Valve opening and closing parts are plug-shaped valve, seal appear before the plane or cone, the valve along the centerline of the fluid in a straight line. Stem form of exercise, (generic name: dark bar), but also lift rotating rod, (common name: the next shot) pieces of cut-off valve is closed (valve) moves along the centerline of the valve seat. According to this form of valve, valve seat and through the mouth of the change is proportional to the valve stroke. As such open or close the valve stem travel relatively short, but very reliable cut-off function, and because changes in the valve seat and through the mouth of the valve stroke is proportional, is ideal for flow adjustment