Primer: Considering gear housings and carriers

Drive designers must make accurate life predictions to ensure reliability. Part of the equation is gear and bearing support flexibility — in the gearbox housing, planet carrier, and ring gear.

Depending on the design and location of a particular gearset and bearing, structural flexing affects both contacting tooth and roller-surface misalignment. Inaccurate calculation of misalignment results in inaccurate calculation of stress distribution and in turn, component life. This is why some manufacturers recommend full system flexibility analysis early as possible in the design process. Here we explore the matter using wind turbines as an example application. Turbine reliability is a pressing issue for the industry, and their gearbox subsystem failures are common, costly, and poorly understood.

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The mathematics for turbine gearbox design are complex, and require both analyses and knowledge of operational loads over design life. The models applied vary in complexity and typically incorporate flexible components for the main elements of the gearbox — for example, planetary gearsets, helical or spur gearsets, shafts, and bearings. Timoshenko or Euler beam formulations are common for shafts, while for gears and bearings, contact mechanics and elastic deformations are considered. That said, current design standards do not explicitly recommend the consideration of gearbox housing or planet carrier flexibility. However, in operation, each element flexes under load — making it important to consider the elasticity of these components.