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Archive for July, 2009

I’ve never really been a car enthusiast. I mean, I like race cars and I sure enjoy the looks of a few models, like this particular one that was parked at some sort of car expo near the amusement park a few weeks ago. Isn’t it a real beauty? I actually like this one better than its younger cousins that were also on display that day.

 Anyway, I like to look at cars and I like to drive them, but I’m certainly not the kind that can remember makes and models, let alone identify all the different pieces that form part of a vehicle.  Last night, however, and out of sheer curiosity after reading a post from the SolidWorks Discussion Forums where someone was asking for examples on how to use the universal joint mate and someone else suggested looking at a drive shaft, I found myself searching the internet, trying to find pictures and information about cars and transmissions and what not. I eventually found some really nice assembly pictures for a 280Z, and my husband,  who  by then had grown curious about my sudden interest for cars, was more than glad to explain to me how each of those parts was supposed to work.   I never thought I would say this, but it was actually very interesting and even kind of fun.  Or maybe he’s just a good teacher, who knows?

This is one of the images I found for the drive shaft.

After taking a long look at it, I went back to SolidWorks to try to find some information on how to use the universal joint mate, but I couldn’t find much in the help. I guess it’s because it’s not really such a complicated mate?  So, I put together my own universal joint assembly to experiment.  The universal joint, also known as Hooke’s coupling, is used to connect two intersecting shafts, and apparently has its widest use in the automotive industry.  A simple model of the Hooke-type universal joint is shown in the following image. The small shaft could be the driver, the long one could be the follower and the third link is a cross piece that connects the two yokes. 

 As the driver rotates, it transfers this rotation to the second shaft.  In SolidWorks, the universal joint mate is useful for those cases when you need to transfer rotational motion around corners or, like in the case of the driver shaft of a car, between two connected shafts that are allowed to bend at the connection point.  

I actually came up with two versions of the same assembly, one with the third link connecting the two shafts, and one where that link is missing. The one without the link uses the universal joint mate, while the other one relies on the link to transfer rotation between the shafts. The results of using one method or the other were pretty much the same.

So, to add a universal joint mate between two components, simply  select  it from the mechanical mates group, and then, under Mate Selections,  select the two components you wish to mate together (in my case the two yokes)  and, as an option, define the joint point.  The joint point (in purple in the image) represents the connection point between the two components. In this case is the point where the axis of one shaft intersects the axis of the other.  Since I didn’t have a physical component connecting both shafts, I actually sketched a point to serve as joint point and located it where the center of the missing link would be.  Just as in the case of other mechanical mates, such as gear mate and rack and pinion mate, this mate will work even when the components don’t actually touch each other or have any other component to come in touch with both and connect them, but you’ll need to add other mates to the mix in order to control the position of the elements in the screen and with respect to each other. In my case I mated the axis of each shaft coincident with a couple of reference axes that intersect each other at the joint point.

I added a rotary motor to the driver shaft and then ran the motion study.  I’m sorry I didn’t actually make a video of this one, but it’s only ten seconds worth of animation, so I didn’t see the reason for it. This is an animated gif; if you double click on it you’ll be able to see the movement of the universal joint.

If you are one of the few that visits at least every week and you are wondering where I’ve been, well, let me just remind you that it’s summertime here in California, and right now I’m home with a sick dog and eight kids. No, I’m not Octomom.  I only gave birth to two of them, but the other six are friends and neighbors that come to play or get “casually” dropped off by their mothers when they need to go shopping or visit the doctor. It never fails! Almost every day I’ll hear the doorbell ring and a boy will be standing there… “Hello, Mrs. Ethan’s mom, um, my mom has to go shopping/to the doctor/to the dentist/to visit a friend, so she said I could stay here with you.”  And how can I say no when the mom is already driving away like a madwoman in a high speed chase?  Sigh…  In all fairness, sometimes my boys go to visit them, but I never drive away (too fast), just in case they want to come back.

Anyway, I’ve also been spending some time exploring SolidWorks 2010 Beta,  preparing for my presentation at my local user group (although I just learned it’s been changed for September), getting ready for a couple of trips (travelling with dogs is complicated), and answering old and new email that has been accumulating.  One of those emails inspired this post.

