As we have seen before, running torque, also called prevailing torque, is the torque required to overcome friction and/or any locking feature in a threaded fastening application, which produces no clamp load, or bolt stretch - in other words, running torque is the torque required to seat the screw head against the component being fastened, before the actual tightening and clamping begins.

While most often torque specifications refer to the total applied torque inclusive of running torque, in certain applications, most frequently in the safety-critical aerospace and automotive worlds, the variation in running torque encountered between joints is considered significant enough that it warrants a special tightening strategy. The final torque specified for these applications is typically indicated as torque above running and that’s where smart torque tools become essential. A smart electric screwdriver in this kind of application would measure the prevailing torque in real-time and then add it to the target torque dynamically, ensuring a consistent clamping torque applied to all joints; in our last article, we covered how to set up Kolver’s K-DUCER for this.

However, there can be situations where there’s additional variables involved and a multi-step tightening strategy is necessary.

One common instance is when we have an initial phase of prevailing torque that’s greater than the final clamping torque. If that were the only variable, then we would just need to use the K-DUCER’s running torque feature, set its interval along with the minimum and maximum torque allowed, and then ensure that the desired clamping torque is reached as expected.
However, in certain cases, the prevailing torque may peak early and then drop considerably during the rundown phase before the screw is seated. In these situations, if we are dynamically measuring the prevailing torque and accounting for it to calculate the final closing torque, we want to use a multi-step strategy.

This situation arises frequently in the case of thread-forming and thread-cutting fasteners, where the screwdriver is finding a high torque for the initial drill or thread formation, and then reverting back to a more standard amount of driving torque until the screw has been seated and final tightening can begin.
In order to tackle this extra variable, a smart tool will need to be set up to follow multiple steps to produce an OK signal.
In the case of the K-DUCER, this can be accomplished by combining the running torque feature with the max-power phase feature, found on the second page of the advanced torque menu screen.
By turning on the max-power phase for the initial interval, say the first 3600 degrees, we will allow the electric screwdriver to go as work in as high of a torque as necessary in order to overcome the initial spike in resistance, needed to form threads or cut through material. Combining this with the running torque feature in compensate-mode for the subsequent phase, say from 3600 to 9000 degrees, will ensure that the screwdriver measures the running torque encountered during the second interval and adds it to the target torque to calculate the final torque applied to the fastener.

For example, let’s say our application has a thread-forming fastener that needs to be tightened to 3.8 Nm above running torque. After running several tests, we analyze the torque-angle curve and note the initial thread-forming phase to be as high as 9 Nm and take about 5400 degrees, followed by a driving torque of, on average, 4.1 Nm up until the fastener’s seating point, which happens pretty consistently at an angle of about 9000 degrees.

We are now ready to set up our K-DUCER. 

Let’s first go to the torque&angle settings screen and set the target torque to be 3.8 Nm, as per our documentation.

We can then open our Advanced Torque settings and, in the second page, turn on the Max-power phase. This setting tells the screwdriver that it can apply as much torque as needed for a specified interval, which we set to 5400 degrees, as that’s the length we measured the thread-forming phase to last. Alternatively, a time interval can be specified.

Back to the first page of the Advanced Torque settings screen, let’s turn on the Prevailing/running torque setting. Because our specifications indicate that the target torque is “above running torque”, we need to work in compensate mode. As we have seen in previous articles, compensate mode will not only measure the prevailing torque encountered but also add it to the target torque; the sum of the target torque plus the measured prevailing torque will make up the final torque applied to the fastener.
As the running torque we’ve analyzed in our earlier graph is relatively flat, we will set the mode to “average” and the interval from 5400 to 9000 degrees. We can also set our running torque boundaries to be 3.5 to 4.5 Nm. 

Now back on the home screen, we are ready to tighten our first screw.

As we can see in the screenshot above, our operation was completed successfully, measuring a prevailing torque of 4 Nm and reaching a target torque of 3.7 Nm. The total torque applied is noted to be 7.7 Nm (the sum of our measured prevailing torque plus our target torque).

If we open the graph screen, we can also see how the screwdriver overcame the initial thread-forming resistance of about 8.8 Nm, before dropping to about 4 Nm throughout the rundown-to-seating phase. We can then observe how the torque curve starts rising again during the tightening phase to a final closing torque of 7.7 Nm.

Because thread-forming screws create the internal threads by displacing material, rather than cutting it, they result in a zero-clearance fit that provides very large binding forces preventing loosening even under strong vibration and even without the use of lockwashers. This characteristic makes them particularly suitable to for the automotive industry, where they are becoming more prevalent.

Thread-cutting screws follow a similar torque curve, with a spike during the initial thread-cutting followed by a lower running torque, but with a lower overall torque profile due to the less energy-intensive cutting instead of forming of threads. This factor makes them suitable for materials where disruptive internal stresses need to be avoided, or when lower-torque tools are preferred.

Other multi-step strategies may be needed when trying to compensate for joint relaxation, a phenomenon that occurs, in varying amounts, in virtually all fastened joints due to the embedment of imperfectly matched surfaces. 
For properly-designed joints, relaxation is small enough that it can be safely ignored (and a 5% loss is often already accounted for); however, in other situations, the operator may want to reduce this effect, and often times a simple downshift step right after the prevailing torque phase but before final torque, can help achieve this purpose. Downshifting will add a 50-100ms time delay right before applying the closing torque, effectively counteracting short-term relaxation effects in the joint. 
Keep in mind that for joints where long-settling parts, washers and gaskets are being used, relaxation will likely occur over a longer period of time, and a different tightening strategy will likely be more appropriate. In those situations, it is also recommended that a thorough testing analysis be done to attempt to quantify the relaxation over time. For critical joints, these simulations become crucial to determine the extent of preload loss and remedial actions required. We will cover this topic in depth in a subsequent article, and Kolver’s K-GRAPH software can be quite helpful in analyzing these joints.

No matter what the application you’re covering, and whether you need a simple torque-angle strategy, a prevailing-torque strategy or a multi-step strategy, Kolver’s K-DUCER line of precision electric screwdrivers will make the setup easy and intuitive and ensure repeatability and precision in your production line.