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Pulley-Rocker

Contents

Belt, Path-Joint, Pulley-Rocker

Add Pulley makes a motion relationships between a Rocker, a Motion-Point on a sketch-loop and the radius of a Pulley.

There are two Pulley types:

•	Drive-Pulley - its motion is given by a kinematically-defined Part.

•	Driven-Pulley - its motion is given by the speed of the Belt.

The linear speed of the Motion-Point along a sketch-loop is equal to the product of the angular velocity of the Pulley and the Pitch-Centre-Radius of the Pulley.

VelocityMOTION-POINT = RadiusPULLEY * ωPULLEY

You can add one Drive Pulley and one or more Driven Pulley to each sketch-loop.

For the Drive-Pulley

Specify a Drive Pulley at an Arc in a sketch-loop with a Rocker joined to its centre.

VMP = Velocity of the Motion-Point - calculated by MechDesigner

RP = Radius of the Drive Pulley - specified by the Pulley dia Example Kinematic-Chains log-box

ωP = Angular Velocity of a Rocker - specified by Kinematic FBs

For each Driven-Pulley

Specify a Driven Pulley at an Arc with a Free-Rotating-Part joined at its centre.

VMP = Velocity of the Motion-Point - specified by the Drive-Pulley

RP = Radius of the Driven Pulley - specified by the Pulley dialog-box

ωP = Angular Velocity of the Free-Rotating-Part becomes a Pulley-Rocker - calculated by MechDesigner

The Pulley-Rocker is a kinematically-defined Part.

The Pulley-Rocker in the Kinematics Tree

Kin-PulleyRocker

A Pulley-Rocker is constructed with a:

•	Pin-Joint – see Add Pin-Joint

•	Part – see Add Part

•	Path-Joint

Add Pulley

Command:

Add Pulley

What for:

Add-Pulley01

To add a Drive Pulley or a Driven Pulley.

•	You usually add one Drive-Pulley and then Driven-Pulleys.

You can also add all Driven-Pulleys. The Motion-Path FB gives the speed of the Belt.

•	The Driven and Driven-Pulleys are associated with one sketch-chain and one Motion-Path FB.

•	A Motion-Point, given by the Motion-Path FB, moves along the sketch-chain.

•	The closed sketch-chain represents the Belt.

•	The Drive-Pulley and Driven-Pulleys rotate with the same peripheral velocity, that is consistent with the angular-velocity of the Drive-Pulley and its Pitch-Centre-Radius.

The Belt can be a complex element to add to the model. Before you can use Add Pulley, other elements must be added to the model in particular ways

Before you can start this command:

Add-Pulley0

Part-Editor

Edit the Base-Part, or an Added-Part, to add a sketch-loop to represent the path of the Belt.

Add these Elements:

•	A sketch-loop, with Lines and Arcs, and possibly Blend-Curves.

•	Each Arc must have a dimension You will add an arc for each Pulley.

When the Arc is intended for a Drive Pulley

Drive-Pulley with sketch-loop in Base-Part

•	Add a Line, or CAD-Line

•	Merge the start-Point of the Line with the centre-Point of an Arc

Add-Pulley-5

Drive-Pulley with sketch-loop in Added-Part

•	Add a new Line or CAD-Line Make the Line collinear with a CAD-Line along the centre of the Part.

•	Merge the start-Point of the Line with the centre-Point of the Arc.

•	Do NOT use the start-Point of the CAD-Line given with the Added-Part.

When the Arc is intended for a Driven Pulley

Driven-Pulley:

•	You do not need to add a Line from the centre-Point of the Arc

Add-Pulley1b

Mechanism-Editor

•	Add a Motion-Path FB to the sketch-loop give a Motion-Point.

Note: The default direction for a Motion-Point is counter-clockwise along the sketch-loop. The initial position of the Motion-Point is at the most clockwise point of the sketch-element you select.

When the Arc is intended for a Drive Pulley

The Drive Pulley gives the peripheral velocity to the Motion-Point(s) along the sketch-loop.

•	Add a Rocker with a Pin-Joint at the centre of an Arc( in sketch-above)

Add-Pulley4

When the Arc is intended for a Driven Pulley

Driven Pulleys rotate with the peripheral velocity of the Belt.

Elements in the Mechanism-Editor

•	Add a Free-Part joined with a Pin-Joint at the centre of an Arc

What to do:

Add-Pulley2

Typically add a Drive Pulley first, followed by Driven-Pulleys at other arcs in the sketch-loop.

Click Add Pulley in the Kinematic-elements toolbar.

Click the Pin-Joint at the centre of the Rotating-Part* and Arc

Click the Motion-Path FB that gives the Motion-Point that moves along the Arc

Click the dimension given to the radius of the Arc when the Part-Editor opens

Click the Rotating-Part* joined with the Pin-Joint at the centre of the Arc

Click OK in the Command-Manager

* Rocker for a Drive Pulley, Free-Part for Driven Pulleys, or Base-Part if the Pulley is fixed to the Frame.

Result:

Add-Pulley3

When you Click OK in the Command-Manager, MechDesigner:

•	Adds a Pulley schematic to the graphic-area.

•	The Radius of the Arc and Pulley Pitch Circle changes to given an integer number-of-teeth.

•	The Radius of the Arc becomes 'Read-Only'.

