I've done a number of stories in Farm Journal Magazine over the years about various types of nuts and bolts, torquing nuts and bolts, and products that help keep nuts and bolts from coming loose. In no particular order, here are some things I learned in talking to engineers who specialize in the nuts and bolts of nuts and bolts:
-A flanged, serrated nut (a nut that has a flared lower edge with serrations that "bite" into the surface the nut is tightened against) locks and holds better than a regular nut and lock washer.
-Loctite or similar products works best on clean metal. Such products "lock" because of a chemical reaction between raw, bare metal and the product, in the absence of oxygen (ie--when a nut or bolt has been tightened and the oxygen has been driven from between the threads). Oil, dirt or contamination decreases the locking ability of thread-locking products.
-Flat washers often have a top and a bottom. Examine a large flat washer carefully and you'll note one side the center hole has a sharp, crisp edge, while the edge on the other side of the washer is slightly rounded. The bottom sides of the heads of bolts, where the head meets the shank of the bolt, is also slightly rounded. The rounded edge of a washer should go toward the rounded edge of the bottom of the bolt head. Theoretically, if the crisp, sharp edge of the washer is forced against the rounded bottom edge of the bolt head, it could produce a stress point that could cause the bolt head to fail.
-Theoretically, no nut or bolt should be re-used after it has been torqued to full value. Torquing to full value microscopically stretches a bolt or nut's threads, and once metal has been deformed, it never again has full strength. Engine builders are religious about never re-using cylinder head bolts, connecting rod cap bolts or any other fastener when re-building engines. Most of us tend to be more agnostic about re-using nuts and bolts on farm equipment...
-There is no truth to the rumor that engineers designed nuts and bolts with hexagonal heads so that no matter how they hit the concrete floor when you drop one, they will always bounce, roll, hop or skitter out of reach. There is evidence, however, that nuts and bolts are proof of the existence of the mysterious "dark matter" that physicists are only now beginning to understand. Nuts and bolts are attracted to "dark matter." The proof is in the near-guarantee that any nut or bolt dropped in a shop with bounce, hop or roll into the nearest dark area under a bench, tire or nearby piece of equipment.