Flex spline and wave gear drive including the same

A wave gear drive, comprising:

• a wave generator;
• a flex spline that is disposed at an outer circumference of the wave generator;
• and
• a circular spline that is disposed at the outer circumference of the wave generator,
• wherein the flex spline comprises:
• a first cylinder portion having a cylinder structure;
• and
• a second cylinder portion that is connected to the first cylinder portion, and is disposed to contact at least a part of an outer surface of the wave generator, and
• an interior diameter of a first end of the second cylinder portion, that is connected to the first cylinder portion, is greater than an interior diameter of a second end of the second cylinder portion that is disposed at an opposite side of the first cylinder portion,
• wherein the flex spline further comprises an external gear that extends from the second cylinder portion to the first cylinder portion, and
• wherein a portion of the external gear extends over an outer surface of the first cylinder portion.