•	If you add a Pulley to a Free-Part, it becomes a kinematically-defined Part, a Pulley-Rocker.

Video:

How to...

Full Video of Add Pulley and Belt

Add sketch-elements, Add Rocker, Add Motion-Path FB, Add a Pulley, Add Driven Pulley.

Notes:

Practical Parameters

Practical Belt Tooth-Pitch, Belt-Length, Minimum number of Pulley-Teeth are in sales literature - not in MechDesigner.

Length of the Belt

You must make sure the Belt has an integer Number-of-Teeth

Number-of-Teeth = Belt-Length / Belt Tooth-Pitch

The Belt-Length is a Read-Only value. The Belt-Length value is given in the Motion-Path FB and the Pulley dialog-boxes.

You can change:

•	Sketch-loop: edit the centre distances between the Pulleys to influence Belt-Length

The radii for the Pulleys in the sketch-loop are finally set by the Number of Pulley-Teeth in the Pulley dialog-box.

•	Belt Tooth-Pitch: edit the Motion-Path FB

•	Number-of-Pulley Teeth: edit the Pulley FB.

Minimum Number-of-Teeth on a Pulley

You must make sure the Number-of-Teeth in each Pulley > Practical Minimum number-of-teeth,

Typically, the minimum is 10 teeth.

Result:

MechDesigner adds a Drive Pulley or a Driven Pulley

Kinematics-Tree:

Add-Pulley-KT

The Kinematics-Tree adds a Pulley-Rocker for each Driven Pulley you add to the model.

A Drive Pulley is a Rocker.

The Red Belt identifies that the Belt drives the Pulley.

Design Arrangements

Fixed Pulley Centres : Two or more Pulley Sets

TwoPulleysSets

Use two Belts to:

•	Give a larger Speed Reduction

•	Reduce Belt Span

The image to the left shows a Simple Belt with the three Pulleys and two Belts

Step 1: First Belt

Step 1: Add first Motion-Path and two Pulleys

MD-17-6

STEP 1:

Edit the Base-Part

STEP 2:

Add a sketch-loop to represent the Path of the Belt

STEP 1:

Add a Line

so that start-Point is at and merged with centre-point of the Arc.

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

STEP 4:

You MUST dimension each Arc

The exact radius of the Arc is not important. MechDesigner uses the dimension to find a Pulley with the nearest radius that also gives an integer number-of-teeth.

STEP 5:

Close the Part-Editor

MD-17-7

STEP 6:

Add a Motion-Path FB

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.

MD-17-8

STEP 7:

Add one Part

for each Arc

STEP 8:

Add a Pin-Joint

between a Point in each Part and the centre-Point of an Arc.

STEP 9:

Add a Motion-Dimension FB

to the Part at the Arc you wish to be the Drive Pulley.

There should be one Rocker that identifies the axis that will be the Drive Pulley.

[Not shown in the image: You can add a Linear-Motion FB to the Rocker]

There should be one Rotating-Part for each Pulley you will add.

The length of the Parts are not important.

Use Add Pulley

Click Add Pulley in the Kinematic-elements toolbar

MD-17-4

Add Pulley opens in the Command-Manager. You must use Add Pulley for each Pulley.

Select these four elements for each Pulley. We add a Pulley to the left Arc first.

Refer to the images above to identify the Elements. Select, from top to bottom.

1.	The Pin-Joint

2.	The Motion-Path FB

The Part-Editor opens with sketch-loop 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 add a Pulley to the right-hand Arc and Part.

MD-17-9

Notes:

•	A Drive Pulley is given to the Pulley on the same axis as the Rocker.

•	The Linear Speed of the Belt = Radius of Drive Pulley *Angular Speed of Drive Pulley

•	The angular speed of the Rocker gives the linear speed to the belt.

•	This then gives the angular speed to the other Rotating Parts.

•	The Drive Pulley can transmit a Torque and tension to the Belt.

Step 2: Second Belt

Step 1: Add second Motion-Path and two Pulleys

MD-17-40

STEP 1:

Edit the Base-Part

STEP 2:

Add a sketch-loop to represent the Path of the second Belt

You must merge the centre-Point of the Arc for the second belt, with the centre-Point of the Arc for the first Belt.

[Add the Arc first, then use Merge-Point tool in the Geometry toolbar].

STEP 3:

You MUST dimension each Arc

The exact radius of the Arc is not important. MechDesigner uses the dimension to find a Pulley with the nearest radius that also gives an integer number-of-teeth.

STEP 4:

Close the Part-Editor

MD-17-41

STEP 5:

Add a Motion-Path FB

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

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

MD-17-42

STEP 6:

Add a Pulley

Make the selections for the Command-Manager:

a.	Select the Pin-Joint at the centre of the Part Pulley

b.	Select the Motion-Path FB

c.	Select the Radius of the Arc (the Part-Editor opens automatically to select the Radius of the Arc)

d.	Select the Part

Click 'OK' in the Command-Manager

The Pulley shows in the graphic-area.

MD-17-43

STEP 7:

Add a different Pulley at the other end of the sketch-loop

Note: You do not need to add a Part for the second belt. We 're-use' the Part and Pin-Joint in the first Pulley.

Tutorials and Reference Help Files for MechDesigner and MotionDesigner 14.2 + © Machine, Cam, Mechanism, and Motion Design Software by PSMotion Ltd