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15.2
Angle validation
Now that the angle can be indicated with great precision, the other challenge is to validate these results against
an external torque/angle transducer with monitor. This is not as simple as setting both the controller and the
monitor to the same snug torque and comparing the resulting angle.
It has been found that a tool's torque trace will never track exactly the same as the external. The calibration is
only the average of a number of readings, generally at a high torque near the maximum capacity of the tool.
When any individual torque reading from the tool's controller is compared to a torque reading from the external
torque monitor, it can easily have a difference of several percent higher or lower. This means that the tool's
controller will start counting angle at a different point than the external torque/angle monitor starts counting. This
could be five to 10 degrees different depending of the hardness of the joint.
The only way to get consistent results when validating an angle reading against an external monitor is to pre-
torque the joint slightly higher than the snug torque. Run the tool on this already-tightened joint, with the snug
torque set to the same value in both the controller and the monitor, even if the tool's transducer and the external
transducer do not exactly agree near the snug torque, they will both start counting angle just before the fastener
starts to rotate, so their zero angle will be synchronized exactly.
For example, if a brake line fitting requires six NM plus 40 degrees, pre-torque the joint to seven NM first. Then
change to an Angle Control strategy, with six NM snug torque, plus 40 degrees angle target, and reset the
external torque/angle monitor. Then as the tool is run in this angle control mode, the tool will start counting
angle as soon as it has six NM (which might have been five or seven NM according to the external transducer),
which is before the joint actually starts to rotate. And the external monitor will start counting angle as soon as it
has six NM which is also before the joint starts to rotate. This way, both meters are reading angle from the same
point, even though the torque readings may differ slightly because of the allowable tolerances in the torque
calibration.
15.3
Torque recovery implementation
After a torque control cycle, typically with multi-spindle applications or on a soft joint, one or more fasteners may
be found to have low residual torque (indicating a loss of clamp load).
This phenomenon can be caused by material flow, component embedment or relaxation within the individual
joints, or by cross-talk. Cross-talk occurs when one fastener arrives at the target torque first, and as the
surrounding fasteners are tightened, they can distort parts of the assembly such that the first fastener can lose
some of its clamp load.
The purpose of this fastening strategy is to re-torque all the fasteners in order to recover any clamp load that
may have been lost during (or immediately after) the previous torque control step. This should then result in
acceptable residual torque values for all fasteners in an assembly as well as consistent values across many
assemblies.
SC Series Controllers
Appendix A
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