Wednesday, June 4, 2008

Bolt torque calculations.

--- In, kannan.sundaram@... wrote:

In continuation of one of the earlier mails on the subject, I would like
to add the below simple approach also.

What is the Proper Torque to Use on a Given Bolt
by Joe Greenslade

"What torque should I use to tighten my bolts?" is a question suppliers
bolts are frequently asked by end user customers. Many times I have been
asked if a chart is published on the recommended tightening torque for
various bolt grades and sizes. I do not know of any. This article
such a chart for "Initial Target Tightening Torque. It See Figure 1. The
formula for generating these values is explained below.

The widely recognized engineering formula, T= K x D x P (to be explained
later in this article), was used to provide the chart's values, but it
must be understood that every bolted joint is unique and the optimum
tightening torque should be determined for each application by careful
experimentation. A properly tightened bolt is one that is stretched such
that it acts like a very ridged spring pulling mating surfaces together.
The rotation of a bolt (torque) at some point causes it to stretch
(tension). Several factors affect how much tension occurs when a given
amount of tightening torque is applied. The first factor is the bolt's
diameter. It takes more force to tighten a 3/4-10 bolt than to tighten a
318-16 bolt because it is larger in diameter. The second factor is the
bolt's grade. It takes more force to stretch an SAE Grade 8 bolt than it
does to stretch an SAE Grade 5 bolt because of the greater material
strength. The third factor is the coefficient of friction, frequently
referred to as the "nut factor." The value of this factor indicates that
harder, smoother, and/or slicker bolting surfaces, such as threads and
bearing surfaces, require less rotational force (torque) to stretch
(tension) a bolt than do softer, rougher, and stickier surfaces. The
formula T = K x D x P stated earlier takes these factors into account
provides users with a starting point for establishing an initial target
tightening torque.

? T Target tighten torque (the result of this formula is in inch pounds,
dividing by 12 yields foot pounds
? K Coefficient of friction (nut factor), always an estimation in this
? D Bolts nominal diameter in inches
? P Bolt's desired tensile load in pounds (generally 75% of yield

The reason all applications should be evaluated to determine the optimum
tightening torque is that the K factor in this formula is always an
estimate. The most commonly used bolting K factors arc 0.20 for plain
finished bolts, 0.22 for zinc plated bolts, and 0.10 for waxed or highly
lubricated bolts.

The only way to properly determine the optimum tightening torque for a
given application is to simulate the exact application. This should be
done with a tension indicating device of some type on the bolt in the
application. The bolt is tightened until the desired P (load) is
by the tension indicating device. The tightening torque required to
achieve the desired tension is the actual tightening torque that should
used for that given application. It is extremely important to realize
this tightening value is valid only so long as all of the aspects of the
application remain constant Bolt suppliers sometimes have customers say
that their bolts are no good because they have started breaking while
being installed. Thorough investigation commonly reveals that the
has started lubricating the bolts to make assembly easier, but
to same torque as was used when the were plain finished

The table in this article shows that by using this formula a 1/2-13
5 plain bolt should be tightened to 82 foot pounds, but the same bolt
is waxed only requires 41 foot pounds to tighten the same tension. A
perfect 1/2-13 Grade 5 waxed bolt will break if it is tightened to 81
pounds because the K factor is drastically lower. The bolts are fine,
the application changed. Suppliers need to understand this and be able
educate their customers to resolve this common customer complaint about
breaking bolts.

The chart is provided for quick reference by fastener suppliers and
for selecting an initial target tightening torque. This chart was
by using the formula shown earlier. An example of the calculation is as

Product: 3/4-10 Grade 5 zinc plated bolt
Formula: T= K x D x P

? K= 0.22 (zinc plated)
? D= .750 (3/4-10 nominal diameter
? P= 23.046 pounds

Hopefully the chart will help suppliers with an initial answer to the
customer's question, "What torque should I use to tighten my bolts?"
in mind this is only an estimated value. It may provide satisfactory
performance, but it also may not. Every application should be evaluated
its own to determine the optimum torque value for each application.
bolt suppliers should have tension indicating equipment necessary to
their customers determine the appropriate tightening values for their
specific applications. Keep in mind that if the lubricant on a bolt and
nut combination is changed, the tightening torque value must be altered
achieve the desired amount of bolt tension.

Joe Greenslade is President of Greenslade and Company, Inc. located in
Rockford, Illinois. His firm specializes in providing manufacturing
tooling and inspection equipment to suppliers of screws, bolts, rivets,
and nuts throughout the world.

Joe is an inventor, author, and lecturer. He holds eleven US Patents.
written over 80 technical articles for industrial trade journals, and
spoken frequently at trade association meetings and technical
on issues related to industrial quality for the past ten years.

He is an Associate Member of the Industrial Fastener Institute and a
member of the American Society of Mechanical Engineers B1 Thread
Specification Committee. In 1992, Joe was recognized for his technical
innovative contributions to the fastener industry when, at age 44, he
became the youngest person to be inducted into the National Industrial
Fastener Show "Hall of Fame. "

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