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A five-year old single reduction conveyor drive was taken out of service for a scheduled preventive maintenance inspection and was disassembled. Prior to the inspection (disassembly), the unit had been in operation with routine vibration based surveillance for a full year since the previous PM inspection. There was no indication of any mechanical defect based on the vibration-based condition assessments.
The craftsman was supposed to disassemble the gearbox, allow for a visual inspection and then reassemble the gearbox. After inspecting the gearbox, a supervisor decided that components were in sufficiently good condition for continued use and instructed the craftsman to clean and reassemble the unit.
The only work performed on the gearbox was cleaning and an oil change. The gasket was replaced with one made in the company shop.
During assembly, the gasket was improperly installed in such a manner that the gasket blocked the vent port. Once the unit was put into operation, internal heat lead to expansion which, with the plugged relief port, lead to the pressurization of the gearbox, rupture of the oil seal and loss of all of the lubricant. The gearbox ran for 48 hours before catastrophic failure of the pinion teeth occurred, as shown in Figure 3.
Figure 3. Severe Wear on the Pinion Gear
As a result of the failure, the site initiated a triple-redundant visual inspection and check-off requirement for all machines disassembled for PM activity, wherein the machine must be inspected by three different personnel prior to final closure and assembly.
In the final synopsis, the site concluded the following:
A “depot level” disassembly is a thoroughly invasive procedure. Under the best of circumstances, start-up failure risk is high following a rebuild. Based on the apparent condition, without the invasive human intervention this unit would probably not have failed. What were the options that could have prevented this occurrence?