bolts

AIRCRAFT BOLT SELECTION

Decoding the Numbers

Whether you are building a kit, modifying an existing aircraft, or designing from scratch, you will have to deal with aircraft (AN) bolts. Much of what you need to know about any specific bolt can be "translated" from the bolt designation:

AN4-27A

"Decoding" the bolt designation is quite simple. The letters AN (AN4-27A) indicate that the designation uses the aircraft numbering system. The first number (AN4-27A) provides the diameter of the bolt in 1/16 inch increments. Bolts most likely to be used in ultralight aircraft include:

	AN3	3/16"
	AN4	1/4"
	AN5	5/16"
	AN6	3/8"

The number to the right of the dash (AN4-27A) signifies the total length of the bolt. If the number has two digits, the left digit (AN4-27A) represents inches, while the right digit (AN4-27A) signifies the remaining length in 1/8-inch increments. Thus a -27 bolt would have a total length, measured from the base of the head to the end of the threaded shaft, of 2 7/8 inches. In the case of bolts designated by a single digit, the bolt is less than 1 inch in length and the total length (in eight-inch increments) is specified by the single digit.

While the total length of the bolt is important, it is equally important to know the grip of any particular bolt - the total length minus the threaded portion of the bolt. The length of the threaded section of AN bolts depends on the bolt diameter:

	AN3	6/16
	AN4	7/16
	AN5	8/16
	AN6	9/16

Thus, the grip of at AN4-27 bolt is the total length (2 7/8 inches) minus 7/16 inches, or 2 7/16 inches. We will get back to the subject of bolt grip shortly.

The final item of bolt nomenclature deals with whether or not there is a final letter A in the bolt designation. If there is no final letter (AN4-27) then the bolt has a hole drilled in the threaded portion of the shaft. Such drilled bolts are used with castle nuts and cotter pins, the only reliable way to secure a nut where the bolt and nut are subject to rotation. If the bolt designation ends with an A (AN4-27A), then the bolt shaft is un-drilled and suitable for use with nylock nuts.

Get a Grip

The importance of bolt grip can be seen with this simple diagram. In this case the "stack" includes the entire sequence of pieces, including washers under the head and nut, that are to be secured by the bolt. The precise thickness of the stack depends on the total thickness of the various components.

Whatever the thickness of the stack, the ideal length for the bolt is one where the grip is just slightly less than the total thickness of the stack - as shown above. When this is the case, the nut can be torqued down to compress the stack, yet the nut is not "bottomed out" on the threads and there are plenty of threads showing where the bolt extends beyond the nut.
If the bolt is too short, there will not be enough threads showing to provide a safe and reliable joint.
If the bolt is too long, the nut will bottom-out on the threads without developing the needed clamping force. In high-shear applications, such as prop bolts, the bolt is certain to shear under load!

It is pretty obvious that bolt length must be selected very carefully. While you can make the selection from a bolt table in a catalog, many of these table are incomplete in terms of precise bolt sizing across the AN3-AN6 range.

This is where the BOLTS PROGRAM comes in! Just download the program (bolts.exe) and copy it anywhere you want on your system. Let's say you copied it to the WINDOWS directory. You would run it as follows:

Click on START

Click on RUN

Type \windows\bolts

Hit ENTER

Click on the full-screen icon on the program window…

Let's say the "stack" consisted of a 2 inch piece of square tubing, a pair of 1/8 inch cheek plates, and a pair of AN960 washers:

Key <1> (one inch) twice

Key <4> (1/8 inch) twice

Key <5> (1/16 inch/AN960 washer) twice

The thicknesses don't have to be keyed in any particular order, but it does pay to organize your thinking so that nothing gets missed. As you press the various keys, you will be provided feedback in the form of a BEEP with each key-press and the current stack thickness will be displayed at the bottom of the page. When all the elements have been keyed in, key <0> and you should see the following list of bolts:

AN3-25

AN4-26

AN5-26

AN6-27

along with the total stack thickness and the grip of each bolt.

Once the bolt selections have been displayed, hitting any key will take you back to the start-up screen. To exit the program, key <0> before entering any stack elements.

I think you will find the program easy to use and very handy in selecting the proper bolt for a particular job or verifying the bolt call-outs on plans.

Ralph E. Taggart (gyrobee@aol.com)