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Torque measurement of rotating shaft

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  • I am [80] years old and just once I wish I could hear the words [I AM WRONG] from an obvious BS’er.
    In a recent and completely unrelated thread I not only admitted my mistake but apologized as well. If I thought I were wrong I would say so.
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    • Evan might not be right but he's never wrong.

      High reverse Scotty..................

      .
      .

      Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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      • 101 and counting...
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        • Originally posted by Evan
          Torsional Rigidity is an intrinsic property of a material. The meaasurement of the deflection vs applied load doesn't measure the intrinsic Torsional Rigidity of the material since it is the geometry of the material that has the greatest affect on the measured result.

          The geometry of a construct of any material is not a property of that material. Note that the Torsional Rigidity of a particular material is a direct function of the Shear modulus of that material per unit area. Geometry is not specified.

          A test that generates a graph of deflection vs load produces a graph that represents the Torsional Rigidity of an UNKNOWN cross section of an UNKNOWN geometry of an UNKNOWN material.

          As such, it does not inform one about the nature of the torque transmitting element nor is it possible to deduce that information from the particular curve generated.
          A very nice (but wrong) answer to a problem which is NOT involved here.....

          We do NOT care what the properties of the MATERIAL are........ in the "abstract"... That isn't relevant. (and torsional rigidity is, of course, not a "property" of a material, but of a material AND a geometry)

          The torsional rigidity of so many square cm of steel, or bone, or balsa wood, will be radically different depending on how that steel is arranged..... hence the dependence on geometry.

          However, you have now made a direct statement, frozen and unalterable above. It is now up to you to defend that, because it is incorrect in the view of most.

          Please explain in detail, showing your work, exactly why the direct measurement of the angular deflection vs torque is INCORRECT..... and why the REAL "torsional rigidity" will have some other value totally unrelated to that measurement.

          This would seem to be rather difficult , since the definition of "torsional rigidity" is the angular deflection vs torque....... But we are always willing to see a good argument as to why "down is really up". "left is right", "blue is really red", etc.
          1601

          Keep eye on ball.
          Hashim Khan

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          • Originally posted by Evan
            A test that generates a graph of deflection vs load produces a graph that represents the Torsional Rigidity of an UNKNOWN cross section of an UNKNOWN geometry of an UNKNOWN material.

            As such, it does not inform one about the nature of the torque transmitting element nor is it possible to deduce that information from the particular curve generated.
            Don't tell that to my torque wrench!

            Carefully chosen words allow you to continue to debate your extremely convoluted points...

            "Nature"?? I suppose that could be debated as doesn't tell me the exact geometry of the torque transmitting member. Agreed but who the hell cares? If it's 1" dia of lead or .1" dia of steel does the mechanic frightening the bolt with the torque wrench care? It does tell me that at this displacement of the torque arm I get this much torque on the bolt, which is the whole point.

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            • A discussion of 'torsional rigidity' is always potentially hazardous, because there are two slightly different definitions.

              The use of the term 'torsional rigidity' for torque divided by the angle of twist seems to be almost universally used in the real world, but in the academic world 'torsional rigidity' is defined as the torque divided by the angle of twist per unit length - i.e. it is influenced by cross section and material only.

              To add to the confusion, some authors then differentiate between 'torsional rigidity' and 'torsional stiffness', and use the latter to refer to torque/twist. So: (torsional stiffness) = (torsional rigidity)/length. - These authors will then define 'torsional compliance' as the inverse of 'torsional stiffness' rather than the inverse of 'torsional rigidity' etc. etc.

              All this is fine and dandy until the unsuspecting Googlist merges results from both camps...

              Don't even start on modulus of...

              Clear - as mud ??

              Cheers

              .

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              • (Edited for mis-spelling typos)
                Well, Barrington seems to bring out the point that Evan is using the academic definition, because it suits his argument AT THIS TIME...... while he apparently used the enginerring definition earlier, BECAUSE IT SERVED HIS POINT AT THAT TIME.

                Here, for his and your delectation and enjoyment, is a compilation of contradictory posts by Evan....

