[Reference] Prevailing Torque Strategy PVT

What is prevailing torque in fastening?

Prevailing torque is defined as the driving torque required to overcome friction in a threaded fastening application, which produces no clamp load, or bolt stretch.   This prevailing 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).

What challenges does prevailing torque present?

In some applications, the prevailing torque may actually be greater than the joint’s seating torque, or even the desired target torque of the fastening process when the fastener is fully seated and not stripped. 

• Note: Seating torque is the torque at which the head of the bolt is “seated” against the work piece after rundown of the fastener or at which consolidation of joint components is achieved. 

Without a solution to accommodate and account for this prevailing torque, the fastening tool would almost always stop the tightening or move on to a subsequent tightening step prematurely due to the tool's measurement of the programmed torque that is caused by this prevailing torque.  A tool is needed that can overcome and/or account for this prevailing torque and still deliver a fully seated fastener with the appropriate torque and clampload applied.

• Normal tightening curve with low prevailing torque

• Tightening curve with Cut-In torque that is higher than final target torque

What is the solution?

Using the Prevailing Torque Strategy from Ingersoll Rand allows the user of a corded or cordless assembly tool to exercise an appropriate level of control and tightening function where the applied torque is not providing clamp load prior to or in lieu of a final fastening strategy. 

How it works:

• The first segment of the prevailing torque strategy is called the Cut-In Zone.  This zone monitors the initial peak prevailing or diving torque to ensure that it falls between expected limits. 
  • 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 second segment of the prevailing torque strategy is called the Prevailing Zone and it is entered only following successful completion of the Cut In Zone. This zone checks the subsequent mean and peak prevailing torque as the thread is formed or the locking feature is engaged to ensure the fastening continues to fall between expected limits. 
  • This zone also determines the seating point of the tightening at which the prevailing torque strategy is complete and the final tightening strategy is invoked.
    • The seating point is determined by the system using one of two methods at the termination of the the prevailing torque zone
      • 1) Fixed Target Angle Method
        • The tool will exit the prevailing zone after a fixed number of degrees of rotation is achieved
        • This method can be challenging if the rundown angle or prevailing torque is very inconsistent
      • 2) Slope Method
        • The tool will exit the prevailing zone when the dynamic slope monitoring of the system indicates that the slope (torque rise / angle run) is increasing successively - indicating seating point has been achieved.
        • This method works well to ensure seating point is consistently detected even with varying amounts of rundown angle and/or prevailing torque
• Finally - as the fastener seats - the final tightening strategy is then invoked which may be a torque controlled, angle controlled, or yield controlled tightening step.

Prevailing Torque specific parameters associated with the Cut-in Zone:

Prevailing Torque specific parameters associated with the Prevailing Torque Zone: