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Question about torque on a shaft

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  • Question about torque on a shaft

    This is probably a simple question. If torque is applied to a shaft, then there must be some measurable twisting of the shaft. Is this correct?

    More generally, is it correct to say that any ridgid body where force is applied must physical deform?

    I'm trying to wrap my mind around the fact that rigid objects like steel even under very low pressure actually do deform albeit very small and perhaps even below the level that we can measure.

  • #2
    Yes, everything that has a force applied to it does deflect. For example, the apparently miraculous ability of a concrete floor to exert an exactly equal and opposite reaction to your body weight results simply from it deflecting minutely, in proportion to your weight.

    As for torsion, it has long been the practice on ships' propeller shafts to measure the amount of twist to determine the torque transmitted. Knowing the speed, the horsepower can then be calculated.
    Last edited by Asquith; 11-09-2007, 03:17 PM.


    • #3
      Do a web search on "strain gage" this should get you some info to read . . .
      "There is no more formidable adversary than one who perceives he has nothing to lose." - Gen. George S. Patton


      • #4
        Consider the lowly allen wrench. The long versions can be felt to deflect under hand effort alone when torquing a screw. Even a steel railroad bridge deflects to some degree when a mosquito lands on it. Our engieering sciences is sufficiently sophisticated that an investigator armed with the bridge's engineering data can tell you how much deflection occurs depending on the weight of the mosquito.

        Thirere's an old saying in engineering: "Everything is made out of rubber." And its true. Everthing deflects in proportion to its load, configiration, and material stiffness characteristics.


        • #5
          I think there will always be an amount of twist in a shaft usually called torsion. Torsion can also be used as a suspension unit as per some old British Leyland and if memory serves a Porsce had it gives a compact means of springing.

          I have tools I don't know how to use!!


          • #6
            Quote "Everything is made out of rubber."

            The way I heard it is that 'Everything is a spring' but what's the diff right.
            Chips Ahoy


            • #7
              It is called "angle of twist". For a circular shaft the angle (phi) is:

              phi = (T*L) / (J*G)


              T = applied torque
              L = length of the shaft
              J = polar moment of inertia
              G = Modulus of Rigidity (material property)


              • #8
                Originally posted by ptjw7uk
                I think there will always be an amount of twist in a shaft usually called torsion. Torsion can also be used as a suspension unit as per some old British Leyland and if memory serves a Porsce had it gives a compact means of springing.

                Yes and even a few American cars used torsion bar suspension.
                And not too many years ago either. :-)


                • #9
                  newer rangers have trorsion bar front suspension, favorable point for many off roaders as they can get a couple inches of lift with a wrench..

                  The most common US user was Chrysler, nice smooth ride and ease of tuning are the nice points of it.



                  • #10
                    Deflections, elasticity & torsion....

                    I think it's strange that people have difficulty accepting that materials deflect under load....but I guess I was raised on it. Even bricks, maountains and the Tower of Babel deflect under load.

                    Given the western persons distaste for mathematics, I suppose that's also behind our disdain for materials science since there is a lot of analysis involved. (Very boring math....) Eastern Europe, Japan, Korea and India have lots going on it the field, though. Canada barely produces a return on the radar.

                    As Forrest alluded, think of the humble Allen wrench, preferably an Unbrako...good illustration of torsion. It happens in all structures subjected to a turning moment. Even coil springs are an example of torsion....
                    Shafts, gear boxes, crankshafts, clutches, engines and compressors are all devices that use or produce torque and due to elasticity and the nature of their construction also induce or suffer from tosional vibration. This can be damaging to sister components in a drive line. (Eg. engine-gear box set up.) Torsional vibration can beat gear sets and bearings to a premature death. Hence the whole interesting field of vibration analysis, flexible couplings, dampers, tuning of drive lines etc.....

                    At least it interested me. I'd still like to get back into it. How though ?



                    • #11
                      Originally posted by dicks42000
                      At least it interested me. I'd still like to get back into it. How though ?

                      Bondage ...................


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


                      • #12
                        Yes even the typical coil spring is nothing but a rolled up torsion bar and in fact is the way it functions, I know some auto manuf. used to paint a line on thier drive axles so you could see if they got "pushed to far" if the straight line turned into a spiral it was time to get another, In general, Iv tightend so many fasteners in my life that Its actually safer practice for me not to use a torque wrench, I still do on things like head bolts and con-rod and mains and stuff --- but generally I just look at the bolt size length hardness and pitch and what its going into and then go by the elasticity of the feel, Iv saved allot of work at stopping short before something not too critical would have stripped out, Iv also gone beyond in certain aplications that allow to were I would not feel comfy with the recomended torque...


                        • #13
                          I can highly recommend this book:-


                          Very readable and entertaining book about structures, strength of components, deflection, etc, etc. It is not a textbook, but if you want the basic formulas, they’re there. Fascinating for everyone from the layperson (my wife enjoyed it) to the professional engineer.


                          • #14
                            One of the side affects of having even a cheap machinist's level
                            is seeing how much you can deflect it by pushing on part of the
                            machine. IIRC I could get a deflection off level in my CI 12x36
                            lathe bed by pushing down on the bed with my hand. Probably
                            not more than a few tenths, but it was detectable. Now maybe
                            it was the heavy sheet metal stands that were moving or maybe
                            the concrete but something moved. My Escort '91 wagon had a
                            big C shaped torsion spring under the rear wheels, discovered it
                            when one of the rubber mounts to the axle worked loose and I
                            got a big creak turning hard one way. Axle part was ~3' wide
                            with 15" arms going forward to clamp to the frame on either side.


                            • #15
                              The most distrubing example of torsional deflection I ever experienced was while riding a Great Lakes ore freighter - a sister ship of the Edmund Fitzgerald - from Duluth to Chicago in February in the mid '70s. The trip was part of a research project to see if winter navigation was possible on the lakes. We got into a storm on Lake Superior with about 50-knot winds, temp was around -30. Ships are basically big box girders, and with large waves at about 45 degrees on the bow, you could watch the ship twist slowly back and forth.