Solidworks Simulation Factor of Safety results

[ajf46]

The plate shown in the attachment has a bearing load of 12500N on the bottom larger hole, the top two holes have fixed hinge geometry.

The min FOS result is 1.34, just at the bearing load point. All around this point there is material where the FOS is over 5.

Would you consider the FOS on this part to be 1.34? or higher because that load is compressive at that point and is supported all around?

It is a 10mm plate and the distance between the 2 smaller holes is 195mm

 

[israelkk]

It depends how are you apply the load. In real life the load is transferred by the pin/bolt inside the hole. The pin always keeps it round shape (except some distortion due to the contact/bearing stresses). In your analysis you may applied the load directly to the lower surface of the hole (how did you know which part of the hole carry the load?).

If the hole distorts and is no longer round, the load still applied evenly while in real life this is not the case. Therefore, to actually know what is happening, you need to use the pin too inside the hole and apply the load to the pin using contact elements between the pin and the hole, thereby resulting in a nonlinear analysis.

Now we come to more interesting questions? How the difference between the actual pin diameter and the hole diameter affect the results. The larger the difference, the larger the contact stress at the contact area between the pin and the hole, etc.

Now you have to decide which failure criterion you consider, bearing, tension, compression, hole dimensional distortion? etc.

More realistic and practical analysis may be to use empirical analysis such as appear in Bruhn book.

No matter which analysis method you use you will still need to test the product under real life testings and see if it correctly functions.

However, be aware the a Finite Element Analysis can become a Finite Elephant Analysis, as some smart person said.

From my long.... experience, to use an FEA analysis you need to have a vast theoretical and practical background and knowledge of material strength and elasticity. Otherwise, garbage in - garbage out.

 

[ajf46]

Those are some interesting things to think about.

I ran a few more scenarios, loading the hole from a rigid (in the case of the study) pin and a contact set. With 0.5mm of slop in the hole the FOS was 0.95, when you bring that down to 0.05mm FOS goes to 1.43. This is with an 8mm plate instead of a 10mm as in the first one.

In my case I am attaching a helicopter cargo hook to the small holes and a machined component to the larger hole. One of these plates going either side of the cargo hook. The cargo hook is rated at 2.5 tonnes and I would like to rate the rest of the components at that as well.

I consider failure to be that something breaks and the load drops. I would want this point to be at 5 times the design load.

For this failure am I correct to disregard the point stresses around the pin? What is a good way of determining this factor of safety from SW FEA if the value given at this point is not relevant?

These components will be destruction tested in the next few weeks.

 

[israelkk]

I believe their should be a standard specifications for helicopter cargo hook load points that you should refer to. When you say that the hook is rated for 2.5 tonnes what does it mean? Does it mean that it will break at 2.5 tonnes? If not, what is the actual allowed breakage load of the hook? The hook too is manufactured to some standard (MIL-STD, etc.). When you say test, how you decide what to test and how to test if you are not working according to some standard specifications? How about fatigue and life cycle of these points? Start with a spec such as MIL-S-8698 STRUCTURAL DESIGN REQUIREMENTS - HELICOPTERS.

 

[Scheffers]

Totally agree with israelkk on this one.
I did some similar research, which showed the effect of a pin in a hole.. see attached pictures.
I am not completely sure what you mean by rigid pin and a contact set...

I would advise on doing some analysis with a solid plate/pin assembly.

 

[Scheffers]

2nd picture.

 

[GregLocock]

If you do a contact stress analysis using Hertz what does it give you as a FoS?

The real issue is will the load distribution alter from your rather arbitrary looking one to something more benign, or will the pin cheesewire through the plate?



Cheers

Greg Locock


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