Put a Coat On
Remember that gold color we
discussed earlier with the grade 8
bolts? That golden color comes from
the bolt’s coating. That gold color
is actually a yellow zinc coating
that helps to protect the bolt from
corrosion. Most bolts have zinc-plating — either silver or yellow.
Bolts can also be galvanized to help
protect the bolt from corrosion.
There are a multitude of coatings
An important consideration in
calculating bolt torques is to
determine whether or not to include
lubrication. In some instances, it may
be desirable to ensure the bolt can
be easily removed at a later date.
Did you put that anti-seize
compound on your spark plugs the
last time you installed them? You
may also want to ensure that the
bolt does not come out easily and
decide to put a thread locker on the
bolt, i.e., Loctite.
The coating and the lubrication
are referred to as the Nut-factor, the
K-factor, or the Friction Factor. Table
1 references some common Nut-factor values based on coatings and
lubricants. Lubricants or coatings
that increase the friction in the
system will have a lower value and
therefore will require less torque to
ensure the bolt remains tight. If you
don’t know what coating you have,
don’t sweat it. Many engineers will
use 0.2 regardless of the hardware
they are using.
At π4 D .9743 p ⋅ − ( ) 2 ⋅ in2 ⋅ = :=
EQUATION 1. Calculation for tensile stress
area for a 1/4-20 bolt.
F .75σproof ⋅ At⋅ 2915 lbf⋅ = :=
EQUATION 2. Calculation for bolt force
for a 1/4-20 bolt.
τpl F K ⋅ D ⋅ 12 ft lbf ⋅ ⋅ = :=
EQUATION 3. Calculation of recommended
torque for a 1/4-20 bolt (dry, no lubrication).
Don’t Get all
Mathy on Me
Now we can get into the meat
of the calculations. We need to
determine the bolt pitch. To get the
bolt pitch, take the inverse of the
thread count. If you have a 1/4-20
bolt, the 1/4 is the diameter of the
bolt and the 20 is the number of
threads per inch. To get the pitch
you divide 1 by the threads per inch,
or, in the case of a 1/4-20 bolt, you
have 1/20 or .05.
From there, we will calculate the
threaded area of the bolt — which is
referred to as the tensile stress area
— using Equation 1, where D is the
bolt diameter and p is the pitch.
Next, begin to calculate the Force
that is required to maintain the
preload using Equation 2, where
theta sub proof is the proof strength
from our reference table and A sub t
is the tensile stress area we
calculated in Equation 1.
Now, calculate the
recommended torque to achieve
that value using Equation 3,
where k is the Nut-factor, D is the
bolt diameter, and F is the Force
from Equation 2. This number is
the amount of torque you must
apply to achieve the preload we
discussed in the beginning. If you
use American standard units
throughout your calculations, this
number will be in in-lbs or ft-lbs.
There are certainly other factors
that can be taken into consideration
to optimize a bolt torque, such as:
the materials that are being
clamped; the number of clamped
pieces; and whether a gasket is
being used, etc. All of these things
will affect how much torque is
required to maintain that bolt
preload, but people much smarter
than me figure that stuff out.
Hopefully, you will be able to use
this article to ensure that those ever
so common wheelectomies become
less common. SV
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