With components’ torque specifications becoming more exact, and fastener requirements changing such that nuts and bolts are more vehicle specific, there is no more running to the backroom if you need a replacement.
This means there are productivity penalties for not properly tightening a fastener using the correct torque tools or procedures. If an oil pan drain plug comes loose or a ball joint stud snaps, it might increase your shop’s insurance premiums and leave you with a social media PR nightmare.
Why Torque is Important
The basic physics of all threaded fasteners are the same. When a threaded fastener is tightened, it stretches. Since metal is elastic, a clamping load is generated. The friction between the threads, shoulders and even the washers creates additional clamping loads and friction to ensure it does not turn and loosen. These loads are generated by the torque from a technician’s tools.
How much the fastener stretches is dependent on how much torque is applied. If too little torque is applied, the fastener won’t generate enough clamping load and the assembly will come apart. If too much torque is applied, the fastener will snap or become elongated to the point where the geometry of the threads changes and it won’t stay torqued.
Your Torque Wrench
Chances are you have a torque wrench or two in your toolbox. It probably resides in the plastic case with the warranty certificate. Ask yourself, how much do you trust your torque wrenches?
In a shop setting, your torque wrench could have been borrowed and returned to its case without being wound down to the lowest setting before being stored, which leaves the mainspring compressed. This can weaken the spring and cause false readings. In addition, going to zero or past zero can affect the calibration of the wrench.
Another common problem occurs when the torque wrench is dropped on the ground. Impacts with the shop floor can damage the internal mechanisms of the wrench. While one fall from a workbench may not damage the accuracy, repeated blows can cause a calibration problem.
So, how do you determine if your torque wrench is accurate? Some tool trucks have a torque wrench tester that can help spot a torque wrench that is malfunctioning. Be sure to test at least several torque settings, including the highest and lowest settings. It is not uncommon to find a wrench that will be accurate at 50 ft.-lbs., but significantly inaccurate at 25 or 150 ft.-lbs.
If the accuracy is off, you should send it out to be calibrated rather than buy another torque wrench. Annual calibration service is recommended by most torque wrench manufacturers. If you have “community property” torque tools at your shop that are used to tighten lug nuts, oil drain plugs or TPMS sensors, calibration on a more frequent basis is recommended.
It is critical that the wrench be calibrated by a service provider that is accredited by a testing laboratory, and that you are provided with documentation on the accuracy of the torque wrench. This can help to cover your liability if a wheel comes off a vehicle or a drain plug falls out.
Adapted from an Underhood Service article.