Getting Started Tutorials - MechDesigner > Tutorial 2A: Four-bar Kinematic-Chains

Build a Crank-Rocker

Summary of this Step

1.Add two(2) more Parts and three(3) more Joints to the model that we completed in Tutorial 1

2.Explain the concept a dyad, and the R-R-R dyad in particular

3.Review the kinematic elements in the Kinematics-Tree

4.Explain why a dyad does not change the Mobility of a kinematic-chain.

Terminology

Term :	Definition
Motion-Part :	A Part whose motion you control with a Motion-Dimension FB.
Rocker :	The name in the Kinematics-Tree for a Motion-Part that rotates.
Crank:	Generic term for a Rocker whose motion is uniform angular velocity.
Dyad :	An assembly of two Parts and three Joints.
R-R-R dyad :	A dyad with three Pin-Joints - this is one of the five dyads that we can construct on a Plane.
Dyad Closure :	A different way to assemble (construct) the two Parts in a dyad.
Four-bar :	A kinematic-chain with one Motion-Part (typically a Crank) and one dyad (typically an R-R-R dyad).
kinematically-defined :	A Part or kinematic-chain whose Mobility=0. The Parts are solved.

Video of Tutorial 2A: STEP 2.1

Add the Parts and Pin-Joints

Note: Add Parts that are approximately equal in length to the Parts that are in the images.

GST-2-1-101

STEP 1: Add a Part

1.Click Kinematic-elements toolbar > Add Part (left of graphics-area)

1.Keyboard Shortcut: Click the INSERT key on your keyboard

a.Drag in the graphics-area to add the Part.

The new Part is in the graphics-area.

Degrees-of-Freedom: Each Part has three degrees-of-freedom.

GST-2-1-102

STEP 2: Add a Pin-Joint

1.Click Kinematic-elements toolbar > Add Pin-Joint

a.Click the start-Point of the new Part that is free

b.Click the end-Point of the Rocker

The Points move to together at the new Pin-Joint.

Degrees-of-Freedom: Each Pin-Joint removes two degrees-of-freedom from the Part. The new Part now has one degree-of-freedom.

STEP 3: Do STEP 1 and STEP 2 again

	1.Click Kinematic-elements toolbar > Add Part
	OR 1.Keyboard Shortcut: Click the INSERT key on your keyboard
	a.Drag in the graphics-area to add the Part.
	2.Click Kinematic-elements toolbar > Add Pin-Joint
	a.Click the start-Point of the new Part that is free b.Click the end-Point of the Line in the Base-Part

The Points move to together at the new Pin-Joint.

Degrees-of-Freedom: The two new Parts have a total of two degrees-of-freedom.

GST-2-1-103

GST-2-1-104

STEP 4: Add a Pin-Joint

1.Click Kinematic-elements toolbar > Add Pin-Joint

a.Click the end-Point of one of the new Parts that is free

b.Click the end-Point of the other new Part that is free

The Points move to together at the new Pin-Joint

Degrees-of-Freedom: The Pin-Joint removes two degrees-of-freedom.

The two new Parts have zero degrees-of-freedom. The Mobility =0.

The kinematic-chain is kinematically-defined, and solved. Perfect.

Save your model - CTRL+S.

You can see that the Part-Outlines are Green (or a type of green). This color indicates to you that the Parts are kinematically-defined and solved.

You MUST make sure all of the Parts in the model are kinematically-defined before you analyze the motions and forces that act on Parts.

Key Information:

A Motion-Part does not change the kinematic state of a kinematic-chain.

A dyad (two Parts with three joints) does not change the kinematic state of a kinematic-chain.

The two new Parts may not assemble in the same way as the image above.

See Step 2.1A to reconfigure the Closure of the Dyad.

Dyad - Kinematics-Tree

Explore the Kinematics-Tree:

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

If you do not see Mechanisms, ...

2.Click Edit toolbar: Rebuild Now

3.Click to the left of Mechanisms

Kinematic-Chain

Explore the Kinematic-Chain

There is one Kinematic-chain

1.Click to the left of Kinematic-Chain

You can see the Rocker and the R-R-R dyad.

Elements in the Rocker - the Motion-Part.

1.Click to the left of the Rocker - there are three elements

a.Pin-Joint

b.Part

c.Motion-Dimension FB

The motion-values at the input to the Motion-Dimension FB are not important.

Elements in the R-R-R dyad:

1.Click to the left of the R-R-R dyad - there are five elements.

a.Pin-Joint

b.Part

c.Pin-Joint

d.Part

e.Pin-Joint

Question:

Why is it an R-R-R dyad and not a P-P-P dyad (Pin-Joints)

Answer:

The standard kinematic-term for Pin-Joint is Revolute-Joint. Therefore, it is R-R-R dyad.

Rocker in the Kinematic-Tree

R-R-R dyad in the Kinematic-Tree

Dyad Symbols:

if the joints in the dyad do not break in a machine cycle

if the joints in the dyad must break in the machine cycle

if the joints in the dyad are broken now

Notes: Crank vs Rocker

Unfortunately, it is easy to confuse the terms Crank and Rocker.

Generic (engineering) terms:

•Crank: a Part that rotates continuously

•Rocker: a Part that oscillates back and forth

Kinematics-Tree terms in MechDesigner:

•A Rocker is a Motion-Part that can rotate continuously, or oscillate back and forth, indexes progressively, ...

Step 2.1: Build a 4-bar Crank-Rocker