Mud motor rotor with core and shell

A method for manufacturing a rotor for a mud motor, the method comprising forming a shell around a core, wherein:

• the shell has a different outer shape than the core;
• the shell defines one or more lobes and one or more cavities, the one or more lobes and the one or more cavities being configured to engage a bore of a stator of the mud motor during rotation of the rotor with respect to the stator;
• and
• a thickness of the shell varies as proceeding around the rotor, from a non-zero minimum thickness to a maximum thickness.

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.