Search Results

During the past five years the use of assembled camshafts in the North American automotive market has grown steadily. For valve-trains that operate with roller followers at relatively high contact stress, assembled camshafts are a cost-effective solution, as compared to camshafts machined from forged bar stock. Forged bearing steels, and, to a lesser extent, powder forged steels, have been the materials of choice for the manufacture of cam lobes for assembled camshafts. Sintered alloys offer an attractive alternative to forged steel because very accurate cam lobes can be manufactured via pressing and sintering. However, the use of sintered alloys has been limited by their relatively low rolling contact fatigue properties. This paper presents the application of sintered lobes in mechanically assembled camshafts that operate in roller follower valve-trains at relatively high contact stress.

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/m3 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 valve-train).

High density is well known to improve both mechanical and magnetic properties. The limited number of applications of PM in automotive starters was mainly due to low cost target not compatible with the obtention of the needed density. This paper will present two successful developments in that field: a starter gear and a magnetic pole. In both cases, the use of SPSS process in cold conditions has allowed the establishment of the cost target. The magnetic pole is produced in pure iron, pressed at a density of 7.50 g/cm3 and sintered at 1120 °C. The starter gear needed high density and high hardness, associated to high toughness (resistance to pitting but also shock resistance). The paper will describe how a density of 7.65 g/cm3 was achieved in cold condition after pressing / sintering / sizing leading to hardness of 62 HRC after case hardening. Dimensional properties at that density level will be also presented and discussed.