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: See: Add Motion-Dimension FB when we want to add a 'Rocker' Motion-Dimension FB to the Pin-Joint

hmtoggle_plus1Theme and Styling of images below:

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]

Pin-Joints in the Graphic-Area


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 :

Kinetostatic Torque-Speed dialog-Box [motor and gearbox sizing only]


Symbols for a Pin-Joint:

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

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

The 'inside circle'[o], which is '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 Command


STEP 1: Start Add Pin-Joint command


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 or Pin-Joint 4R-s15




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


Dyads [also termed Assur Groups]


Belt and Pulley


VIDEO : Add Pin-Joint - Simple-Case:


How to...

Add a Pin-Joint - Simple-Case



tog_minusSpecial-case - Add two coincident Pin-Joints

Frequently, a machine design has two or more Pin-Joints at the the same position, between three or more Parts

When you add a Pin-Joint at the same place as another, you must decide which two Points from which of two Parts you want to select for each Pin-Joint.

Example options:

A total three Parts , we would like to join all three Parts at one position. We must add two Pin-Joints.

If, say, Pin-Joint#1 is between Points 1 & 2 [in Parts 1 & 2], 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]

When to do Option A or Option B becomes important when you need to also add Motion-Dimension FBs to specify the motions of the two Parts.


Preparation for this example

The image to the left shows:

A Line1s-red in the Base-Part . It is a kinematically-defined Part – and it is coloured Green.
A completely-free Part2s – you can drag the Part-Outline freely.
A free Part3s which is joined with a Pin-Joint to a Point at one end of the Green Line1s-red.

We want to join the completely-free Part2s with a Pin-Joint to become coincident 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



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

There is ambiguity as to which Point you want to select at the Pin-Joint 5s.

MechDesigner sees a total of three[3] Points:

one[1] Point4s in the Completely-Free Part


two[2] Points5s at the Pin-Joint


Select-Elements dialog-box

When there is ambiguity as to which two Points you want to select for a Pin-Joint, the Select Elements dialog-box opens.

In this model, the three possible Points are:


Notes: The Owner-Elements are also three different Parts.


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

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

Point11 and Point5






- or -

Point11 and Point7.







Point5 and Point7 is not possible.

Note: Red Circle[O] 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 when the selections are complete and 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 to add the Pin-Joint.

Top-Tip: Select Points in this order:

Always click a Point in the completely-free Part before you click a Point that is more ambiguous.

If the Select-Elements dialog opens, the point in the Completely-Free Part is at the top of the list.

You know you must select the first Point in the list, plus one more Point from the list to add the Pin-Joint.

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