Gearbox
All gearboxes should be the product of one manufacturer and specified to be capable of withstanding an overload torque of 300% of full-load motor torque.
The GB reduces rotation speed from the input shaft to the output shaft by a factor of about 30:1 and increases available torque by about the same amount. It is designed to not to be overloaded. The unit most be maintained fairly well balanced (weights adjusted to keep motor and GB loads about the same on up/downstroke) to make sure the GB is not overloaded in opeation. Also 4maintenance includes checking the condition of the lubricant in the GB (Fig. 10.13). Older GB’s may need the addition of a wiper if they are to turn less than about 5 SPM (strokes per minute). Newer GB’s are sold with the wiper already installed.
Gearbox design
Gearbox designs are classified as either bevel designs, where gears are perpendicular to each other, or spur designs, where two gears rotate parallel to each other. Bevel gears are used for on-off valves like gate and globe valves and spur gears are used for quarter-turn valves like ball valves. One type of spur gearbox is designed by a gearbox manufacturer in which 6-18 gears are involved at the same time instead of one to three gears, which can increase efficiency by 75% and prevent the use of actuators.
The gearbox housing material can be either SS316 or painted cast iron ASTM A536. It is better to select SS316, shaft minimum 13Cr, and stop pins as 316SS. Stop pins, also called set screws, are covered by caps and serve as stops for quarter-turn movements of the stem and ball. The gearbox bolts are normally A4-70 or 80 equivalent to SS316 or A2-70, which is equal to SS304. Gearbox 316SS has better corrosion resistance, is not brittle, and is not exposed to cracking due to dropping objects, unlike cast iron. Cast iron surfaces are rough, making the paint adhesion difficult on the metallic surface. Fig. 19.1 shows a corroded gearbox in cast iron.
28 авг 2024
