SC Series Controllers
Appendix A
A simple solution is to wait a short time to allow for any relaxation to occur, and then run another torque control
step. To not impact the fasteners, the torque should be increased at a controlled rate. This is done by ramping
up the current limit to the level necessary to deliver the target torque. This re-torque step ends when the target
torque is reached.
The fastener may or may not rotate, depending on if it did, or did not, experience relaxation. Any fastener that
did relax will have its lost torque recovered during this torque recovery step.
In order to report the peak dynamic torque from this multi-step fastening cycle, the controller monitors if the
fastener actually advances during the torque recovery step.
If the fastener does rotate, then the peak torque from the torque recovery step should be reported as the peak
dynamic torque for that cycle.
If the fastener does not rotate during the torque recovery step, then the peak torque from the previous step
should be reported as the peak dynamic torque for that cycle.
To report the final tightening angle beyond the snug torque, we need to report the total angle for both the torque
control step and the torque recovery step.
15.4
Fastener yield control implementation
The process of tightening a fastener involves stretching, or preloading, the bolt to allow it to store enough force
to hold the assembled parts together. Preloading the bolt to a higher load will hold the assembled parts together
with more clamp force. Preloading a fastener to the yield point of the bolt material will provide the maximum
clamp force possible from each fastener.
Preloading a fastener to its yield point can also assure a static loading condition for the fastener when the
service loads may exceed the preload available with other fastening methods, thereby reducing the risk of
fatigue failures. A bolt acts like an extension spring. Within its elastic region, any increase in deflection will
produce a proportional increase in load. But once the bolt is stretched beyond its elastic limit and into the plastic
region, the same incremental amount of deflection will produce a proportionally smaller increase in load. As long
as the bolt is preloaded within its elastic limit, no permanent deformation of the bolt will occur. When unloaded, it
will return to its original length.
But once the bolt is deflected beyond its elastic limit and into the plastic region, permanent elongation will occur.
The yield point of a material is traditionally defined as the point at which 0.2% permanent elongation occurs.
When tightening a fastener, the applied Torque is directly proportional to Load, and the Angle of rotation is
directly related to the Deflection through the thread pitch. By monitoring the dynamic Torque and the Angle of
rotation during a fastening cycle (beyond the initial free run-down and pull-up phases of a fastening cycle), the
rate of change of Torque vs. Angle is directly related to the rate of change of Load vs. Deflection of the bolt
material, thereby providing a convenient method for monitoring the onset of the elastic limit of the bolt material.
The QPM Controller software can now detect this fastener yield point and stop the fastening process when this
occurs.
136 / 1032
