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).

Terminology 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. We do not know the velocity or acceleration of Points in these Parts.

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.
3.Use the Change Dyad Closure
4.We review the Kinematics-Tree to help us understand the elements we need for Motion-Parts and Dyads.

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 - 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 a R-R-R dyad.

Video of this Step


Tutorial 2: STEP 2.1


2 Parts and
3 Pin-Joints (R-R-R Dyad)
Change Dyad Closure


Add the Parts and Pin-Joints


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

To add a Part:

1.Mechanism-Editor: Click Kinematic-elements toolbar > Add Part
2.Mechanism-Editor: Drag to add the Part (Left-mouse-button-down, Move your mouse, Left-mouse-button-up)

The Part has a derived name of free-Part. - it  has a Blue Part-Outline


To add a Pin-Joint:

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


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

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

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


STEP 4: Add a Pin-Joint
1.Mechanism-Editor: Click Kinematic-elements toolbar > Add Pin-Joint
2.Mechanism-Editor: Click the end-Point of the Part we added in STEP 1
3.Mechanism-Editor: Click the end-Point of the Part we added in STEP 3

The two Parts are kinematically-defined Parts and have Green Part-Outlines.

The Parts may not be in the configuration as shown to the left.

Even if the Parts are in the configuration, please read and do the next step.

The Concept of a Dyad - Please Read


I have added a 'red shape' around the new Parts (labelled Part 1 and Part 2) and new Pin-Joints (labelled R, R and R)

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

Dyads always have exactly:

Two(2) Parts


Three(3) Joints

Each Dyad always has three letters, which represent the type of Joints.

The two types of Joint in planar Dyads are R and P.

R - is a Revolute-Joint. This is identical to a Pin-Joint

P - is a Prismatic-Joint. This is identical to a Slide-Joint. (we will add a Slide-Joint in Step 2.3).

We have added three Pin-Joints. (see above) Thus, it is the R-R-R Dyad - or the Revolute-Revolute-Revolute Dyad.

If the Part-Outlines are Green before you start to add a Dyad, the Part-Outlines are Green after you add a Dyad. Why is this?

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

2 × Parts = + 6 degrees-of-freedom.

Each new Joint removes 2 degrees-of-freedom.

3 × Joints = – 6 degrees-of-freedom

Thus,, we add then subtract 6 degrees-of-freedom. Thus, if the Part-Outlines were Green before, they are Green after.

See Degrees-of-Freedom and Mobility of Motion-Parts and Dyads, for more information.

Is the Dyad in the Correct Closure?

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

After you add the last Pin-Joint, MechDesigner may put the Parts in a closure that you do not want in your machine. In this case, you must use the 'Change Dyad Closure' tool to put the dyad in the closure you do want in your machine.

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


Kinematic-elements toolbar > Change Dyad Closure


1.Click one of the two Parts in the Dyad, NOT the Motion-Part (Crank).
2.Click the Kinematics toolbar > Change Dyad Closure
3.Click OK-tiny-11-15 in the Command-Manager.

Each time you do 1 – 3, the closure of the dyad will change its Closure.

Cycle the kinematic-chain in each closure

1.Click Run menu (or toolbar) > Cycle , or ALT+C on your keyboard, to start the cycling.
2.Press ALT+C again on your keyboard again to stop.

We will learn more about Dyad Closures in Step 2.4. Change Closure of Dyad.

The Kinematics Tree


To explore the in the Kinematics-Tree:

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

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

The Rocker is Motion-Part.  Motion-Parts laways have three elements:

c.Motion-Dimension FB

The R-R-R Dyad - All dyad have five elements:

d.Pin-Joint (Revolute Joint)
f.Pin-Joint (Revolute Joint)
h.Pin-Joint (Revolute Joint)

Dyad Symbols:

solved is next to the R-R-R Dyad when the joints do not break at all MMA and Machine Cycle.

broken shows if the Dyad 'breaks' for a period (even if an infinitesimally small period) within the machine cycle.

Brocken replaces broken when, as you cycle the mechanism, the MMA is at an angle such that the Dyad is actively 'broken'.

Tutorials and Reference Help Files for MechDesigner and MotionDesigner 14.2 + © Machine, Cam, Mechanism, and Motion Design Software by PSMotion Ltd