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 from we completed Tutorial 1

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

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 graphic-area)

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

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

The new Part is in the graphic-area.

Degrees-of-Freedom: The 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: The 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 graphic-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 also indicates to you that the Parts are now kinematically-defined and solved.

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

Key Information:

Add a Motion-Part to kinematic-chain that is kinematically-defined or solved, and the kinematic-chain is also solved with the Motion-Part.

Add any dyad (two Parts with any three joints) to a kinematic-chain that is kinematically-defined or solved, and the kinematic-chain is also solved with the dyad.

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)

Anaswer:

The standard kinematic-term for Pin-Joint is Revolute-Joint. Thus, the 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:

•Rocker: a Motion-Part that can rotate with ANY motion. As required, it may rotate continuously or oscillate back and forth.

Step 2.1: Build a 4-bar Crank-Rocker