A transmission device is provided in which a first speed change portion and a second speed change portion are formed. The first speed change portion includes an eccentric shaft having a main shaft portion on a first rotary axis and an eccentric shaft portion on a second rotary axis disposed eccentrically with respect to the first rotary axis; an eccentric unit that is rotatably supported on the eccentric shaft portion, and has external teeth formed on an outer peripheral portion thereof; and a housing having an axial center on the first rotary axis, and having internal teeth that mesh with the external teeth of the eccentric unit. The second speed change portion includes curved wave grooves respectively formed on the eccentric unit and a rotary unit; and rolling elements sandwiched between the curved wave grooves and moving along an orbit defined by the curved wave grooves.

A harmonic drive assembly and fluid-powered linear motors with both axial pistons are rotary piston arrangements incorporating the harmonic drive assembly are disclosed. The motors may be used in downhole drilling applications, but the drive assembly and/or motors may be used in other applications. The assembly, motors and methods use advanced harmonic drives, advanced helical drives, and combinations thereof with 1) motors with axial pistons and reciprocating linear rings to convert reciprocative axial motion to continuous rotary motion, and 2) motors with rotary pistons and reciprocating linear rings to rectify reciprocative rotary motion to continual rotary motion to improve performance over prior configurations. Axial pistons provide a robust simple solution for generating rotation; Rotational pistons provide increased torque generation as the torque generated is proportional to motor length. Since downhole drills are long, a high-torque motor can be produced using this method.

A logarithmic hollow shaft speed converter comprising a compact, modular two-stage rotary system with independently selectable coupling paths. The first stage produces coaxial outputs directed to either the ring, sun, or carrier of a standalone second-stage differential planetary gear. This architecture enables multiple torque-speed modeshigh-speed/low-torque, balanced, and high-torque/low-speedwithout altering drive element configuration. The converter's modularity and clarity support cost-effective manufacturing and flexible deployment.

A logarithmic hollow shaft speed reducer comprising a compact, modular two-stage rotary system with independently selectable coupling paths. The first stage produces coaxial outputs directed to either the ring, sun, or carrier of a standalone second-stage differential planetary gear. This architecture enables multiple torque-speed modeshigh-speed/low-torque, balanced, and high-torque/low-speedwithout altering drive element configuration. The speed reducer's modularity and clarity support cost-effective manufacturing and flexible deployment.