Mud motor rotor with core and shell

A method for manufacturing a mud motor, comprising:

• forming a core having a first outer shape;
• forming a shell having a second outer shape around the core, the first outer shape being different from the second outer shape, wherein the second outer shape comprises one or more lobes and one or more cavities, wherein the shell and the core at least partially form a rotor of the mud motor, and wherein a thickness of the shell varies as proceeding around the core, from a non-zero minimum thickness of at least about 1 mm to a maximum thickness that is greater than the minimum thickness and at most about 25% of a maximum cross-sectional dimension of the rotor;
• and
• positioning the rotor in a stator, such that the shell is configured to engage an inner bore of the stator during rotation of the rotor relative to the stator.

A rotor for a mud motor, comprising:

• a core having a first outer shape and a core feature comprising a groove;
• and
• a shell positioned around the core, the shell having a second outer shape that is different from the first outer shape and the shell comprises a radially-extending ridge, wherein the radially-extending ridge comprises a circumferentially-facing leading surface and a circumferentially-facing trailing surface with respect to a direction of rotation of the shell, the leading surface is larger than the trailing surface and the radially-extending ridge is configured to interface with the core feature to transmit torque to the core, wherein the leading surface is configured to receive more torque from the shell than the trailing surface;
• wherein the second outer shape defines one or more lobes and one or more cavities that are configured to engage a bore of a stator during rotation of the rotor relative to the stator, wherein a thickness of the shell varies as proceeding around the core, from a non-zero minimum thickness to a maximum thickness.