Roller crank question
Roller crank question
(OP)
I've made a three piece crank that will use two sets of Corilla rods for a 750 Harley
That said my question is will the rod journals made of 4340 HT to R/c 48-50 and nitride to 60c hold up as well as an 8620 HT to R/c 60-62.
My concern is that I have not seen a set of roller cranks or rods without the high carbon steel, even the Carrillo rods I have are inserted and they are made out of Chromyl.
Thanks in advance
That said my question is will the rod journals made of 4340 HT to R/c 48-50 and nitride to 60c hold up as well as an 8620 HT to R/c 60-62.
My concern is that I have not seen a set of roller cranks or rods without the high carbon steel, even the Carrillo rods I have are inserted and they are made out of Chromyl.
Thanks in advance
I don't know anything but the people that do.





RE: Roller crank question
Typical spec for bearing race hardness is RC 60 to a depth of 0.060" or so, although conventional description means the hardness at 0.060 is down to maybe RC 40 something.
Deep hardening via carburizing starts with a process to put extra carbon into the 8620 steel. More time spent in the bath means deeper penetration of the carbon and possibility of deeper hardening when all done.
RE: Roller crank question
RE: Roller crank question
I think I will remake the crank pin out of 8620 or is there something better.
Thanks again
I don't know anything but the people that do.
RE: Roller crank question
As Tmoose correctly points out, race surface case depth is very important with rolling element bearings. Hertzian contact theory tells us that the most likely failure mode in your particular example, is a subsurface shear failure at the most highly loaded sector on the inner race (ie. the crank pin). The TDC crank pin surface has the combined effects of high radial loads, small radius of curvature, and a high number of load cycles.
You can determine the approximate case depth/hardness profile your crank pin needs with a Hertzian contact analysis. The critical value is the depth of max subsurface shear stress. The depth of the max subsurface shear stress should lie well within the case structure. If you know the loads and geometries of your components, a good text like Roark's will give you the equation for depth of max subsurface shear stress.
Hope that helps.
Terry