It depends on the point of load application. You are right if and only if the load is applied along the common axis. If not, you have a more complicated case where simple adding is not the case. Viktor
Inertia is basically a property of an object that resists a change in motion. It is dependent on the mass and shape of the object. The greater the object's mass, the greater the inertia. The greater the inertia the more force/torque is neccessary to accelerate/decelerate the object.
Read up on Newton's Laws of Motion.
For a Hollow Cylinder/Tube System, the formulas for calculating Inertia are.
W
J = ---- x (ro^2 + ri^2)
2g
or
(pi)(L)(p)
J = ----------- x (ro^4 + ri^4)
2g
J = inertia (lb-in-sec^2)
W = Weight of Load (lbs)
ro = radius to the outside of the tube (in)
ri = radius to the inside of tube (in)
g = gravity (386 in/sec^2)
pi = Pi = 3.141592654
L = Length of tube
p = material density of tube
Every shape/object will have an associate inertia formula tied to it so we can calculate.
Once you figure out how to do calculate Inertia, it is fairly simple, just can get time consuming.
Cameron Anderson
Sales & Applications Engineer
Aerotech Upper Midwest
"It depends on the point of load application."
"How does the load affect the moment of inertia?"
It has nothing to do with it. As was stated in the beginning Servocam's post, "It is dependent on the mass and shape of the object." However, I think Servocam is mixing up Mass Moment of Inertia and Moment of Inertia. In the first equation, the W/g portion is the mass of the object, hence W is NOT applied load as was inferred by Viktor.
Plain and simple....moment of inertia is a function of geometry only. Mass moment of inertia is a function of mass/density and geometry.
Thank you, Fred. My point exactly. Instead of asking a question intended to promote some re-thinking, I should have simply pointed out that load application has no effect whatsoever on moment of inertia or mass moment of inertia.
