Fixed Centre Pulleys

This is a typical Multi-Pulley system. Drive one Pulley with a motion, and let MechDesigner find the angular motion of other Pulleys.



STEP 1: Edit the Base-Part
STEP 2: Add a closed sketch-chain (sketch-loop) to represent the Path of the Belt

We must define which Arc we will use to add the Drive-Pulley. The rotation of the Drive-Pulley is given kinematically-defined Part.

We will add Line, so that we can add a Rocker in the Mechanism-Editor.

STEP 3: Add a Line 1s-red so that start-Point is merged with centre-point of an Arc.

The start-Point of the Line MUST be merged with the centre-Point of the Arc.
[It is easier to Add the Arc, then Hover+Drag the Line from the centre-Point of the Arc].

STEP 4: You MUST dimension each Arc

The exact radius of the Arc is not important.

MechDesigner uses the dimension to add a Pulley with an integer number-of-teeth that is near to the radius. (You can edit the number-of-teeth later with the Edit Pulley dialog).

STEP 5: Close the Part-Editor


STEP 6: Add a Motion-Path FB2s

Select the sketch-loop that represents the path of the belt.

The Motion-Point shows at the start-Point of the sketch-element you select in the sketch-loop.


STEP 7: Add a Part for each Pulley.

Add the Parts near to the centre-Point of each Arc
Mouse-button-down and drag from near to the centre-Point of each Arc.


STEP 8: Add a Pin-Joint between the centre-Point of each Arc and the start-Point of each Part

There should be one rotating Part (that is, a Part with a Pin-Joint) for each Pulley you will add.

Usually, one rotating Part at the centre-Point of each Arc.

The length of each Part is not important.


STEP 9: Add a Motion-Dimension FB to the Part you wish to be the Drive Pulley.

There should be one kinematically-defined Part to show which rotating Part is the DriveR Pulley.

Use Add Pulley


You must use Add Pulley for each Pulley.

Click Add Pulley in the:

Add menu > Mechanism sub-menu > Add Pulley

- or -

Kinematic-elements toolbar


The Add Pulley dialog starts in the Command-Manager.

Add a Pulley to the kinematically-defined Part and Arc first.

Click, in the graphic-area, from top to bottom.

1.The Pin-Joint
2.The Motion-Path FB

The Part-Editor opens with sketch-chain to which you add the Motion-Path FB

3.The Dimension of the Arc sketch-element

The Part-Editor closes

4.The Part joined with a Pin-Joint at the centre of the Arc.

MechDesigner adds the Pulley.

Do Add Pulley again to the four Rotating Parts.



Drive Pulleys are Cyan
Driven Pulley are Magenta
The radius of each Arc changes to give an integer Number-of-Teeth.
Edit the Number-of-Teeth with the Edit Pulley dialog.

Use Edit Pulley dialog box


1.Open the Pulley dialog-box

Double-click a Pulley in the:


- or -


- or -



In the Pulley dialog-box

2.Edit the Number-of-Teeth

Repeat 1 and 2 for each Pulley.


3.Edit the Motion-Dimension FB to 0º.

When the DriveR-Pulley is at 0º, the other Pulleys will also be at 0º.

Edit the Length of the Belt

Belts have an integer number-of-teeth. Also, belt manufacturers have preferred lengths of belts.

To make the Motion-Path give an integer number-of-teeth and a preferred belt length, you can use the:

'Adjust Path Length' tool in the Motion-Path FB dialog-box

The Adjust Belt Length tool in MechDesigner moves a pulley, or another dimension, to make the length of the belt equal to a target belt length.


Frequently, you will include a belt tensioning pulley in the layout of the belt design. The centre of the Belt Tensioning Pulley moves to remove slack and to give the belt the correct tension.

1.Make sure you add a dimension to the sketch-chain that can change the length of the Belt, such as a tensioning pulley.

In the design to the left, we will make the 'Belt Tensioner Pulley'. the pulley at the top the sketch-chain.

Therefore, I will use the vertical dimension in the sketch-chain. See the dimension - in RED.

When we apply the 'Adjust Path Length' tool, MechDesigner makes a change to this dimension.

2.Close the Part-Editor


3.Double-click the Motion-Path FB dialog box top open the dialog
4.Expand the 'Path Length using dimension' separator

You will see the 'Actual Path Length'.

In this case, the 'Actual Path Length' is 497.26mm

5.Click the 'Controlling Dimension' selection-box
6.Edit the Base-Part with the sketch-chain
7.Click the dimension you want to adjust.

The dimension value shows in the box

The dimension element name shows above the box

8.Close the Part-Editor
9.Enter the Target Path Length for the Belt

In this case, I have entered 500mm it is a standard Belt Length for a 5mm tooth pitch belt.

It is best to make the length near to the Actual Path Length.

10.Click the Adjust Path Length button

If successful, the dimension, DimL, will change to make the Actual Path Length equal to the Target Path Length.

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