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