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Need some info and help about torque!

  1. Jan 27, 2007 #1
    First of all, let me tell you I don't know much about physics, engineering etc. I know a little bit of math, through Calculus. I also know a little bit about Electronics. Anyways on to the questions!

    Let's say I have a hollow steel shaft. Strain gages have been applied for measuring torque. Now if I know the Zero load microstrain, Gage Factor, Inner and Outer Diameter of the shaft, Gage resistances.

    Am I able to calclulate and estimate what my total bridge output, and Actual strain is at a certain clockwise torque in NM applied to the shaft. How can I determine breaking points for X amount of cycles.

    These questions may be a little vague, and for that I'm sorry. I'm trying to figure out how a piece of software I have, calculates this type of information.
    Any help or information about this would be greatly appreciated.
    Thank you!
  2. jcsd
  3. Jan 27, 2007 #2


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    As long as you know what all of the particulars are like gauge factor, excitation voltage, etc...you can indeed calculate the estimated output assuming you are familiar with basic mechanics and how to do the caluclations for pure torsion.

    As a start, take a look here for a nice primer on Wheatstone bridges.
  4. Jan 27, 2007 #3


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    Homework Helper

    You can break this down into a few subproblems:

    1. Converting electrical signals from the strain gauges into measurements of strain. That depends mainly on the type of strain gauge, not on what the gauge is attached to.

    2. Converting the individual strain measrements from a rosette of strain gauges (or whatever type of gauge you used) into the shear strain in the shaft. The software might combine this with the previous step.

    3. Converting the strain to stress in the shaft and hence to a torque. You need the material properties (the shear modulus of elasticity at the relevant temperature) as well as the geometry.

    4. Converting the stress (presumably it's not a constant stress, since you said " the number of cycles") into a life to failure. Basically you compare your stress value against a curve fitted to test measurements of the material behaviour at different stress levels. If you have different stresses in the shaft at different operating conditions, you need to combine the amounts of "damage" done at the different stress levels to estimate the time to failure.

    For 1, most likely see the technical data sheets for your particular strain gauges.

    For 2 and 3, see any "strength of materials" or "mechanical engineering 101" type of text.

    For 4, try Googling "S-N curves" and "Miners Law".
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