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Tightening tools are
controlled in a number of different ways. In
general, they may be classified as direct
control, using a measuring device like a torque
transducer or angle encoder, or indirect using a
clutch in an air tool, or current measurement in
a DC Electric tool.
In order to ensure that the tools are set
correctly to output the required torque or angle
parameters, frequent periodic calibration is
necessary.
Why is this necessary?
...Because mechanical parts wear, lubrication
dries up, electronic devices may drift or fail,
maintenance schedules are missed, tools get
contaminated with dirt etc, etc.
Most manufacturing companies, particularly those
with recognized certification like ISO 9000,
have strict procedures requiring the periodic
calibration of assembly tools. Normally
calibration is required at least once a year.
Ingersoll-Rand recommends every 6 months or
100,000 cycles.
Calibration is normally addressed in one of the
following ways:
-
The tool is
returned to the manufacturer for
re-certification.
-
The tool is
re-calibrated in the user's own lab or tool
crib using a traceable transducer and a test
joint kit.
-
The tool is
calibrated on line with an add-on external
transducer that is traceable to a Master
transducer.
The Calibration
Process
The process is to run an appropriate number of
tightening cycles, usually 10-25, record the
values from the tool and the external
transducer. For calibration of tools with
indirect control devices like clutches, adjust
the control device until the required torque is
achieved.
For DC Electric tightening systems, when the
data is collected, calculate the mean of the
external transducer and the tool transducer. If
there is a variation between the two values,
adjust the transducer value (TR) until
correlation is obtained.
The new adjusted TR value is calculated as
follows:
Repeat the calibration tightening cycles until
the two transducers match within the
specification of the tightening tool.
The equipment used for tool calibration is shown
below:
Calibration can be conducted using stationary or
rotary transducers:
The above shows a DC Electric tool being
calibrated on a test joint using a rotary
in-line transducer.
The above shows a DC Electric tool being
calibrated on a test joint using a bench mounted
stationary transducer.
In between the scheduled calibration of
controlled tightening tools, it is recommended
that torque audit plans are implemented.
This will insure that the equipment has not been
mal-adjusted or mis-applied between
calibrations.
Torque auditing is best achieved by measuring
the torque dynamically.
Torque auditing using the Ingersoll-Rand ETW
Dynamic Torque Audit Wrench
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