A torque-to-failure study offers valuable information about your threaded joint. It should always be conducted when determining the torque specifications for a new application and when troubleshooting joint failures.
In fact, anytime you’re dealing with assembly of threaded fasteners, it’s a good idea to have the torque study data and documentation available.
Fortunately, the K-Ducer smart torque driver by Kolver makes executing a torque study and analyzing the results quick and easy. Follow along to see how!
This is a destructive test, and you should have several parts and screws ready (ideally 10 or more) to have statistical significance in your results. However, even from a single test you’ll be able to extract a lot of useful data.
As we have seen previously, every fastener stretches slightly when tightened. If the load is removed while it's still in its elastic range, it will return to its original shape. However, if tightened beyond this point—into the plastic range—it won't return to its original shape and it will become permanently stretched. Any further tightening beyond this point will eventually cause it to break.
The idea is to tighten the screw past the point where the screw (or the part) breaks or strips.
To do this, configure a program with an appropriate speed (300 RPM if unsure), angle control with torque monitoring strategy, and maximum torque value equal to the maximum torque of the screwdriver.
Safety note: if the maximum torque is higher than 3 Nm (25 inch-lbs), you should use a torque reaction arm, especially if you don’t have a right-angle model screwdriver.
Note: depending on the model (and the fastener's strength), your screwdriver may or may not have enough torque capacity to cause failure of the screws or parts. Even if you run into this situation, you can still gather useful data from the study.
The angle target should be a few revolutions greater than the number of threads in the screw. If unsure, use this formula:
Angle target (in degrees) = TPI x L x 360 x 1.5
Where:
TPI is the threads per inch value of your screw
L is the length of your screw in inches
360 is the conversion from threads to degrees
1.5 is a safety factor to ensure you take the screw past the stripping or breaking point
To ensure you don’t lose data in case you accidentally release the trigger during tightening, you should also activate the “lever error” from the menu program => other.
For example, for a fastener with 20 threads per inch, 0.67” long, we would set an angle target of:
(0.67) x 20 x 360 x 1.5 = 7200°
In the Torque & Angle program menu on the K-DUCER controller, the configuration would look like this:
After configuring the Torque & Angle parameters, connect the KDU-1A to a PC using our free K-Graph software.
Alternatively, plug in a USB drive onto the KDU-1A controller to automatically record data for your study, that you can later load onto K-Graph or a spreadsheet program for analysis.
Perform the first test.
Make sure to tighten all the way until you see the “Screw OK” screen on the KDU-1A.
In the following example, a thread-forming screw was tightened onto plastic and taken past the point where the threads strip.
Torque vs Angle graph as seen on the K-DUCER controller:
Torque vs Angle graph as seen on the free K-Graph software for PC:
Right away, we can identify the following features:
- Seating torque: approximately 0.30 Nm
- Seating point: approximately 2000 degrees
- Strip/failure torque: approximately 2.20 Nm
In order to gain statistical significance in our data, we should repeat this study many times, with new screws and new parts, in order to get averages, minimums, and maximums of all the points of interest.
And that’s all there is to it!
Armed with this data, we now know the failure points of the joint, and in the absence of other specifications from the design engineers, we can follow some rules of thumb to establish our target torque, rotational angle limits, and prevailing torque compensation settings.