WHICH NUT FIRST?
Although it’s not a hard and fast rule, propeller nuts are frequently available in two thicknesses or heights: full height and half height. The American Boat and Yacht Council, using Society of Automotive Engineers’ references, refers to these components as the “jam nut (thin)” and “plain nut (thick).”
The reasoning behind this differing height nut approach is straightforward enough. The full-height nut, because it has more thread engagement, is designed to carry the lion’s share of the load, while the half-height nut is designed to act as a locking mechanism. That’s the aspect of propeller nuts most folks understand and with which they agree. The tricky part, and where the controversy comes in, involves their order of installation. Contrary to commonly held beliefs, when assembly is complete, the half-height nut should be against the propeller hub, having been installed first, while the full-height nut is farthest from the propeller hub, having been installed last. The logic for this seemingly counter-intuitive approach goes like this: when the first nut is installed and torqued down, it carries all the load or tension. When the second nut is installed against the first nut and it is torqued, much of that load or tension is transferred to the second nut. Thus, it only stands to reason that the second nut (the one that carries the majority of the load), should engage more of the propeller shaft with more threads.
I know for all you gear-heads out there this appears to be heresy or worse, and you are wondering if I’ve lost my nut-and-bolt mind. When I first began proliferating this information several years ago, it was met with the hue and cry of many in the boat-owning and marine repair and boat-building communities, proclaiming it sacrilege, barbaric, and just plain stupid. Some even claimed that I had intentionally misled folks to drum up business for myself and my colleagues in the industry by propagating a spate of lost propellers. Even today, whenever I share this approach, the skeptics come out of the woodwork.
Rest assured, not only is this approach one I wholeheartedly endorse, but it’s also backed up with good engineering and it’s endorsed by propeller manufacturers, the Society of Automotive Engineers, the American Boat and Yacht Council, the U.S. Coast Guard, and the U.S. Navy. The latter spells this procedure out in excruciating detail in chapter 75 of their sublime (if you like this stuff) NavShipsTechManual (Chapter 075-5.3.4, Fasteners). It can be downloaded as a pdf, courtesy of Uncle Sam; I keep a copy on my iPad for “light” in-flight reading.
075-5.3.4 JAM NUTS (LOCK NUTS). Jam nuts are an older variation of the prevailing torque concept. They
are not usually recommended for new installations due to the tendency to use an improper thickness for the jam
nut and to install them in the wrong relative positions.
075-126.96.36.199 Jam Nut Assembly. The jam nut assembly requires a regular or main nut and a thin jam nut, as
shown in Figure 075-5-5. The assembly is installed with the thinner nut between the thick nut and the bearing
surface. The main nut has to be as thick as if no jam nut were being used, because the main nut carries all the
working load. The jam nut is usually about 2/3 as thick as the main nut. If the jam nut is too thin, however, the
threads in the jam nut area will be damaged as the main nut will pull the bolt threads partially through the jam
nut. Conversely, if the jam nut is too thick, the main nut cannot distort the threads enough.
075-188.8.131.52 Tightening the Jam Nut. At assembly, first tighten the jam nut to the same or slightly less percentage
of the preload torque specified for the main nut, based on the relation the jam nut thickness bears to the
thickness of the main nut. Then hold it in position with a wrench while you tighten the main nut. For example,
if the jam nut is 2/3 as thick as the nut, tighten the jam nut to 1/2 to 2/3 of the torque used for the main nut.
Then, when the main nut is tightened to the preload torque specified for the bolt, it stretches the bolt (stud),
thereby tending to pull it through the jam nut. Any vibration or load that tends to loosen the bolted joint will
allow the bolt to shrink back to its original length, leaving the jam nut tight against the main nut. This creates
the necessary prevailing torque to prevent the jam or main nut assembly from rotating on the bolt.
075-5.3.5 SETSCREWS. Setscrews are seldom used in the U.S. Navy. Setscrews can be used in a variety of
ways to lock threads (see Figure 075-5-6). A setscrew can:
a. Jam a plug of softer material (plastic, copper, or lead) against the threads to be locked.
075-5-4 Distorted Collar Nuts