A mechanical assembly for a power tool, and a power tool incorporating the mechanical assembly are provided. The mechanical assembly includes (i) a first member having a first axis and a first engagement portion inclined relative to the first axis, (ii) a second member having a second axis and a second engagement portion inclined relative to the second axis, wherein the first and second engagement portions engage each other by movement of the first and second members relative to each other in the direction of at least one of the axes; and wherein the first member is a ring gear and the second member is a central gear and the first and second engagement portions engage and disengage each other by movement of the second member within a space defined within the first member.