Kinematic Closure of Dyads

It is possible to assemble the Parts in a Dyad so that the Parts are in different positions.

The different positions are called 'Dyad-Closures', or simply 'Closures'.

When you first add a Dyad, MechDesigner can only guess which closure you want in your design.

Therefore, we have a tool - 'Change Dyad Closure' - to move the dyad to the closure you want.

Use 'Change Dyad Closure' more than one time when the Dyad has more closures.

After you have set the Dyad to the correct closure, it will not change again.

Objective of this Step.

Use the Change Dyad Closure tool
Review how to identify 'Solved' and 'Not-Solved' Dyad-Closures.

tog_minus        Video Showing Solved and Not-Solved Closures of an Offset Slider

Change a Dyad-Closure.

tog_minus        Solved Closures - RRR Dyad

The RRR Dyad has zero or two Closures.


This Image shows a four-bar mechanism.

Fixed Rocker

The input part to the mechanism. Its position will not move when we use the 'Change-Closure' tool.
It is a Motion-Part. Its position is defined by the angle in the Motion-Dimension FB.

The Dyad

Part 1
oPart 1 has two Joints: J1 and J3.
oJ1 is fixed by the end of the 'Fixed Rocker'.
Part 2
oPart 2 has two Joints: J2 and J3
oJ2 is defined by its position in the Base-Part, the Frame.

Kinematic or Dyad Closure- four-bar Mechanism

In this image, I have added two circles to the mechanism.

Circle 1

Centre-Point is at J1. Radius: equal to the length of Part 1.
The Circle must pass through J3

Circle 2

Centre-Point is at J2. Radius: equal to the length of Part 2.
The Circle must pass also through J3

In this kinematic-chain, the circles interact at two positions: one position is at J3, the other at 'J3'.

These are the two possible Closures. Closure 1 [J1–J3–J2] OR Closure 2 [J1–J3–J2].

It is also possible that the circles do not intersect. Theoretically, they can intersect at one point.


This schematic sketch shows the two possible ways you can assembly the two Parts in the RRR Dyad.

Closure 1 is in Blue

Closure 2 is in Green

tog_minus        Closure that are Solved and Not-Solved - 'Offset' RRP DYAD


Four Closures : Four Solved Closures

'Offset RRP Dyads' have four closures. This images shows the four Closures (C1, C2, C3 and C4).

You can see that the:

Circle shows possible positions of the end of Part 1.
Two parallel Lines show all of possible position of the Point that is offset to the sliding-part - Part 2.

The sliding-part must 'flip over' to give the two parallel lines.

The Circle can intersect two parallel Lines at four Points - shown as C1, C2, C3 and C4 in the image.


Four Closures: Two Solved and two Not-Solved Closures

In the image, the rotating-Part is shorter, and and it can intersect with only one Line and not two Lines.

Closures: C1 & C4: It is possible to find a solution for these two closures

Closures: C2 & C3: It is NOT possible to find a solution for these two closures.

As you use the 'Change Dyad Closure' tool, MechDesigner will 'attempt' to make all four closures. However, when it attempts to find a solution for Closure C2 and then C3, it will not move from C1. But it will then move to Closure C4. Then it will move to Closure C1, again.


The clue is in the Kinematics-Tree.

If you see the Dyad with a red cross1s-red, then it is not-solved.

tog_minus        To change the Closure of a Dyad

Dyad Closure2

The order of these two Steps is important.

STEP 1: Click one of the two Parts in the Dyad.
STEP 2: Click the Change Dyad Closure icon in the Kinematic-elements toolbar.

Note: If you click the 'Change Dyad Closure' icon before you click the Part, then deselect the icon, and then follow the two steps.

tog_minus        More than two Closures: Four(4) Valid Closures of the Offset RRP Dyad


STEP 1: Click one of the Parts in the Dyad that you want to change closure.

Here, I have clicked the 'Connecting Rod' in the RRP Dyad.

Dyad Closure2

STEP 2: Click the Change Dyad Closure icon in the Kinematic-elements toolbar.
Dyad Closure 1- RRP

Dyad Closure 1- RRP

Dyad Closure 2 - RRP

Dyad Closure 2 - RRP

Dyad Closure 3 - RRP

Dyad Closure 3 - RRP

Dyad Closure 4 - RRP

Dyad Closure 4 - RRP

To change the closure of a Dyad


Change Closure with the Change Closure tool

Procedure 1

Most useful when you can easily identify the Dyad in the graphic-area.

STEP 1: Click the Part-Outline of a Part in a Dyad you want to change the closure
STEP 2: Click the 'Change Dyad Closure' icon in the Kinematic-elements toolbar

Change Dyad in Kinematics-Tree

Procedure 2

If it is difficult to identify which Parts are in a Dyad, you can explore the Kinematic-Tree.


STEP 1:Click the Kinematics-Tree tab in the Element Explorer
STEP 2:Click each kinematic-chain to identify each in the graphic area.

When you identify the kinematic-chain with the Dyad

STEP 3:Expand the kinematic-chain to explore it.


The next steps are important! Three clicks!!! Left, Right, Left.

STEP 4:Click: Left-Click the Dyad in the Kinematic-Chain.

The icon for the Dyad now has a red square round it.

Step 2 is Complete.


STEP 5:Click: Right-click the icon with a red square to show the contextual-menu
STEP 6:Click: Left-Click 'Change Dyad Closure' in the menu

The Dyad changes its closure.

Note: The first time you use this command, you may need to repeat these three clicks to change the closure. If you do, click the graphic-area to clear all elements from the Selection-Window.

Do 3–5 again, to change the closure again.

Some Dyads have four closures. You may need to repeat Steps 4-6 four times to return to the original closure.

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