Worm gear actuators have the advantage of easing the manual opening and closing of valves, generally quarter-turn valves, without opting for more expensive pneumatic, hydraulic or electric actuators. Worm gears are intertwined in such a way that each gear reduces torque, and the combined torque reduction across all worms can be so substantial as to allow the manual operation of even quite large ball valves.
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My interests include duplex valves from The Alloy Valve Stockist, Barcelona, Spain at
http://alloy-valves.com