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ADVANCED TORQUE CONTROL STRATEGIES

While basic target torque & angle control are often sufficient, some applications often require more advanced tightening techniques. 

Fortunately, the K-DUCER can handle even the most complex and sophisticated applications thanks to features such as Sequential Torque & Angle Control, Running Torque (Compensate and Monitor), Prevailing Torque, and multi-step program options. To find out how to turn on any of these features, read on!

Note: If you’re not familiar with any of  these terms, check out the terminology section of K-DUCER manual. Also, remember that our support team is always available to help you with your application needs. If you have any questions or need help, give us a call.



SEQUENTIAL TORQUE AND ANGLE CONTROL


Sequential Torque & Angle Control is a strategy where, as part of a single tightening, the screwdriver first targets a torque, and, after reaching it, rotates for a specified number of degrees (target angle).


For example, you can program the driver to tighten to 20 lbf-in, and then tighten for an additional 180° degrees, all seamlessly and as part of the same program.


To do this, in the TORQUE & ANGLE menu:

1) set the ANGLE CONTROL / TORQUE MONITORING flag to “Angle Control and Torque Monitoring”
2) set the TARGET Angle to the number of degrees to rotate for after reaching the threshold torque. Set the Min/Max angle error bounds accordingly, if desired.
3) set the Torque Min/Max error bounds, if desired. These will be the error bounds on the torque value reached at the very end of the tightening, after the target angle has been reached. The final torque will typically be higher than the initial torque target.
4) set the STARTING AT parameter to the Torque value that you want to target for the first phase of the rundown, before switching to the angle targeting mode


Make sure to follow the “Determining the appropriate program settings” guidance, adding a downshift threshold if necessary. This is especially important for low target angle values and/or hard joints!


RUNNING TORQUE


Use running torque to apply the clamping torque, letting the closing torque vary according to the running torque encountered and measured during the running torque window.

The goal in this case is to apply a consistent amount of clamping force on the assembly, as opposed to a consistent amount of closing torque.

If the running torque value is expected to be higher than the target clamping torque value, the application may also require the use of a prevailing torque phase, to be used in conjunction with and during the running torque phase.


Note: if you simply want  to measure and monitor the running and clamping torques, applying a fixed closing torque, use the "Running Torque Monitoring" mode instead of  "Running Torque Compensate" mode.


Seating point determination


First, it is necessary to understand the morphology and torque rate of the joint. Perform a series of at least 10 tightening operations with a target torque near the maximum closing torque allowed by the application (inclusive of running torque), without using any running/prevailing/downshift settings. Follow all precautions necessary and note that this may damage the assembly item.


Mark down the following information from torque and angle graphs and/or graph data provided by the K-DUCER. Remember that you can also take advantage of the free K-Expand software to readily analyze the graphs point-by-point.

* Seating point, in terms of both torque & angle, and of their variability
* Running torque


Test with running torque monitoring only


In the RAMP, TIME, & PV TORQUE menu, configure the running torque WINDOW by angle (or time), such that it always ends before the seating point. Use a window max value of at most the lowest seating angle point of the test tightening.

Select appropriate running torque MIN-MAX bounds, such that the running torques measured fall within the bounds, while retaining the ability to detect incorrect values.

Select peak or average VALUE depending on the requirements of the application and shape of the graph.

If the running torque slopes upward, it may be appropriate to use the peak value.

If the running torque oscillates but is generally trending flat, it may be more appropriate to use the average value.


Finally, select monitoring mode for a second round of testing without compensating for the running torque.

Perform another batch of tightenings, and check if the detected running torque value matches expectations.



Note: if the running torque value is higher than the desired clamping torque value, you will also need to superimpose a prevailing torque phase to the running torque phase, to enable the screwdriver to finish the running torque phase at a higher torque value than the TARGET Torque, as the running torque value is not added to the target torque until the end of the running torque window.


Final test with running torque compensation

After fine-tuning the running torque detection, switch to compensate MODE. The system will now add the running torque value measured on each tightening to the target torque.

Perform another batch of tests and analysis to verify the desired result.


PREVAILING TORQUE


Self-threading applications may require a prevailing torque strategy, if the initial torque required to overcome the self-threading action is higher than the target closing torque.

Applications where the running torque value is expected to be higher than the target clamping torque may also require a prevailing torque phase, ending at or after the end of the running torque phase.


In the RAMP, TIME, & PV TORQUE menu, activate the prevailing torque mode with torque and angle 


Warning: for the duration of the prevailing torque phase, the screwdriver will only stop if it reaches its maximum torque possible.




A tightening with target torque 0.21 Nm, and prevailing torque phase for 2.0 seconds.

Note how the maximum torque is higher than the final torque. This type of profile is only possible through the prevailing torque setting.


CROSS-THREAD DETECTION


To detect cross threading without stripping the fastener, use the following settings:


RUNNING TORQUE: ANGLE or TIME. Select a window where to detect cross threading.


VALUE: select PEAK, so that the screwdriver stops immediately if it encounters the running torque MAX torque during the running torque window


MIN - MAX: set MIN to zero, and set MAX to a torque high enough to disregard normal friction, but low enough to detect cross threading without stripping the screw


MODE: select Monitoring, to avoid adding the running torque value to the torque target



MULTI-STEP AND COMBINED PROGRAMS


To reverse the screwdriver at the beginning of the tightening, or after reaching the closing torque, or to change the direction of tightening, refer to the corresponding options in the REV & PRE-REV menu.


To combine multiple programs in a single tightening, you can configure a sequence with the Program transition box set to “auto”.

The sequence will then transition between two or more programs automatically while the operator continues to hold the screwdriver lever. The screwdriver will stop momentarily in-between programs. Activate the LEVER ERROR setting for each program involved to raise an error if the operator stops holding the lever during or in between the programs.


This allows for multi-program tightening strategies.