Fatigue Life of Diesel Engine Cams in Accelerated Test Environment 2003-01-0052
Surface distress involving pitting, scuffing, frosting, and high friction leads to failure in fuel injector cams. The relationship amongst relative features of these failures is extremely complex. They comprise local plastic straining, cyclic softening/hardening, crack initiation/propagation, impact, skidding, and third body formation. From an industrial application point of view determination of failure probability and service life expectancy are important. Although factors such as operating loads, speeds, number of loading cycles, surface roughness, lubrication conditions, and third body particles are known to affect the life of gears and bearings, the effect of such factors on the life of cam-follower contact is little known. In particular, failure of cams due to pitting under conditions of rolling/sliding friction is unclear. The objective of this research was a) to investigate premature failure of injector cams in Diesel Engines and b) to determine the pitting life of fuel injector cams in terms of number of cycles of the cam, through laboratory test simulation and analysis. Towards the accomplishment of this goal, an instrumented cam-follower test rig facility was designed and fabricated, followed by experimentation to reproduce field failures in a laboratory environment under accelerated test conditions. Different cams were run at contact stresses ranging from 3000 MPa to 3800 MPa. In the first cam, after the initial running in at 3000 MPa, the load was increased to produce a base circle stress of 3800 MPa. Within 5 minutes of running at this loading condition catastrophic failure of the cam occurred, as characterized by squeal, chatter, smoke, and significant increase in the contact loads and accelerations. Examination of the cam surface indicated severe gouging and scoring of the surface, particularly on the ramp-down side of the cam. Similar failures on the ramp-down side of the cam can also be seen in field failures, but the exact mechanism of back sliding in field failures is not clearly known. This experiment was very valuable in terms of replicating the damage that was commonly found on the cams failed in the field. It is noteworthy to mention that it is the first time this backside-sliding phenomenon on cam has been observed in a laboratory environment. A second and third cam were tested for approximately 700 hours (<50,000K loading cycles) before the first signs of failure (pitting) was observed. Results from these tests indicate that normal contact fatigue failure occur after 50 million loading cycles are applied to the cam. Forces, acceleration, power spectral density (PSD) function, surface roughness, and wear measurements at different locations of the cams clearly indicate an increase with increase in loading cycles. Cam surface distress observed from this test is basically mild wear on the base circle with severe pitting on ramp-down side occurring after 50 million contact cycles. The lab tests having been conducted on full size components and operating at or near the field conditions have significant practical value. The findings of this research are useful in extending the understanding of failure of diesel engines primarily due to failure of cams roller-follower system.