﻿ Getting Started Tutorials - MechDesigner > Tutorial 2A: Four-bar Kinematic-Chains > Step 2.1: Build a 4-bar Crank-Rocker

# Step 2.1: Build a 4-bar Crank-Rocker

## Build a 'Crank-Rocker' Kinematic-Chain

A Crank-Rocker is the term for a mechanism. Its input rotates (the Crank) and its output oscillates (the Rocker).

Reminder:

To model a machine correctly, we MUST make sure all of the Parts in the model are kinematically-defined.

The color of the Part-Outline of a Part that is kinematically-defined (usually a type of Green) is different to the colour a Part that is NOT kinematically-defined (usually a type of Blue).

### Summary of this Step

1.Add two(2) more Parts and three(3) more Joints to the model

2.Explain the concept of the R-R-R Dyad

3.Use the Change Dyad Closure tool

4.Review the Kinematics-Tree

### New Terminology

 Rocker : The name in the Kinematics-Tree for a Motion-Part that rotates, and usually oscillates back-and-forth. We use the derived name: Crank If the Rocker rotates continuously. Motion-Part : A Part that is directly driven by motion-values at the input to a Motion-Dimension FB. Dyad : An assembly of two Parts and three Joints R-R-R Dyad : A Dyad with 3 Pin-Joints - this is one Dyad of five(5) possible Dyads that we can construct on a Plane. Dyad Closure : Different ways to assembly (construct) the same Dyad with Parts of the same Length. Four-bar : A kinematic-chain with one Motion-Part and one Dyad - most typically an R-R-R dyad.

### Video of Tutorial 2A: STEP 2.1

Video: Add two PArts, three-Joints- Four-Bar Mechanism

### Add the Parts and Pin-Joints

STEP 1:

 1.Mechanism-Editor: Click Kinematic-elements toolbar > Add Part, or click the INSERT key on your keyboard. 2.Mechanism-Editor: Drag to add the Part (Mouse-button-down, Move your mouse, Mouse-button-up)

The Part has a derived-name completely-free Part. Its Part-Outline is a Blue (or type of blue).

As you drag in the graphic-area to add the Part, try to make its length approximately 1.5 × the length of the Rocker.

STEP 2:

 1.Mechanism-Editor: Click Kinematic-elements toolbar > Add Pin-Joint 2.Mechanism-Editor: Click the start-Point of the free-Part 3.Mechanism-Editor: Click the end-Point of the Rocker

The Part has a derived-name free Part.

<<< The image shows the click at the end-Point of the Rocker. Then, the Pin-Joint is in the model.

STEP 3: Do STEP 1 and STEP 2 again

 1.Mechanism-Editor: Click Kinematic-elements toolbar > Add Part 2.Mechanism-Editor: Click Kinematic-elements toolbar > Add Pin-Joint

The two Parts are free Parts and have Blue Part-Outline.

See image - there is a Pin-Joint at the other of the Line in the Base-Part ( see Tutorial1, Step 3).