Prevailing Torque allows the tool controller to exercise an appropriate control and tightening function where the applied torque is not providing clamp load prior to or in lieu of a final fastening strategy. Prevailing Torque is the driving torque required to overcome friction in a threaded fastening application, which produces no clamp load, or bolt stretch. In some applications, the Prevailing Torque may be greater than the joint’s seating torque. This driving torque may be required to overcome intentional interference designed into the bolted joint (Nylok patches, squeeze nuts, Tri-Lobe fasteners, etc.) or unintentional interference that is a by-product of component part variation and quality (weld spatter, cross-threading, misaligned holes, contamination). The Prevailing Torque step must always be followed by a final Tightening step (Torque Control, Angle Control or Yield Control).
Note: Seating torque is the torque at which the head of the bolt is “seated” against the workpiece after rundown of the fastener or at which consolidation of joint components is achieved. In the Slope method of Prevailing Torque zone exit it is detected by recognizing when the Torque/Angle slope starts up the linear portion of the Tightening curve.
There are two zones associated with the Prevailing Torque step: The Cut-in zone and the Prevailing Torque zone. The Cut In zone is the first segment of the step. It provides an inspection zone during the start-up or “Cut-in” torque area encountered by the tool during a fastening. It is used to identify poor quality conditions of a bolted joint (e.g. undersized hole, tightening obstruction). A failure (High torque) in the Cut In zone stops the tightening step with a fault. The Prevailing Torque zone is the second segment of the step and is entered after successful completion of the Cut-In zone. It provides a monitoring zone (for Peak & Average Prevailing torque) while the tool is encountering prevailing torque during a fastening. It also identifies the “Seating Torque” point at which the final tightening strategy is invoked.
The Prevailing Torque step exits into the final tightening zone upon achieving a preset Target Angle or detection of the Seating Torque point. You must select the exit method.
Note: Average Prevailing torque is one of the data values determined from the Prevailing Torque zone. It is calculated using a moving average of up to the last 360 degrees of running torque prior to the Seating point.
The Prevailing Torque strategy setup has a checkbox labeled EOR Data Enable. When checked, the system outputs an End of Run message at the completion of this Tightening Strategy.
The Prevailing Torque strategy setup also has a checkbox labeled Include Cycles Below Torque Threshold. When checked, the system outputs an End of Run message for cycles below torque threshold.
Prevailing Torque specific parameters associated with the Cut-in Zone:
|Snug Torque||Identifies the measured Torque where the Prevailing Torque step begins.|
|Cut in Torque High Limit||Identifies the maximum torque that is allowed during the Cut-In zone. If measured torque is equal to or exceeds this value the step is halted and a fault declared.|
|Cut in Angle||Identifies the amount of socket rotation to complete the Cut-In zone. Measurement starts once Snug torque is identified.|
Prevailing Torque specific parameters associated with the Prevailing Torque Zone:
|Target Prevailing Torque||Identifies the programmed amount of tool socket angle rotation during the Prevailing Torque zone. It is one of the two options for exiting the Prevailing Torque zone and entering the Final Tightening step. Measurement starts once the Cut- In zone has been exited.|
|Prevailing Angle High Limit||Identifies the maximum amount of tool socket angle rotation allowed during the Prevailing Torque zone. If socket rotation equals or exceeds this limit the step is halted and a fault declared. This limit is used when the “Slope Detection” method of Prevailing Torque zone exiting is selected. Measurement starts once the Cut-In zone has been exited.|
|Prevailing Angle Low Limit||Identifies the minimum amount of tool socket angle rotation that must be turned during the Prevailing Torque zone. When the Prevailing Torque zone is exited the measured Prevailing Angle is checked to make sure it is greater than this limit. If it is not then the step sequence is halted and a fault declared. Measurement starts once the Cut-In zone has been exited.|
|Prevailing Torque High Limit||Identifies the maximum amount of torque allowed during the Prevailing Torque zone. If measured torque equals or exceeds this limit the step is halted and a fault declared.Measurement starts once the Cut-In zone has been exited.|
|Prevailing Torque Low Limit||Identifies the minimum amount of torque that must be encountered during the Prevailing Torque zone. The minimum torque encountered during the Prevailing torque zone is stored while in the zone. When the Prevailing Torque zone is exited the stored minimum is checked to make sure it is greater than this limit. If it is not then a fault is declared. Measurement starts once the Cut-In zone has been exited.|
|Slope Enable||When this checkbox is checked, the Prevailing Torque Zone will be exited using the Final Tightening Slope detection method.|
|Slope Chord Length||The number of tool socket degrees of rotation over which the Torque/Angle gradient is calculated in the Slope method of Seating Point detection. A larger number results in more Torque noise filtering but a delay in Seating Point recognition.|
|Slope Deviation||The Slope value threshold that identifies the beginning of Seating Point Detection.|
|Slope Deviation Persistance||Defines the number of consecutive degrees over which the Torque/Angle gradient must be greater than the Slope Deviation in order to identify the Seating Point and exit the Prevailing Torque zone.|
If enabled, the Torque Control’s Target torque is added to the Average Prevailing torque to create a new modified Target torque (to compensate for the Prevailing torque). The modified Target is checked against the Torque Control’s High Torque Limit for violation. If the limit is exceeded, the step declares an error.
Note: Tare Compensation is only available if the Final tightening step is Torque Control.