Application of Orbiting Pulley center

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Application of Orbiting Pulley center

This application creates a simple and cheap indexer. I would not use it for a high-speed machine unless the components have very low inertia properties and the belt is stiff.

This application also provides an Exact Straight-Line Mechanism.

Dwell Mechanism


STEP 1: Do Orbiting Pulley center

STEP 2: Add a Trace-Point to the Motion-Point.

1.Do Kinematic-element toolbar > Add Trace-Point

2.Click the Motion-PointRed-14-1b

<< Do ALT+C (run menu > Cycle) to see the Motion-Point move along the Trace-Point.

To construct the simple indexer, we need the Trace-Point to make an integer number-of-lobes (flower-petals) around the fixed Pulley.


STEP 2: Add a Gearing FB to increase the number of rotations of the Crank

1.Do Kinematic-FBs toolbar > Add Gearing FB | Click the graphic-area.

2.Edit the Gearing FB | In the dialog-box, enter Gearing Ratio = 4

<<< Now we can see the Trace-Point make more lobes (flower petals) around the fixed Pulley. However, it does not finish where it starts.

We can edit these parameters:

Number of Teeth on the fixed Pulley and the rotating Pulley.

Length of the Belt.

We edit the Pulley to edit the Number-of-Teeth

We can edit the dimension between the two Pulleys to change the length of the belt.

However, that dimension does not directly edit the length of the belt.

But, we can directly edit the length-of-the belt  the Motion-Path dialog-box with the Target Belt Length parameter.

STEP 3: Edit each Pulley to edit their number-of-teeth

1.Double-click the each Pulley to open the Pulley dialog-box (or see How to open a dialog-box)

2.Fixed Pulley dialog-box > Number of Teeth =120

3.Moving Pulley dialog-box > Number of Teeth =60

Note: If Length of the Part on the rotating-Pulley = Distance between the Pulley-centers, the end-Point of the rotating Part will be a straight-line.

(I happen to know these dimensions are near to what I need)

The Pulley dialog also provides as Read-Only:

Pitch-Circle Diameter = (Tooth Pitch × Number-of-Teeth) ÷ π

Tooth Pitch and Belt Length



STEP 4: Edit the Motion-Dimension FB of the Crank, In the Motion Dimension dialog-box | Enter Base-Value = 0

STEP 5: Edit the Length of the Belt.

1.Double-click the Motion-Path FB (or see How to open a dialog-box)

2.Motion-Path dialog-box > Length Control tab > Belt-Tooth Pitch  > Belt Tooth Pitch =5 (mm)

3.Motion Path dialog-box > Length Control tab > Path Length using dimension

In my case, you will see the Actual Path Length = 815.731mm.  It is Read-Only.

4.Click the Controlling Dimension box

The Part-Editor opens to edit the Part in which the belt is defined by the sketch-path.

5.Click the dimension you want to use to change the length Belt

In our case, we will click the dimension that controls the distance between the two Pulleys



The Part-Editor closes

The dimension value NOW shows in the Controlling Dimension box = -176.362mm (- do not worry that the dimension is a negative dimension).

Also, you can now see that the Target Path Length = Actual Path Length

The Maximum and Minimum Dimension Values are Soft-Limits of the Controlling-Dimension.

Edit the Maximum and Minimum Values if you need to edit the Controlling Dimension outside the Soft-Limits

6.Edit the Target Path Length SLOWLY with the Spin-Box tool to increase or decrease the Actual Belt Length.

I want the Actual-Path-Length = 800mm.

Why is this?

Fixed (Moving) Pulley Circumference = Tooth-Pitch (5mm) × Number-of-Teeth 120 (60) = 600mm (300mm)

300mm × 8 = 2400 (the moving pulley) will rotate 8× relative as the Crank rotates 4×

The belt will make 3 x full rotations as it rotates 4-1 times relative to the Fixed Pulley..

The number of lobes (flower petals) = (Number of Crank Rotations -1)

The Motion-Point will move  2400mm÷800 = 3 belt lengths



This will give 3 lobes (flower petals) to the motion of the Motion-Point relative to the Base-Parts



STEP 4: Add a Dyad - RPR (Remember - a Dyad has 2 × Parts + 3 × Joints).

1.Click Kinematic-elements toolbar > Add Part | Drag to add 2 × Parts

2.Click Kinematic-elements toolbar > Add Pin-Joint | Click the start-Point of a Part and a Point at the center of the fixed Pulley.

The Select-Elements dialog opens because there is more than one Point at the center of the fixed Pulley

3.In the list of Points in the Select-Elements dialog: CTRL+CLICK the start-Point of the Part and the Point with the Base-Part as an owner (parent)

4.Click Kinematic-elements toolbar > Add Pin-Joint | Click the start-Point of a Part and a Motion-Point on the Belt.

Note: When the MMA =0, and the Motion-Dimension = 0 , the Motion-Point is at the start-Point of a sketch-element on the belt-path. To make it easier to click the Motion-Point, edit the MMA = ~5


We must add one more Joint: a Slide-Joint between CAD-Lines of the two new Parts.

5.Click Kinematic-elements toolbar > Add Slide-Joint | Click the two CAD-Lines along the centers of each new Part.

The Slide-Joint will make the two CAD-Lines, and Parts, become collinear.

Cycle the mechanism.

You can see the new output Part that has the Pin-Joint with the center of the fixed Pulley moves then dwells 3x as the Crank  rotates 4x

Because the Motion-Point traces three lobes, the output Part is a three stop indexer.