An apparatus including a first portion comprising a projection extending in a first direction; a second portion comprising an aperture extending in the first direction, where the first portion and the second portion are configured for relative movement in at least the first direction; and wherein the projection extends from the first portion in the first direction between the first portion and the second portion through the aperture and wherein the aperture surrounds the projection with clearances configured to enforce limits on relative movement of the first portion and the second portion while enabling reciprocating movement of the first portion and the second portion in the first direction towards each other and away from each other.