Getting Started Tutorials - MechDesigner > Tutorial 2A: Four-bar Kinematic-Chains > Step 2.4: R-R-P Dyad Closures: Valid, Invalid, or Broken

Objective of this Step.

We will edit the Crank-Slider to be an Offset Crank-Slider.

We explore the Kinematics-Tree and the elements in the :

•Rocker (Motion-Part)

•R-R-P dyad

Use the Kinematic elements toolbar > Change Dyad Closure command to see:

•4 × Closures with the R-R-P dyad

Understand why and when:

•Dyad-closures can solve

•Dyad-closures cannot solve

Terminology

Term :	Definition
Machine-Cycle :	The MMA changes by 360.
Motion-Part :	A Part whose position we control with a Motion-Dimension FB.
Rocker :	A Motion-Part whose angle we control with a Motion-Dimension FB.
Crank :	A Rocker whose angle changes with a uniform angular velocity.
Rotating-Part :	Any Part that you join with a Pin-Joint to a different Part.
Sliding-Part :	Any Part that you join with a Slide-Joint to a different Part.
Dyad :	A dyad is a kinematic construction of: •2 Parts, and •3 Joints
Dyad Closure :	A different way to assemble the two Parts in a dyad.
Crank-Slider Mechanism:	A kinematic-chain that has continuously-rotating Rocker (Crank) and a dyad that has two Pin-Joints and one Slide-Joint. The output is the sliding-Part that reciprocates back and forth.
Offset Crank-Slider Mechanism :	A Crank-Slider Mechanism in which the Pin-Joint (x-R-x) of the R-R-P dyad is offset from the Slide-Joint (x-x-P)

Kinematics-Tree Symbols

	Unrestricted Dyad Closure :	The dyad can solve throughout the machine-cycle.
	Restricted Dyad Closure :	The dyad cannot solve for a period within a machine-cycle.
	Broken dyad :	The dyad cannot solve now at this machine-angle.

Videos

There are two videos. They show the dyad-closures when they are broken and not broken

As you look at these videos, identify the Motion-Part. The Motion-Part ALWAYS moves with the motion-values that are at the input-connector of the Motion-Dimension FB.

The videos show: Unrestricted and Restricted Dyad Closure and Invalid Closures of the R-R-P dyad.

Video 1: CRANK + R-R-P dyad - 4 VALID Closures

Video 2: CRANK + R-R-P dyad - 2 VALID & 2 INVALID Closures

Kinematics-Tree and the R-R-P dyad

GST-2-316

Expand the Kinematics-Tree

•1 × Kinematic-Chain

•1 × Rocker

•1 × R-R-P dyad

Dyads always have five elements:

1.Pin-Joint (Revolute Joint)

2.Part

3.Pin-Joint (Revolute Joint)

4.Part

5.Slide-Joint (Prismatic Joint)

Change the Crank-Slider to an OFFSET Crank-Slider

GST-2-401

STEP 1: Edit the sliding-Part

1.Double-click the Part-Outline of the sliding-Part

The sliding-Part opens in the Part-Editor

GST-2-402

STEP 2: Add Geometry - a short Line

1.Click Geometry toolbar > Add Line

Drag up from near to start-Point of the CAD-Line

STEP 3: Add Constraints

1.Click Constraints toolbar > Add Coincident

Click the start-Point of the Line and the start-Point of the CAD-Line.

2.Click Constraints toolbar > Add Vertical

Click the Line

3.Click Geometry toolbar > Add Dimension

Click the Line, OR Click the end-Point of the Line and the CAD-Line or X-axis

4.Exit the Part-Editor

Double-click the Y-axis (see image)

You are now in the Part-Editor.

STEP 4: Delete the Pin-Joint

1.Click the Pin-Joint only - if you can select the Pin-Joint only - see image

2.Click the Delete key on your keyboard.

The two Parts are again not kinematically-defined, or not solved.

If you cannot click the Pin-Joint only, then, as described in detail in Step 2.3:

•use the Selection-Window to delete the Pin-Joint

•delete the Pin-Joint from the Assembly-Tree.

GST-2-403

GST-2-404

GST-2-405

STEP 5: Move the sliding-Part; Add the Pin-Joint again.

1.Drag the sliding-Part along the Slide-Joint.

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

Click the end-Point of the CAD-Line in the rotating-Part

Click the end-Point of the Line in the sliding-Part.

The R-R-P dyad is again complete.

The three Joints in the dyad are:

Revolute - Pin-Joint

Prismatic - Slide-Joint

Now we use the term Offset Crank-Slider.

Is this the dyad-closure you want?

Save your Mechanism : CTRL+S

GST-2-406

Offset Crank-Slider: Valid and Invalid R-R-P Dyad Closures

Four Valid Closures
	In the image you can see: 1 x Arc with a center at the Pin-Joint (R-x-x) 2 x Lines that are parallel to the Slide-Joint (x-x-P) In the image you can see that the Arc and Lines intersect in four places. 4 x indicate where the Arc and Lines intersect. These are the positions of the Pin-Joint in the four dyad-closures.
Closure 1 Closure 2 Closure 3 Closure 4
Two Valid Closures and two Invalid Closures
	In the image you can see 1 x Arc with a center at the Pin-Joint (R-x-x) 2 x Lines that are parallel to the Slide-Joint (x-x-P) In the image you can see that the Arc intersects only one of the two Lines. 2 x indicate where the Arc and Lines intersect. These are the positions of the Pin-Joint in the two VALID dyad-closures.