Roller crank question 2

[thundair]

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


I don't know anything but the people that do.

 

[Tmoose]

Generically nitriding hardness is pretty shallow. Roller bearings need the hardness to extend pretty deep, as the Hertzian stress from line contact are high to a significant depth below the surface, and the material down there needs high yield and ultimate strength too. there are formulas on line from INA for case depth required based on loading.
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.

 

[swall]

If this is a race bike, it might work for a few races. It probably would have worked with the Rc 48-50 crank pins without the nitride. As Tmoose indicates, the nitride case won't support much load. This would lead to spalling of the case and then fatigue cracking into the 4340 crankpin.

 

[thundair]

Thanks all; I knew it was an issue if every other roller bearing I looked at was with inserts

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.

 

[tbuelna]

thundair,

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