﻿ 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 a convenient name we give to a mechanism that has an input that rotates continuously (the Crank), and an output that oscillates (the Rocker).

Reminder:

Kinematically-Defined Parts have Green Part-Outlines. We know the position, velocity, and acceleration of all Points in a Part that is kinematically-defined.

Parts that are NOT kinematically-defined have Blue Part-Outlines.

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

At the end of all Tutorials, the Parts should have Green Part-Outlines.

### Summary of this Step

1.Add two more Parts and three more Joints to the model.

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

4.We review the Kinematics-Tree

### New Terminology

 Rocker : The name in the Kinematics-Tree for a Motion-Part that rotates - it may index progressively, rotate continuously, or oscillate back-and-forth. Motion-Part : A Part that is directly driven by 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 5 possible Dyads that we can construct on a Plane. Dyad Closure : Different ways to assembly the same Dyad Four-bar : A kinematic-chain with one Motion-Part and one Dyad - most typically an R-R-R dyad.