In previous articles, we have illustrated how to run a torque study to obtain important data about a fastener and joint. In particular, we saw how to determine the rundown phase, the seating point (where the head of the screw fully touches the material at the end of the rundown phase), the torque and angle values at the seating point, and the yield point at which the fastener fails.
Here we are going to use this information to set the appropriate parameters in Kolver’s K-DUCER precision fastening system. But first, let’s briefly review joint types and how they affect our selection of the right parameters.
The importance of the joint type
Understanding the characteristics of your joint type is critical to maximize the precision of the K-DUCER system, minimize the wear-and-tear on the tool, and minimize the assembly cycle time for your application.
According to the ISO standard 5393:2017, the type of joints you can find will fall in between these two “extreme” definitions:
- Soft, elastic, low torque-rate joint: after the fastener is seated, the tightening reaches its target torque in more than one full revolution ( > 360°).
Examples: plastic screws; fasteners with split washers; self-threading fasteners.
- Hard joint, inelastic joint, or high torque-rate joint: after the fastener is seated, the tightening reaches its target torque in a fraction of a revolution ( < 30°).
This is due to the materials being fastened compressing very little when subjected to a clamping force, resulting in a rapid increase in torque with minimal rotation of the fastener after initial contact between the parts being joined.
Examples: metal screws on metal socket with simple washer.
As seen before, a simple approach to determine the joint type is to perform a tightening of your fastener with the K-DUCER and observe the slope of the Torque vs Time graph. A soft joint will show a moderate slope from the seating point to the final torque, while a hard joint will show a very high slope (almost vertical).
Above picture: Torque vs Angle graph for a soft joint on the K-DUCER. This joint takes approx 450 degrees to reach the final torque.
Above picture: Torque vs Angle graph for a hard joint on the K-DUCER. This joint takes approx 20 degrees to reach the final torque.
Starting with v40 of the K-DUCER (remember you can always upgrade to the latest version for free!), the controller will automatically detect hard joints and it will display a warning below the status bar if it is not able to stop at the target torque within the 5% due to suboptimal parameters.
Above picture: the K-DUCER detected a hard joint and will suggest more appropriate settings for this application.
Determining the appropriate program settings
As explained before, the K-DUCER is a highly accurate system, but it is critically important to select the appropriate settings to ensure that the desired torque is correctly applied, and that the screwdriver motor works effectively and efficiently.
Determining the appropriate program settings for your application requires careful consideration and is ideally done by a trained engineer with knowledge of the torque specifications and of the mechanical characteristics of the assembly joint.
Please take advantage of the free support provided by your Kolver representative throughout this process.
What follows are some general guidelines, but they are not meant to substitute a careful examination of the application. Each application is unique and may require deviations from these guidelines.
These guidelines are always superseded by the specifications of the assembly joint and by all safety requirements of the operator and work environment.
Kolver is not responsible for damages or injuries resulting from following these guidelines.
Hard/inelastic joints
These joints are best finished at low speed, to improve precision and avoid a high velocity impact at the end of the tightening.
Choose a low FINAL SPEED such as 100 RPM, and if desired, use a two-speed approach by activating the DOWNSHIFT setting using either an angle threshold or a torque threshold equal to 20-50% of the target torque.
In general, the lower the torque target is, relative to the range of the screwdriver, the higher the effect of motor inertia will be, requiring a lower DOWNSHIFT speed to avoid overshooting the downshift threshold through and past the final torque target.
In other words, a more powerful motor will have a harder time stopping at a very low torque unless we slow it down before reaching the target torque. The downshift function does exactly that.
Soft/elastic joints
Just like workout resistance bands are a lot more difficult to stretch slowly than in one fast motion, tightening an elastic joint requires more motor effort at lower speeds.
Therefore, these soft joints are best executed at high speed, to avoid keeping the motor under tension for a prolonged time and overheating the screwdriver.
Choose a higher FINAL SPEED for these joints, and if a two speed approach is required, activate the DOWNSHIFT setting using either an angle threshold or a torque threshold of at least 80% of the target torque to ensure most of the torque is still applied at higher speed.
Very elastic joints, or semi-elastic joints with a high target torque relative to the range of the screwdriver, may benefit from a higher FINAL SPEED and from avoiding the use of the DOWNSHIFT function altogether.
A practical example
In our torque study example, we had identified the following:
- The seating point at approximately 2000 degrees
- A seating torque of approximately 0.30 Nm
- A failure point at approximately 2200 degrees
- A failure torque of approximately 2.20 Nm
These values point to a hard joint.
Assuming that repeated tightenings give us similar results, we would want to set the following parameters:
Torque
In the Torque and Angle settings screen, we want to set a target torque close, but not too close, to the yield point. We do not recommend going above 90% of the failure torque; in fact, a number around 80% is usually a safe choice, to give ourselves a reasonable margin of safety.
Therefore, let’s go ahead and set 1.76 Nm as our target torque (2.20 Nm * 80% = 1.76), with the lower and upper bounds set to 1.54 Nm and 1.98 Nm respectively.
Angle
If we also want to monitor our angle, we know that the fastener fails approximately 200 degrees after the seating point, which happens at approximately 0.30 Nm.
Therefore, let’s set our MAX ANGLE to 180 degrees (200 * 90%) and STARTING AT 0.30 Nm.
Note that in the event of a running, or prevailing, torque of or above 0.30 Nm, our angle monitoring would get triggered too early and give an error. In that case, we can start counting the angle at the beginning of each tightening and set STARTING AT to 0 Nm.
In our example, we would set the MAX ANGLE to 2180 degrees (2000 + 180) and STARTING AT to 0 Nm.
Final Speed
Finally, with this being a hard joint, we want to ensure that we set an appropriately-slow final speed.
In this case, for our production line we would likely select a KDS PL3 or PL6. If we want to get through the rundown phase at close to the max speed of the tool, let’s turn on the downshift function and set the final speed to 100 RPM and the AT parameter to either 2000 degrees or to 0.40 Nm (slightly above our seating torque but well below our target torque). We can then set the rundown speed until downshift to, say, 1600 RPM (if using a KDS PL3).
Image above: our final settings. The screwdriver will run at 1600 RPM for 2000 degrees, then slow down to 100 RPM until it reaches the target torque of 1.76 Nm and ensure it tightens the fastener to no more than 2180 degrees.
Need help?
For more complex fasteners and joints, more advanced torque strategies may be needed, which we covered in our dedicated article.
We always recommend you take advantage of the free support provided by Kolver, so if you have any questions, reach out to your Kolver representative or write us at [email protected]