by Mantosh Bhattacharya, FAFG Chair
In API 617 / API 672 compliant centrifugal blowers, axial Thrust Management is executed either by thrust bearing or by thrust collar inbuilt into HS shaft (pinion shaft). Thrust collar is applied as thrust bearing of each pinion shaft. In this system, the thrust force of the compressor is transmitted to the thrust bearing of LS shaft (bull gear), shaft by the pinion.
The High-speed shaft of API 617 centrifugal blower compliant faced repeated failure of thrust collar with operating speed of 12000 RPM at OEM (Original equipment manufacturer) test bed.
During 1st full speed spin test, thrust collar of pinion shaft rubbed with bull gear, which was initially attributed to probable excitation of pinion lateral speed affecting axial movement by helical profile of mesh. In this arrangement there was no axial probe to protect machine with high axial thrust and this is common in single stage centrifugal blower .With controlled start up on 2nd internal test Thrust collar of pinion rubbed with bull gear again ( however with lesser severity ).
Upon detailed investigation using Fish Bone chart including coupling pre stretch and LS and HS shafts , it was found that the taper of gear wheel and pinion thrust collar were reversed during manufacturing by gear unit manufacturer which was overlooked by OEM quality control department. The tapers are direction oriented to create an oil film wedge to enable lubrication between pinion and bull gear thrust collar faces. If the direction and orientation of taper is not matched, severe rubs can occur on thrust collars.