Tangency problems in Wankel motor model assembly 1

[JamesMcD]

I'm an Industrial Design Technology / Technical Graphics major currently taking my 2nd Solidworks class. I don't usually get stumped all that easy but the project I've chosen for my midterm is giving me problems.

I'm making a model of a wankel motor (like in the RX7) and am having troubles with my assembly. As you may know the wankel motor consists of a triangle-ish rotor whose all three corners are constantly tangent with the interior surface of the Epitrochoid housing. To get my assembly to work, these three corners must be able to slide along this surface as the rotor goes through it's eccentric motion.

The problem is: I can make 2 corners mate tangentially to the housing, thus forcing the 3rd to always be tangent as well, but when this is done, the two corners will no longer slide along the surface. I can mate ONE corner to the housing, and it will slide (like a cam can spin against a rocker arm in SW), but of course the rotor will fall out of the housing unless I have at least 2 corners mated tangent to it.

How can I mate 2 corners tangent to a surface AND allow both those corners to slide along it like a cam?

Any help is greatly appreciated!!

MazdaJim

 

Why don't you try modelling the eccentric shaft as well as giving one rotor apex a mate . I'm not sure why SW doesn't let the rotor slide with two tangencies though.

Could you tell me how you modelled the trochoid housing too, is it geometrically correct or just an approximation? I'd like to try an RX after I finish my V8


Oliver Manhire

engin_eer@hotmail.com

 

[JamesMcD]

Oliver, thanks for the reply.

To be honest I've just been doing some initial model studies, and haven't yet modeled the housing EXACTLY. I anticipated having troubles in this area, so I didn't want to get too far into the model and find that it was impossible.

How to model the housing... That was going to be my 2nd question! I'm guessing I'd have to make the sketch in some other program, and import it into SW.
The ability to make sketch curves based on trig functions is something more CAD packages should feature. (If anyone knows how to do this feel free to chime in!!)

Even if I model the eccentric shaft, the rotor will tilt outside to housing. I want to be able to rotate the shaft and have the rotor spin correctly on the gearset. I don't think this can be accomplished except for having 2 tangencies w/ the housing. With only one tangency there's nothing to keep the rotor oriented properly. (Correct me if you see it differently...)

This may be something that SW can't do. I'm going to try it in Autodesk Inventor...I'll let you know how it goes.

thanks,

James McDonald

 

How about writing an equation relating the degrees of rotation of the rotor on the shaft and the degrees of rotation of the shaft in the housing. Just create an angle mate on both pairs of parts.
That's all I can think of

Ollie

 

[meintsi]

Why not mate your two inner pitch diameters tangent? Since the eccentric motion of the triangular rotor is generated by the motion of its inner circle moving around another circle (the camshaft gear I believe in this case).

Keeping these two circles tangent wiil keep all three outer edges tangent if you have done your modeling correctly.

For your other problem, create you irregular curve shapes by using the insert curve through free points command and browse to where you have your datafile stored for these points. Worked great where I had to show a group of students how to model an airplane wing. Two years ago when we did this, the data had to imported as a tab-delimited file (excel to notepad). I think they corected that flaw now but am not entirely sure.

 

[meintsi]

Should read more carefully....before I answer (half-a**ed).

Keeping gears meshed during mating poses a problem. Have you tried downing one of the free trial offered by SolidWorks support partners. I've recently gotten a few of them but haven't had time to try any of them out except the tolerance stackup program. That work great.

Try the downloads of MotionWorks or Dynamic Motion Designer, they mihgt just solve your problem.

As for the curve, if you can generate the numbers in excel, save as a tab-delimited text file and import curves as free points will generate the spline shape you're looking for.

 

[JamesMcD]

Thanks a lot for the help guys. I havn't tried Motionworks yet, though I've heard of it. I'll look into it. The idea for the curve is great. I didn't know you could do that. Could you maybe go a little more in-depth on what a de-limited text file is? By free points do you just mean coordinates?

thanks
James McDonald

 

[meintsi]

Yes, the free points will be inserted a three-dimensional coordinates generated fron your excel file.

When doing a "save-as" in excel, you have two ways to save it as a text file, space delimited and tab delimited. Try saving an excel spreadsheet in both methods, open them up and play with the data. The differences will be easy to see. Much easier than me trying to explain here.

 

[MichaelPaauwe]

I tried to do this with SW99 but kept running into difficulties.
The trochoid surface of the rotor housing was easy enough.
You generate the trochoid coordinates in an excel spreadsheet and save the sheet as a text file.
You can import this as a curve and then use the curve to do a sweep which becomes the wall of the rotor housing.
Next mate the apex of your rotor coincident to this surface you will be able to mate two apexs ok. And you will be able to move the rotor OK. If you want to drive the rotor from the e-shaft you will hit problems. a concentric mate causes the rotor to be overconstrained.
Another approach is to use an equation to link position of rotor to e-shaft rotation. You can't jog the assembly using move part because the equations are not solved until you rebuild.
I got this far and could not find a way to animate the thing. I can send you the model to give you some ideas if you like.
Mike Paauwe

 

[JamesMcD]

I really appreciate you going through the trouble of trying this out...I would like to see you model too..

you can send it to mcdonald@clas.net

thanks.