Parallel shaft gears offer optimum reliability and lower MTTR and are preferred for ease of maintenance and minimum components. Parallel axes are the most common gear arrangement, consisting of a meshing pinion and gear. Parallel arrangements can be simple, or compounded with other parallel gear sets to obtain high gear ratios. The conventional way to describe such a gear is ‘double increaser’ or ‘reduction’ or triple increaser’ or ‘reduction gear’. Parallel axis gears can use spur, helical, double helical or herringbone elements.
Parallel axis gear design reliability factors to optimize gear train field reliability are:
Ensure uniform face width tooth loading
Compensate for torsion (windup) of low stiffness shafts
Equalize torque transmission through each shaft (multiple shaft design)
Consider ease of assembly/disassembly
Minimize the axial thrust (helix angle selection)
Tooth hardness considerations
Limit face width to pitch (L/D) diameters to proven values
A planetary or epicyclic gear has multiple gear meshes and bearings, and has greater mean time to repair and exposure to lower reliability than parallel shaft gears owing to its larger number of component parts.
The principal advantage of planetary gear units is that they allow both the input and output axes to be concentric thus providing a very compact gear arrangement. They are used for aircraft prop engine drives, automotive and truck transmissions, power generation units and some critical equipment (pump and compressor) drives.
Their main disadvantage is that they require more assembly and disassembly time than a conventional parallel shaft gear. They can be configured in simple and compound arrangements depending upon the required gear ratio.