Rebuilding a Coal Pulverizer Gearbox

Friday, December 16th, 2011

Case Study: The Portland General Electric (PGE) power plant

The Portland General Electric (PGE) power plant in Boardman, Oregon has been providing low-cost, reliable electrical energy to the greater Portland area since 1975. At full production, the facility delivers 610 megawatts per hour from one huge generator driven by a massive steam turbine. The steam is generated by a boiler that is fueled by 350 tons of coal per hour. The plant utilizes multiple, large pulverizers that operate simultaneously to grind the coal into highly combustible powder. In the last few years, the Company has invested millions in emissions scrubbing technology. To keep the plant running without interruption 24 hours a day, the Company has developed an extensive preventative maintenance program and a working relationship with The Gear Works for gearbox overhauls.

Pulverizer Application

Six pulverizers operate simultaneously to feed the boiler with coal, each driven by a 700 H.P. motor through a right angle gearbox of 28:1 ratio. The entire 350,000 pound weight of the loaded pulverizer is carried by the gearbox. Periodic monitoring is conducted to assess the on-going condition of the gears, bearings and lubrication.

Gearbox Rebuild

On a scheduled rotation, each gearbox is prioritized for rehabilitation based on operating cycles and condition monitoring assessments. PGE has developed an extensive gearbox rebuild procedure requiring numerous inspections, measurements and non-destructive testing. A typical gearbox repair will include a complete disassembly, housing repairs, re-machining and hand scraping of thrust surfaces, refurbishment or replacement of all gears, new bearings, lube system inspection, and a no-load spin test.

Thrust bearing assembly

To support the weight of the 175 ton pulverizer unit, each gearbox is outfitted with a custom thrust bearing with football size rollers.

Gear assembly

The tooth flanks of the low speed gear and pinion are ground to AGMA class 13 tolerances and then magnetic particle inspected per ASTM E709 – MIL S1907. The gear set is checked for tooth contact in a precision roll stand prior to assembly and then duplicated in the gearbox housing. Critical fits and clearances are measured and recorded.

Spin test

Each assembled gearbox undergoes a no-load spin test prior to delivery. A 150 H.P. variable speed dynamometer brings the gearbox up to operating RPM in multiple stages. Noise, vibration, tooth contacts and lubrication flows are monitored and recorded.


At the conclusion of successful testing and compiling of the QA document package, the 25 ton gearbox is loaded on the truck for quick delivery to Oregon. The lights of Portland will continue to burn bright.

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