Originally Posted by
aixaix
I'll try. If you screw two nuts onto a bolt, then tighten them together, you can lock them anywhere on the threads you like. Since the inner nut on the shifter is trapped in the housing, the outer nut locks the screw in place after it has been tightened the right amount. The two nuts push together when the outer one is tightened. They pull and distort the thread on the screw very slightly, just enough to keep it from turning when the lever is moved. The distortion is temporary: as soon as you loosen the nuts, the screw returns to normal and can be turned.
Right! Phrased another way, as the two nuts are tightened against each other, they are forced to push against the threads of the bolt in opposite directions from each other. The tighter they are tightened, the more friction there is between the bolt threads and each nut. That friction prevents either of them from rotating in either direction. Without a locknut, the friction between the bolt threads and the one nut is very small, and that's why the bolt will continually work its way loose.