Gear arrangement

An internal-external gear pair comprising:

• an internal gear, arranged about an internal gear axis and having a first number of internal gear teeth;
• and
• an external gear having an external gear axis arranged to orbit the internal gear axis and having a second number of external gear teeth, the external gear being arranged to engage with the internal gear teeth;
• wherein the internal gear teeth and the external gear teeth have respective pressure angles, measured at midpoints of working portions of the gear teeth, that differ by less than 20% of the pressure angle of the external gear teeth;
• wherein at least one of the internal gear teeth and the external gear teeth has an addendum coefficient of less than 0.8;
• and
• wherein the internal-external gear pair has an instantaneous centre of rotation, ICR, defined as a point at which a radius drawn through the internal gear axis and the external gear axis intersects pitch circles of the external and internal gears, and wherein all tooth pairs transferring load between the external and internal gears are contained within an arc of twenty degrees centered about the ICR.

An internal-external gear pair comprising:

• an internal gear, arranged about an internal gear axis and having a first number of internal gear teeth;
• and
• an external gear having an external gear axis arranged to orbit the internal gear axis and having a second number of external gear teeth, the external gear being arranged to engage with the internal gear teeth;
• wherein the internal gear teeth and the external gear teeth have standard modules and no profile shift;
• wherein at least one of the internal gear teeth and the external gear teeth each has an addendum coefficient of less than 0.8;
• and
• wherein at least one of the internal gear teeth and the external gear teeth have an addendum coefficient greater than 0.3.