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TractorGuy14-Alexandrite
14-Alexandrite
October 3, 2018
Question

Pinned joint idealization?

  • October 3, 2018
  • 2 replies
  • 3439 views

I am trying to determine the best method for simulating an assembly with pinned joints and hydraulic cylinders that will give reasonable results with minimal run time. I have seen 4 different methods used and I wanted to get some advice and pros/cons for each one:

 

  1. The method I have used in the past is to setup contact interfaces. This gives very good results that correlate well with measured data but it can take hours to run in some cases. When you have 6-12 loading conditions to check this is very time consuming.
  2. The second method I have seen used is to define weighted links and springs. My understanding is that this won't necessarily give an accurate result for the pins stresses, but it will give good results for the assembly stresses. If anyone has a link to a tutorial on this method I would love to see it.
  3. The third method was something I saw in documentation for an older program (I-DEAS). This method used beam elements to represent the pins and cylinders and then define rotation for the pins. I'm also not entirely sure how to setup this method so any information would be helpful.
  4. The final method is to simply run the FEA with the pins bonded to the other parts. This is ok in very simple cases, but my assembly has a good bit of deflection and twisting which causes very high stresses at the pins because they can't rotate.

2 replies

G
gfraulini17-Peridot
17-Peridot
October 4, 2018

Hi,
can you share with us the assembly? Maybe some pics with notes that show the zones where you want very clear results aor other things?
Bye

T
TractorGuy14-AlexandriteAuthor
14-Alexandrite
October 4, 2018

Unfortunately I can not share the model but I can tell you that it is a piece of heavy equipment. So think along the same lines as a wheel loader or excavator.

S
SweetPeasHub17-Peridot
17-Peridot
October 4, 2018

Great question. 

 

There are some examples of pin joints using weighted links, etc. in the community from the user skunks, within the last few months.

 

So far, my work concurs with your findings, that the local stress at the joints are best as contacts, they are just really time consuming.  Some other methods with beams/beam releases or springs with low torsional stiffness are OK but they need weighted links (preferred) or rigid links to the expected contact areas (like a 90 degree segment of the pin cylinders).  Also when trying as a bonded connection, it is more accurate to bond over the expected contact area, not 360 degrees of the pin.  I tend to go with this bonded method as a good compromise.

 

There is another possible method which is the "free body diagram" method, for lack of a better term...

A: using rigid body mechanism dynamics to analyze and then transferring the loads to structure (loads-mechanism loads) to evaluate each link separately.  Use bearing loads (or other non-uniform distribution) and inertia relief to get accurate stresses at the joints.

B: free body diagrams by hand and evaluate each component separately, again distributed loads and inertia relief for more accurate stresses at the boundary conditions.  (St Venant's Principle does not apply in these areas)

 

I recognize that your assembly could be statically indeterminate, in which case a more complete model must be analyzed.

 

Does anyone think fastener idealization could be used to represent the pins?

    G
    gfraulini17-Peridot
    17-Peridot
    October 5, 2018

    I heard about using weighted links (or springs) instead of contact reagions to make modal analysies over parts clusters, not to make structural analyses. But it makes sense in order to improve the velocity (linear analysis with multi-thread instead of non-linear one with single thread).

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