There is a gear set. The gear set has a) a first gear having a first surface and b) an intermeshing second gear having a second surface. The first and second surfaces each, independently, have an isotropic arithmetic mean roughness, Ra, of about 0.0762 micrometers/3 microinches or less and are lubricated. There is also a method for increasing the contact surface-fatigue life of a gear set.

A flexible gear coupling includes two external gears and two internal gears meshing with the respective two external gears. A tooth root crowning radius of each external gear is smaller than a tooth tip crowning radius of the external gear. The external gear is formed such that a reference tooth height that is a tooth height at a tooth width direction middle position is smaller than an end tooth height that is a tooth height at a tooth width direction end position. A ratio of the end tooth height to the reference tooth height is set to 1.21 or more, and/or a ratio Rc/Rb of the tooth root crowning radius to the tooth tip crowning radius is set to 0.37 or less.

A flexible gear coupling includes two external gears and two internal gears meshing with the respective two external gears. A tooth root crowning radius of each external gear is smaller than a tooth tip crowning radius of the external gear. The external gear is formed such that a reference tooth height that is a tooth height at a tooth width direction middle position is smaller than an end tooth height that is a tooth height at a tooth width direction end position. A ratio of the end tooth height to the reference tooth height is set to 1.21 or more, and/or a ratio Rc/Rb of the tooth root crowning radius to the tooth tip crowning radius is set to 0.37 or less.

A drive assembly for driving an implement is provided. The drive assembly includes a gear drive mechanism comprising a first drive gear rotatable about a first axis of rotation; a second drive gear rotatable about a second axis of rotation and engaged to the first drive gear; a driven member driven by the second drive gear, engaged to the implement, and rotatable about a third axis of rotation; a lobe; and a flexible link engaged to the first drive gear and the lobe, the flexible link comprising a first loop engaged to the lobe and a second loop engaged to the first drive gear. The drive assembly also includes a handle member engaged to the gear drive mechanism and configured to drive the lobe. The implement is rotated about the third axis of rotation as the handle member is moved from an unfired position to a fired position.

A gear of a gear device includes a plurality of teeth having tooth surfaces. The tooth surfaces are shaped such that contact length ratios of three or more of the teeth are outside a predetermined range centered on an average value of the contact length ratios of all of the teeth, where the contact length ratio is obtained by dividing a contact length of a tooth contact face by a diagonal length of a plane of action.