Nuts and Bolts: The Basics You Should Know PDF Print E-mail
Written by Jacqueline Shipe   
Thursday, 24 February 2011 09:41

Airplanes are made of sheets of metal and a numerous variety of parts which must be held together securely. The many bolts, screws, nuts, rivets and other fasteners which make up the aircraft hardware can seem like small, relatively insignificant items but they are very important. Learning the basics about them will help you truly appreciate them.

Aircraft-grade hardware is much stronger and is manufactured to a higher standard than items for automotive or household use. Aircraft bolts, for example, have threads which are created with a roller that puts pressure on the metal and pushes it up to form the threads. There is no loss of material in this process and the threads are very strong. Automotive bolt threads are cut out, resulting in a loss of metal and strength. Aircraft-grade bolts are also heat treated and manufactured to be corrosion-resistant.

Aircraft hardware is usually identified by part numbers that were used by the military. The abbreviations “AN” stands for Air Force-Navy, “NAS” means National Aircraft Standard and “MS” indicates Military Standard. These abbreviations refer to the specifications to which the parts are manufactured. Specialized fasteners are identified by part numbers used by the manufacturer (Dzus, Southco, etc.). Bolts, screws and nuts are also identified by the type of thread they have, which can be fine or course. Course thread bolts and nuts are generally used on engine applications. Most of the bolts and nuts on the airframe are fine thread.

Each bolt has a head which is the hex-shaped top portion and a shank which is the lower portion of the bolt from the bottom of the head to the bottom of the bolt. The top unthreaded portion of the shank is the grip.

Bolts are generally made of cadmium- or zinc-plated steel, corrosion-resistant steel (stainless) or aluminum. Most of the bolts used are the plated steel variety. These bolts are identified with a raised “X” or an “X” with an extra line through it, depending on the manufacturer, which is on the head of the bolt. Stainless steel bolts have a single raised dash on them. These are used often on exhaust systems where corrosion is prevalent. Aluminum bolts (which are hardly ever used) have two dashes opposite each other on the head.

The bolt is numbered according to its size. An example of a bolt part number is AN5-16A. The 5 refers to the diameter of the shank in sixteenths of an inch. The 16 refers to the shank length in eighths of an inch. An “A” at the end means that there is an absence of a hole in the shank for a cotter key. The letter “H” after the “AN” would mean that the bolt head is drilled so it can be safety wired. This particular bolt would be 5/16 inches in diameter and 2 inches long. These types of bolts are considered to be general purpose bolts.

Close tolerance bolts are manufactured to be slightly oversized than standard, general purpose bolts. The shank is fatter and they are used in areas where there is no movement or space allowed in the bolt hole. A hammer generally has to be used to tap them into place. This is called a tight drive fit.

The nuts used on different areas of the airplane are varied according to their application. They each have some means of preventing them from loosening as the airplane vibrates in flight. The most commonly used are the lock nuts which are manufactured to require a wrench to thread the nut onto the bolt or screw. These come in two varieties, the fiber lock nut and the all metal lock nut.

Fiber lock nuts have a fiber insert which has a smaller diameter than the threads on the nut and it squeezes around the threads of the screw or bolt to prevent movement. These are very reliable and can be re-used until the fiber loses its grip. If the nut can be spun on by hand past the point where the threads of the bolt start contacting the insert, it should be discarded. Fiber lock nuts can’t be used in areas where the temperature exceeds 250 degrees Fahrenheit.

Metal lock nuts are used in higher temperature areas such as forward of the firewall and in other areas designated by the airplane manufacturer. The threads of a metal locknut are narrowed on the end to provide the locking mechanism.

Castle nuts have slots in them and are designed to be used with a bolt that has a drilled shank and a cotter key. These are used often with clevis bolts on cable attachments or on control surface attachments where some swivel movement is needed.

Plain nuts are used with lock washers to prevent loosening. Star lock washers with internal or external teeth can be used under the nut to hold it securely and maintain the torque. A flat washer should be used between the star lock washer and the surface of the part to protect the surface. Star lock washers should be used only once and discarded at re-assembly. Split lock washers are spring loaded washers with a split in them that allows one end to be curled up. The curled up end flattens out when torqued and puts tension on the nut to prevent movement.

Wing nuts are designed to be tightened by hand and are used on covers or areas where access is required often. Some wing nuts have holes in them to allow them to be safety wired.

Bolts should be installed so that the grip length is a touch longer than the thickness of the parts that it is holding together. Plain washers are used to protect the mating surface and to ensure that the nut does not tighten against the grip of the bolt and become “thread bound.” Bolts should be installed with the head up or forward whenever possible.

Lock nuts should always be installed so that at least one thread of the bolt or screw is protruding through the end once it is tightened. Thin nuts designed for a sheer (side to side) load should not be installed in a tension load. (A tension load pulls against the threads, as if to pull the nut off the bolt.)

Catalogs from different aviation suppliers can be valuable tools when looking for part numbers for hardware items because they usually have a picture or drawing of the piece and a chart with the dimensions in them. There are also bolt gauges that can be purchased and used to determine the part number of a particular bolt. Hardware can be expensive and is usually sold in set quantities so you want to be sure you have the correct part number before ordering.

Nuts and bolts are small items in themselves but a failure of even one of them can have serious consequences. Knowing all that you can about nuts and bolts will help you avoid preventable accidents.

From the March 2010 issue of Cessna Owner