We often hear questions about breakaway torque. More importantly, when discussing torque measurement, we encounter a lot of misinformation about breakaway torque.
Torque application is considered to be "Dynamic Torque" or torque in motion that is changing. Measuring torque on a previously tightened fastener is considered "Static Torque" where a fastener at rest, is being placed in motion.
When you are thinking about breakaway torque, you really want to know, how much clamp load is in the joint, or how tight is this already tightened fastener? Another term for breakaway torque is Residual Torque. We have tools that measure Residual Torque and these tools have multiple options for the audit that you are performing.
When measuring residual torque, you are measuring the amount of torque required to place the previously static fastener into motion. Placing the fastener in motion requires more force to overcome friction to move from static to dynamic torque. Once the fastener begins to move, there is a momentary drop in torque. Our tools look at the peak torque prior to the drop that results in placing the fastener in motion. It also looks at the drop in torque.
Residual torque is the amount of torque required to move relative to the mating threads. In setting up the parameter for auditing fasteners, we give you two options for measuring residual torque. One option is to enter zero as the degrees of rotation. Since it requires more force to place the fastener in motion, there will be a slight drop in torque once the fastener begins to move. With zero selected as the target angle, the tool will sense the drop in torque and report the peak prior to movement.
The other option is to set a number of degrees of rotation and then the tool will read the value at that point. The default setting when using this method is 3 degrees. With the DTC, 1350 Series Exacta 2, 1350 TD, and the 1250 Series Exacta 2, when the tool is in Residual Mode, the upper left-hand side of the tool display will show the degrees of rotation. The larger numbers in the main part of the display will show torque.
As you apply torque the display will backlight. When you achieve the target angle, the display or light pipe on the tool side will turn green and the tool will emit the acceptance tone. If you are set the target angle to 3 degrees and pull past it, the tool reads torque at the point at which you stopped pulling.
Once torque has been applied to a joint, an audit function measures exactly how tight that fastener was torqued. Please note, due to surface conditions, joints can relax. There can be changes in the clamp load between the time the torque was applied and the time that the torque value audit is conducted.
While a good tool with residual mode will provide an accurate assessment of the clamp load, there is no way to tell if the joint relaxed, or by what percentage it relaxed. The tool can only tell you the torque at the time of the audit.
Using the correct parameter settings is imperative in creating an accurate Residual Torque reading. If your operators are highly skilled, they typically can use a P-SET where the target angle is zero and they can get accurate results. If your operators are inexperienced, we recommend using 3 degrees as the TARGET ANGLE setting. Our tools are fully capable of getting an accurate reading at 3 degrees of rotation. If the operator pulls through 3 degrees of rotation and stops at 4 degrees of rotation, the tool will read torque at 4 degrees of rotation.
In filling out the remaining elements in the P-Set, if you have the original specifications on MINIMUM and MAXIMUM torque, they can be added to the P-Set. This will help determine if the fastening is compliant or a reject.
It starts with selecting the appropriate torque wrench. Our tools with Residual Torque functionality include:
When auditing previously tightened fasteners, using the appropriate torque wrench is imperative. You'll want to use the most accurate wrench you can find. The 1250 Series wrench is accurate to +/- 1% of the indicated value. The remaining tools on the list above are all accurate to +/-2% of the indicated value. Even the 1350 TD is accurate to +/- 2% of the indicated value.
Sturtevant Richmont digital torque and angle wrenches come equipped with a radio and there are models without an embedded radio. In the tools that come radio-equipped, the radio transmits the torque and angle values to the controller. The data is stored in the controller or sent to a data repository. The DTC does not have a radio. It comes with a mini-USB port and you simply connect the DTC to a computer and use the DTC CONNECT freeware to download the date/time-stamped results from the wrench.
Digital torque wrenches with a radio are designed to communicate with the Global 400 and Global 400mp torque controllers. To find more information about those wrenches look for our 1250 Series Exacta 2, 1350 Series Exacta 2, 1350 Series TD digital torque, and angle wrenches.
We also make suitable audit tools without a radio. Those wrenches are the DTC digital torque and angle wrenches.
Once you have the appropriate wrench selected, the process is the next point of focus.
In which direction should you apply force to create the movement needed to establish breakaway torque?
For this example, consider a fastener that is turned clockwise to tighten and is turned counterclockwise to loosen.
Which direction should the fastener be turned to establish "breakaway torque"? The direction in which the fastener was originally tightened. While most of those will be clockwise, there are fasteners that have left-handed threads, so they are tightened in the counter-clockwise direction.
Remember that breakaway torque is that moment in time when the previously torqued (static) fastener begins to move again (dynamic torque). Once that movement happens, it cannot be created or captured again.
If you loosen the fastener, you've just changed the joint that someone worked so hard to create.
Given reaction times, we've seen too many cases where someone trying to establish breakaway torque simply loosens the fastener to the point where the joint is no longer in the condition it was when the fastener was tightened.
That is a problem. You were just trying to solve for the established torque value of a previously tightened fastener, and now you are creating the problem of a joint that is no longer in the condition it once was.
If you tighten the fastener, catching the moment of movement is easier and you will get a more accurate breakaway torque reading.
Most importantly, in your audit to validate accuracy, you won't destroy the joint that you are auditing.
To learn more about how Sturtevant Richmont tools with Residual Torque functionality can solve your auditing challenges, contact your local Sturtevant Richmont Torque Expert.