Does the sequence in which a jam nut and a standard nut are installed make a difference? Definitely.
A jam nut is a thinner-than-average nut tightened against a standard-height nut to lock the two nuts in place. Installed correctly, a jammed nuts resist loosening due to vibration better than a single lock nut.
The correct way to install a jam and standard nut on, say, a long, threaded bolt that tensions a drive belt on a combine or baler, is to install the thinner nut first. Tighten that thinner nut until the belt is properly tensioned. Then install the second, thicker nut, hold the thinner nut with a wrench, and tighten the thicker nut against the thinner nut.
Tightening the two nuts together locks them in place because the thinner nut’s threads wedges against the top side of the bolt’s thread, while the thicker nut’s threads wedges against the bottom of the bolt’s threads. The resulting tension locks those two nuts in position on the bolt’s thread.
Be cautious when using two full-thickness nuts as jam nuts. The extra threads inside each full-thickness nut, when jammed against each other, can create so much wedging force that they become nearly impossible to disassemble with manual tools. Jam nuts can be adequately tightened by squeezing two wrenches in one hand. Using two hands and arms to tighten jam nuts risks nearly welding them to the bolt’s thread.
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