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gpommeranz10-Marble
10-Marble
June 3, 2021
Question

Engine Rubber Isolator Modal Analysis

  • June 3, 2021
  • 1 reply
  • 1774 views

Hi all,

 

I have a few general questions about an analysis I'm attempting.  I'd like to determine the natural frequencies of an (IC) engine isolation system.  The engine and associated components are mounted on typical cylindrical rubber isolators that are connected to the chassis.  

 

My plan is as follows:

 

1. Setup the engine as a point mass at the CG location of the engine with the mass moments of inertia.

2. Create points for the start and end of the isolators.

3. Connect the point mass to the isolator ends using rigid links.

4. Determine the rubber isolator properties and set up springs with those.

5. Run a pre-stress static to determine the deflection/stiffness of the system under the effect of gravity.

6. Use that to do the modal analysis to determine the natural frequencies of the system.

 

It seems pretty straightforward but I want to make sure I don't miss anything.

1 reply

R
rrabe12-Amethyst
12-Amethyst
June 24, 2021

Late response, but it may still be useful in the future.

 

1. Setup the engine as a point mass at the CG location of the engine with the mass moments of inertia.

_ Ok if you think that your structure will move as a single body. I would assume that is true as the isolators are the dominant weak spring in the system. 

 

2. Create points for the start and end of the isolators.

_ Is it is required? In some case you can use a ground spring (but that causes a lot simulation restrictions) 

 

3. Connect the point mass to the isolator ends using rigid links.

_ Here is also ok but maybe, in some situations, a weight link would give you a better approach not creating an infinite stiff structure. But they have some tricks. Try to find a post in this forum where I compare the influence of the distance between the CG and the fixation - and in this case combining rigid and weight links. But it may not be your case since the isolators may be very weak. 

 

4. Determine the rubber isolator properties and set up springs with those.

_ Yes, specially multi layer rubber isolators are very stiff axially but compliant in the plane (shear) directions. The problem here is, a rubber isolator stiffness will change with the amplitude of the deformation and the strain rate. For small vibrations amplitude may not be an issue but the rubber will stiff as the frequency increases. 

 

5. Run a pre-stress static to determine the deflection/stiffness of the system under the effect of gravity.

_ If you are using virtual linear springs I don´t think pre-stress will change the modal analysis result. You can simply use gravity to visualize the deflection in each isolator but not for a more "accurate" modal result.  

 

6. Use that to do the modal analysis to determine the natural frequencies of the system.

_ As explained before the rubber isolator should change its stiffness with the frequency. If you have the stiffness response of the isolator by frequency you can try to use non-linear springs.

 

Good luck and have fun in the simulation!

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