Maximize the first resonance frequency of a plate. 2

[vidiii]

Hi

I have a plate made of a certain material (plastic, cannot be changed). I need to maximize the first resonance frequency of the plate. The availible methods are: making some places thinner and using ribs.

When using ribs on the plate with fixed dimensions (100mm x 75mm x 2mm) you can't increase the net weight of the plate with more than 10 percent.

Is there somebody with experience with such problems? What is according to you the best way to increase the first resonance frequency to the maximum.

Thanks in advance!

Kindest regards,
Vidi

 

[floattuber]

1. When you say maximize the 1st resonance, do you mean maximize the response or do you mean change the frequency?

2. What are the boundary conditions?

3. Is this a project for school?

 

[GregLocock]

The answer will be limited by two-ish things (a) your minimum wall thickness (b) you maximum section depth and (c) are you allowed to create holes (in which case there is one obvious solution)?

The answer will change depending on your boundary conditions.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[vidiii]

The boundary condition is free clamping, but the rigid body modes are neglected. It is not allow to make holes, you must respect a certain minimum thickness of 2mm.

floattubber: Its the frequency, not response, that needs to be maximized. Why? The plate is used in a design where it operates at certain frequencies where it must not resonate. So the idea was to use techniques to maximize the first eigen frequency.

Thanks for help!

Regards,
Vidi

 

[GregLocock]

Are you only interested in the first flexural mode?

Are you expecting to solve this analytically, or by FEA?

Do you have a maximum section depth, or is that effectively controlled by the 'no more than 10% mass increase' and 'walls must be at least 2mm thick' requirements?




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

also - "making some places thinner"

What do you mean by that- how thin can you go across the main face of the part?

What do you get as your first mode (frequency, mode shape)?





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[vidiii]

Thanks for your reply GregLocock! I'm interested in increasing and maximizing the fundamental resonance frequency. The idea is to change the geometry (using ribs or other techniques) in a way that the natural frequency increases while the constraints are respected.

The depth dimension is not a constant, but it must respect the mass constraint and the minimum 2mm constraint.

The first mode:

http://files.engineering.com/getfile.aspx?folder=81073892-0af3-4a9a-80f7-0e8214bcc7d4&file=a.avi

The problem is being solved using FEM.

Thanks alot!

 

[GregLocock]

OK, I got torsional as well. I don't know what optimisation tools you have available, I must admit that adding ribs by hand gave very disappointing results compared with simply thickening the entire section up, which gave 10% as expected.

Ribs are generally rather useless for torsion.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BobM3]

For torsion use diagonal ribs. If your manufacturing process allows, try a skin on the lower surface to create a closed box section.

 

[Spirit]

vidiii,

considering the shape of the mode of interest and the constraints you mentioned, I would suggest you to consider a plate with a C-shaped border frame; if this is not enough, I would also evaluate adding two diagonal ribs.
I think this is a tradeoff solution between the starting design and the pseudo-sandwitch solution proposed by BobM3.

Regards,

Spirit




'Ability is 10% inspiration and 90% perspiration.'

 

[GregLocock]

Spirit - the 2mm/10% restriction is a killer, there is not enough spare material to build a decent looking reinforcement. I must confess I suspect the 10% requiremment is pulled from thin air, it sounds like a manager's number.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.