Tutorial 12: Design a Spatial Mechanism

Design of a Spatial Mechanism - very common to Packaging Machines

We will design a kinematic-chain that is frequently found in packaging machines. The input and output parts of the kinematic-chain are on Mechanism-Planes that are perpendicular[⊥] to each other.

Design a Spatial Mechanism

About the Ball-Joint and Connecting-Part
Add a Part and two Ball-Joints to make the Part a Connecting-Part
Edit the Length of the Connecting-Part
Edit the 'diameter' of the Connecting-Part
Edit the 'diameter' of the Ball symbol
Edit the offset of the Ball-Joints along the Z-axis of each Mechanism-Plane

Note-Pencil-20 : In a machine, the Ball-Joint is usually a Rod-End bearing or a Ball and Socket bearing. A Rod-End bearing is called a 'Heim-Joint' in North America or a 'Rose-Joint' in the UK.

See Ball-Joint Mounting Configurations.

tog_minus        The Ball-Joint and Connecting-Part

MechDesigner - Ball Joint icon

A 'Ball-Joint' is a joint we use to constrain a Point in one Part to align with a Point in a different Part.

Image of Connectiing Part in Spatial Mechansim

When you use a Ball-Joint:

You must add a Ball-Joint to each end of a Part.
We call the Part with the two Ball-Joints a Connecting-Part.
In this tutorial, the Connecting-Part is equivalent to the 'coupler' Part - see also Tutorial 2.1.
The Connecting-Part is one Part in a Dyad. The Dyad must be an SSR or SSP. These are very similar to an RRR and RRP Dyads

The image shows a Connecting-Part. There is a Ball-Joint at each end.

The Connecting-Part does not look like an Added-Part


Frequently, a kinematic-chain in a packaging machine may have levers that are offset from the Mechanism-Plane. However, if you can 'project' the Parts onto the Mechanism-Plane, do NOT add Ball-Joints. In this case, you should offset the Solids from the Mechanism-Plane.

tog_minus        Preparation

Model Prepared - ready for a Connecting Part and two Ball-Joints

Before we add the Connecting-Part and Ball-Joints we must prepare a kinematic-chain in two different Mechanism-Editors. In this case, the Mechanism-Editors are perpendicular[⊥] to each other.

Kinematic-Chain 1

Is a kinematically-defined chain; it has Green Part-Outlines.
There is a Rocker1s-red joined to the Base-Part2s with a Pin-Joint3s .

Kinematic-Chain 2

Is not a kinematically-defined chain; it has Blue Part-Outlines.
There is a Free Part4s that is joined to the Base-Part5s with a Pin-Joint6s .

tog_minus        Prepare two Kinematic-Chains

Prepare Kinematic-Chain 1


1.Add a new Mechanism-Editor to Plane (the front Plane in the Model-Editor).

[Call it 'Mechanism A]

2.Edit the Base-Part
3.Add a Line

Step 3 is complete

4.Close the Part-Editor


5.Add a Part
6.Add a Pin-Joint between the Part and the Base-Part
7.Add a Motion-Dimension FB – to make it a Rocker
8.Edit the Motion-Dimension FB and edit the Base-Value to -15º. (this may show as 345º in the graphic-area)

Step 8 is complete

Also, add a Motion FB [Edit the motion to give a rotation of 30º] and Linear-Motion FB and connect them to the Motion-Dimension FB.


9.Edit the new Part
10.Edit the length of the Base-Part to 190mm

Step 10 is complete

11.Close the Part-Editor

Prepare the Kinematic-Chain 2


We need  a new Mechanism-Editor [Mechanism B] that is added to a new Plane.

We need the new Plane to be offset from the Right Plane [Plane3]. Plain3 is a child to the Model-Editor.

1.Click the Model name-tab to show to the Model-Editor
2.Click the Model elements toolbar > Add Plane

The Add Plane dialog-box opens.

3.Click Plane3 in the Assembly-Tree, or in the graphic-area.


4.Edit the Distance to New Plane = 200mm1s-red

Step 4 is complete

5.Click OK-tiny-11-15 in the Add Plane dialog-box


Spin the model view to show the new Plane1s-red [Plane4] in the Model-Editor

6.Click Model toolbar > Add Mechanism
7.Click Plane41s-red in the Assembly-Tree or the graphic-area
8.Click 'OK-tiny-11-15' in the Command-Manager.


MechDesigner 'jumps' immediately to the new Mechanism.

In this tutorial, we call this Mechanism-Editor 'Mechanism B'

9.Click 'Show or Hide Other Mechanism Sketches'

Visibility toolbar > Show/Hide other Mechanism Sketches.

10.Use the left/right and up/down arrow keys to spin the view.

Mechanism A shows in Grey.

Step 10 is complete.


11.Edit the Base-Part in Mechanism B

To edit the Base-Part, double-click the rectangular Part-Outline.

12.Add a short horizontal Line (with the left Point of the Line at 100,1001s-red in the Base-Part)
13.Close the Part-Editor

Step 13 is complete.


14.Add a new Part
15.Add a Pin-Joint to join the Part to the Point at 100,100.
16.Edit the Part, and edit its length to be approximately 105mm long.

This is the Part that is Not Kinematically Defined.

It has a Blue Part-Outline. It is also a Free Part.

Step 16 is complete.


Now, make Mechanism A the active Mechanism-Editor.

17.Click the 'Mechanism A' tab
18.Spin the model again

Now, you can see the Part1s-red in Mechanism B , but in grey.

tog_minus        Add a Connecting-Part and two Ball-Joints

Make Mechanism B the active Mechanism-Editor again.

Summary of this Step:

Add a new, Completely Free Part to Mechanism B
Add a Ball-Joint to each end of the new Free Part.
oAdd one Ball-Joint between the Free Part and the end of Rocker1s-red in Mechanism A
oAdd the other Ball-Joint between the Free Part and the end of the Free Part2s in Mechanism B.

We call the Part joined to other Parts with Ball-Joints a Connecting-Part.


1.Make Mechanism B the active Mechanism-Editor
2.Toggle-On the 'Show/Hide other Mechanism Sketches' tool to show the Rocker in Mechanism A again.
3.Use the keyboard arrow keys to spin the view.


4.Use Front-View, to view normal to the plane again.
5.Add a new Completely-Free-Part3s
6.Edit the Part and edit its length to be approximately 50mm.

Step 6 is complete


7.Spin the model again slightly to see the:
The two mechanisms
The Parts in each Mechanism
The Points at the end of each Part.

MechDesigner - Ball Joint icon

Click Kinematic-elements toolbar >Add Ball-Joint

The pointer changes to the 'Add Ball-Joint' pointer.


8.Click a Point at the ends of the:
Completely-Free Part1s-red

- and the


In the image, Point1s-red has been selected – it is red. We are about to click Point2s.


When you click the second point in the Free-Part, to add the first Ball-Joint:

The Points DO NOT snap together
The end of a Part has a Ball-Joint3s symbol in Black

You must add two Ball-Joints to one Part before the Part becomes a 'Connecting-Part'.


9.Click Add Ball-Joint again, to make the command active, if necessary
10.Click a Point3s in the Completely-Free Part in Mechanism B

Step 10 is complete

11.Click a Point4s in Mechanism A

Note: This Point is near to the origin of the Base-Part  It is at the end of the grey Part.


MechDesigner replaces the Completely-Free Part' with a 'Connecting-Part'
The Connecting-Part has a symbolic rod5s and two Ball-Joints6s7s.
The symbol of the Connecting-Part and the Ball-Joints are now Green
Ball-Joints in Kinematic-Tree

Ball-Joints in Kinematic-Tree

The 'Kinematics-Chain' now includes an S-S-R Dyad.

Each 'S' is a Spherical-Joint. They are the Ball-Joints.


The Connecting-Part was added in Mechanism B.

Mechanism B began with a Free-Part and a Completely-Free Part.

The Free Part and the Completely-Free Part became the two Parts in the S-S-R Dyad.

tog_minus        Edit the Length of the Connecting-Part

The Connecting-Part is in Mechanism 1 in the images below. However, you should select Mechanism 2 to continue with this Tutorial.

It is quite easy to edit a Connecting-Part.

It is easier to leave the 'Show / Hide Sketches' Toggled IN' to show the sketches of other Parts, even in the Part-Editor.


Edit the Length of the 'Connecting-Part'.

1.Hover above the Symbol for the 'Connecting-Part'

MechDesigner opens the 'Connecting-Part' in the Part-Editor.

