An OS&Y Valve (outside screw & yoke) functions by opening and closing via a gate, which lowers both in and out of the valve. When the handle is turned clockwise the gate will close preventing any water from traveling through the valve. When the handle is turned counter clockwise, the gate will open allowing the water to flow through freely. The screws of the valve are held by a yolk on the outer portion of the valve, a wheel in the valve is attached to the nut which turns, allowing the shaft to lower or rise from the lower portion of the valve.
What is the difference between NRS and OS&Y gate valves?
NRS means Non-Rising Stem. OS&Y means Outside Stem and Yoke. In an NRS valve, the stem will turn to open and close the gate, but the stem does not move up or down as it turns. With a OS&Y valve, as the stem turns, it moves into or out of the valve, which also moves the gate to open or seal the valve.