I need a rotational stress formula 1

[Techno97]

I have one from Marks book of engineering (5-55) but it also uses terms that leads to further computation. p= mass density, lb.s^2/in^4 This has stumped me. I have no idea of what I should place in for these values. What is inches raised to the fourth? Is this mulitplied by gravity?

Don't laugh but my dog thought the leather binding was tasty. I no longer have the first chapter or the end of the index.
I only need to find the stress in a rotating disk so I can begin selecting a material.

I have tried finding this information with no result. My searches led me to this forum.

 

[brad]

I can only presume that you are searching for the mass moment of inertia (MOI), which may show up in equations as Ixx, Iyy, Izz. MOI is to rotational acceleration as mass is to translational acceleration (the higher it is, the more it resists applied 'loads'). It is essentially a function of mass and the distribution of the mass (i.e. it's harder to spin a 10 foot 2x4 than it is a 10 lb bowling ball, even though they weigh about the same--this is MOI effects).

The details of MOI are beyond a simple posting. Please reference an introductory dynamics book. Look for 'moment of inertia', 'mass moment', or 'rotational acceleration'.
Brad

 

[prex]

From Roark's Formulas for stress and strain.
The maximum radial stress and maximum tangential stress in a solid homogeneous circular disk of uniform thickness are equal and occur at the center:
max sigma_r=max sigma_t=delta*omega²*(3+ni)*R²/8/386.4 lb/in²
delta=mass density lb/in³
omega=angular velocity rad/sec
R=disk radius in
ni=Poisson's ratio (0.3 for steel)
The Roark has also formulas for an annular disk and for disks with variable thickness.
prex
motori@xcalcsREMOVE.com

http://www.xcalcs.com

Online tools for structural design

 

[brad]

WHOAH!
I missed the boat on that one! Additionally my brain misfired on the units. I really shouldn't post late at night.

Go with what Prex said; please ignore my previous post, as I was definitely offbase.

Brad