Moment of Inertia
Moment of Inertia
(OP)
I'm trying to calculate the total system mass for a friction hoist.
My question is in the conversion of the moment of inertia of a hoist drum and pulleys.
If a catalogue gives me the moment of inertia I, then would I be able to calculate the equivalent mass (or mass required to turn the drum from rest) using the inertia and radius of the drum?
I=1/2 m r^2
m=(2) I/r^2
I would think this is true if the drum was a solid cylinder. But since it has a different shape, how do I calculate this?
Thanks
My question is in the conversion of the moment of inertia of a hoist drum and pulleys.
If a catalogue gives me the moment of inertia I, then would I be able to calculate the equivalent mass (or mass required to turn the drum from rest) using the inertia and radius of the drum?
I=1/2 m r^2
m=(2) I/r^2
I would think this is true if the drum was a solid cylinder. But since it has a different shape, how do I calculate this?
Thanks





RE: Moment of Inertia
It sounds like you want inertia, and they give you inertia.
Or do you want mass? In that case, call the manufacturer?
RE: Moment of Inertia
Thanks
RE: Moment of Inertia
You can get closer by breaking the shape up into segments and using the moment of inertia transfer formulas to get your total inertia. Then solve it for mass.
(Look up 'Inertia Transfer Formula', or 'Parallel Axis Theorum')
An alternative would be to draw the drum in a 3d cad program (solidworks, inventor, etc) and then tweak the material density until the inertia matches the catalog value. Then read off the mass.
RE: Moment of Inertia
Err... or by this are you actually looking for total inertia of your system as seen by X component (say, the drum) taking into account pulley ratios? I don't really grasp what "mass required to turn the drum" is supposed to mean.
Try googling "reflected inertia". Maybe some of this will get you near the right track.
RE: Moment of Inertia
RE: Moment of Inertia
Inertia cannot be converted to mass.
An inertial system can be converted into a simpler equivalent inertial system only.
For eg. MMOI about a point O is simply m*r^2, where r is the distance to the mass "m".
But, what are you really trying to do?
Fe
RE: Moment of Inertia
RE: Moment of Inertia
T=I*alpha,
T= the torque required
this generates an acceleration "alpha"
given mass moment of inertial "I"
This is dynamic, ie. if it is spinning, that relation holds always.
If you are trying to figure out how much torque (Force*distance) from a stop, you need to know how much friction is present ect.
Fe
RE: Moment of Inertia
Fe
RE: Moment of Inertia
It is fundamental dynamics.
Fe