brain! ;-)
Easy for you to say Royce
Now that makes sense.
That sure clears things up.
You can get a 15,000 psi or so compressive stress in your crankshaft by nitriding after grinding. You will have to polish the journals and may have to straighten the crank but you have far better fatigue strength as well as a hard journal surface.
Simplifying Royce's information, residual compressive stress lowers the maximum tensile stress and increases fatigue strength. For example if the maximum tensile stress is 30,000 psi, a 5000 residual compressive stress would reduce it to 25,000 psi. Rolling the fillets induces a compressive stress in the fillet area only.
Need to remember that these are Babbitt engines and is not necessary to cut the crank to the next .0010. All that is needed is to cut enough clean up the crank and bore the Babbitt to size. If the mains are not all the same size, that's OK too. The grinder MUST get the radius right are it's disaster in the making.