What is a Ball-Joint?

A Ball-Joint makes a Point in one Part coincide with a Point in a different Part. The Parts can be in two different Mechanisms.

You must do Add Ball-Joint two times, with a Ball-Joint at each end of one Part.

We use the derived name 'Connecting-Part' for a Part that has a Ball-Joint at each end.

See: How to Edit a Ball-Joint', How to Edit a Connecting Part, Ball-Joint configurations, Ball-Joint dialog-box

Add Ball-Joint

Add Ball-Joint icon

Kinematic Elements Toolbar : Add Ball-Joint icon


Add menu > Mechanism sub-menu > Add Ball-Joint


Ball-Joint dialog-Box

You must know:

How to add Mechanism-Editors to different Planes.

The difference between a Free-Part and a Completely Free Part.



It is best to add the two Mechanisms-Editors, Parts, Pin-Joints and Slide-Joints, Motion-Dimension FB, before you add the Ball-Joints.

Do Visibility toolbar > Show other Mechanism Sketches.
Mechanism-Editor A should include a kinematically-defined chain - in the model, below, it is a Rocker. It can be ANY kinematically-defined Part , with a Green Part-Outline.
Mechanism-Editor B should include a Free-Part [typically, joined to the Base-Part with a Pin-Joint or Slide-Joint, and a Completely Free Part. [Blue Part-Outlines].

Below, An example model prepared before you add Ball-Joints:

The two Mechanism-Editors - A and B - are typically perpendicular[] to each other. But this is not necessary. They should not be parallel.


Mechanism-Editor A

A kinematically-defined Chain3s(in GREY) in Mechanism-Editor(A).

Mechanism-Editor B [the active Mechanism-Editor]

A Kinematic-Chain that is not yet a kinematically-defined chain4s. The Parts have Blue Part-Outlines.
A Free-Part4s [with a Pin-Joint or a Slide-Joint].



Always do Add Ball-Joint to each end of the Part that is a Completely Free Part.

IT is often a good idea to Click Edit toolbar > Auto-rebuild to 'off'. After you have added the two Ball-Joints, then again click Edit toolbar > Auto-rebuild to 'on'.

Add Ball-Joint #1

1.Make Mechanism-Editor B active.
2.Click Point1s-red at one end of the Completely Free Part
3.Click Point2s in a Free Part in the active Mechanism-Editor B.

Note: if you cannot select the two Points to add a Ball-Joint, Toggle-Off the Auto-Update tool.


When you add Ball-Joint 1, the Ball-Joint shows in the graphic-area as a 'rough' Ball-Joint8s.

Points1s-red2s do not above image snap together until you add Ball-Joint #2.



Add Ball-Joint #2

4.Click Point3s at the other end of the Completely Free Part

Do NOT make Mechanism A active. You click the two Points when Mechanism B is active

5.Click Point4s - the Grey Point - in the kinematically-defined Part4s that is in Mechanism-Editor(A)

Note: Point4s is not a sketch-element in the Base-Part.



When you add Ball-Joint 2, the two Ball-Joints6s7s snap the Completely Free Part to the two ends of the Points in Mechanisms (A) and (B).

The Completely-Free-Part becomes a Part we name the Connecting-Part5s
The Free-Part and the Completely-Free-Part become kinematically-defined in the active Mechanism-Editor(B) as a new Dyad.
The new Dyad is an S-S-R Dyad. It is also possible to add an S-S-P Dyad
The centre of the Ball-Joints are on the two Mechanism-Planes - one on each.

Use the Ball-Joint dialog-box to move the Ball-Joint off the Mechanism-Plane.


Notes: After you add the Ball-Joints you can edit its visibility.

1.Edit the Connecting-Part to edit its length and 'Symbolic' shaft diameter.
2.Edit the Ball-Joint to edit the 'Symbolic' ball diameter.

In the Kinematics-Tree, each Ball-Joint is a letter 'S', which is for Spherical-Joint.
The two Ball-Joints6s7s are the two 'S' Joints in the S-S-R and the S-S-P Dyad. See Dyads and Kinematics-Tree.
A Connecting Part5s is a Part with two Ball-Joints.
Do not add two Ball-Joints to Parts that are in one Mechanism-Editor.

Other Skills:

Add Part, Edit Part, Add Plane, Add Mechanism, Add Motion-Dimension FB, Add Pin-Joint and /or Add Slide-Joint, Spin View.

tog_minusHow to Edit the Ball-Joint


To edit the Ball-Joint, you must open the Ball-Joint dialog-box.

To open the Ball-Joint dialog-box:

1.Click the Ball-Joint 'rings' in the graphic-area1s-red or the Ball-Joint element in the Assembly-Tree

The Ball-Joint should show in the Selection-Window.


In the Selection-Window,

2.Right-click the Ball-Joint element to open the contextual-menu
3.Click 'Edit Element' in the contextual-menu



Ball Diameter

To edit the symbolic diameter of the ball.

Ball Offset [±]

To move the Ball-Joint along the Z-axis of the Mechanism Plane

Ball-Joints frequently are not on the active Mechanism-Plane. See also: Ball-Joint configurations

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


Mass of the Ball-Joint. MechDesigner calculates Planar Forces. The mass is referred to the Mechanism-Plane.


Example of Ball-Offset: Z = 10mm

1s-red Mechanism-Plane

The Ball-Joint is on the Mechanism-Plane when the Ball-Offset is '0'.

2s Ball-Offset.

The Ball-Joint moves along the Z-axis in the +Z-axis or –Z-axis direction.

In the image, the Ball-Offset is 10mm in the +Z-axis direction.

tog_minusEdit the Connecting-Part: Length and Symbolic Part/Rod Radius

You cannot add Solids to Connecting-Parts.

However, it is possible to add and edit a symbol to give the diameter of the 'rod' between the Ball-Joints.


Edit the Connecting-Part

MechDesigner uses the first dimension you add to the Part in the Part-Editor as the symbolic radius for the Connecting-Part.

To add and edit the Dimension, you must edit the Connecting-Part

1.Double-click the symbol for the Connecting-Part

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


2.Add a Line or Circle to the Part
3.Add a dimension to the Line or Circle

Note: when you edit dimensions in a Part and the view is not the Front View, the CAD-Line, Axes, sketch-elements and dimensions may show in unusual places!

If you cannot add or edit a dimension, then click View | Front Plane.

4.Edit the Dimension to 7mm, with the dimension tool
5.Edit the Length of the Connecting-Part to 100mm, in the usual way.


6.Close the Part-Editor

Use Visibility toolbar > Show Model in Mechanism' tool

It still is not very attractive!

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