Torque Tightening
Torque Tightening
What is Torque?
Torque Tightening and Preload
Turning movement
It is a measure of how much force acting on
an object which causes that object to rotate.
The amount of preload created when torqueing
is largely dependant on the effects of friction.
What is Torque Tightening?
The application of preload to a fastener by the
turning of the fastener’s nut.
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).
Force
Stretch of Fastener (Pre-load)
Preload (residual load) = Applied Torque minus Frictional Losses
Friction points should always be lubricated when
using the torque tightening method.
Residual Bolt Load (kN) ▶
Lubrication Reduces Friction
Lubrication Reduces Friction
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.
Lubricant or anti-seizure compounds should be
applied to both the nut bearing surface and the
male threads.
Frictional Losses
10% converted to preload
50% at nut
face
40% in
threads
Frictional Losses (dry steel bolt)
Torque (Nm) ▶
(Applied Load)
Lubricated with Moly grease (µ = 0,10)
Light oil (µ = 0,15)
Dry unlubricated (µ = 0,20)
Example of how a lubricant can reduce the effect
of friction and convert more torque to bolt preload.
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Torque Tightening
80%
Manufacturer’s rating of pressure and torque are maximum safe limits.
Good practice encourages using only 80% of these ratings!
Torque Procedure
80%
Select the Right Wrench
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:
Choose your Enerpac torque wrench
using the untightening rule of
thumb:
Torque Sequence
1
• 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
Step 1 Spanner tight ensuring that 2 - 3
threads extend above nut
Step 4 Increase the torque to full torque
following the pattern shown above.
Step 2 Tighten each bolt to one-third (1/3)
of the final required torque following
the pattern as shown above.
Step 5 Perform one final pass on each bolt
working clockwise from bolt 1, at
the full final torque.
• Humidity corrosion (rust) requires up to 2 times the torque required for tightening.
• Sea water and chemical corrosion requires
up to 2½ times the torque required for
tightening.
• Heat corrosion requires up to 3 times the
torque required for tightening.
Minimum Output Torque
• The recommended minimum torque value of
a hydraulic wrench is 10% of the maximum
rated value.
Step 3 Increase the torque to two-thirds
(2/3) following the pattern shown
above.
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.
Breakout torque cannot be accurately
calculated, however, depending on conditions
it can take up to 2½ times the input torque to
breakout.
The use of penetrating oils or anti-seize
products is always recommended when
performing breakout operations.
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