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Do torque values equal compression

  1. Jul 1, 2016 #1
    Greetings All,
    We have a stack of parts which needs to have the height measured with 100 lbs of force applied. The plan is to put the stack in a vise and, using a torque wrench, tighten to handle to 100 ft/lbs. This doesn't seem right to me. Will 100 ft/lbs of torque equal 100 lbs of compressive force on the part between the vise jaws?
    Many TIA, CH
  2. jcsd
  3. Jul 1, 2016 #2
    I would lean towards using a Hydraulic Press (is; bench mounted bearing press)
    Your torque wrench only provides and accurate measurements at the centre of its pivot point!
    As note fastener torque specifics can vary depending on environmental factors.
  4. Jul 1, 2016 #3


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    No - not with vice of any conventional construction .

    There is a relationship between torque input and axial force generated though :

    Axial force = 0.6 * input torque * 2 * Pi / screw pitch .

    The 0.6 is a factor which allows for friction in the screw thread . 0.6 is best guess for a usually crude vice screw and nut .

    So if screw pitch = 0.2 inches and input torque = 100 lbs-ft then :

    Axial force = 0.6 * 100 * 2 * 3.142 * / 0.2 = 1885 lbs .

    To get your 100 lbs axial force the input torque needed would need to be reduced to about 5.5 lbs-ft .
  5. Jul 1, 2016 #4


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    VTSC87 is right about the hydraulic press though - it will give more certain results and an easier way of working .
  6. Jul 1, 2016 #5
    In addition if you are to use a press please use all the appropriate PPE a misaligned object in a hydraulic press can quickly becoming a projectile!
    Safety first!
  7. Jul 1, 2016 #6
    Thanks for the input, but due to the budget for this, and the fact that we already have a milling vise, hydraulic isn't an option.
  8. Jul 1, 2016 #7
    Thanks for the data. I'm thinking about putting a load cell in between the vise jaw and test part so we have a definitive compression reading.
  9. Jul 1, 2016 #8


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    Definitely a good thing to do - eliminates uncertainty about exact values of input torque and of effect of friction .
  10. Jul 1, 2016 #9


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    Hydraulic press pressure will work, but the pressure will not be 1:1 unless the area on the piston is equal to the area over which you are applying the compression.

    The hydraulics I work with generally run (up to) 200 Bar or ~3,000 psi. Applying 100 lbs could well be right at the bottom of the control range - unless you have a very small press.

    I would make a jig to stack the parts vertical and apply physical weights. Or Google a "compression test machine".
  11. Jul 1, 2016 #10
    Thanks, but due to the budget and harsh environment (next to a punch press), we need something fairly stout and rugged, hence the milling vise.
  12. Jul 1, 2016 #11
    Could you just stack them up on a bench and put a hundred pound weight on top?
  13. Jul 5, 2016 #12
    Since the diameter of the part is rather small, it would be difficult to balance 100 lbs on top of it & still get an indicator in there somewhere to measure the height.
  14. Jul 5, 2016 #13


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    What about using a compression spring in the vise and compress to a known compression distance? It's easy enough to measure compression using a caliper, and so using the spring's spring rate you can determine how much it needs to compress to achieve 100 lbf.

    Take for example this spring: http://www.centuryspring.com/compression-spring-11864.html


    The above example has a 2.25" free length and 299 lbf/in spring rate. So to achieve 100 lbf, compress it by 0.334" (in series with your part) and voila, 100 lbf compression load.
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