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Case hardening layer running out in fillet

Case hardening layer running out in fillet

Case hardening layer running out in fillet

(OP)
Hi All,

I have inherited a design of camshaft that has induction hardened cam surfaces with a case depth of ~2mm. The shaft diameter lies ~3mm below the base circles of the cams with a generous fillet radius transitions between the shaft and cams that run out just below the base circles. This means that the case layers run out (transition back to parent metal) part way through the fillets. Is this an acceptable condition or could this give rise to a risk of failure? Is there anything that can be done in manufacturing to mitigate any risk?

Many thanks,

Dave

RE: Case hardening layer running out in fillet

The hardened case is under compressive residual stress as a result of the induction hardening. If there is residual compressive stress, there must also be residual tensile stresses. These exist in the base material at the edges of the induction hardened areas. If, as you say, these edges are located in the fillet between the cams and the base circles, then these fillets can have residual tensile stresses, which could reduce fatigue life.

The operative word here is "could". If these cams routinely fail at this location, this could be a reason. However, is it quite possible that the fatigue loading in this area does not reach critical levels, so while in some designs this situation may be dangerous, in others it is not.

You say you have inherited this design. Do you have any history with the longevity of this design? Are fatigue failures common? If not, I wouldn't worry about it. Even if this currently is not a problem, it could become one if the loading of the cams increases (which is why design changes should be fully analyzed before putting into service).

rp

RE: Case hardening layer running out in fillet

If I was faced with this situation I would either shorten the IH pattern so that it terminates ahead of the radius, or extend the pattern so that it terminates past the fillet onto the base diameter.

RE: Case hardening layer running out in fillet

I agree with red picker and dbooker, but would proceed as dbooker suggested.

RE: Case hardening layer running out in fillet

The runnout of the IH case is not uniform around the perimeter of the cam lobe edges. It will be different at the base circle sector than it is at the nose sector. I've attached a crude sketch to illustrate this situation. Typically the case produced with IH cams tends to conform to the lobe surface profile, so at the sector around the nose of the lobe the case will runnout along the axial faces of the lobe, rather than into the fillet transition to the shaft like it does at the base circle sector. Also, there should only be a minimal amount of material removed from the working surface of the cam lobe during finish grinding. The material in the case runnout areas will mostly be left intact.

As others noted, this condition does not seem to present serious problems since many camshafts are successfully produced using this approach. However, as with any other case hardening process involving heating and quenching, you should pay attention to details in your part design, process and heat treating equipment.

RE: Case hardening layer running out in fillet

(OP)
Hi all,

Thanks for all your replies. I'm sorry I've not thanked you all sooner, I've been off work. The information you've provided will be really useful.

Dave

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