Outside screw and yoke. The handwheel turns a nut in the yoke and the threaded stem rises as the valve opens, so the threads stay dry and lubricated and the exposed stem shows valve position at a glance. That visible indication is why fire sprinkler mains use OS&Y gate valves. The alternative, NRS or non-rising stem, keeps the threads inside the body and suits buried or cramped installs.
Stem's sticking way out on that OS&Y, so it's open, no matter what the tag says.
Also heard as
- OS and Y
- outside screw and yoke
Do not mix it up with
- Gate valve: OS&Y describes the stem and yoke arrangement; most OS&Y valves are gate valves, but globe valves come in OS&Y too.
Related terms
Gate valve
A multi-turn isolation valve that lifts a flat or wedge-shaped gate clear of the flow path to open.
Yoke
The frame on top of a valve bonnet that holds the stem nut or mounts the actuator above the packing.
Stem
The rod that carries motion from the handwheel, lever or actuator through the bonnet to the closure member.
Backseat
A seat on the underside of the bonnet that a stem shoulder seals against at full open, isolating the packing.