Moment of inertia

[Ringo]

Dear all,


I need to understand the formula


4j = MD2


M is mass and What is D? and how to calculate it


I need to calculate for a cylindrical shaft of mass 9.5Kg


I tried with max diameter in shaft, which gave a wrong answer.


Thanks inadvance.


Ringo

 

[Beam]

Try with this


If it is a hollow shaft the fromula is the following:


J=1/2 (R2+r2)


or palin shaft:


J=1/2 mr2


J being imertia


r=inner rad, R outer rad


m=mass, and 2= square


Your formula is derived from the previous one


Martin

 

[soccer8888]

Dear all,


I need to understand the formula


4j = MD2


M is mass and What is D? and how to calculate it


I need to calculate for a cylindrical shaft of mass 9.5Kg


I tried with max diameter in shaft, which gave a wrong answer.


Thanks inadvance.


Ringo

Ringo,


What you have there is a WR^2 formula, but instead of R (radius), you have it in D (diameter). Since the R is squared, to make a D value equivalent, you must divide it by 4, which has been carried over to the left side of the equation.


The problem you are having, I imagine, is finding the correct D value to use, as it is not the max diameter. It is the radius of gyration, which can be found by multiplying the radius by the square root of two, and then dividing that by 2. To usea diameter just multiply the square of two by the diameter and divide that by 4.