Spring rate and torque: In my spring replacement above, the wire size was d = 0.2253 inches, and the ID was 2 inches, giving a mean diameter D = 2.2253 inches. The number of coils is L/d = 24 inches / 0.2253 = 107, less about 5 dead coils on the winding cones, or 102 active coils. Thus the spring rate is K = (π*28.5*10^6 * (0.2253)^4) / (32 * 102 * 2.2253) = 31.8 in-lb/turn (IPPT). Winding 7.5 turns * 31.8 in-lb/turn yields a torque of 238 in-lbs per spring.
If you're clever and equipped with a welder, you might think you could get away by welding a broken spring back together. At least two factors make this extremely risky. First, the weld itself may fail, either due to insufficient basic strength, or weakening of the nearby parts of the spring. Second, the fact that the spring was old and fatigued enough to break once, means that it is likely to break again soon at other location(s).
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And for some extra features, you'll appreciate accessing the system through the MyQ smartphone app. You can set up this Chamberlain opener to automatically close the door after 1, 5, or 10 minutes, which is great for people who are a bit forgetful. We also really like the motion-detecting control panel, which turns on lights whenever it records nearby movement.