Powder metal alloys with high contact fatigue properties: application to cam lobes and bucket tappet shims

摘要

Forged bearing steels and powder-forged steels (e.g., AISI 52100, SAE 5160 and MPIF FL-4680) have been used to make cam lobes for assembled camshafts operating with roller followers. Application of powder metal (pressed and sintered) alloys to this and other components that operate under high rolling contact stress has been limited by relatively poor rolling contact fatigue (RCF) properties. This paper introduces developmental sintered steel alloys with high RCF strength. The density of these alloys is 7.4-7.6 Mg/m~3 and the macrohardness is 500-800 HV. Endurance limits are in the range 1,700-2,280 MPa. The RCF endurance limit at 200 million stress cycles was determined using a testing rig. Camshafts for a 4.6 L V8 engine and a 4.0 L V6 engine have been assembled. These engines have a single overhead camshaft with end pivot rocker with roller follower (type 2 valvetrain). The maximum static normal contact stress on the lobe is approximately 1,200 and 1.500 MPa for the V6 and V8 engines respectively. The camshafts were tested in a motorized cylinder head and in fired engines. After engine testing the sintered cam loves exhibited no pitting and, in the case of the V8 engine, the cam lobe wear was very similar to the wear observed on powder-forged lobes. Initial testing of powder-made tappet shim materials suggests that these alloys may provide a useful alternative for addressing tribological issues that arise between the different combinations of sliding pairs currently used for mechanical bucket tappet designs.