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Working on the roof, several J-bolts appeared to have been fastened with more than a single nut. Without exception each such bolt was almost flush to the cement sheet.

I saw this done on a car wheel aeons ago; the original nuts holding the wheel were almost thrice as long as the four nuts.

  • Is a bolt fastened to more than a single nut less likely to pop open?
  • Is it better to use two nuts, rather than a single long nut on the bolt?
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1 Answer 1

up vote 5 down vote accepted

Two nuts can be tightened against each other so that they won't come loose. A single nut can be tightened against whatever it's holding, and if that's wood, the nut can loosen as the wood changes size with changes in humidity.

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Additionally other factors such as vibration, temperature changes and rotation can cause single nuts to come loose. In some applications lock washers are used in an attempt to keep nuts from loosening. Other times nuts with castellated tops or a castellated nut cover are used with a cotter pin to keep a nut in place. Although this latter application is used when the nut is not fully torqued down such as when adjusting bearings or spring tension. The double nut trick can be very useful but does require that both nuts be accessible to a pair of wrenches or a wrench and socket. –  Michael Karas Feb 23 at 13:26

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