﻿ Pulley-Rocker

# Pulley-Rocker

### 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.

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 dialog-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

A Pulley-Rocker is constructed with a:

 • Pin-Joint – see Add Pin-Joint

 • Part – see Add Part

 • Path-Joint

Command:

What for:

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:

Part-Editor

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

 • 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

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

 • Add a new Line or CAD-Line Make the Line co-linear 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

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)

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:

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:

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

Video:

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:

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.

Pulley dialog-box

### Design Arrangements

#### Fixed Pulley Centres : Two or more Pulley Sets

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: Add first Motion-Path and two Pulleys

 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

 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.

 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.

Click Add Pulley in the Kinematic-elements toolbar

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.

Do Add Pulley again, to add a Pulley to the right-hand Arc and Part.

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 1: Add second Motion-Path and two Pulleys

 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

 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.

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.

 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.

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