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bhorn1-Visitor
1-Visitor
May 1, 2016
Solved

Metric system: Calculation of torque

  • May 1, 2016
  • 3 replies
  • 6675 views

Hello,

I have just started using MathCAD. I am trying to get very simple Torque formula to work good but no luck (like in attachment). I should get around 64 Nm but I get 3,8 milion Nm.

I am using formula: Torque=9550*P(kW)/n(rpm).

I know:

P=2.7 kW

n=400 rpm

What I am doing wrong?

Thank you!

Best answer by Fred_Kohlhepp

Two things:

1)  k is a unit conversion value you're used to if you've not been using Mathcad.  Mathcad is unit aware, it will deal with your units, changing the numbers to make the values match the units you put in (and adding units to make your equations balanced.)

2) ne isn't right.  6.667 Hz is NOT 6.667 radians/second, To deal with the need to have both cycles per second and radians per second, Mathcad has "Hza," (angular  Hertz?) and rpm.  Once you get past that, everything's fine:

3 replies

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RichardJ19-Tanzanite
19-Tanzanite
May 2, 2016

Torque is power divided by angular speed. Angular speed should be in degrees/second or radians/second. The 9550 in your formula is probably a conversion factor for some specific unit system, and you should get rid of it.

F
Fred_Kohlhepp23-Emerald IAnswer
23-Emerald I
May 2, 2016

Two things:

1)  k is a unit conversion value you're used to if you've not been using Mathcad.  Mathcad is unit aware, it will deal with your units, changing the numbers to make the values match the units you put in (and adding units to make your equations balanced.)

2) ne isn't right.  6.667 Hz is NOT 6.667 radians/second, To deal with the need to have both cycles per second and radians per second, Mathcad has "Hza," (angular  Hertz?) and rpm.  Once you get past that, everything's fine:

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    bhorn1-VisitorAuthor
    1-Visitor
    May 2, 2016

    Thanks Fred,

    This is good. I did not know that there is unit Hza. It looks ugly in calculation, since it is not standard, but it works!

    Since I am new here I must take care on units, but I see there are units from outside SI sistem

    W
    Werner_E25-Diamond I
    25-Diamond I
    May 2, 2016

    The formula you use is from times where unit aware calculation tools were not available.

    Mathcad is unitaware and it considers the units, too, not just the numbers in front.

    You may simulate what engineers in former times did by dividing every variable by the unit the formula is expecting (kN and 1/min) and then at the end add the unit the formula is claiming it result to be.

    The other alternative would be to use a unit-aware formula which in your case simply means to divide all by 60000. Doing so you can provide your variables in any (correct) units you like and will get a correct result.

    EDIT Just noticed that Fred had beaten me by 1 Minute and has provided the better solution.

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