Mechanism | Change Dyad Closure

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Mechanism | Change Dyad Closure

What is a 'Dyad Closure'

A Dyad is always two(2) Parts and three(3) Joints. When it is possible to assemble the two parts in a different way, such that the motion of the Parts is different, then the Dyad has two or more Closures.

When you first add the Parts and Joints, the Parts will frequently assemble in a closure you do not want. You use this tool - Change Dyad Closure - to change the closure to the closure you do want.

The two Parts that form the Dyad must be kinematically-defined before you can use this command.

See Dyads

Note: Dyads are also called Assur Groups

Change Dyad Closure

Change Dyad Closure icon

Kinematic Elements Toolbar : Change Dyad Closure icon

Menu :

Add menu > Mechanism sub-menu > Change Dyad Closure

RRR Dyad - Closure 1

RRR Dyad
Closure 1

In the Graphic-Area

1.Click Change Dyad Closure.

2.Click a Part-Outline of one of the two Parts in the dyad with the closure you want to change.

3.Click OK-tiny-11-15 in the Command-Manager

The dyad re-assembles in a different closure.

Dyads have 2 or 4 Closures.

Thus, if necessary:

4.Do 1 to 3 again and again to find the closure you want.


RRR Dyad - Closure 2

RRR Dyad
Closure 2


If there are many Dyads in the model, you can explore the Kinematics-Tree to help you identify each Dyad.

In the Kinematics-Tree

1.Click and explore the Kinematics-Tree

2.Click each Dyad

The Parts and Joints become red in the graphic-area

tog_plusChange the Closure of the RRP Dyad


The RRP Dyad name indicates the kinematic joint names: Revolute - Revolute - Prismatic

In MechDesigner, the joint names, in the same order, are: Pin-Joint - Pin-Joint - Slide-Joint.

The Motion-PartRed-14-1b of this model is a Rocker/Crank. We have fixed its position at 90º to the inclined Line.  In this position,

STEP 1: Click a Part-Outline in the Dyad (Red-14-2 or Red-14-3)

In this case, I have clicked the Part-Outline of PartRed-14-2

The end of PartRed-14-2 can only lie on the red circle unless I edit its length.

This means the Pin-Joint that joins the PartRed-14-2 to the Sliding-Part must also be on the red circle.

Change Dyad Closure icon

STEP 2: Click Kinematic-elements toolbar > Change Dyad Closure

STEP 3: Click OK-tiny-11-15 in the Command Manager

The Closure will usually change immediately. However, it is possible the Dyad cannot connect the joints when in a different closure.




This image shows the four possible positions of the middle Pin-Joint. They lie on the red circle.


It is possible that the Dyad is in a closure that cannot physically connect the Joints. In this case, when you cycle the mechanism, the Joints will 'brake'. In this case, do Steps 1, 2 and 3 again to find if the joints can assemble.

If after you change the closure of a Dyad it is still not in the correct closure, Change Dyad Closure again.

The RRR Dyad has 2 closures.

The offset RRP Dyad has 4 closures. (see above for schematic)