                Note that early on, up to post 29, he said you could do it all by measurement, and NOW, somewhere after post #29, he reversed his position, he has taken up the "Albigensian heresy" and says that using measurement is "impossible" (his word)

                It does not seem very necessary to take anything he says about the matter very seriously. An excellent example of why "the shoemaker should stick to his last"......

                For the purposes of full disclosure, I mention that I have in some cases added bold and italic to egregious points... but not every bold or italic item is bold etc by my action.

                Post #11
                Originally posted by Evan
                Meauring the twist doesn't require knowing anything about the shaft, only the

                load.
                Post #20
                Originally posted by Evan
                Nope. All you need to know is how much twist is produced by a particular

                amount of torque on the output. That can be a purely relative value that is determined by an

                experienced operator who can tell when the system is close to it's limit. All the other possible

                methods face exactly the same issue if an absolute value of torque is required to be known and

                that is somehow measuring the actual torque on the output.
                post #23
                Originally posted by Evan
                Nope. All it is is knowing the relative alignment of each end of the shaft to the other. That does not reveal the properties of the shaft nor does it matter. That isn't what is being measured. It is a value that is directly coupled to the torque load and can be calibrated in absolute terms without even knowing what size or shape the shaft is.
                Post #25
                Originally posted by Evan
                In case you missed it here it is again.

                All you need to know is how much twist is produced by a particular amount of torque on the output. That can be a purely relative value that is determined by an experienced operator who can tell when the system is close to it's limit. All the other possible methods face exactly the same issue if an absolute value of torque is required to be known and that is somehow measuring the actual torque on the output.


                It still doesn't require knowing anything about the properties of the shaft.



                By the way, we don't even need to know that much. All we need to know is that there is a timing difference between when the marks pass the sensors and that also doesn't need to be quantified. The most we need to know is that X timing difference corresponds to N torque.
                Post #27
                Originally posted by Evan
                You can measure the torque on the output end of the shaft with a dyno. That is all you need to know to calibrate the system. If you don't understand that I can't make it any simpler. That doesn't give you information about the properties of the shaft since it doesn't even tell you the amount of deflection that takes place. You don't need to know that.

                The only information that you obtain is a phase difference between input and output. That is enough.
                Post #29
                Originally posted by Evan
                I said no such thing. Go back and read the postings Jerry. The dyno is for calibration which is precisely what I said in two different posts.

                For the third time:

                All the other possible methods face exactly the same issue if an absolute value of torque is required to be known and that is somehow measuring the actual torque on the output.



                Yes it was. It's called a propellor. That is exactly how some dynomometers work.
                copied from Post #145
                Quote:

                1) Evan proposes a "torque box" which has no description of the innards AND THEREFORE NO WAY TO CALCULATE ANYTHING, LET ALONE THE TORSIONAL RIGIDITY.

                Precisely. That is the entire point.

                Quote:
                2) Then he says this: "It's really simple. There is no known solution to calculating Torsional Rigidity for most possible cross sections. It must discovered empirically with full knowledge of the shaft and it's materials and cross section.'.....

                And of course we cannot do that so that avenue is closed.

                Quote:
                Of course, the obvious course of simply measuring was denied and denied and evaded.....

                No it wasn't. Simply meauring from outside tells you nothing specific.

                Quote:
                But suddenly it is back....... now it is EVAN's idea, and it has blossomed into "THE ANSWER" (to what it isn't clear)................ ""It's really simple. There is no known solution to calculating Torsional Rigidity for most possible cross sections. It must discovered empirically with full knowledge of the shaft and it's materials and cross section."


                As is often the case your understanding of the discussion is inverted.
                Measuring isn't back, it's impossible. That is my point.


                Quote:
                of course we really don't need teh cross-section if we have a lever and a weight,

                Yes you do if you want to quantify the characteristics of the torque member. You see, the point is that you cannot do so without knowing the cross section and since you don't know that you cannot describe it no matter what you do externally. That of course follows because many instances of the problem cannot be solved even if you do have full knowledge.

                Your post is completely meaningless since you failed entirely to grasp my point and have actually taken it to mean the exact opposite. That of course would make no sense. Do you stand on your head often?
                Last edited by J Tiers; 04-22-2010, 11:02 PM.
                1601

                Keep eye on ball.
                Hashim Khan

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