Gear reducers can be complex devices that apply the science of gearing and mechanical benefit to run thousands of complex operations in many different industries. Gearbox manufacturers have designed a number of gearboxes in multitudes of different configurations and equipment ratios. When failures 
happen it is critical to understand how to repair the failed units and how to prevent future failures to keep production ready to go.
Three things that may cause premature failure are poor lubrication, misalignment and overloading. Failure modes can involve bearing failures or gear failures, or both.
Lubrication is critical for both bearing and gear life. Important areas of lubrication are the level of lubricant that is delivered to each gear mesh and bearings, and also the properties of the lubricant. The lubricant forms a slim film that prevents metal-to-metal contact between gears and between bearing components. Modern industrial gears make use of an involute tooth form and tooth engagement, which really is a mixture of rolling and sliding. The essential oil film is a slim barrier between moving parts that allows the rotating drive to turn the gears very easily without damage to the metal surfaces. Contamination in the lubricant can lead to scuffing and far faster put on for both gear reducer box bearings and the gearing in a gearbox, so that it is imperative that maintenance mechanics verify gearbox lubricant for contamination periodically, once for 12 months as the very least. Each gearbox would have a recommended oil level as well as a method to lubricate both bearings and the gear set. With bath lubrication, all shifting components dip down below the essential oil level. With splash lubrication, essential oil can be splashed around in the gearbox housing by fast moving components, covering all shifting parts. With pressure lubrication, essential oil is certainly pumped to each gear mesh and bearing through spray nozzles or essential oil passages from the gearbox oil sump or from and exterior reservoir.