What is a Pin-Joint?

A Pin-Joint makes a Point in one Part become co-axial (coincident) with a Point in a different Part. The two Parts rotate relative to each other with the centre-of-rotation at the Pin-Joint.

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


Mechanism Local Toolbar : Add Pin-Joint icon


Mechanism menu > Add Pin-Joint


The Pin-Joint symbols in the graphic-area:

1s-red PointSymbol the Symbol for a Pin-Joint that joins a Part to the Base-Part.

2s PointandPinJoint-Symbol the larger circle is the Symbol for a Pin-Joint , the smaller circle, inside, is the two 'Points' that join one Part to a different Part.




Before you can add a Pin-Joint, you must have a minimum of two Points, with one Point in two different Parts.

A minimum of one Part must be a Free-Part.

Thus, in preparation, you need a:

1.Point that is a child to a Free-Part

and a

2.Point that is a child to:
a different Free-Part

- or -

Add Pin-Joint:



STEP 1: Click Add Pin-Joint


Mechanism menu  > Add Pin-Joint


Mechanism Local toolbar > Add Pin-Joint

STEP 2: Select two Points:

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


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


STEP 3: Complete the command:

If the Points do not snap together, it may be necessary to:

1.Click the graphic-area

The Points 'snap' together.



MechDesigner adds a Pin-Joint to the Assembly-Tree.


'R' is the letter we use for Revolute-Joint. It is identical to our Pin-Joint.

In the Kinematics-Tree , a Pin-Joint is the:

'R' Joint in a Dyad or a

'R' Joint in a Rocker




How to...

Add a Pin-Joint

See Also:

Add Pin-Joint, Add Ball-Joint

Add Motion-Dimension FB

tog_minusMore Details: Special - To add two or more co-axial Pin-Joints

'A Pin-Joint makes a Point in one Part to be co-axial (coincident) with a Point in a different Part. The two Parts can rotate relative to each other with the Pin-Joint as the centre of rotation'.

Frequently a design has two co-axial Pin-Joints.

When we add a 'Pin-Joint 2', we must select one of the two Points that already belongs to 'Pin-Joint 1'.

Thus, the Point in the two Pin-Joints may be:

Option A: Pin-Joint 1: Points 1&2 /and/ Pin-Joint 2: Points 3&1

Option B: Pin-Joint 1: Points 1&2 /and/ Pin-Joint 2: Points 3&2

Option A or B becomes important when you also need to add one or more 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.
A Free-Part3s joined with a Pin-Joint to a Point on the Green Line1s-red.
A Completely Free Part2s – you can drag the Part-Outline freely.

We want to join the Completely Free-Part2s with a Pin-Joint at the same position as the existing Pin-Joint.

Note: A minimum of one Part must be a Free-Part.

One Part, two Parts or all three Parts may be Free-Parts.


STEP 1: Click Add Pin-Joint in the Mechanism Local-Toolbar


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


Top-Tip: Select the Point that is in the Completely Free-Part before any other Point. This becomes useful when the Select Elements dialog-box opens - see below.


STEP 3: Click the Point5s in the Pin-Joint.

Note: Point5s is Red. It is at the centre of the Pin-Joint Symbol.

There is ambiguity at Point5s.

MechDesigner sees a total of three Points:

the Point in the Free-Part, and


two Points at the Pin-Joint.


Select-Elements dialog-box

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

The element at the top of the list is always the first element you select. See Point4s above in STEP 2.

This is why we should select the Point that does not have ambiguity before the elements that do have ambiguity. it is at the top of the list in the Select-Elements dialog-box.


To select the Points

1.Ctrlb key 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 two selections that are possible:

Point 11 and Point 5


- or -

Point 11 and Point 7.

MD-Dialogue-SelectPoints-not valid

Point 5 and Point 7 are not possible.

Point 5 and Point 7 are joined with a Pin-Joint, already.


STEP 4: Select two Points.

To select the Points:

Ctrlb key down
Click the two Points that you want to join with a Pin-Joint.
STEP 5: Click OK in the Select-Elements dialog-box.

The dialog-box closes. The Points snap together.

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

Help Files for MechDesigner and MotionDesigner 11.1 + © Machine, Mechanism, Motion and Cam Design Software by PSMotion Ltd