In a previous article, I discussed Grade 8 versus Grade 5 fasteners
and I received a few requests right away from loyal RC readers
for a follow up article about fastener torque. Your wish is my
command…sometimes. Either way, it seemed like the next logical
step. After you have calculated/decided which fastener size and grade
to use, how do you know what to torque the fastener to so you can
take advantage of its strength? Some hardware installation instructions
include torque specifications, but unfortunately most do not, which
leaves you hanging by a fastener thread (pun intended).
Some people take a wild guess at what torque they should use and
others don't even use a torque wrench. They just go by feel. I
personally wouldn't be going by gut feel if I were attaching a
$1000 winch or $2000 suspension lift to my multithousand dollar
vehicle. That just doesn't make sense, but it happens all the time.
It is extremely important to properly torque fasteners or you may
see your winch flying past you or your suspension lift rattling
your fillings loose when you least expect it.
You can undertorque
or overtorque fasteners. Undertorquing a fastener is just as
bad as overtorquing it. If the fastener is undertorqued, that
will cause it to cycle many more times than normal and eventually
shorten the fatigue life drastically, which in turn leads to
an early failure.
Overtorquing risks shearing the fastener head
off (break out the EZ out) or, even worse, it will actually yield
the fastener. If the fastener yields, then there is NO preload
on the fastener. No preload means the nut will most likely fall
off due to vibration along with whatever it was supposed to hold
on your vehicle.
Now, hopefully, you can see how important it is to torque the
fastener to the correct level.
Put On Your Science Hat
There is some science
behind how much to torque a fastener. It mainly comes down to bolt
preload/fastener tension. When you're applying torque to a fastener,
you are essentially stretching or preloading the fastener. This
is done to prevent the fastener from relaxing too much and the
nut from coming loose  or even worse, falling off. The big question
is “how much preload do I need?”
Where I work, the basic rule we use is that for structural applications,
the fastener preload should be 67% of the yield strength. This
value was chosen because a fastener (i.e. bolt) has a proof load
value for each grade and diameter. The proof load is usually 90
percent of the yield strength. The proof load guarantees that the
fastener will not permanently yield or stretch. If it did, you
will loose your preload and the fastener will eventually fail or
fall out.
So now you are probably asking yourself right now then why don't
we just use the proof load level for establishing the preload.
You don't because of a little something called friction. For example,
the coefficient of friction between a bolt and nut can vary tremendously.
Fastener coefficients of friction vary as much as +/20% of nominal.
Therefore, if we used a value close to the proof load (i.e. 80%)
and had a high variance in coefficient of friction (+18%) from
our nominal calculation, we would end up going beyond the proof
load and yielding the fastener, thus applying no preload.
The next thing you need to know is what value for the coefficient
of friction you should use. If the fasteners are dry it can vary
the coefficient of friction from 0.15 to 0.25. If either is lubed
with some sort of lubricant (ie. dry film lube, WD40), etc.),
it changes the coefficient of friction quite a bit. We use 0.20
for the unlubed coefficient of friction and 0.09 for the lubed
coefficient of friction. As you can see again, there is a pretty
large difference, so please note if the fasteners you are about
to put the torque wrench on are dry or lubed with any sort of lubricant.
Now you have all the tools to calculate the torque required, so
here is the basic formula to use.
T = K x U x D x P where K is a constant that equals
1.33
U is the coefficient of friction
D is the basic diameter of the fastener
P is the preload you need
T is the torque required
Let's look at an example of where someone has a ½ inch
diameter, unlubricated Grade 8 bolt and needs to calculate the
torque setting.
D = 0.500 inch
U = 0.20 (unlubricated coefficient
of friction)
K = 1.33 (constant)
We need to calculate the preload P. Going back
to my earlier Grade 8 versus Grade 5 fastener technical discussion,
you can see that a Grade 8 fastener has an ultimate strength of
150 ksi and a yield strength of 130 ksi (130,000 lb per square
inch). The thread area (minimum diameter of the fastener) of a ½ diameter
bolt is 0.1599 square inches according to MILS8879C. Therefore,
the full yield strength is 130,000 lb per square inch X 0.1599
square inches which equals 20,787 lb. 67% of the full yield strength
is 13,927 lb.
Inputting this gives you get T
= (1.33) x (0.20) x (0.500 inch) x (13,927 lb) = 1852 INCH LBS
or 154 FT LBS
Remember that this assumes you know what grade fastener you are
using and whether or not it is lubricated. If we had chosen the
lubricated case, the result is much different. Here is the same example, but this time using a lubricated fastener.
T = (1.33) x (0.09) x (0.500 inch) x (13,927 lb) = 834 INCH LBS
or 70 FT LBS
Notice that the torque required for a lubricated fastener is LESS
THAN HALF that of an unlubricated fastener. I hope you note how
important the coefficient of friction is and how it affects the
required torque setting to achieve the same preload.
We have built a chart with the nominal torque
value for each size and grade of fastener for unlubricated and
lubricated cases. This should make it much easier than calculating
it yourself, plus you get to revisit Rockcrawler.com every time
you need it! Click on the link here to try it out.
Hopefully, the next tool you ask for for your birthday
is a torque wrench. If you already have one then you are now fullyequipped
mentally and physically to torque those fasteners correctly. Remember,
it's your safety at stake and your hard earned pesos, so please
torque those fasteners right the first time!
Always torque to and
follow the instructions recommended by the manufacturer. If no
torque instructions come with your hardware, contact the manufacturer
for the appropriate torque levels. Since the coefficient of friction
plays such a large role in the total torque required, make sure
to use either clean dry threads or fully lubricated threads and
use the corresponding coefficient of friction when calculating
the total torque.

Based
near Denver, Colorado, David Potter is a new technical
writer here at Rockcrawler.com and is
also the proud new owner of the Project TJ "Mad
Cow." David is a genuine rocket engineer for Lockheed
Martin.
Contact
David at dpotter@rockcrawler.com 
Disclaimer: Though we have taken steps to ensure accuracy of this calculator,
please always check with the appropriate manufacturer(s) before choosing the
torque level for your project. Rockcrawler.com will not be held responsible
for errors. This calculator is for informational use only. 