Strain wave drive

A strain wave drive comprising:

• an elliptical wave generator shaft rotatable about an axis (X);
• a flexible flex spline mounted around the wave generator shaft;
• and
• a ring gear assembly mounted around the flex spline, the ring gear assembly comprising an output ring gear having a circular inner periphery and being sandwiched between two earth ring gears each having a circular inner periphery;
• wherein the flex spline has a first set of a first number of radially outwardly extending teeth around its outer periphery;
• and
• wherein each earth ring gear engages with the first set of teeth of the flex spline;
• and
• wherein the flex spline has a second set of the first number of radially outwardly extending teeth around its outer periphery
• wherein the output ring gear has a second number of radially inwardly extending teeth,
• such that:
• as the wave generator shaft rotates, it causes the flex spline to take up an elliptical shape of the wave generator shaft;
• and
• the teeth of the flex spline engage with second set of teeth of the output ring gear at a major diameter of the elliptical shape, causing the output ring gear to rotate,
• wherein the wave generator shaft is provided with a radially outer surface defining the elliptical shape and arranged to engage with an inner surface of the flex spline;
• the strain wave drive further comprising:
• roller bearings that engage the flex spline, and
• wherein:
• the teeth of the flex spline define a longitudinal barrel profile,
• to define a convex curved surface at the interface between the ring gear and the flexspline.