While the Part is open in the Part-Editor, it returns to the position it had before we added the Ball-Joints.


3.Double-Click the dimension

Remember, to edit a dimension, double-click the extension lines or arrowheads, but NOT the dimension number itself.


To edit the value, use your keyboard to enter a new value, use the spin-box tool, or use the Zero/Round contextual-menu. For example:

4.Right-Click the dimension box
5.Click 'Round'
6.Round it to exactly 50mm


MechDesigner changes the length of the Part - if it is not already 50mm.

7.Close the Part-Editor

The Connecting-Part is now exactly 50mm long.

tog_minus        Edit the 'Radius' of the Connecting-Part

When you want to add Solids to the Parts that represent the kinematics, you can import a Solid from SOLIDWORKS®, or use MechDesigner to add a sketch-loop and then show an Extrusion.

However, you cannot import Solids or add Extrusions to Connecting-Parts.

It is possible, though, to add and edit a symbol for the diameter of the rod that is between the Ball-Joints.


Edit the Radius of the Connecting-Part

Add and then dimension a sketch-element in the Connecting-Part.

MechDesigner uses the dimension of the 'first' sketch-element in the Connecting-Part as the 'radius'.

1.Use the 'Show Model in Mechanism' tool
2.Hover + Double-click  the symbol for the Connecting-Part

MechDesigner opens the Connecting-Part in the Part-Editor.

While the Connection-Part is open in the Part-Editor, it returns to the position it was at before we added the Ball-Joints.


3.Add a Line or Circle to the Part

Note: when you add and dimension sketch-elements and the current view is not the Front View, the sketch-elements and dimensions may show in unusual places! Use the Front View if necessary.

4.Dimension the Line or Circle
The dimension is 6mm in this example.

Step 4 is complete.



5.Close the Part-Editor

When you close the Part-Editor, the Dimension you add to the Line or Circle becomes the 'radius' of the Connecting-Part.

The Connecting-Part is still not very attractive!

tog_minus        Edit the Ball-Joints

Use the Ball-Joint dialog-box to edit parameters that apply to the appearance and the kinematics of the Ball-Joint.

1.Diameter of the Ball-Joint

It is possible to add and change the symbol for the diameter of the Balls.

2.Offset the Ball-Joint from the Mechanism-Plane

Ball-Joints frequently do not lie on the Mechanism-Plane. See Ball-Joint configurations

When Ball-Joints are not on the Mechanism-Plane, the kinematic analysis changes.

3.Mass of the Ball-Joint

MechDesigner calculates Planar Forces.

Enter the mass of the Ball-Joint. This mass is 'referred to' the Ball-Joint from the kinematic-chain in the other Mechanism.


Open the Ball-Joint dialog-box.

As with most dialog-boxes, you can open a dialog-box to edit an element in at least three ways:

Option 1: Graphic-Area: If you can double-click only the element:

1.Double-Click the Ball-Joint1s-red in the graphic-area

- or -

Option 2: Selection-Window: If you cannot click only the element:

1.Click the Ball-Joint in the graphic-area so it shows, with other elements, in the Selection-Window
2.Double-click the Ball-Joint in the Selection-Window

- or -

2.Right-click the Ball-Joint in the Selection-Window
3.Click 'Edit element...' in the contextual-menu

- or -

Option 3: Assembly-Tree: Use the Assembly-Tree

1.Right-Click the Ball-Joint in the Assembly-Tree
2.Click 'Edit...' in the contextual-menu



Use Option 2.


The Ball Joint dialog-box


Enter these values:

Ball Diameter [mm]

This is the diameter of the Symbol for the Ball. The symbol only shows when you use 'Show Model in Mechanisms'.

20mm - This is bigger than the diameter of the Connecting-Part

Ball Offset from Plane [mm].

-7mm [+ve is in the +Z direction of the Mechanism-Editor]

See Also: Ball-Joint configurations

Ball Equivalent Mass [kg]



MechDesigner does not yet calculate forces for all of the Mechanism-Editors together. MechDesigner only calculates the forces of individual Mechanism-Editors.



Edit the two Ball-Joints

Here, the two settings for the Ball-Joints are:

Ball-Joint Offset = -7mm
Ball-Joint Diameter = 16mm

Use Visibility toolbar > 'Show Model in Mechanisms' to see the symbols for the Ball-Joints.

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