A reader had a problem trying to create something like a spring. I know, you can make that easily with a helix and/or a sweep, right? Yes, but the tricky part was that he needed the ends of the spring to bend towards the axis of the spring.  I wasn’t quite sure at first what he wanted to do, so I referred him to a couple of tutorials from Matt Lombard’s SolidWorks 2007 Bible that I remembered going through a while ago. In one of them a spring with a little hook attached to one of its ends is modeled using projected and composite curves. The other one is a helical sweep where the profile appears to grow larger as it travels along the helix. Just as I was getting proud of myself for saving those old files “just in case”, he wrote me back saying the tutorials were good, but he still couldn’t find the way to do what he needed to do. He sent me a picture (that I won’t publish here) and the practice part he was working on, and my first thought was to start fixing the sketches for the projected curve he needed, but after taking a closer look to those pictures, it occurred to me that what he was really trying to do could actually be accomplished in a much simpler way, just by taking better advantage of what splines and 3DSketches have to offer.

See, you start with a circle centered on the origin and a simple helix, not tapered and constant pitch. I only needed one revolution or so for this one, but that’s not really important.

Next, you are going to open a 3DSketch, select the helix and convert it into the 3DSketch by using Convert Entities. See, unlike what happens when you convert a helix into a sketch, where you end up with a projection of the helix on the sketch plane, when you convert a helix into a 3Dsketch, you end up with a sketch entity that is just like the helix you had before, all 3D and no flat, it’s actually a spline, and you can drag the ends and add other entities and relations inside the same 3DSketch, as well.

My reader needed the ends of the helix to go back to the center.  Imagine a wire that runs vertically along the axis of the helix, then bends around to form the spring and comes back to the center to continue running vertically along the axis. That’s what he needed.  In order to do that, and still inside the same 3DSketch, change to a Front view and sketch a vertical line along the Y axis. You should see the relation AlongY added to this line, if you don’t have it, add it through Display/Delete Relations, Add Relation.  Also add a coincident relation between this line and the origin.

Still inside the same 3DSketch, change to an isometric view, or at least rotate the view a bit so you can see better what’s going on, and sketch a two point spline joining one of the ends of the helix with one of the vertical lines. At first, the spline will look like it’s just a line, but you can make it look curvy by dragging the handles.  Select the spline and the helix and add a relation (Display/Delete Relations, Add Relations). This relation can be tangent or equal curvature, whichever works better for your purposes.

Now, in order to smooth out the transition from the spline to the vertical line, select both the spline and the vertical line and use Tools, Spline Tools, Fit spline, to fit both entities into one single spline.  Make sure the option to close the spline is unchecked.

Notice that you can still adjust the transition even more by dragging the spline handle, like in this image.

Then you simply use this 3DSketch as the path for the sweep and that’s it. No need for complicated ways for this one, just to remember what can be done with the tools we have.

I had mentioned in a previous post that I was working on a motion study using a model of a pair of friction grabs. Here I have a short video of that motion study, demonstrating the use of friction when defining 3D contacts for motion analysis.  

When defining 3D contacts for motion analysis in SolidWorks Motion, we have the option of acknowledging there’s friction between the components or not. Sometimes, no friction works fine for the kind of model we are working with, but there are other times when our simulation won’t work at all unless we add friction to the 3D contacts between the components.  My friction grabs, as you will see in the video, are one of those models that need the friction in order to work.

Dry friction, also known as Coulomb friction force is calculated based on a couple of coefficients:  the static coefficient and the kinematic coefficient.

The static coefficient is a constant related to the force necessary to overcome friction when the body is at rest, as expressed in this formula.

Fm = µs N

Where Fm is the maximum value of the static friction force, µs is the static coefficient and N is the normal component of the reaction force acting on the body.

The kinematic coefficient is a constant related to the kinetic friction force, which is the one acting on the body once it’s already in movement and its magnitude is usually much smaller than that of the static friction force. The magnitude of this force remains pretty much constant as the body’s velocity increases.

