High temperature drilling motor drive with cycloidal speed reducer

A bottom hole assembly has a drill bit that is driven by a downhole turbine. The turbine speed is reduced by cycloidal gearing that requires no temperature sensitive seals when operating temperatures in some applications exceed 350 degrees F. The output shaft of the cycloidal gear reducer goes through a bearing before connection to the drill bit or associated reamer. The motive fluid can be the drilling mud. The bit can be driven at desired speeds such as 50-300 RPM while the speed reduction ratio can be in the order of 10 to 1 or more. This drive assembly can replace Moineau type downhole motor drivers that have temperature limitations due to use of rubber in the stators.