15.5
Limitations & recommendations for yield control
The type of fasteners used and the make-up of the assembled components can have a significant effect on the
success of implementing a fastener Yield Control strategy. If the fastened components can yield throughout the
assembly process, this can be interpreted as fastener yield. Since the cross-sectional area of the threaded
portion of a bolt is less than the area of the shank portion, all the yield will occur in the threaded area. Bolts with
reduced areas in the shank portion will distribute the yield over a greater length. The Yield Control strategy
tends to work better on joints with longer bolt grip lengths which allow greater total bolt elongation.
Since the angle of rotation is a key element in this strategy, it is recommended this fastening strategy not be
used with hand-held assembly tools. Understand that the nature of some applications will require a back-up
wrench to be hand-held on the opposite end of the assembly. Tests showed no adverse effect when
handholding the backup wrench.
Since this fastening strategy will tighten any fastener to its yield point, a different grade fastener will yield at a
different Load value. Take caution to assure that other grade fasteners are not mixed with the fasteners
intended for this application. The One Box controller Yield & Torque Rate Control consistency of fastener
material properties within a given Grade is also critical for good Clamp Load control.
This fastening strategy is not a substitute for good fastener quality control. Slight variations in frictional
properties of fastener components will not affect the Clamp Load control. But certain applications will exhibit the
occasional "stick-slip" condition which can affect the ability to sense the onset of fastener yield.
A fastener yield strategy can be difficult to verify in a production environment. During laboratory tests, measure
each fastener before and after each assembly to verify that yield has occurred. Disassembling product parts is
not practical, but recommend an offline test fixture to allow the production tooling to be run on production
fasteners and fastener elongation measured on a statistical sampling basis. Occasionally observing the Torque
vs. Angle trace in production is recommended and can provide a quick indication that fastener Yield is indeed
occurring.
Any embedment or joint relaxation which occurs after the fastening process can affect the final clamp load. A
complete laboratory analysis of the joint is important to understand any characteristics that may affect the final
clamp load before implementing this fastening strategy.
15.6
Monitor torque window
This strategy is mainly used as prevailing torque monitoring. Monitor torque window monitors torque during an
angle window somewhere within the rundown phase with reference to Snug Torque.
Once the fastening cycle achieves snug torque of the step with Monitor torque window enabled, the One Box
controller then "looks back" to see if the torque at any time violated the Monitor torque window defined by Upper
and Lower Torque and Angle limits.
The achieved torque must enter the window between the Upper and Lower Torque values at the Upper Angle
and must leave the window between the Upper and Lower Torque values at the Lower Angle. If the torque rose
above the Upper Torque limit, or fell below the Lower Torque limit at any time in the monitoring window step, the
SC Series Controllers
Appendix A
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