Fk = µk N

Where Fk is the kinematic friction force and µk is the kinematic coefficient.

Both coefficients usually depend on the nature of the surfaces that are in contact, but not really on their shape or size. Some values have been obtained experimentally and organized in tables. To use these coefficients all we’d need to do would be to determine what kind of materials would be in contact and find the coefficients for that particular combination from the tables. SolidWorks simulation has a small collection of materials for contacts available. When we choose materials from this list for our 3D contacts and mark the option to use friction in the 3D contact property manager, the coefficients for that particular combination are already determined for us. However, we can always clear out the option of using materials from the list and enter our own values for the coefficients, if we know them from some table or from experimentation.

In this little video of the simulation that I put together, I run a simulation first without friction and then with friction added to the 3D contacts between the grabs and the block.  Notice that, in my model, the tongs and block are made out of steel; however, since I choose rubber (dry) for the material in my contacts, it is the coefficients for this kind of material that will be used in calculating friction forces and not the coefficients that would apply for a combination of steel on steel.

Enjoy the video! And in case you have trouble seeing it, it will also be availabe at Viddler.com, just follow the link: http://www.viddler.com/explore/lainge96/videos/20/

Let me start by making it clear that this is somehow off-topic, and by that I mean that it’s not about how to do something in SolidWorks, or how to work around a limitation or known issue.  Yes, I know, how dreadfully disappointing! But, you see, every now and then I like to claim this blog as my personal space for reflection and, well, rambling too.  So, anyway, that little person that is always giving me burns at Novedge no matter what I write about (yes, now I know who you are) can run like a mad soul and give me a hundred burns right this instant, for all I care.

So, let’s see…  I think only one or two people knew about it, but truth is in the last couple of weeks I came really close to giving up and even shutting down this blog. I’m serious.  It wasn’t out of whim, either.  I think it was a combination of different factors, mainly the frustration that built up over quite some time. I guess you can say I was burnt out; somewhere down the road, what started as so much fun somehow didn’t feel as much now, and all I could think about was how little I had accomplished and how far I still am from reaching that goal I had set up myself from the beginning, to resurrect my engineering career.  

I felt overwhelmed whenever I thought about it, so I tried not to, but you know how these things have the nasty habit of coming back to haunt you when you less need them, like in the middle of the night, when you’re trying to get some sleep, or precisely at those moments when you’re trying to have fun and enjoy yourself. From wherever I tried to see it, it just seemed like it was taking way too long to be even reasonable. You know, often people tell you if you don’t see results within a certain time, then you’re probably wasting your time, so you’re supposed to have something to show for all your trouble and fast. Then, on the other hand, it just couldn’t go any faster than this.

I began doubting if this was even a goal I was supposed to have in the first place.  I am a mother and that comes first for me, because I want to be the kind of mother that is really significant in the life of her children, not the one that is never there, that simply lets other people raise them (even if that other people is a grandma or an aunt) and influence their character. And no, this is not to add to the debate of the working mother versus the stay at home mother, believe me, because at some point I want to be a working mother myself, just not a working-all-the-time mother, but it appears almost impossible to achieve that balance  when it seems to me, from what I’ve observed in my male peers, that putting long hours and sacrificing family time is almost expected in this kind of profession, otherwise you’re not seen as dedicated enough. And I also know that some mothers really have no other choice, so this is not to condemn anyone. I guess it’s simply the way I was raised, and I know many other women were raised the same way. Let’s face it, we are still the nurturing caregivers, and most of us will at some point have to make a pause in our career or at least slow down enough to raise our children or care for our ailing parents.  I know there are some stay at home dads, but they are few. And while it’s true that nowadays fathers are more involved in their children’s lives than they were when I was a kid myself, for most families it’s still the mother who carries the bigger load in making a home and caring for the kids, whether she works or not. I’m not saying this is fair or unfair. It’s just part of life.

