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Attaching and fixing object to internal gear

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I use Autodesk Inventor 2016 (I'm trying to draw a Wankel engine)

I have a (blue) large internal gear that revolves around a stationary (green) small normal gear. It works fine and the teeth mesh.

I also created a (red) square block with a hole in it that has the same diameter as the outer diameter of the (blue)internal gear. I used the "insert constraint" to attach the (red) square block to the (blue) internal gear. Now the square block and the internal gear revolve around the small gear.

There is only 1 problem : the internal gear rotates within the square block.

North, south, west and east of the square block never changes...

I would like the square block to turn with the internal gear. The square block should be fixed on the internal gear and move exactly the way the internal gear moves...


I tried using "mate constraint" between the internal gear and the square block, but I always get an error message "The assembly cannot be solved"

I tried an "angle constraint" between the side of the square block and a side of one of the teeth of the internal gear which also results in an error.

I tried nearly everything I could think of.


The principle I want to create is that of a Wankel engine.


The specs of the internal and small gear are :

small gear

gear ratio : 1,5 Module : 2,75 Center Distance : 17,5 #teeth : 26 ul


internal gear

gear ratio : 1,5 Module : 2,75 Center Distance : 17,5 #teeth : 39 ul


I used a "mate constraint" between :

z-axis of small gear and z-axis of assembly

centerpoint of small gear and centerpoint of assembly

YZPlane of small gear and YZplane of assembly


Is there anyone who has an idea to fix this problem ?

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I arrived at the answer myself.

The problem was a chronological one apparently.

Should you be interested, follow these steps exactly :


nota : + the gear settings are specified in the question

+ learn to draw gears first, by watching this video :

+ Gear1 is the small gear

Gear2 is the larger internal gear


1. Bring small gear1 into assembly(unground it!) and "mate constraint" z-axis of

assembly(World) and z-axis of Gear1

2."mate constraint" Center Point of assembly and Center Point of Gear1

3."mate constraint" face of Gear1 and XY Plane of assembly (if you get an error,

try the back of the Gear1, just flip the gear around)

4. Bring in Gear2(internal) "mate constraint" z-axis of Gear1 and z-axis of Gear2

5. Create offset -> rightclick last mate(the one in point 4) choose Edit and enter

"center distance" (17,5) click ok

6.Make sure gears are on same plane by "mate constraint" click face of Gear1 and

Gear2 and choose "Flush" ok

7."Motion Constraint" fill in 1,5 and click between the teeth of both gears

8. Don't forget to ground Gear1



9.Bring in Rotor(make it first, diameter hole in rotor = 124,784),

"Constraint Insert" click on inside border of the hole(of rotor) and outside border

of Gear2, click ok

10. "mate constraint" YZ Plane of Gear2 and YZ Plane of the Rotor



11. Bring in the Shaft, "Mate Constraint" Z-axis of Gear 1 and Z-axis of the shaft

12. "Constraint Insert" between round edge of shaft and round edge of rotor



Instead of drawing circles around a bigger circle like others have done, which is a valid approach none the less, use the "Equation Curve" tool(SketchTab in ribbon-Line(click dropdawn arrow here)-Equation Curve), which is a lot faster.


Sadly the "Equation Curve" has some errors in it

If you make a Wankel 8 chamber, the axis points on the positive Y axis and the negative X axis are not correct !!

The points on the negative Y axis and the positive X axis however, are correct.


To remedy this :

Only take the correct points and mirror the whole thing twice.

This is what you enter into the equation :


17,5*cos(3*t) + 130,621778264911*cos(t)

17,5*sin(3*t) + 130,621778264911*sin(t)

(t)min = 270

(t)max = 360


You will get 1/4 of a Wankel 8 chamber

You mirror this to the left and then you mirror to the top

You will now have a WankelChamber with the correct axis points.


As many of you will know, the mathematical equation of the Wankel 8 chamber is :


X = E cos3B + R cosB

Y = E sin3B + R sinB


E = the eccentric throw of the rotor

R = the distance from the centre to the tip of the rotor

B = the angle of rotation of the rotor in degrees


Use Excel to draw a chart with this information and you will find the correct axis points (X, -X, Y, -Y), should you have the inclination to further inquire into this matter...

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Interesting reading, welcome to the forum Fred.

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Step 10 seems to not be working😕 Gears' teeth can not act together anymore... Do you have any solution ?

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