Inertia units conversion
Inertia units conversion
(OP)
I feel as though this may have been explained before, but my searching was not successful. My apologies if I have missed the answer...
I am an electrical engineer struggling with the units of inertia.
I have a specification that states that the inertial load is 0.65 lb-in-sec^2. I can't think of inertia in those units, and so I want to convert to kg-m^2.
I have found several online calculators that can perform this conversion. Here are two:
http://www.magtrol.com/support/inertia_calc.htm
http://www .linengine ering.com/ LinE/conte nts/stepmo tors/Calcu lator.aspx
So, all seems good, but I can't work out how on earth the conversion is happening without any additional information given the change in units.
My question is this: what is the theory behind converting an inertia from lb-in-sec^2 to kg-m^2?
Many thanks,
BPELEC.
I am an electrical engineer struggling with the units of inertia.
I have a specification that states that the inertial load is 0.65 lb-in-sec^2. I can't think of inertia in those units, and so I want to convert to kg-m^2.
I have found several online calculators that can perform this conversion. Here are two:
http://www.magtrol.com/support/inertia_calc.htm
http://www
So, all seems good, but I can't work out how on earth the conversion is happening without any additional information given the change in units.
My question is this: what is the theory behind converting an inertia from lb-in-sec^2 to kg-m^2?
Many thanks,
BPELEC.





RE: Inertia units conversion
one meaning is inertial force. lbm*in*sec^-2 has the dimension of force. it is an unconventional unit, a "proper" lbf = 32.174lbm*ft*sec^-2 = 384lbm*in*sec^-2.
another in rotational mass moment of inertia, that you may be more familiar with from rotating machinery.
RE: Inertia units conversion
corus
RE: Inertia units conversion
1lb = 4.45N
1N = 1kg-m/s^2
so
1lb = 4.45kg-m/s^2
1in = 0.0254m
Put them together and you get:
1lb-in-s^2 = 4.45kg-m/s^2 * 0.0254m * s^2
Multiplying and cancelling dimensions, you get:
1lb-in-s^2 = 0.113kg-m^2
If you type this into the converter on your first link, you get the same answer...
If you "heard" it on the internet, it's guilty until proven innocent. - DCS
RE: Inertia units conversion
i guess that's aprt of the problem of having both pound force and pound mass.
RE: Inertia units conversion
Kind regards,
BPELEC.
RE: Inertia units conversion
1 lb = 1 slug-ft/s^2
Using the lbm is stricly bush-league. It is an imaginary unit that is extremely misleading...much like the kgf. I avoid their use entirely where possible.
I guess they were dreamed up for people whose talents are too modest to understand the concepts of mass and force.
Don
Kansas City
RE: Inertia units conversion
RE: Inertia units conversion
I am based in the UK and last time I checked, SI units were all metric:
http://physics.nist.gov/cuu/Units/units.html
Also, I have never come across Kgf or Kgfs before.. What is the 'f'?
Ok, just found it on Wikipedia, but I have never come across Kilogram-Force before, and I work in a mechanical engineering company despite being electrical myself...
RE: Inertia units conversion
RE: Inertia units conversion
RE: Inertia units conversion
RE: Inertia units conversion
1N = 1kg-m/s^2
What on earth is this? I would be grateful if swearingen or anybody else can explain this one to me.
The real problem is the mixing of pounds force with pounds mass. The pound force (lbf) is the unit of force in the British Engineering System of units, and in this system the unit of mass is the slug. The pound mass (lbm) is the unit of mass in the Foot-pound-second system of units, and in this system the unit of force is the poundal. Both of these systems of units (i.e. BES and F-P-S) are consistent sets of units, but things go horribly wrong when people insist on mixing the two systems (as most Americans do unfortunately).
If you mix the two systems you have to introduce the proportionality constant, which is usually written as gc. This has the value 32.174 ft/s2, or 386.09 inch/s2.
In the original value of 0.65 lb-in-sec^2 these are pounds force and not pounds mass. The lbf value has to be multiplied by gc, ie by 386.09 to bring it to pounds mass, and then divided by 2.205 to bring it to kg. And there are 1550 inch2 in a meter2.
Now the conversion becomes
(0.65 x 386.09) / (2.205 x 1550) = 0.0734 kg.m2
which is the value given by the first link.
If this is not a good reason to learn to use SI units then I don't know what is.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Inertia units conversion
- Steve
RE: Inertia units conversion
But the root cause of the problem is still using pounds force where they should be pounds mass.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Inertia units conversion
One of the problems is that a lot of catalog specs still use hybrid, old-timey units, like oz-in for torque or kilogram-force. As long as you diligently boil them down to MKS or lb-ft-s, everything works out without having to use those crazy formulas with magical conversion constants in them.
Don
Kansas City
RE: Inertia units conversion
T = J*a
where:
T - torque (system unit specific)
J - Mass moment of inertia (system unit specific)
a - rotational acceleration in radians/sec^2
This results in J = T/a.
In the English unit of torque would be in pound-inches and rotational acceleration would be in radians/sec^2. This works to a unit of:
pound-inch-sec^2/radian
Technically, a radian isn't a unit, it's a ratio (that should get a discussion going) so it is dropped from the unit definition, leaving lb-in-sec^2. As stated above, given this unit, pounds are a unit of force, not the (imaginary) unit of mass.
Keith
RE: Inertia units conversion
So, yes, there are people citing kgf.
TTFN
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RE: Inertia units conversion
I have tutored math and physics for many years (specifically to stay up on the basics so I can answer my kids' questions) and these questions come up time and again. I always tell my students to break them down into the base units and then there's never a problem...
If you "heard" it on the internet, it's guilty until proven innocent. - DCS
RE: Inertia units conversion
The equations for inertia are written out using both force units and mass units. The resulting inertia values are not equivalent.
I suggest you systematically use mass unit equations, and use...
m = w/g
The weight in pounds is a unit of force, and it must be divided by g to get mass. Kilograms are, of course, a unit of mass, and you do not divide by g.
The alternative is to be arbitrary and unmethodical where you stick the g value in your equations, and hope for the best.
JHG
RE: Inertia units conversion
TTFN
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RE: Inertia units conversion
http://www.katmarsoftware.com/uconeer.htm
RE: Inertia units conversion
Harvey
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Inertia units conversion