﻿ Option 2: Orbiting Pulley Centres

# Option 2: Orbiting Pulley Centres

### Orbiting Pulley centres

Orbiting Pulley Centres

There is one fixed Pulley, joined to the Base-Part.

The belt path is on a rotating Part. The Orbiting Pulleys rotate with 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.

#### Preparation

 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 Arc

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-Joint 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

Use the Pin-Joint, the Line in the Base-Part and the Line in the Part for the

 STEP 13: Add a Motion-Path FB

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

#### Use Add Pulley

Click Add Pulley in the Function-Block Toolbar

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-Joint
 2 Click the Motion-Path FB

The Part-Editor opens. It shows the sketch-loop to which you have added the Motion-Path FB

 3 Click the Dimension of the Arc sketch-element

The Part-Editor closes

 4 Click the Base-Part. 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: Add Pulley again

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

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