Most of you have no idea and perhaps don’t even care about the fact that I adopted a little kitten about a month ago. What can I say? If you are a smart person, unlike moi, you’ll avoid visiting the pet store while the local cat rescue is showing off their adoptable cats. But I admit I would’ve probably ended up adopting the kitten anyway, eventually…

I named him Troubles because it suits his personality. He’s always in the mood for mischief and looking for ways to get into all sorts of places.  Unfortunately for me, one of his favorite places to explore is inside my kitchen cupboards, where I keep the aluminum foil, the sugary cereal, and other goodies. Up until a couple of days ago, I used to think I had the situation under control thanks to the leftovers of the childproof latches I had installed on those cupboard doors to keep my own kids out of them. That’s when I contemplated in horror how the cat managed to push the latch down and swing the cupboard door open.  Wait a minute?  I thought those things were supposed to be hard to open even for a small child! Not that it requires a lot of effort, but, I mean, how strong is a cat, anyway?

Motivated by this question, I decided to make a simple model of a childproof latch and use SolidWorks Simulation to estimate the force that is required in order to push the latch down and open the cupboard door.  First of all, the kind of latch I’m talking about is a simple vinyl one, such as the one in this picture.

The long narrow piece goes attached to the inside top corner of the cupboard door and there’s a small piece that goes secured to the frame of the cupboard, and that will serve as a stop for the latch. When the child attempts to open the door, the latch will get trapped by the other piece, allowing the door to open only partially, unless the latch is pushed down enough for its tip to pass underneath the other piece.  I’m not so good at explaining this, but I’m sure most everyone has seen one of these before.

So this is what I did… I made a very simple model of the latch, as you see here. My model included some filleted edges, but they are not really necessary or useful for this analysis, as you will see in a bit, so I decided to suppress the fillets and run an analysis without them.  Doing this usually makes the calculations easier and faster, and the results aren’t affected, unless, of course, there’s a concentration of stress in the corners and you are interested in knowing   the stresses precisely in the filleted areas.

Next thing I needed to do was create a new Simulation study using this configuration without fillets, and establish some boundary conditions.  I applied a fixed geometry fixture to the back of the rectangular plate, to simulate how it would be securely attached to the cupboard door, unable to rotate, slide or move in any direction. This is done simply by right clicking on Fixtures and selecting Fixed Geometry from the menu.

I applied a second fixture to this study. This fixture makes the study slightly unusual, because what I was used to do was to apply some boundary conditions (usually some fixed geometry) and then a force and that’s it, let SolidWorks calculate stresses, displacements, etc. due to that force. In this case, however, I’m trying to find the magnitude of a force that will generate a certain known displacement, and this second fixture is going to help me in that task.

I knew I needed the very tip of the latch to displace some 5 mm down, so I used an advanced fixture to specify this translation.  If you right click on Fixtures and select Advanced Fixture, you’ll open a property manager where you’ll be able to choose from several different advanced fixtures available. In this case, I used Use Reference Geometry.  At first, I made the mistake of thinking that what I wanted was for the that small rectangular face on the tip of the latch (shown in pink) to displace down 5 mm along the vertical face adjacent to it (shown in green), and so I used those two faces to define the fixture, as you can see in the image. 

This, however, was a mistake because, after meshing the model and running the simulation, it produced the following result.  Notice something funny about this image? Look closely. If you were paying attention, you probably noticed that both faces remain parallel to their original positions throughout the deformation process, which is not the way you expect the latch would deform when pushed down. You can see it clearly in the image, as the original model has been superimposed on the deformed one.

So, I tried again, only this time I used different entities to define the fixture. Instead of a face, I used an edge on the tip of the latch.  I specified that I needed that edge to translate 5 mm down in a direction normal to the Top plane, as you can see in the following image. 

Well, that seemed to do the trick! After meshing the model and running the simulation, I obtained results that were more like what I was expecting.

By the way, in case I haven’t mentioned it before, I don’t have Simulation Premium, I was running this analysis in SolidWorks Simulation, but even though SolidWorks Simulation is usually limited to the small displacement kind of analysis (linear analysis), where the deformation of the model is so small it really can’t be noticed by the naked eye, it is also possible to solve some large displacement, non-linear problems, as well,  and obtain some accurate results, provided that there is no permanent deformation.  This one is a large displacement kind of problem, since 5 mm is an extremely noticeable deformation, however, this deformation doesn’t appear to be permanent, since the maximum stress is way below the yield point for this material.  To run an analysis making use of the large displacements option, simply right click the analysis name on the tree, select Properties, Options, and check the option Large Displacement, as you see in this image.  However, if you don’t select this option yourself and, while running the simulation, SolidWorks Simulation detects that this is a problem where large displacements are involved, it will give you a warning about it and ask you about running the simulation using this option. Don’t ignore the warning, since it can lead to incorrect results.

Once the stress distribution was calculated, I was able to estimate the force necessary to push the latch down 5 mm by right clicking on the Results folder and selecting List Result Force from the menu. I selected the rectangular face of the tip (in green), clicked Update, and found that the magnitude of the force should be approximately 5.5 lbs, applied normal to this face. 

I checked these findings by running an analysis the “typical” way, applying a force of 6 lbs normal to that same face, and the displacements plot showed the kind of large displacements I was expecting, once again with a maximum stress way below the yield point.  One thing to notice here is that if you look at the stress distribution plot for this problem I just talked to you about, you’ll see that the magnitude of the stress appears to be higher on the particular edge that was used to define the second fixture, when compared to the stress on rest of the latch’s tip, that is. This, I think is a consequence of applying the fixture using the edge, and not necessarily relevant, but I could be wrong.