What is a Pin-Joint?

Typically, use a Pin-Joint to model a rotary bearing.

MechDesigner definition: a Pin-Joint makes a Point in a Part become co-axial (coincident) with a Point in a different Part. The two Parts can rotate relative to each other.

IFTOMM Kinematic definition: a Pin-Joint makes an axis in one part become collinear with an axis in a different part, but allows the axes rotate relative to each other.

IMPORTANT: ...when we want to add a 'Rocker' Motion-Dimension FB and the Pin-Joint is to become the rotational axis joint, then the Pin-Joint MUST be between two Points that are at the ends of two Lines, which are in two different Parts.
See also: Add Motion-Dimension FB


Edit menu > Application-Settings > General tab > Theme Options > Active Theme: Charcoal Dark Slate,

Edit menu > Application-Settings > Graphics tab > Display Colours :  [Background : White] ;  [Part-Unsolved : Blue] ; [Part-Solved: Green] ;[ Joint-Solved : Green] ; [Joint-Unsolved : Blue]



Kinematic Elements Toolbar : Add Pin-Joint icon [left of the graphic-area, in Mechanism-Editor]

Menu :

Add menu > Mechanism sub-menu > Add Pin-Joint

Dialog-Box :



The two different Symbols for a Pin-Joint:

1s-red PointSymbol the symbol for a Pin-Joint between a Added-Part and the Base-Part.

2s PointandPinJoint-Symbol the larger circle is the symbol for a Pin-Joint between two Added-Parts

The small circle[o], which you can see 'inside' each Pin-Joint symbol, is the symbol for the two Points that have become coincident, and co-axial, when you add the Pin-Joint.

<<< Pin-Joint1s-red between a Part and the Base-Part

<<< Pin-Joint2s  between two Added-Parts

See Also:

Add Slide-Joint, Add Ball-Joint

Add Motion-Dimension FB

Add Pin-Joint - Prepare

Before you can do Kinematics-elements toolbar > add Pin-Joint, you need a minimum of one Point in two different Parts.  A minimum of one Part must be a Free-Part [Blue Part-Outline]

GA-AddPin-Joint Prep1

<<< A Line in the Base-Part, three Parts [with CAD-Lines], and a Point in two of the Parts.

1.Point1s-15, 3R-s15, or 5R-s15 are each a child to a Free-Part - with a Blue Part-Outline


2.Point2R-s154R-s156R-s15 are each a child to:
a different Free-Part

- or -

The Base-Part is always a kinematically-defined Part. All sketch-elements you add to the Base-Part are also kinematically-defined.

Add Pin-Joint


STEP 1: Add Pin-Joint


1.Add menu > Mechanism menu  > Add Pin-Joint

- or -

1.Kinematic elements toolbar > Add Pin-Joint

STEP 2: Select two Points:

1.Click a Point in a Free-Part
2.Click a different Point in a different Free-Part

- or -

2.Click a Point in a kinematically-defined Part.

Note: If the Select-Elements dialog-box shows, see Pin-Joint Special Case

STEP 3: Complete the command:

If the Points do not snap together

1.Click the graphic-area

The Points 'snap' to become coincident to give the Pin-Joints 3R-s15, and Pin-Joint 4R-s15 when you repeat the Add Pin-Joint.


VIDEO : Add Pin-Joint - Simple-Case:


How to...

Add a Pin-Joint - Simple-Case





The Pin-Joints in the Kinematics-Tree may be a sub-element to different Machine-Elements:


Dyads [also termed Assur Groups]


Belt and Pulley


tog_plusSPECIAL CASE - two or more coincident Pin-Joints

Frequently, a design has two, or more, coincident[co-axial] Pin-Joints.

We always need to select two[2] Points [from different Parts] for a Pin-Joint.

However, we must be careful as to which two Points we select for Pin-Joint #2 when we want it to be co-axial with Pin-Joint #1,

From a total three Parts and Points, we must decide which two Points we want to select for each Pin-Joint.

Assuming that Pin-Joint#1 is between Points 1 & 2 [in Parts 1 & 2], then there are two options for Pin-Joint#2

Option A: Pin-Joint#2: between Points 3 & 1 [in Parts 3 & 1]

Option B: Pin-Joint#2: between Points 3 & 2  [in Parts 3 & 2]

Option A and Option B becomes important when you need to add Motion-Dimension FBs to specify the motions of the two Parts


The image to the left shows:

A Line1s-red that is a child to a kinematically-defined Part – it is coloured Green. [e.g., a Line in the Base-Part]
A Free-Part2s joined with a Pin-Joint to a Point at one end of the Green Line1s-red.
A Completely Free Part3s – you can drag the Part-Outline freely.

We want to join the Completely Free-Part2s with a Pin-Joint that is co-axial with the existing Pin-Joint.


STEP 1: Click Kinematic-elements Toolbar > Add Pin-Joint



STEP 2: Click the Point4s at the end of the 'Completely Free Part'



Select as the first element the Point in the Completely Free-Part

The first element you click become the TOP element in the Select-Elements dialog. This Point does not have any ambiguity. This becomes useful when the Select Elements dialog-box opens - see below.


STEP 3: Click the Point5s at the centre of the Pin-Joint.

There is ambiguity at Point5s.

MechDesigner sees a total of three[3] Points:

one[1] Point4s in the Completely Free-Part, and


two[2] Points5s at the Pin-Joint


Select-Elements dialog-box

When there is ambiguity about which two Points you can select, the Select Elements dialog-box opens.

In this model, the three possible Points are:

Point5 &

Notes: The Owner-Elements are three different Parts. Elements with the same element-name are appended with a number.



To select the Points:

1.Ctrlb key-button down - do this before you click the Points in the box.
2.Select all of the Points in the list to do the Add Pin-Joint command


There are three possible selections of two elements from the three Points that are possible:

Point11 and Point5



- or -

Point11 and Point7.




Point5 and Point7 are not possible.

Note: Red Circle at the bottom of the dialog - left.


Why? Because Point5 and Point7 are joined with a Pin-Joint, already.


STEP 4: Select two Points.

Select the Point11 and Point5:

Ctrlb key-button down
Click the two Points that you want to join with a Pin-Joint.

Note: The OK-tiny-11-15 becomes colourized whee selections are compatible.

STEP 5: Click OK-tiny-11-15  [top-right of Select-Elements dialog-box]

The Select-Elements dialog-box closes. The Points move together.

Now, there are two co-axial Pin-Joints.

VIDEO - Add Pin-Joint - Special-Case:


'Expand' then 'Play'

How to...

Add a Pin-Joint - Simple



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