Vibrating Block

[joesm519]

A rectangular block has a rod attached off-center. At the end of the rod there is a weight. The rectangular block is vibrating about a small angle. The axis of rotation is through the center of the rectangular block and parallel to the axis of the rod. In my simulation I have applied the weight and centrifugal force at the point where the load is. I feel there should be another load in order to take care of the shock as the weight moves to one end and then the direction of the momentum changes suddenly. Is my thinking right if so how can I apply this load. Thanks

 

[GBor]

I'm having a little trouble visualizing the "real-world" arrangement, but it sounds as though you would need to do a dynamic analysis if you are not already doing so. Depending on your software, the inertial effects should be accounted for either through an inertial load transfer capability in the software or through your load curve that you apply to the oscillating force representing the block.

What software are you using and can you post an image of either the FEA pre-processed model, or a CAD image?

 

[joesm519]

Thanks GBor,
I am using pro/mechanica linear static analysis, do not have the advance version. I am trying to apply the shock load as a static load as a work around. Please guide me as to how I can calculate the magnitude of this inertia loading and is it fair to do so in the absence of non-linear dynamic capability. I have attached a simple sketch to explain my geometry. Thanks

http://files.engineering.com/getfil...d-e027-4286-bd5a-4e94d9a7873c&file=sketch.bmp

 

[GBor]

I think you should probably do a kinetic energy calculation to determine the inertial loads. You could just add a dynamic load factor, but it may take some thought to come up with what that factor should be.

 

[joesm519]

GBor,
I found some vague reference for the shock factor as between 2 and 3. I am not sure how much I can trust it. Attaced is a sketch showing my stress analysis methodology. Please let me know if I am missing something. I want to use the shock factor to the resultant moment shown in the image. Thanks

Your file's link is:

http://files.engineering.com/getfil...-8eb7-4e02-a781-9cff2620c198&file=sketch1.JPG

 

[GBor]

Have you calculated the natural frequency of the beam/weight combination? The question may fall in to the category of "am I exciting a mode of the weight/beam"? Do you know the frequency of vibration of the block? And, what stress are you concerned about...stress in the beam? the block?

There are other questions related to the modeling: What boundary conditions do you plan on using? How will you apply the load (centrifugal, moment, force, etc.).

I know I keep coming up with more questions than answers, but I'm really trying to grasp what it is you are trying to do...

If you are trying to determine the stress in the block due to the vibration of the block and the inertia of the beam/weight combination, you need to check the vibration absorption due to the elasticity of the beam (AE/L is the spring constant) and see if it will even move the weight. If it does, the weight could be positioned at a node if that is important (a vibratory node...not an FEA node) then it would't move despite the vibration.

If your dimensions are set and weights determined so that there is no additional design involved, you still need to know the flexibility of the beam to see how much elastic energy will be "whipping" your weight. It's not unlike a fishing pole...a rigid rod won't "whip" your bait over the pond, but a flexible one will.