Application of Orbiting Pulley center

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Application of Orbiting Pulley center

This application creates a simple and cheap indexer. However, the output has motion-discontinuities.  Therefore, I would not use it for a high-speed machine unless the load inertia is very low.

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

3.Run menu > Cycle)

The the Motion-Point move along the Trace-Point.

To construct a 3-Stop Indexer, the Trace-Point must trace exactly three Lobes (flower-petals) around the fixed Pulley.


STEP 3: 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.Delete the wire between the Linear-Motion FB and the Motion-Dimension FB

3.Connect a wire between the Linear-Motion FB and the Gearing FB, and also between the Gearing FB and the Motion-Dimension FB

4.Edit the Gearing FB,

5.Enter Gearing Ratio = 4

<<< The Trace-Point traces more lobes. However, the Trace-Point does not finish where it starts.

Edit these elements to edit these parameters:

Pulley : Number of Teeth

Motion-Path FB : Length of the Belt, Tooth-Pitch

STEP 4: Edit each Pulley

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

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

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

Pulley dialog-box also has (Read-Only):

(Read-Only ) - Pitch-Circle Diameter ( = (Tooth Pitch × Number-of-Teeth) / π )

(Read-Only ) - Tooth Pitch (Edit Belt-Pitch in Motion-Path dialog-box)

(Read-Only ) - Belt Length (Edit Target Path Length in Motion-Path dialog-box)



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 the image, you can 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 = -168.176mm (- do not worry that the dimension is negative).

STEP 7: Edit the Target Path Length SLOWLY with the Spin-Box tool to increase or decrease the Actual Belt Length.

You should see that Target Path Length = Actual Path Length

If this is not the case, then edit the Minimum and/or Maximum Dimension Values

One of these dimensions may be constraining the limits. You may even need to increase the Minimum or decrease the Maximum Dimension Value.

MD16 - the parameters are now Control-Dimension Min / Max)

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

Ignore below

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

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

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 8: 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.