Displacement simulation with load?

[shaische222]

I'm trying to create pretty much what is shown in this tutorial: https://www.youtube.com/watch?v=3YmLorbJh_0&t=186s

I have a clip that hooks onto a pin as shown in the attached images. However, instead of just simulating displacement of the clip, I would like to see at what pull force the clip gives way. Is there a way to do that? At $15 per simulation that Fusion 360 charges... I would like to get it right on the first try 🙂

[jhackney1972]

You would probably get quicker responses if you would attach your model.  No one wants to have to recreate the wheel just to try and give you an answer plus if we modeled it, we may change some parameter that is not what you desire.

[John_Holtz]

Hi @shaische222 

Based on my 30+ years of experience with simulation, I think these two things are safe assumptions:

1. I have done more than 100 simulations. 🙂
2. Very few work as intended on the first try. ☹️ Either because I forgot to enter something, or I did not anticipate something occurring, or some other problem.

In other words, you are not going to get it right on the first try, especially if you are trying to do an event simulation.

The most successful way is to do a static stress analysis (or nonlinear static stress). You know how far apart the legs of the clip need to move; they spread apart a distance equal to the diameter of the pin. So model the clip only and apply a prescribe displacement necessary to separate the legs the diameter of the pin. The result will be the force required to separate the legs. From knowing that force, you can do a hand calculation to estimate the force required on the pin.

The more complex simulation would be the event simulation. Model the pin and clip. You know how far the pin needs to move, so apply a prescribe translation to the pin. (Also apply an initial velocity to the pin that is equal to the prescribe translation/duration.) The only question is whether the event simulation gives the reaction force to move the pin. It should, but I do not know if that update has been released yet.

[shaische222]

@John_Holtz "From knowing that force, you can do a hand calculation to estimate the force required on the pin."

This definitely sounds like an easier solution than doing a mechanical event simulation, but what's the formula if I may ask? I assume the force to push out the clip's legs isn't the same force to pull out the pin? Haven't been able to find anything on google for a clip of this type for calculation formulas.

[John_Holtz]

You solve the forces on the pin using a form of F=m*a (one of Newton's law). Specifically, you create a free body diagram of the forces on the pin and use (sum of vertical forces) = 0.

• Solve for the angle of contact between a from 2*R*sin(a) = the opening of the clip. (This is an approximation because it assumes the pin makes contact with the narrowest opening of the clip. What you really want to do is use the geometry to determine the tangent point between the pin and the clip. Just make a sketch with the pin at different positions and measure the angle a to the tangent points if you want a better approximation.)
• The prescribe translation on the clip to open it gives a reaction force. Use that reaction to calculate Fp from Fp = (reaction force on clip)/sin(a). (You use the reaction from one side of the clip. The factor of 2 in the next equation takes care of there being 2 sides to the clip.)
• Then the downward force on the pin Fd can be calculated from Fd = 2*Fp*cos(a).
• Increase the force Fd by some factor (50%?) to account for friction and a safety factor.

Statics free body diagram on the pin.

Also, you need to use a much finer mesh than the default that I used, at least in the area of highest bending. Since no one knows ahead of time what mesh size is required for sure, you will probably want to run the analysis 2 or 3 times with a finer mesh each time until the reaction force to open the clip becomes constant. (This is another reason to start with a linear static analysis. You can run multiple analyses on your local computer instead of the cloud to get a better idea of what mesh size is required for the Event Simulation analysis, if you decide to run that to check your hand calculations.)

[shaische222]

Hi @John_Holtz I tried following your formula several times, but I get lost in the explanations, sorry. I think I understand the first part of it on how to get a more accurate reading by finding the tangent angle of the opening, which you can see below (if I got it right?).

I ran the simulation, albeit too early before seeing the recommendation about making the mesh setting finer. Here are the results below. Since I already ran it, what's the proper way to read the force required to pull out the pin from the results? Is the "Reaction Force (Y)" the correct selection? And if yes, would that mean it takes about 9 lbs of pull out force based on this reading? The pin is being moved in the Y axis.

I'm still trying to understand about how to figure out how fine the mesh should be to get accurate results. You mention to run static stress analysis, but how would you set that up properly? I tried running some simulations, but all I got was a deformed clip spring that is "stretched out".

I appreciate all the help you're giving!

[John_Holtz]

Hi @shaische222 

"What's the proper way to read the force required to pull out the pin from the results? Is the "Reaction Force (Y)" the correct selection?"

No, that is not the correct way. That is graphing the one node (of 10, 100, maybe 1000 different nodes) that has the largest reaction force. What you want it the sum of the reaction forces, using either the nodes that are constrained on the clip, or the nodes with the prescribe translation on the pin. You cannot get a graph of this. You need to use "Inspect > Reactions" and step through the output steps 1-by-1 to find when the largest result occurs.

If you did not apply an initial velocity to the pin, then some fraction of the reaction force on the pin is the load that it takes to accelerate the pin from 0 velocity to Y velocity.

There will probably be a lot of vibration occurring, so the reaction force may jump up and down. You probably want to use some type of average over several steps to "smooth out" the vibration.

"You mention to run static stress analysis, but how would you set that up properly?"

You know how far apart the legs of the clip need to move; they spread apart a distance equal to the diameter of the pin. So model the clip only and apply a prescribe displacement necessary to separate the legs the diameter of the pin. The result will be the force required to separate the legs. From knowing that force, you can do a hand calculation to estimate the force required on the pin.