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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:

'Expand' then 'Play'

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