Non-chattering ball detent torque limiter

A torque-limiting assembly comprising:

• a shaft rotatable about a shaft axis;
• a gear mounted on the shaft so as to be rotatable about the shaft axis relative to the shaft, the gear including a driving surface having a plurality of primary ball pockets therein and a plurality of secondary ball pockets therein, the plurality of primary ball pockets in the driving surface being angularly spaced about the shaft axis at a first radius and the plurality of secondary ball pockets in the driving surface being angularly spaced about the shaft axis at a second radius different from the first radius;
• a backing plate mounted on the shaft so as to rotate with the shaft, the backing plate including a detent surface opposing the driving surface, the detent surface having a plurality of primary ball pockets therein, the plurality of primary ball pockets in the detent surface being angularly spaced about the shaft axis at the first radius;
• at least one of the gear and the backing plate being axially displaceable along the shaft;
• at least one spring arranged to provide an axially-directed load opposing axial separation of the gear relative to the backing plate;
• a cage mounted on the shaft between the driving surface of the gear and the detent surface of the backing plate, the cage including a driven surface facing the driving surface and a braking surface facing the detent surface, the cage further including a plurality of primary ball openings angularly spaced about the shaft axis at the first radius, the driven surface of the cage having a plurality of secondary ball pockets therein, the plurality of secondary ball pockets in the driven surface being angularly spaced about the shaft axis at the second radius;
• a plurality of primary balls of a first diameter respectively received in the plurality of primary ball openings, wherein the first diameter is greater than an axial thickness of the cage from the driven surface to the braking surface such that protruding spherical caps of each primary ball project into a corresponding one of the primary ball pockets in the driving surface and an opposing one of the primary ball pockets in the detent surface;
• and
• a plurality of secondary balls of a second diameter respectively received partially in the plurality of secondary ball pockets in the driving surface of the gear and partially in the plurality of secondary ball pockets in the driven surface of the cage;
• wherein torque is transmitted between the gear and the shaft such that the gear and the shaft rotate together about the shaft axis when the transmitted torque does not exceed a torque limit, and wherein there is relative rotation between the gear and the shaft when the torque limit is exceeded causing the plurality of primary balls to roll out of the primary ball pockets in the driving surface and the detent surface, whereby the primary balls separate the driving surface and the detent surface against the urging of the at least one spring by an axial separation distance corresponding to the first diameter;
• wherein continued relative rotation between the gear and the shaft after the primary balls have rolled out of the primary ball pockets causes the plurality of secondary balls to deploy by rolling out of the secondary ball pockets in the driving surface of the gear and the driven surface of the cage, whereby the secondary balls and the cage separate the driving surface and the detent surface against the urging of the at least one spring by an axial separation distance at least as great as the first diameter to relieve axial spring force on the primary balls.