Option 2: Orbiting Pulley Centre


Orbiting Gear Centre: There is one fixed Pulley, joined to the Base-Part.

The belt path is on a rotating Part. The Orbiting Pulleys rotate with a rotating Part around the centre of one fixed Pulley on a rotating Part.

Each Part can be:

DriveR Pulley
The DriveR Pulley is kinematically-defined with a motion before you use Add Pulley.
The DriveR Pulley rotates to move the Belt.

Linear Motion of Belt = Drive Pulley Radius x Drive Pulley Angular Motion

DriveN Pulley
The DriveN Pulley becomes kinematically-defined by Add Pulley.
The Belt moves to rotate the DriveN Pulley.

Angular Motion of Pulley = Linear Motion of Belt / Pulley Radius.



STEP 1: Edit the Base-Part
STEP 2: Add a Line
STEP 3: Make it Horizontal
STEP 4: Close the Part-Editor

Step 4 is complete


STEP 5: Add a Part

This Part will be the rotating frame for the Orbiting Pulley.


STEP 6: Edit the new Part
STEP 7: Add an two Arcs and two Lines to give the Path of the Belt
STEP 8: Add a Line, drag from the Centre of the Arc near the Part Axes
STEP 9: Add a dimension to each Arc3s

Step 9 is complete

Note that the centre of the Arc is not at the Origin of the Part.

A Drive Pulley has one important rule -

'You must merge the centre-Point of the Arc and the start-Point of the Line'

However, a rule that you might not know is:

'You cannot merge the centre-Point of the Arc with the start-Point of the CAD-Line along the centre of the Part'.

This means you cannot use the CAD-Line for the centre-Point of the Arc.

This is why we added a new Line to merge the centre-Point of the Arc with it.  This is the only reason.

STEP 10: Close the Part-Editor


STEP 11: Add a Pin-Joint1s-red between the Line in the Part and the Line in the Base-Part

Note: the Pin-Joint does not use the Point at the Origin of the CAD-Line.



STEP 12: Add a Motion-Dimension FB and Linear Motion FB

To add the Motion-Dimension FB, click the Pin-Joint, the Line in the Base-Part and the Line in the Part


STEP 13: Do Kinematic FB toolbar > Add Motion-Path FB2s

Select the sketch-loop around the Crank.


STEP 14: Tidy up the Line in the Part!

Now, we can :

make the start-Point of the Line coincident with the Origin of the CAD-Line - add coincident constraints between the point and the X-axis and Y-axis of the CAD-Line
make the end-Point of the Line coincident with the ray of the CAD Line

tog_minusUse Add Pulley


Click  in the Kinematic-element toolbar > Add Pulley


Add Pulley opens in the Command-Manager.

Refer to the images above to identify the four Elements you must select to add a Pulley. From top to bottom:

1.Click the Pin-Joint1s-red
2.Click the Motion-Path FB2s

The Part-Editor opens. It shows the sketch-loop for the which becomes the Belt

3.Click the Dimension3s of the Arc sketch-element

The Part-Editor closes

4.Click the Base-Part4s.
Select the Base-Part because the Pulley is fixed to the Base-Part.

MechDesigner adds the Pulley. It is Cyan colour. This identifies it is a 'Drive' pulley.


You must use 'Add Pulley' for each shaft centre where you want to add a Pulley.

STEP 1: Add a Part with a Pin-Joint to the centre-Point of the Arc at the other end of the sketch-loop.

Do not add a Motion-Dimension to the Part - leave it as a Free-Part with a Blue Part-Outline.

STEP 2: Do Add Pulley again

This time the Pulley has a Magenta colour. This means it is a 'DriveN' pulley.

Tutorial and Reference Help Files for MechDesigner and MotionDesigner 13.2 + © Machine, Mechanism, Motion and Cam Design Software by PSMotion Ltd