Unequal load sharing in planetary gearboxes 7

[Windward]

I have no particular problem here, just requesting general comments on the subject. Unequal load sharing among the planet gears seems to be the main disadvantage with planetary gearboxes. Is this a myth? Cases of damage from unequal load sharing, design rules for improving load sharing, all such information is requested. Thanks for the help.

 

[MattRR]

I have designed both "multiple path" and planetary gearing for turboprop gearboxes. My guess is that simple gearing is used for the first stage or two, and the planetary used for the final stage, based on gears being sized by torque. The planetary is used for the high torque part of the geartrain, and standard parallel axis gears are used for the first stages where torque is low, and consequently size is low. Most of the weight and size savings occurs at the high torque part of the geartrain.
For interrupted and very low speed operation, rolling element bearings would be a good choice for the planetary. The choices for gearing would then be high contact ratio spur (noise reduction) or single helical gearing. I have not found anyone happy with double helical gears/rolling element planetaries (two many floating members for load sharing creates dynamic problems)(double helical/journal bearings seems to work well). As the axial loads from the sun and ring mesh for single helical planet would be equal and opposite, a pair of cylindrical bearings would be used to react the resulting moment. A nice feature of the planetary is that there are no resulting radial or tangential gear reaction loads on the bearings on the input or output shafts.
With precision ground gearing, and correct means of load sharing, there really is no reason to derate a planetary by more than a few percent. Usually the planet bearings will size the unit, not the reverse bending of the planet gear. Clearly the planetary is smaller and lighter.
Cincinati Gear was developing a unit for Enron that used a planetary. Their Chief Engineer, Octave Labath, is now a consultant. He is listed on the AGMA website, I believe he could save you many years of development problems.