Torque Tightening
Tightening Methods
Principally there are two modes of tightening: "Uncontrolled" and "Controlled".
Uncontrolled tightening
Uses equipment and/or
procedures that cannot be
measured. Preload is applied to
a bolt and nut assembly using
a hammer and spanner or other
types of impact tools.
Controlled tightening
Employs calibrated and
measurable equipment, follows
prescribed procedures and is
carried out by trained personnel.
or further information
F
on Torque Tightening or
other controlled tightening
methods, please visit
www.enerpac.com.
Advantages of Controlled Tightening
Known, controllable and accurate
bolt loads
Safe operation following
prescribed procedures
Employs tooling with controllable
outputs and adopts calculation to
determine the required tool settings.
Eliminates the dangerous activities of
manual uncontrolled tightening and
requires that the operators be skilled
and follow procedures.
Uniformity of bolt loading
Reduces operational time resulting in
increased productivity
Especially important on gasketed joints
as an even and consistent compression
is required for the gasket to be effective.
What is Torque?
It is a measure of how much force
acting on an object which causes that
object to rotate.
What is Torque Tightening?
The application of preload to a fastener
by the turning of the fastener’s nut.
Reduces tightening time and operator
fatigue by replacing manual effort with
the use of controlled tooling.
Torque Tightening and Preload
The amount of preload created when
torqueing is largely dependant on the
effects of friction.
Principally there are three different
“torque components”:
• torque to stretch the bolt
• torque to overcome the friction in
bolt and nut threads
• torque to overcome friction at the
nut spot face (bearing contact
surface).
Reliable and repeatable results
Using calibrated, tested equipment,
following procedures and employing
skilled operators achieves known
results consistently.
The right results first time
Many of the uncertainties surrounding
in-service joint failures are removed
by ensuring the correct assembly and
tightening of the joint are carried out
the first time.
Torque Tightening
Turning movement
Force
Stretch of Fastener
(Pre-load)
Visit www.enerpac.com to access our free on-line bolting software application and
obtain information on tool selection, bolt load calculations and tool pressure settings.
A combined application data sheet and joint completion report is also available.
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Torque Tightening
Preload (residual load) = Applied Torque minus Frictional Losses
Lubrication Reduces Friction
Friction points should always be
lubricated when using the torque
tightening method.
Frictional Losses
Lubrication reduces the friction during
tightening, decreases bolt failure during
installation and increases bolt service
life. Variation in friction coefficients affect
the amount of preload achieved at a
specified torque. Higher friction results in
less conversion of torque to preload. The
value for the friction coefficient provided
by the lubricant manufacturer must
be known to accurately establish the
required torque value.
10% converted
to preload
40% in
threads
50% at
nut face
Frictional Losses (dry steel bolt)
Lubricant or anti-seizure compounds
should be applied to both the nut bearing
surface and the male threads.
Torque Procedure
When torquing it is common to tighten only one bolt at a time, this can result
in Point Loading and Load Scatter. To avoid this, torque is applied in stages
following a prescribed pattern:
Torque Sequence
1
1
12
5
8
1
5
9
8
4
3
3
4
Select the Right Wrench
Choose your Enerpac torque
wrench using the untightening
rule of thumb:
• When loosening a nut or bolt more
torque is usually required than
when tightening.
• For general conditions it can take
up to 2½ times the input torque
to breakout.
• Do not apply more than 75% of the
maximum torque output of the tool
when loosening nuts or bolts.
Conditions of bolted joints
• Humidity corrosion (rust) requires
up to twice the torque required
for tightening.
•Sea water and chemical corrosion
requires up to 21⁄2 times the torque
required for tightening.
• Heat corrosion requires up to 3 times
the torque required for tightening.
4
3
10
6
2
7
2
Step 1 Spanner tight ensuring that
2 - 3 threads extend above nut
Step 2 Tighten each bolt to one-third
of the final required torque
following the pattern as
shown above.
Step 3 Increase the torque to twothirds following the pattern
shown above.
7
6
2
11
Step 4 Increase the torque to full
torque following the pattern
shown above.
Step 5 Perform one final pass on each
bolt working clockwise from
bolt 1, at the full final torque.
Breakout Torque
When loosening bolts a torque value
higher than the tightening torque is
normally required. This is mainly due to
corrosion and deformations in the bolt
and nut threads.
The use of penetrating oils or anti-seize
products is always recommended
when performing breakout operations.
Breakout torque cannot be accurately
calculated, however, depending on
conditions it can take up to 2½ times
the input torque to breakout.
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