Gearboxes have had a troubled reputation from reliability problems that often required replacement rather than repair. But since the late 1990s, the reliability of gearboxes in GE wind turbines has improved many times over. For instance, in the 1990s, about four significant tests qualified a gearbox. These included spin testing, nominal-load tests, contact-pattern inspections, and noise-emission measurements. Since then, many more requirements, processes, qualifications, and tests have been added, requirements such as clean steel processes, 100% Nital etching (for inspections to spot grind burn), hardened ring gears, and torque limiting controls. These efforts are being applied to what could be called an ultra reliable universal gearbox for service replacements.
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