Step 2.1.A: Dyad and Change Dyad Closure

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Step 2.1.A: Dyad and Change Dyad Closure

Video of Tutorial 2A: STEP 2.1


The Concept of a Dyad


Dyad - the Concept


In Step 2.1, we added two(2) Parts and three(3) Pin-Joints.

In the image to the left, I have added shapes around the two(2) new Parts (Part 1 and Part 2). The Pin-Joints are labelled as R, R and R.

The two Parts and three Pin-Joints are called a Dyad.

Dyad is an IMPORTANT kinematic term.  A dyad is also called an Assur Group in some kinematic journals.

Dyads always have:

Two(2) Parts


Three(3) Joints

Dyad - Degrees-of-Freedom

If a kinematic-chain is kinematically-defined before you add a dyad, it is also kinematically-defined after you add a Dyad.

Why is this?

Each new Part adds three(3) degrees-of-freedom (when on a Plane)

3 x 2 Parts = + 6 degrees-of-freedom.

Each new Joint subtracts two(2) degrees-of-freedom.

2 × 3 Joints = – 6 degrees-of-freedom

Therefore, we add 6 degrees-of-freedom and we also subtract 6 degrees-of-freedom.

Dyads - Dyad Closures

Frequently, it is possible to arrange the two Parts that are in a dyad into different configurations. Each configuration is called a Dyad Closure, or Closure.

After you add the last Pin-Joint, in Step 2.1, MechDesigner may arrange the dyad in the closure that you do not want!

When the dyad is not in the closure you want, you must use the Change Dyad Closure tool to re-arrange the dyad to the closure that you do want.

We can change the Closure again and again, Thus, do this exercise, even if the Closure is correct.


Kinematic-elements toolbar > Change Dyad Closure


Change the closure of a dyad:

1.Mechanism-Editor: Click Kinematic-elements toolbar > Change Dyad Closure

2.Mechanism-Editor: Click one of the two PartsRed-14-1 in the dyad, NOT the Motion-Part (Rocker/Crank).

3.Mechanism-Editor: Click the Kinematics toolbar > Change Dyad ClosureRed-14-2

4.Mechanism-Editor: Click GST-TICK-GREEN-22X22 in the Command-Manager.

Do 1 - 3 again and again.

We will use the Change Dyad Closure again in Step 2.4.


Dyad - Kinematics-Tree

Rocker in the Kinematic-Tree

Rocker in the Kinematic-Tree

To explore the Kinematics-Tree:

1.Click the Kinematics-Tree tab in the Elements Explorer

2.Click + to the left of Mechanisms, Kinematic-Chains, Rocker and R-R-R Dyad to expand and collapse the tree.

The Kinematics-Chain includes a Rocker and an R-R-R Dyad

A Rocker is a Motion-Part (different to a Dyad). It always has three elements:



C)Motion-Dimension FB

R-R-R dyad in the Kinematic-Tree

R-R-R dyad in the Kinematic-Tree

There are always five(5) elements in the R-R-R Dyad

A)Pin-Joint 1

B)Part 1

C)Pin-Joint 2

D)Part 2

E)Pin-Joint 3

Q: Why R-R-R.

A: The standard kinematic-term for a Pin-Joint is a Revolute-Joint. Thus, we use R in the dyad name.

And, the dyad name does not need to include the Parts, as there are always two Parts.

Thus, in the Kinematics-Tree, the R-R-R Dyad - or Revolute-Revolute-Revolute Dyad.

Please remember, that the R-R-R Dyad is identical to Pin-Pin-Pin Dyad.

We will look at Dyads with Slide-Joints later.

Dyad Symbols:

Icon-KT-SolvedAlways if the joints in the Dyad do not break in a machine cycle.

Icon-KT-SolvedBreaks if the joints in the Dyad must break in the machine cycle.

Icon-KT-SolvedbutBroken if the joints in the Dyad are broken now