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I wanted to write this before the fact and publish it Saturday evening or Sunday morning. At least, that was the plan, but of course thanks to Murphy things don’t usually go according to plan, so…

Anyway, by the time you read this I’ll be somewhere in Colorado… maybe further along on the way to Kansas, who knows…  In case you haven’t figured it out just yet, I’m taking some time off and going with my family on a road trip to visit relatives in Ohio. Why by road? Well, because we’re glutton for punishment, because we used a big chunk of our airline miles for previous trips, and because, believe it or not, we really do love to drive and sightsee.  Oh, and did I mention that we’re bringing our dog along?

I brought my laptop along to try and do some writing in my quiet time and check email every now and then, but I won’t really be posting. Sorry!

See you back in a couple of weeks! Enjoy your summer or what’s left of it!

It’s past midnight here in Lexington, Massachusetts, and I guess I should be in bed, but after the great time I had today, I really felt like blogging, even if just for a bit.

I’m here as one of DS SolidWorks’ guests.  They were so kind to invite yours truly and a few other bloggers to visit their offices in Concord  for a couple of days, chat with several of the key people that work here, and take a peek at what’s in store for 2010… plus a few wild guesses at what the future may hold.  This is not really a press event. It’s just for us bloggers, a lot more informal and more enjoyable too.

Because we are all under an NDA, I can’t really tell you (just yet) about all that was discussed here today, but the right time will come very soon, and you will probably agree with me that it’s all really exciting stuff.

For now, I’ll leave you with a couple of pictures of the blogger team (I’m not there because I’m taking the picture)…

And one more of all of us at F1 Boston, the racing track.  That was really cool, scary, but cool!  We all got three races of ten laps each. I came in last place and continuously slammed my kart against the walls when trying to make the curves, but it was fun.  Of course, now I’m discovering all these new bruises I didn’t have before. Hmmm… 

I’m really glad to be here, very thankful to SolidWorks for inviting me this time and to Nancy Buchino for the amazing job she does in putting this together and making sure we all have everything we need to enjoy a great time… even a pair of shoes. Thanks Nancy! You are the best!

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/