O S & Y Valve
O S & Y (outside stem and yoke) gate valves operate by opening and closing via a gate, which lowers into or raises out of the valve. Lowering the gate prevents water from entering and flowing through the casing of the valve.
Elements
O S & Y valves contain a screw nut held by a yolk, or beam, on the exterior of the valve. A wheel in the valve attaches to the nut, which turns via external screw threads, allowing the shaft to rise from or lower into the valve body. This shaft action either pushes or pulls the valve disc, enabling the valve to open or close.