artsifrtsi said:
find a calibration shop, any other 'redneck' methods may and will get you into trouble fast. You know how the handles usually always have a ring cut into about the center? That's where the optimal accuracy will come from.... IE, put the center of your hand over that ring.
Artsi ... thanks for sharing your opinion, but rest assured I'm not a "redneck." (The predudice some people have about Montana can be mind-boggling.)
I just finished the Pre-Med sequence at UM, and on my way to becoming an optometrist. As such, I had to take a lot of calculus-based physics to get accepted to a decent optometry school.
I can assure you there is nothing magical or mystical about measuring torque. Torque is simply a rotational force placed upon a central object. It's generally defined as force x radius. In our case the force is pounds of exertion, and the radius is the distance (in inches or feet) the force is being applied from the center of the object being turned. We were able to measure it very accurately in physics lab with simple devices.
You brought up a good point about the handle mounting. Yes, most beam-type torque wrenches I've ever seen either had a round ball for a handle, or a haft mounted on a center pin. The reason for that was to get a more accurate reading. If you tilted the handle while pulling, you would be pushing with your thumb, and pulling with the heal of your hand. That would cause the bar to bend more than if you gave it a straight pull, giving you an inaccurate reading. (Picture a strong-man bending a re-bar). THAT is why they tell you to pull evenly about the pin.
And yes, most newer torque wrenches have the ring in the center of the handle. In my example above, I should have made it more clear that the 1.5-foot measurement was just for illustration ... it would have been better to measure from the center of the square drive to the center of the handle, and mount the weights from there. It's simpler to give 1.5 feet as a nice round number. I was explaining the general concept.
BUT ... It's entirely possible to get an accurate measurement from the center of the square drive (which corresponds to the center of the bolt) to the center of the handle. Any known force applied at that distance from the center can be used to calculate torque. Feet from center x number of pounds applied. That's torque, plain and simple.
Now ... of course there will be some room for error, even in lab situations. If you've ever worked in a lab, I'm sure you know that all lab reports have to discuss possible sources of error. In my example above the error should be minimal. A decent tape measure is quite accurate. And even if the distance measurement is off by 1/16" (which is a pretty big mistake for anybody who knows how to use a measuring stick), the inaccuracy would be roughly 1/3 of 1% if we were measuring 1.5 feet.
The weights could also be a source of error. But ... I have a baby scale at home which can measure 30 lbs, +/- .5 ounce. So I can know with good precision, what force is being applied at what distance.
A far GREATER source of error with torque wrenches is the actual bolt you are tightening. How clean are the threads? How much lube is in there? How old and worn is the bolt and nut? How tight were the manufacturing tolerances of the bolt and nut? Those factors have a much, MUCH greater effect on the final "tightness" of the bolt than minor errors in the torque wrench itself. When manufacturers provide torque specs for bolts, they do that knowing full well there are gross inaccuracies in the process of tightening bolts -- including bolts for "steering linkages" and "wheels."
Now ... if we were working on the Space Shuttle, where all hardware is probably manufactured to extreme specs, and assembly conditions are very tightly controlled, perhaps it would be more necessary to shave every .1% -- I dunno. But for cars, any torque wrench can be accurately tested within the 4% you mentioned above.
And by the way ... the method I described above wasn't given to me by some "redneck" around a cave fire, it's been used and accepted for decades by many certified, professional mechanics who produce quality results.
It's easy to dismiss something as ignorant, without thinking it through. And it's easy to feel afraid when thinking of steering linkages breaking, or wheels falling off. But ... the logic you used above could be used against doing anything ourselves. If we wanted to rebuild their own engine, you could say "have you ever seen a piston knock a chunk out of an engine head?" If you want to walk down a long flight of stairs, wouldn't it be terrible to go tumbling head over heels? Whenever you want to do something, there will always be some "expert" there to tell you that you can't do that, and describe all kinds of scary scenarios.
I prefer to trust my own head. The method above is a time-tested, accurate way to measure the torque applied to a torque wrench. It's been used by many professional, certified mechanics over the years, and will work well for the home mechanic too.
Anyway,this post is getting WAAY too long. That's because you forced me to re-evaluate what I wrote -- which is a good thing. Debate can be good and productive, and I would look forward to hearing other opinions.
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