Nuts fall into two distinct categories: plain nuts and self-locking nuts. As their name implies, plain nuts have no built-in means of locking into place. Plain nuts are generally used in situations where the nut is installed in tension (versus in shear). In cases where there is significant tension in the joint (such as cylinder base nuts), the goal is to torque the nut down until the bolt is stretched with a specific “pre-load” of tension that is greater than the forces trying to pull the joint apart, and that also produces enough friction to prevent the nut from loosening. Nuts used in these situations are specialized and often have one side that is specially designed to face the mating surface. It’s very important to make sure you are always using the correct nut, oriented in the correct direction.
The majority of plain nuts on an aircraft, however, are not secured solely through pre-load. Most plain nuts use a helper of some kind to ensure they stay in place. The most basic form of this is the lock washer. The most common star-type lock washers are made of spring steel and have small, angled teeth that bite into the metal of the nut and use spring tension to prevent it from coming loose. These washers are available with internal teeth (Type A) or external teeth (Type B). Star washers are one-time-use devices. Anytime that you loosen or remove a fastener that utilizes a star lock washer, be sure to replace it before re-tightening.
Another type of lock washer is the split washer. These washers are a single, split circle made of carbon steel that is bent to create a spring that is compressed when the fastener is tightened. Unlike the thin star washers, these lock washers can be reused as long as they still retain their shape and tension ability. That said, at a cost of only a few cents apiece, I usually just opt to replace them when I’m working with critical components. Another method of securing plain nuts is to add a second nut, tightened against the first one. These secondary nuts are commonly referred to as jam nuts and are often thinner and lighter than the main nut. Their job is simply to exert tension on the bolt against the main nut, preventing it from coming loose.
The last method of securing plain nuts is to use a mechanical locking device to hold it in position. The most common type of locking device is the cotter pin. Plain nuts that are castellated are designed to be used with drilled bolts, axles, etc. and have slots machined into the nut that capture a cotter pin passing through the nut and hole in the bolt. This design is used almost exclusively in shear situations, although some joints such as axle nuts do employ some degree of pre-load to hold bearings in place. There are also drilled nuts, designed to use safety wire to secure them in place. Lastly, you may come across situations where a plain nut is locked in place by a special washer with a locking tab that is bent up against a flat side of the nut, ensuring that it cannot rotate. These are also designed for one-time use and should be replaced anytime that the fastener is disassembled.
The second category of nuts is self-locking nuts, commonly referred to as “lock nuts.” These nuts are designed to resist turning and coming loose without relying on pre-load or friction against the surface metal. Lock nuts can be either elastic or all metal. The elastic versions have an embedded fiber insert that is designed to stretch around the threads of the bolt to create the friction necessary to prevent it from coming loose. Because of the low melting point of the elastic material, it’s very important that this style of lock nut only be used in low-heat environments. You should never use an elastic lock nut in an engine compartment, or in a high-current electrical assembly. Both cases can lead to high temperatures that can (and will) melt the elastic and allow the lock nut to come loose. Years ago, I encountered a situation where an electrical bus bar had a loose wire that caused the bar to heat up, which in turn melted an elastic lock nut and made a bad situation much worse.
The alternative to elastic lock nuts is all-metal lock nuts. These nuts are manufactured with either a slightly oval shape or flanged teeth that look a little like a castle nut but are angled inward to provide tension on the bolt. They do not have the same temperature restrictions as elastic lock nuts. When in doubt, always use all-metal lock nuts.
There are all sorts of variations on the lock nut concept, including clip nuts that slip over the edge of metal, low profile, and even combinations of lock nuts and castle nuts. Regardless of the specific design, the rules for using lock nuts are simple. Most lock nuts can be reused as long as they still provide enough resistance to turning on the bolt such that you cannot turn the nut by hand without using a wrench or similar tool. If it’s too tight for you to move with your hands, it’s probably OK to reuse.
There are many more types of nuts and details about their use than we can cover here. However, understanding why certain nuts are used in different situations will make you a more educated owner the next time you swing a wrench on your airplane. In addition, it may help you identify issues on the ground during your next preflight before they become problems in the air. As always, refer to the manufacturer’s maintenance manual for the correct parts for every fastener and never substitute without consulting your mechanic first. Until next time, I hope you and your families remain safe and healthy and wish you blue skies.
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