Investigation Of Diesel Engine Cam Life Using An Accelerating Fatigue Test Rig

摘要

Surface distress involving pitting, scuffing, frosting, and high friction leads to failure in fuelrninjector cams. The relationship amongst relative features of these failures is extremely complex. Theyrncomprise local plastic straining, cyclic softening/hardening, crack initiation/propagation, impact,rnskidding, and third body formation. From an industrial application point of view determination of failurernprobability and service life expectancy are important. Although factors such as operating loads, speeds,rnnumber of loading cycles, surface roughness, lubrication conditions, and third body particles are knownrnto affect the life of gears and bearings, the effect of such factors on the life of cam-follower contact isrnlittle known. In particular, failure of cams due to pitting under conditions of rolling/sliding friction isrnunclear. The objective of this research was a) to investigate premature failure of injector cams in DieselrnEngines and b) to determine the pitting life of fuel injector cams in terms of number of cycles of therncam, through laboratory test simulation and analysis. Towards the accomplishment of this goal, anrninstrumented cam-follower test rig facility was designed and fabricated, followed by experimentation tornreproduce field failures in a laboratory environment under accelerated test conditions. Different camsrnwere run at contact stresses ranging from 3000 MPa to 3800 MPa. In the first cam, after the initialrnrunning in at 3000 MPa, the load was increased to produce a base circle stress of 3800 MPa. Within 5rnminutes of running at this loading condition catastrophic failure of the cam occurred, as characterized byrnsqueal, chatter, smoke, and significant increase in the contact loads and accelerations. Examination ofrnthe cam surface indicated severe gouging and scoring of the surface, particularly on the ramp-down sidernof the cam. This was typical of the severe field failures that mainly occur on the ramp down side of therncam. 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-slidingrnphenomenon on cam has been observed in a laboratory environment. A second and third cam wererntested for approximately 700hours (<50,000K loading cycles) before the first signs of failure (pitting)rnwas observed. Results from these tests indicate that normal contact fatigue failure occur after 50 millionrnloading cycles are applied to the cam. Forces, acceleration, power spectral density (PSD) function,rnsurface roughness, and wear measurements at different locations of the cams clearly indicate an increasernwith increase in loading cycles. Cam surface distress observed from this test is basically mild wear onrnthe base circle with severe pitting on ramp-down side occurring after 50 million contact cycles. The labrntests having been conducted on full size components and operating at or near the field conditions havernsignificant practical value. The findings of this research are useful in extending the understanding ofrnfailure of diesel engines primarily due to failure of cams roller-follower system.