Collets for coupling rotary actuators to valves are disclosed. An example collet includes a plurality of flexible members configured to be coupled to an elongated member and each having an inner surface that forms at least a portion of a substantially rectangular bore configured to receive a rectangular shaft. The plurality of flexible members form an involute outer surface for engaging an involute inner surface of an opening of a lever. The opening of the lever is configured to cause the plurality of flexible members to be displaced toward an axis of the elongated member to cause the inner surface of each of the plurality of flexible members to engage one or more surfaces of the rectangular shaft.