The Prevailing Torque Strategy (PVT) is designed for fastening applications where torque is required to overcome resistance during rundown before the fastener is seated.
This resistance, known as prevailing torque, does not produce clamp load or bolt stretch. Instead, it may result from intentional fastener features or conditions within the joint.
The PVT allows supported Ingersoll Rand precision fastening systems to monitor and account for this resistance before completing the final tightening step.
Note: Available tightening strategies and programmable parameters vary by tool, controller, and programming platform. Refer to the documentation for your fastening system to confirm available settings.
What Is Prevailing Torque?
Prevailing torque is the driving torque required to overcome friction or interference in a threaded fastening application before the fastener is seated.
Prevailing torque can result from intentional features such as:
- Locking patches
- Squeeze nuts
- Thread-forming or interference-style fasteners
It can also result from unintended joint conditions, including:
- Weld spatter
- Cross-threading
- Misaligned holes
- Contamination
- Component variation
Because this torque occurs before seating, it does not represent the torque producing the final clamp load.
Why Prevailing Torque Can Affect Tightening
In some applications, prevailing torque can be greater than the joint's seating torque or even greater than the desired final target torque.
Without a tightening strategy designed to accommodate this condition, the tool may interpret the prevailing torque as reaching its programmed target. This can cause the tool to stop or advance to another tightening step before the fastener is fully seated.
Seating torque: The torque at which the fastener seats against the workpiece or consolidation of the joint components is achieved.
Normal tightening curve with low prevailing torque:
Tightening curve with cut-in torque that is higher than final target torque:
How the Prevailing Torque Strategy Works
The PVT divides the fastening process into distinct zones so the system can monitor resistance during rundown and determine when to begin final tightening.
1. Cut-In Zone
The Cut-In Zone monitors the initial prevailing or driving torque to verify that it remains within the expected limits.
This zone can help identify abnormal joint conditions, such as:
- An undersized hole
- An obstruction
- Unexpectedly high initial resistance
If the measured torque reaches or exceeds the programmed Cut-In Torque High Limit, the tightening step stops and reports a fault.
2. Prevailing Torque Zone
After the Cut-In Zone completes successfully, the tool enters the Prevailing Torque Zone.
During this portion of the fastening process, the system monitors the prevailing torque as the thread is formed or a locking or interference feature is engaged.
The system also determines when the prevailing portion of the tightening process is complete and the fastener has reached the point where final tightening should begin.
Depending on the supported fastening system and configuration, the seating point can be determined using one of the following methods.
Fixed Target Angle
The tool exits the Prevailing Torque Zone after a programmed amount of socket rotation is achieved.
This method can be more difficult to use when rundown angle or prevailing torque varies significantly between fasteners.
Slope Detection
The system monitors the torque/angle gradient and detects the increase in slope associated with the fastener reaching its seating point.
Slope detection can help identify the seating point when rundown angle or prevailing torque varies between fasteners.
3. Final Tightening
After the Prevailing Torque Zone is complete, the fastening process proceeds to the programmed final tightening step.
The final tightening step completes the fastening process and produces the required clamp load.
Prevailing torque strategy tightening curve/diagram:
Cut-In Zone Parameters
The following parameters may be available when configuring a PVT. Available parameters can vary by fastening system.
| Parameter | Description |
|---|---|
| Snug Torque | Identifies the measured torque at which the Prevailing Torque step begins. |
| Cut-In Torque High Limit | Identifies the maximum torque allowed during the Cut-In Zone. If measured torque reaches or exceeds this value, the step stops and reports a fault. |
| Cut-In Angle | Identifies the amount of socket rotation used to complete the Cut-In Zone. Measurement begins after Snug Torque is identified. |
Prevailing Torque Zone Parameters
| Parameter | Description |
|---|---|
| Target Prevailing Torque | Identifies the programmed amount of socket angle rotation during the Prevailing Torque Zone when this method is used to determine when to enter final tightening. |
| Prevailing Angle High Limit | Identifies the maximum socket rotation permitted during the Prevailing Torque Zone. |
| Prevailing Angle Low Limit | Identifies the minimum socket rotation required during the Prevailing Torque Zone. |
| Prevailing Torque High Limit | Identifies the maximum torque permitted during the Prevailing Torque Zone. |
| Prevailing Torque Low Limit | Identifies the minimum torque that must be encountered during the Prevailing Torque Zone. |
| Slope Enable | Enables slope detection to determine when the tool exits the Prevailing Torque Zone. |
| Slope Chord Length | Defines the number of degrees of socket rotation over which the torque/angle gradient is calculated. A larger value provides more torque-noise filtering but can delay seating-point detection. |
| Slope Deviation | Defines the slope threshold used to begin seating-point detection. |
| Slope Deviation Persistence | Defines how long the torque/angle gradient must remain above the Slope Deviation threshold to identify the seating point. |
| Tare Compensation | When supported and enabled, compensates the final Torque Control target for the measured prevailing torque. |
Note: Parameter names, available settings, and configuration options can vary by tool, controller, software, and firmware version. Refer to the programming documentation for your fastening system.
When to Use the Prevailing Torque Strategy
Consider using a PVT when an application has significant torque during rundown that does not represent final joint tightening.
Examples include applications with:
- Locking or interference features
- Thread-forming fasteners
- Significant or variable rundown resistance
- Prevailing torque that approaches or exceeds the final target torque
The strategy helps the fastening system distinguish between torque required to overcome resistance during rundown and the final tightening used to achieve the required joint condition.
Prevailing Torque can also be useful for detecting abnormal conditions during rundown, such as thread defects or cross-threading, when appropriate limits are established for the application.
Important: Fastening parameters should be established and validated using the actual joint. Prevailing torque, rundown angle, seating behavior, and appropriate limits vary by application.
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