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. Cam lobes were pressed and sintered to densities in the range 7.6-7.65 g/cm3 and case hardened to a macrohardness of 58-60 HRc. The lobes were assembled on a low alloy steel tube. An interference stress was generated between the tube and the components through mechanical expansion of the tube with a mandrel.
A key functional aspect of assembled camshafts is the strength of the assembly. Specifically, the torsional static and fatigue strength of the bond between the shaft and the cam lobe-or any other assembled component e.g. driving sprockets, journal bearings and cams for unitary Diesel injectors. Torsional static and fatigue tests show that with relatively lightweight construction (tube wall thickness in the range 2 - 3 mm) it is possible to achieve static torque strengths in the range 200-300 Nm and torsional fatigue endurance limits in the range 190-210 Nm. For heavy-duty applications, camshafts with tube wall thickness of 6 mm can increase the static strength to 1,500 Nm. The static and fatigue failure modes are different. Static failure results from displacement of the lobe relative to the tube. In contrast, under torsional fatigue, the failure is due to fatigue of the tube, with a crack initiating in areas of high stress concentration. Through proper camshaft design, the necessary combination of static and fatigue strength can be achieved.
The sintered alloys developed exhibit a rolling contact fatigue endurance limit that can exceed 2,500 MPa. Camshafts manufactured with these alloys have been validated in fired engine tests, including a 3.4 L V6 engine with a type 5 valve-train (push-rod with roller follower) that operates at a contact stress of approximately 2,000 MPa. Three 300-hour power tests and one 1,000-hour idle test were completed. The sintered cam lobes exhibited no pitting and the cam lobe wear was negligible.