How To Use A Torque Wrench
Torque wrench is a very important tool that should be the part of everyone who does any assembly, especially where broken bolts and stripped nuts can cause equipment coming apart and design has large safety margin or large “reserve of strength”. There are much more uses for torque wrenches than engines put together or transmissions and proper torqueing lug nuts on your wheels.
Unfortunately, it is “common sense” that the harder you crank that bolt, the tighter it will be, in case of bolts and nuts which can lead to thread stripping. The thing about thread stripping is that it may start very slowly. Right after assembly, it may feel nice and tight but after several hours of operation and vibration the assembly will fail. The “simple” nuts, studs, and bolts designed and fitted into applications by engineers are specialized in a very complex and boring subject – tribology. I am not going to bore the reader with the specifics of tedious calculations. That is what we pay engineers for. I want to provide you with some common situations to look for when putting things together right the first time.
To establish the size of the fastener, the engineers have calculated the tensile area and appropriate safety factor. If the same grade of material bolt and nut are used, it means that the thread stripping force is similar to the breaking point of the bolt provided that the threads have not been damaged before. Things quickly get more complicated when you put a bolt into lower strength material like bolting a generator onto an aluminum engine block for example. Before bolting it on, you need to check the depth of the threaded hole to ensure sufficient thread engagement.
The following are easy rules of thumb to follow:
- For steel material: The length of thread engagement must be at least one time the diameter of the bolt or stud.
- For cast iron, brass, or bronze: The thread engagement must be at least one and a half of the bolts diameter.
- For aluminum, zinc, or plastics: The thread engagement must be at least 2 times the diameter of the bolt.
One needs to realize that the thread engagement relationship to the specified torque is based on “nominal” condition of the threads and materials. To ensure the “nominal” conditions for the thread, you need to check the conditions of the thread first before putting things together. I’ve always been a proponent of “keeping in the toolbox a good thread chasing set”.
Second important thing: the first 2-3 threads into the threaded hole play a disproportionally large role in clamping power. Unfortunately, those 2-3 first threads also have the highest risk of being cross-threaded. If you cross-thread the first few threads, especially whenthe bolt goes into softer material, you will strip the bolt even when you’re applying the proper torque. That is especially a big problem when putting back the oil pan back onto aluminum block. Want to know my own secret in preventing this? I always start threading with my fingers. First, I rotate the bolt counter to the thread tightening rotation, until I feel/hear a click. (The click means that the internal and external threads begin at that point and it will safely engage). Then I make sure that the bolt turns 3 freely without noticeable resistance. Extra few seconds spent on ensuring a good fit will go a long way.
Now that you have learned all you need about the threads, you can go on bolting the real stuff. When using power tools, you need to make sure that the power torque setting of the tool is set to at least 30% down from the specified torque. Even certified torque settings on power tools are notoriously imprecise. On top of that, the power supplied by air or electricity can fluctuate due to the air pressure or voltage. Now you can use your torque wrench to set right torque values. Make sure you follow the manual instructions on how to use it. A piece of advice to remember: use the torque wrench in one smooth and confident motion. Don’t be insecure and double-click when you reach the proper value and you are using click type torque-wrench.