We all do the best we can, but somehow it seemed to me that my best wasn’t going to be good enough for me to accomplish my dream, not now, not ever, so what was the point of even having that dream? I mean, was there any point in doing all this? Was all my effort really going to take me somewhere or had it just become some sort of hobby?  For me, it seemed that I had a lot going on against me and nothing going on for me: on one hand, not being able ( or not willing) to put in as many hours as a man would do in a job limited my options tremendously, and on the other, the lack of experience due to having been at home and apart from anything related to engineering for so many years made me still ineligible for many jobs out there where a more experienced individual was often required, no matter how much I could learn from books or how many classes I took.  The current situation of the economy doesn’t seem to help much, either, but that almost sounds like an excuse, because I know that truly valuable professionals don’t stay out of a job for long.

At some point, it appeared to me that the only way I could ever succeed at this attempt was if I gave up my parenting goals for the career goals, and I wasn’t about to do that. Still, a woman needs a way to provide for herself and some financial independence, as well.  As I read somewhere, even when happily married, a man is definitely never a plan. So, I began considering changing gears and going for a more “mother friendly” career… Trouble is I couldn’t think of anything else I would really like to do.  That is a problem because if you don’t love what you do, then you’re likely to do an awful job and resent it too. More sleepless nights, more stress. I hardly could bring myself to write much in this blog, because it only reminded me of how impossible my situation seemed…

But right when I was about to give up (or at least run around screaming out of frustration and messing my hair like the crazy woman I’ve been told I am) is when I mustered the courage to seek for advice, this time not from a man like I’d done before, but from a real woman engineer and mother that’s been there and done that.  Wow! What a difference! I’m glad I did because, unlike the men who often measure the worth of your efforts by how fast and how much you can deliver, she understood exactly where I was coming from and what’s important for me.  It was liberating to say the least!  She made me realize that life is not a race, but a journey, and that as long as I have a goal and work hard to achieve it, it doesn’t really matter how long it takes to get there. I will get there in my own time. That even if I have to knock on many doors, one will eventually open for sure.  That it doesn’t matter what everybody else may think about me or about my goal, my priorities in life or my efforts.  It doesn’t matter because they don’t know me and because it is my own, personal journey and they don’t even know where I’m headed to, but I do. That you can really be the mother you want to be for your kids and still give yourself a chance to achieve your dream; it may just take a little longer, but it’s perfectly fine. All you need is a plan, hard work, and some faith. The best of all is, for the first time in many, many years I felt it was OK to be all that I am: a woman, a mother, an engineer.  Probably for the first time in my life I realized that one didn’t have to take from the other or negate the other like some men seem to imply, that I didn’t have to choose, that I didn’t have to apologize to anyone for being all woman, for not thinking or behaving like my male peers or vice versa, for pursuing this kind of career instead of a more “lady like” profession, as my grandmother used to say, that I don’t need to prove anything or keep up with anyone, that it’s OK to be just where I am, and there’s nothing wrong with that. Hey, nobody is ever born knowing it all. It’s a process and there’s nothing to be embarrassed about if you’re not an “expert” in such and such. A lifetime is a long time to spend full of conflict over who you are and what you want, believe me, so for me this is an incredible feeling, to finally let go of that conflict.  It’s thrilling and invigorating, like a giant wave crashing on the shore, taking it all away to start anew.

I’m really thankful that there are still people out there willing to mentor others and provide some much needed advice to those like me.  I’m not giving up anymore, but you know, I’ll get there in my own time. Of course, I will continue to write in this blog, but also don’t expect to see a blogging frenzy. I write about my own experiences and what I learn as I go. I feel fine whenever what I’ve written helps others in their own efforts to learn, but I’m no expert, and that’s OK too.

Thank you very much for your patience, if you’ve made it this far. I promise next post will be about SolidWorks.  By the way, if you are in the area on August 5^th, consider attending the meeting of the Tri-Valley SolidWorks user group. If anything goes according to plan, I will be presenting about animation and/or rendering at that meeting, and you’re of course invited to come watch and ask questions, as long as you promise to go easy on me. This presentation is intended for beginners and is my first presentation at a user group and my first time speaking in public since 1996, so you bet I’ll be a bit nervous.

Hope to see you there!