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P&H 4100 Shovel Crowd Gearcase Input Shaft Bearing Case Study

Background
The gearcase is monitored with vibration on a 2 monthly cycle. The drive is a reversing, variable speed, variable system and the gearcase is belt driven. The shovel operates approximately 6,300 hour per year with a minimum down time cost of $480 per hour. The gearcase had approximately 20,000 service hours.
Detection

The first survey of the machine showed evidence of an outer race defect corresponding to the input shaft drive end bearing. This bearing is a double race tapered roller bearing designed to take the axial loads applied to this shaft. The figure to the right shows the outer race defect frequency harmonics and the gear mesh harmonics (M1).

The second survey, shown lower right confirmed the bearing defect but also a dramatic increase in the 1st stage gear mesh (M1). This change was evident in the high frequency an low frequency domain.

These changes combined with knowledge of the equipment led analysts to conclude that not only the drive end bearing damaged but it was likely that the non drive end cylindrical roller bearing had deteriorated to the point where it was loosing geometry and it was recommended that the bearings be replaced.

Strip Down Inspection
The gearcase was repaired in conjunction with a scheduled outage. Inspection of the components confirmed that the non drive end bearing had a broken cage and had lost several rolling elements (thus the loss of geometry). The drive end bearing had brinelling through the outer race and was lightly spalled in the load zone area. The defect was detected and corrected before it caused any damage to the input pinion and mating gear.

Non-Drive Cage



Drive Outer Race

Cost Benefit Analysis
  Production Downtime Saving Repair Cost Saving
  The planned change out without any secondary damage is typically 12 hours unplanned 24 to 48 hours depending on the secondary damage. Thus saving between $11,520 and $23,0400. In service failure would have resulted in a combination of shaft, housing, and 1st stage gear set damage. Typically resulting in between $30,000 to $95,000 to the repair cost.
Mid Range Savings $USD 17,000 $USD 62,000
ROI 4300% (based on 1 event per 5 year period)