﻿ Getting Started Tutorials - MechDesigner > Tutorial 2A: Four-bar Kinematic-Chains > Step 2.1.A: Dyad and Change Dyad Closure

### Video of Tutorial 2A: STEP 2.1

 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 and •Three(3) Joints

 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.

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 Parts in the dyad, NOT the Motion-Part (Rocker/Crank). 3.Mechanism-Editor: Click the Kinematics toolbar > Change Dyad Closure 4.Mechanism-Editor: Click in the Command-Manager.

Do 1 - 3 again and again.

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