Oh boy, Eddy - this horse will never run again, will he? <img src="/forums/images/graemlins/lol.gif" alt="" /> -
I think you're partly correct - but I'm not sure it's for the right reason. <img src="/forums/images/graemlins/lol.gif" alt="" />
I have come to the conclusion that the size of the fastener is also a variable.
I believe that's true - but primarily because of difference in thread pitch and friction loading (more on larger diameter fasteners). I think at least some torque wrenches are calibrated with a set coefficient of friction assumption of .14 to .16. If you notice in this next paste, there is a reference:
In the original design of a fastener which will be subject to external loading, whether static or dynamic, it will be necessary to establish bolt size and allowable stress in accordance with current engineering practice.
The mathematical relationship between torque applied and the resulting tension force in the bolt has been determined to be as follows:
LET: T = Torque required (inch pounds)
F = Bolt tension desired (pounds).
D = Nominal bolt diameter.
EQUATION: T = .2 D F
This relationship is based on the assumption that commercial, steel, semi-finished, regular series nuts with rolled threads, semi-finished steel bolts are used, acting on steel surfaces without lubrication.
Tests have shown these conditions will result in a coefficient of friction of .16 at both the thread and head bearing surfaces.
For the usual application this equation can be used to approximate the required torque. Tests can then be conducted to determine a more exact value, if required. In any case, it is important to maintain conditions on the job as consistent as possible to those used at the time of determination.
The strange thing to see on the CDI site is that when you consider that clamping force is what the purpose of torquing a fastener is all about, it seems hard to believe that about 90% of the applied force is used to overcome friction - 50% head friction/40% thread - and only 10% results in clamping force. I still don't understand all of that, but I suppose I'll accept it until I see something different.
I do think (but I could be wrong) that the diameter of the fastener matters (particularly the diameter of the nut) but the difference between sockets is very little as a percentage of the overall handle length, so the wrench may very well be calibrated using a mid-range size socket. I'd have to do some more research on that before I'd come close to understanding the possibilities I'm considering.
I did ask the engineer at CDI about my hypothesis (maybe someone else's rule?) that mechanical advantage could be found by using the Ratio of Circumferences between the handle length and either bolt diameter or perhaps mean thread diameter as a method of determining applied torque without benefit of a torque wrench. He said he thought it probably could be done, but wasn't certain which base number reference should be used.
As luck would have it, one of my torque wrenches just blew up - can't read the number in the window any more - so I'm now off to Sears for a replacement (I hope). The warranty says they repair AND recalibrate within 3 months, but only repair (no free re-calibration) for the next 9 months. I don't think it'd be worth much without a calibration - maybe not even with one..... <img src="/forums/images/graemlins/lol.gif" alt="" /> so I'm off to argue for a replacement.
Frank <img src="/forums/images/graemlins/cheers.gif" alt="" />
