Rotating One Piece Stem
A one piece valve stem rotates and translates along its axis as it is being driven axially into the orifice. The rotational motion of a one piece stem can produce friction as the stem interacts with the seating area, which could result in galling at the seating surface. This is less of an issue in a soft seat valve or if the media provides lubrication.
Non-Rotating Two Piece Stem
A non-rotating stem tip rotates independently from the stem and stops rotating as it is driven axially against the orifice sealing surface while the stem continues its rotational and axial movement.