"This torque wrench series offers accuracy of ±3% "
When my butt is strapped to a helicopter, I want to know that those bolts holding all that stuff together over my head are tightened properly. So I purchase a torque wrench advertised to be +-3% accurate. Are my torques going to be within +-3%?
No, don't count on it. But I had my wrench "calibrated" and it came back within the +-3%. Isn't it accurate?
Yes, in the lab under carefully controlled conditions, your wrench is accurate to +-3%. But it looses this accuracy when it leaves the lab. Calibration shops have become artificial islands of reality, taking measurements with extreme accuracy using the tools in ways that will never be duplicated by the user.
During the 20 years or so that I had my FAA approved torque wrench calibration shop I calibrated lots of wrenches. One thing that I learned is that; where the wrench snaps depends an awful lot on how I snap it - especially if the wrench is worn.
You set the wrench to 40 lb. ft. now where do you want it to snap? Lets try for 37. A small side load on the wrench or apply the pressure quickly and 37. Now remove the side load and pull it nice and slow and it snaps at 47. Place the wrench vertical and snap it at 40. Place the same wrench horizontal and it snaps at 42.
Any measurement instrument is affected by the CONDITIONS or environment in which it takes measurements. Torque wrenches are no different. In the lab the conditions are carefully controlled - a better term is "artificial". Temperature is controlled. Wrench position is controlled - horizontal. Pull speed is controlled. The wrench is accurately oriented so there are no side loads. Have two different calibration shops calibrate your wrench and they both should report back the same numbers because the conditions are standardized.
These carefully controlled measurements do not accurately reflect how the wrench will perform in your hand. Test the wrench horizontally, vertically; side load it; test it warm; test it cold. This is how torque wrenches are used . Now how accurate is it? It's a lot less than +-3%. Think of accuracy statements like you would credit card interest rates; somehow, someway, your rate is always higher than the advertised ones.
You can make your torque wrench more accurate by controlling conditions of torque. Don't side load the wrench; pull the wrench smoothly; take care of your wrench and don't abuse it; have the wrench repaired and calibrated. Most inaccurate wrenches are that way because they are worn out and in need of repair.
Torque wrench extension software with bolt torque equation, bolt torque chart standard, effect of phosphate coating on bolt torque, grade 8 bolt torque value, is included in Mechanic's Toolbox software.
