Genuine Aircraft Hardware Co.
Fastener Math and Terminology
There are some things about dealing with fasteners that will be easier if you understand
the terms and mathematics associated with them. To many of you this will be a review,
to some of you it will be new. It is my hope that it will help all of you, at least with your
fastener aspects.
Terminology
Grip Length:
The grip length of a fastener is always the unthreaded (solid) part of the shank. The grip
length does not include the head on a protruding head fastener, however the grip length
does include the head on a countersunk head fastener. The Grip of a fastener starts at
the head (included or excluded as applicable) and continues to the full cylindrical portion
of the shank. The Grip Length does not include the Transition Area or the Threaded part
of the fastener. In the case where the fastener has no threads but has a cross-drilled hole
for retention, such as a clevis pin, then the grip ends at the beginning of the hole. Some
fasteners such as Hi-Shear Rivets or Lockbolts with collars have neither hole nor
threads. In this case the grip ends at the end of the full cylindrical portion of the shank
or a little earlier if the manufacturer allows room for collar installation. Fasteners that are
fully threaded do not have a grip length.
Shank:
The unthreaded part of a fastener. The shank is always a part of, if not all of the Grip
Length of the fastener. The shank does not include the head of the fastener.
Transition Area:
The part of the fastener where the Shank starts to reduce in dimension to allow the
threads to be formed. The Transition Area includes the imperfect threads and extends
just until the threads are fully formed.
Inf1
Genuine Aircraft Hardware Co.
Threads:
This is the part of the fastener that is best described as a helical formed triangular strip.
On a male fastener such as a bolt or screw it usually starts at the end of the grip or just
under the Head of the fastener. The threads are formed by a process called rolling or by
an older process called cutting. The threads are the part of the male fastener that the nut
screws or threads onto by rotating its matching internal threads until both the nut and the
fastener have fully engaged.
Thread Area:
This includes the Transition area (between the Shank and the Threads) and the Threads.
This is measured from the end of the full cylindrical portion of the shank (where the Grip
ends), to the end of the fastener.
Overall Length:
The overall length is the sum of the Grip Length, The Transition Area and the Thread
Length. The Overall length of a fastener starts at the head (included or excluded as
applicable) and continues to the end of the fastener. The Overall Length does include the
Grip, the Transition Area, and the Threaded part of the fastener. In the case where the
fastener has no threads but has a cross-drilled hole for retention, such as a clevis pin,
then the Overall Length ends at the end of the fastener regardless of the hole.
Decimal/Fractional Charts have been made in many styles and forms. Sometimes the
Chart helps to put things into perspective. Remember, people think about values in
many different ways.
Inf2
Genuine Aircraft Hardware Co.
Fastener Math
For some of you this will be a review, for some it will be a numerical revelation. So in
either case be patient, with yourself or with me whichever the case may be.
It may help some people to refer to the Summary on page 7 for the formulas without
the instructions.
Some of us were raised in times when math was stressed more in school and some of
us only learned how to add and subtract. For this reason we will do all of our exercises
using a calculator or Decimal/fractional chart.
The first thing is to realize that there are so many different ways a number or a fraction
may be written or looked at that it surprises most people. For example:
One quarter could be looked at in so many ways, and some people are accustomed to
relating to it in only one way. See below:
Inf3
Genuine Aircraft Hardware Co.
Fractional Terms
The number in a fraction that is to the left of the division sign ÷ , or / , or on the top of
the division line as in
1
is called the numerator.
16
The number on the bottom or the right is called the denominator.
Examples of one 16th are:
1 ,
1/16 or 1 ÷ 16.
16
Converting Numbers with Decimals into Fractions
1)
Select the desired denominator you are looking for such as 16ths of an inch,
which is a useful denominator because most of the MS and NAS bolts grip
lengths are measured in 16ths.
2)
Using the calculator multiply your decimal figure by your fractional denominator.
For an example we will use .8125 for our decimal figure.
On the calculator input .8125, then the x key, then input 16, then the = key.
The answer you should have come up with is 13 on the display of the calculator.
Or as we will complete the mathematical statement on paper, 13/16ths of an
inch.
Inf4
Genuine Aircraft Hardware Co.
Converting Fractions into Decimals
1)
Using the calculator enter the numerator, then the ÷ key, then enter the
denominator, then the = key.
This will give you a decimal number.
2)
As an example we will convert the fraction 13 or 13/16 or 13 ÷16.
16
Using the calculator enter 13, then ÷, then 16, then =
3)
The answer you should have come up with is .8125, that is the decimal
equivalent for 13/16ths.
Decimal/Fractional Charts
For the ease of use and to cover situations where a calculator is not handy,
Decimal/Fractional Charts have been made in many styles and forms. Sometimes the
Chart helps to put things into perspective. Remember, people think about values in
many different ways.
Please see the Decimal/Fraction Chart on the following page.
Notice that it goes all the way to 1/64ths.
Inf5
Genuine Aircraft Hardware Co.
Inf6
Genuine Aircraft Hardware Co.
Summary
The Overall Length of a fastener is:
+
+
Grip Length
Transition Area
Thread Length
Overall Length
To convert decimals into fractions, multiply the decimal by the desired fractional
denominator. This will become the numerator.
Decimal x Fractional Denominator = Numerator. Or...
Multiples of the Denominator = Numerator
To convert fractions into decimals, divide the numerator by the denominator.
Numerator
Denominator
=
Decimal Number
Additional Information:
Whenever, in a fraction, the numerator is larger than the denominator, then the value is
greater than one. This is called an Improper Fraction.
You will need to simplify it by separating the Whole numbers out and simplifying or
converting the remaining proper fraction.
I.e. 24/16 = 1.5
Compare
or
24/16 = ((16/16) + (8/16))
1.5 =
1
+ .5
Inf7
Genuine Aircraft Hardware Co.
Notes
Inf8