Vibration simulation

[sklar]

Dear All
is there any guidance please on how to proceed when simulating the vibration load on a plastic pin pushed into a metal housing. where would the desired output be if the plastic part "shakes off" from the case when gravity and vibrations are applied?
Thank you for your help 

[John_Holtz]

Hi @sklar 

This sounds like a tricky simulation. Here are some things you need to figure out.

1. What is the theory that says the pin "shakes loose" or does not shake loose?
2. What type of vibrations are occurring, and does that analysis type provide the result needed for the theory in step 1?

Let's think of some simple examples.

• If the pin is a loose fit and gravity is trying to pull the pin out, the pin will fall out. (No calculation needed. 😊)
• Same arrangement, but now include an interference fit on the pin. Gravity can still cause it to fall out if the friction force is too small. That could be determine from the press-fit contact force (Nastran analysis) and friction coefficient using a hand calculation.
• Now flip the arrangement around so that gravity is trying to keep the pin in the hole. (Let's change the pin to a bolt with no nut. The head prevents the "pin" from falling through the hole.)
• If it is a loose fit, the bolt could "fly out" of the hole if the vibration is too extreme.
• If it is a tighter fit, the bolt can still fly out of the hole if the friction force is too small compared to the force needed to accelerate the bolt.
• Other possible considerations: thermal expansion if present and viscoplastic effects if the plastic material has long-term creep or stress-relaxation.

Assuming the theory will handle the arrangement, now you can determine if the vibration analysis provides the results that are needed. For example, the analysis types that use a modal analysis (shock/response spectrum, modal frequency response, random response) may only provide a "positive" result. See this article for an example: Reaction forces are unexpected in a Shock/Response Spectrum analysis in Nastran. If you need the total contact force between the pin and hole, is there any limitation on the sign of the contact force output by the vibration analysis? You may want to do a test model or two to confirm the results that you need are available.

Good luck.

John