Better Your Maintenance Skills by Better Understanding Torque
By Mark EdmundsIndustry
“Click!” That’s the magic sound many technicians wait to hear when using a torque wrench. When you hear it, the proper amount of torque is applied and you’re finished. But was it applied correctly? How do you know?
Understanding the principles of torque is much more involved than simply cranking down on a fastener until you hear the torque wrench click. There is a right way to safely and accurately apply torque. Recognizing the physics of torque, as well as the dos and don’ts when using a torque wrench, will make you a more efficient technician, while also ensuring the integrity of your work.
The Physics Behind Torque
A good starting point for understanding torque is to appreciate what exactly occurs when torque is applied to a threaded fastener.
The physics of torque can be understood by first defining the term torque. Simply put, torque is rotational force. Torque is calculated (not measured directly) by multiplying applied force by the distance from the centre of rotation, or the pivot point. This is why the units of torque always have a distance component (inch, foot, metre) and a force component (pound, ounce, newton).
When torque is applied to a threaded fastener, that fastener is stretching ever so slightly. The fastener is being elastically pulled (or stretched) in one direction as the threads force the joint together. Tension in the fastener increases as torque is applied and friction holds the fastener in place.
As torque is being applied with a torque wrench, technicians can feel when the targeted torque setting is nearing due to the amount of effort needed to turn the wrench. It is at this point in the torque process where technicians can get into trouble by applying too much torque.
For example, if a technician keeps applying force to a torque wrench after the targeted torque setting is reached, the fastener can be plastically deformed (meaning, the fastener will not go back to its original shape) and elongated, i.e., stretched beyond its limits. This is called the yield phase. If a fastener yields, the clamp force will likely drop, and the joint will be compromised. Damage to the fastener’s threads, as well as crushing of the installation material, can also occur during over-torquing.
Over-torquing is common, especially when using mechanical torque wrenches. That’s because some technicians still apply force after they hear the click. Once they hear the click, a customary old-school practice is to give it one last little pull for good measure. That can result in the fastener being overtorqued. Even if a technician stops when he or she hears the click, the time it takes to release pressure on the wrench can result in over torquing and the fastener being stretched beyond its limits.
Why does this occur? Often, it’s a speed issue. Technicians sometimes rush to get the job done faster, and, in doing so, they’re not applying the steady, methodical force of torque
that they need. One way to reduce this occurrence is to slow things down and turn the torque wrench slowly, especially when the targeted torque setting is in range, so as not to overtorque the fastener.
It’s important to remember that an under-torqued fastener can also pose problems. Vibrations can occur when a fastener is under-torqued and there is not enough clamping force to hold the joint properly. The fastener could start to come loose over time, compromising the integrity of the joint. Vibrations resulting from an under-torqued joint can also cause cracks in the fastener, and those cracks will grow and could eventually lead to catastrophic failure of the fastener and assembly.
Mechanical vs. Electronic
Mechanical torque wrenches are popular within many industries because they are relatively inexpensive, and it is the style many technicians grew up with. While mechanical wrenches represent an older technology, they perform well if the technician uses the tool correctly, stores it properly, and keeps it accurately calibrated. Electronic torque wrenches tend to offer greater accuracy than mechanical models. The chance of over-torquing is reduced with electronic wrenches as illuminating LEDs provide operational guidance, signalling the user that he or she is approaching the targeted torque.
Data collection is another benefit of electronic torque wrenches. For each use, instant data on the exact torque applied is displayed and stored, which can be downloaded and analyzed for tracking and auditing purposes. These wrenches come with built-in calibration factors for different head lengths and adaptors, and alert the user when the wrench is due for calibration based on pre-set date and cycle count reminders.
It is difficult to obtain accurate results from either a mechanical or an electronic model if the tool isn’t properly used. That’s why correct hand position on the wrench is critical.
Your hand should fit squarely on the steel or plastic-covered handle. Positioning your hand on any other part of the wrench, or even just off-centre on the handle, can lead to injury and result in a false torque reading. If two hands need to be used, place one hand on top of the other. The length of a torque wrench is designed and calibrated in such a way that your hand(s) needs to be properly positioned on the handle during use. Changing the location of your hand negatively affects the tool’s performance.
Tips for safe torque wrench use include the following:
• Wearing safety glasses.
• Always pull rather than push the wrench when applying torque.
• Never use sockets that are worn or showing signs of cracking or fatigue.
• Ensure fasteners and bolts are well cleaned and dry (thread condition actually changes the amount of torque required to achieve proper fastener tension and clamping force).
• Avoid using accessories or handle extensions unless specifically allowed by the torque wrench manufacturer.
Some tooling manufacturers offer torque training to technicians and students to become certified in all things torque. These training seminars cover understanding torque theory, learning the proper procedures for applying torque, identifying bolt hardness and metal grades/thread pitch, fastener lubrication, appropriate safety measures, and selecting the right tool for the job. Reputable manufacturers host these seminars on the job site, while many technical schools offer them as part of their training programs.
There is a perception that applying torque to a fastener is intuitive, that anyone can do it. Sometimes that is not the case. Ensuring torque is applied properly takes an in-depth understanding of the physics behind the action. Using the right type of torque wrench and following sound safety practices are a must when applying torque. Fortunately, there are resources available to help learn more about the fundamentals of torque, which will go a long way toward making technicians more resourceful in their jobs.
Mark Edmunds is a Product and Natural Resources Program Manager at Snap-on Industrial. He focuses on managing the torque and bolting solutions product lines and has helped develop and launch several products designed to solve issues in critical bolting applications. Mark has worked in the mining and the oil and gas industries in capacities ranging from drill rig labourer to engineering and product management. He can be reached at firstname.lastname@example.org.
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