Improved self-grinding valve

In a valve of the type having (1) a casing having (2) an inlet opening and (3) an outlet opening and (4) a valve seat intermediate the inlet and outlet openings, (5) a disc adapted to sit on and seal the valve seat thereby preventing flow from the inlet to the outlet, (6) a longitudinal stem attached to the disc at one end and having a threaded portion intermediate its ends, (7) stem rotating means operatively attached to the stem to cause it to rotate about its longitudinal axis, (8) nut means operatively attached to the threaded portion of the stem, and (9) means operative either to hold the nut means fixed with respect to the casing or to rotate the nut means, thereby moving the stem longitudinally, said stem rotating means being operative in a first operating mode, wherein the nut means is fixed with respect to the casing, to move the stem longitudinally, thereby moving the disc toward or away from the valve seat, and said nut operative means being operative in a second operating mode wherein said stem rotating means prevents rotation of the stem, to move the stem longitudinally, the improvement wherein the stem rotating means and the nut operative means are each driven independently of the other and wherein the stem rotating means and the nut operative means are fixed relative to each other and relative to the valve casing so as to prevent their longitudinal motion along the axis of the stem relative to each other or to the casing, and control means operatively connected to the stem rotating means and the nut operative means to operate them (a) individually, so that the stem and the disc move longitudinally with or without rotation, depending on the mode chosen, or (b) concurrently in the same direction so that the stem and the disc move more rapidly toward or away from the valve seat than when the stem rotating means or the nut operative means are operated individually or (c) in opposition so that the stem and the disc rotate but do not move longitudinally whereby the valve can be continuously self-grinding after the disc is seated on the valve seat.