Learning Forum

[rmaur29]

Apologies I'm starting a second discussion but this forum has been really helpful.

In my model to reduce the natural frequency, I have added revolute joints to 2xsupports to allow DOF around pins bring my natural frequency down, although my frequency still needs to be lower (currently 6.8Hz to 3.5Hz). Model has been drawn to exact geometries. Additional mass to components is not an option as I have checked all components and they are correct mass properties. The supports are all set to fixed (2x torque arms & 4x base plates) and since the structure is actually sitting on an upper mezzanine framework, I believe there would be some flexibility in the lower framework supporting the structure ( so maybe using different supports(elastic) with set stiffness representing the lower frame flexibility will reduce natural frequency. The only other thing I can think of is looking at my contact regions to reduce the natural frequency? Will this be effective in reducing natural frequency? 

 

Below is a screenshot of the model isometric and contact region pane.

[parkersheaffer]

If you don't mind sharing I would be interested in taking a look at the model itself, might be able to provide better feedback this way.   

[rmaur29]

Do you have an email I can share a link to?

[parkersheaffer]

This should work foransysmodel@gmail.com. 

[peteroznewman]

Here is how you can Attach a .wpbz Archive file after you post. The file size must be < 120 MB.  If it is larger than that, you can reduce the file size by deleting the mesh (clear generated data).

If the file size is still > 120 MB and you have a Gmail account, you can upload it to an email on Gmail, which will create a link to the file on Google Drive. You can copy and paste that link into your reply.  Similar functionality is available with Microsoft Outlook and OneDrive.

[rmaur29]

Thanks, a link should have sent through. 

[parkersheaffer]

When you setup a model you need to keep in mind the impact a contact will have on stiffness. Bonding two plates together in place of using joints can over-constrain a model. A shaft going through a gear box is not going to have the same stiffness as a shaft sliced then bonded to the outside of the gear box.

In addition to that you need to review the contacts you have in the model, there is large amount of redundancy(both friction less and bonded contact being used between two surfaces).  

A properly sized mesh is important for accuracy, when I make the element size smaller the natural frequency changes. I didn't perform a complete mesh convergence study on this due to limited time but I suggest you do. 

Boundary conditions are also something i looked into. The fixed support is not an accurate representation of the bolt pattern you have at the base of the I-beams. For the time being i just used the holes to fix to ground, double check though that what this is attaches to is a rigid body(concrete floor, etc). If not you might need to add in more to the model.

One question i have is how are these boxes secured on the physical machine and what are these pads they sit on.

 

 

I was able to get to 5.6hz for the first natural frequency with the few changes i have made, I don't know how easy it will be to get to your measured 3 hz but i will take another look when i get a chance on Monday.

[rmaur29]

Really appreciate the feedback, those changes have made a clear difference to Hz. 

The pads that the motor and gearbox are sitting on represent the mounts that connect to the below base and trying to represent how they are fixed. 

Fixing the base plate holes instead of the hole plate has reduced frequency significantly. 

I tried refining the mesh to finer element size but hasn't seemed to make much of a difference.

I'm currently reviewing the bonded and frictionless contact regions to see which a re redundant and this has reduced frequency. 

The most detailed adjustment is the pulley shaft going through the gearbox, so im planning to extend the shaft through the gearbox with 1mm gap, and planning to create a bonded contact region between the pulley shaft and the circular gearbox cutout for the pulley shaft, and additionally creating a cylindrical joint between the pulley shaft and gearbox allowing rotation around only the z-axis. 

I managed to get 5.8Hz excluding extending the pulley shaft and applying a joint, but will see what results I get.