﻿ Getting Started Tutorials - MechDesigner > Tutorial 17: Belts and Pulleys > Option 2: Orbiting Pulley Centre > Application of Orbiting Pulley Centre

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:

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

 1.Do Kinematic-element toolbar > Add Trace-Point 2.Click the Motion-Point 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

(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.

Select the CONTROLLING DIMENSION

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.