What is a 'Dyad Closure'

When it is possible to assemble the two parts in a dyad in different ways, such that their motion becomes different, then the dyad has two or more 'Closures'.

Dyads are also called Assur Groups [but not in this help].

The two Parts in the dyad must be kinematically-defined Parts [default Green Part-Outlines].

See Dyads

Change Dyad Closure

Change Dyad Closure icon

Kinematic Elements Toolbar : Change Dyad Closure icon [left of graphic-area]

Menu :

Add menu > Mechanism sub-menu > Change Dyad Closure

What to do :

In the Graphic-Area

1.Click a Part-Outline of one of the two Parts in a dyad whose closure you want to change.
2.Click Kinematic elements toolbar > Change Dyad Closure.
3.Click OK-tiny-11-15 in the Command-Manager

The dyad re-assembles in a different closure.

If required:

4.Do 1 to 3 again to change the closure.
KINEMATICS TREE RRR Dyad 2 Parts, 3 Pin-Joints

RRR Dyad
2 Parts, 3 Pin-Joints

In the Kinematics-Tree

1.Click and explore the Kinematics-Tree tab - to the right of the Assembly-Tree tab
2.Click each dyad - they expand to show you the two Parts and three Joints.

Also, the Parts and Joints in the dyad become red in the graphic-area. When you find the dyad whose closure you want to change:

3.Click one of the two Part in the Kinematics-Tree
4.Click Kinematic Elements Toolbar > Change Dyad Closure.
5.Click OK-tiny-11-15 in the Command-Manager

Dyads have up to four closures. Thus, you may need to:

6.Do 1 to 5 again to change Closure.

tog_plusExample: Change the Closure of the RRP Dyad

Use the Kinematics-Tree to identify the Kinematic-Chain and the Dyad that is in a closure you want to change.



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-Part1s of this model is a Rocker/Crank. We have fixed its position at 90º to the inclined Line.  In this position,

STEP 1: Click one of the two Part-Outlines in the Dyad [2s or 3s]

In this case, I have clicked the Connecting Rod2s


One end of the The end of the Connecting Rod Part2s can only lie on the red circle unless I edit its length.

This means the Pin-Joint that joins the Connecting-Rod 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]


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