Motorcycle Engine Crankcase Pressure Induced Roller Bearing Noise

摘要

During the later stages of development of the Twin Cam 88 Engine, an objectionablenoise was identified in the motor. The noise was characterized as a 'ticking' noise and wasmost prevalent at idle. The time domain traces showed two distinct impact events percrankshaft rotation. The occurrence of the noise had no impact on engine durability and waspresent, at a measurable but varying level, at all operating temperatures and ambientconditions. Also, the timing of the noise versus engine timing did not correlate with anyvalve train events, engine firing events, or primary drive events. The engine right side mainbearing became the primary suspect in the investigation even though the noise signature didnot show typical bearing noise characteristics such as ball pass, ball spin, or cage passfrequencies. The in-depth investigation lead to the discovery that crankcase pressurefluctuation did match the timing of the noise. The pressure transition points within thecrankcase caused the roller elements of the bearing to slide axially within the availableclearance and impact the walls of the bearing races. Working with samples from the bearingmanufacturer revealed that reductions in the axial clearance between the rollers and racessignificantly reduced the energy of the impacts and thus reduced the noise to an indiscerniblelevel.