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

# Step 2.4: Dyad Closures: Valid & Invalid or Broken

This is an important topic about

The three joint that identify each type of Dyad.

We use Kinematic elements toolbar > Change-Dyad-Closure again and again until the closure is correct for your design.

### Objective of this Step.

To understand that Motion-Parts move (are forced to move) with the motion-values at input to a Motion-Dimension FB.

To explore the Kinematics-Tree to see Motion-Parts, the R-R-R-Dyad and R-R-P Dyads.

To understand why there are 2 Closures for the R-R-R Dyad and 4 Closures for the R-R-P Dyad.

To use the Kinematics toolbar > Change Dyad Closure command.

Note: Derived terminology and terms in this step:

Valid Dyad Closure: the state in which the joints in a dyad do not break over a machine cycle.

Invalid Dyad Closure: the state in which the joints in a dyad do break over the complete or a period of the machine cycle. When the joints are in the invalid dyad closure state, they are Broken.

### Videos

I have prepared four important videos. They help us understand Closures, and when they become valid or invalid.

The videos show:

Valid and Invalid Closures

As you look at these videos, identify the Motion-Part. The Motion-Part ALWAYS moves with the motion-values at the input-connector of the Motion-Dimension FB. , or Motion-Path FB, Pulley-Rocker, Geared-Rocker, ...

#### Video 1: CRANK + R-R-R Dyad - 2 VALID Closures

 Tutorial 2; Step 2.4 R-R-R-Dyad & 2 VALID Closures. The joints do not break.

#### Video 2: CRANK + R-R-R Dyad - 2 INVALID Closures (Broken Closures)

 Tutorial 2; Step 2.4 R-R-R-Dyad & 2 INVALID Closures. The joints do break.

#### Video 3: CRANK + R-R-P Dyad - 4 VALID Closures

 Tutorial 2; Step 2.4 R-R-P Dyad & 4 VALID Closures. The joints do not break.

#### Video 4: CRANK + R-R-P Dyad - 2 VALID & 2 INVALID Closures

 Tutorial 2; Step 2.4 R-R-P Dyad 2 VALID Closures. •The joints do not break R-R-P-Dyad & 2 INVALID Closures •The joints do break

### Kinematics Tree and the R-R-R Dyad

 I have increased the thickness of the Part-Outlines for the two Parts in the R-R-R Dyad. Motion-Part - the Fixed Rocker •The position of a Motion-Part is specified by the angle from the Motion-Dimension FB. •It will not move if do Change-Dyad-Closure. Dyad Part 1 •Part 1 has two Joints: J1 and J3. •J1 is fixed by the end of the Motion-Part •If J3 is not made, then Part 1 can rotate freely about J2 - indicated by the ARC Part 2 •Part 2 has two Joints: J2 and J3 •J2 is defined by its position in the Base-Part, the Frame. •If J3 is not made, then Part 2 can rotate freely about J2 - indicated by the ARC J3 can only at the two intersections of the two Arcs. The two ARC intersections indicate the position of the two Parts in the two Closures.

#### Change the Closure of the R-R-R Dyad

 To change the closure of a dyad: STEP 1: Click the Part-Outline of one of the two Parts in the dyad. In this image, we have clicked Part 2 in an R-R-R dyad. Remember: R is the abbreviation for Revolute. Revolute is identical to the Pin-Joint. Kinematic-elements toolbar > Change Dyad Closure   STEP 2: Click Kinematic-Elements toolbar > Change Dyad Closure in the  (left of the graphic-area) The Part should be in the selection-box in the Command-Manager. If it is not, then click the Part-Outline again in the graphic-area. STEP 3: Click in the Command-Manager The dyad changes to a new closure. Do STEPS 1 – 3, again and the closure of the dyad will change again from Closure 2 > 1, and then Closure 2 > 1.

 It is possible that when you cycle the Input Motion-Part (in this case a Rocker or Crank,) the combined length of the Parts cannot reach the end of the Motion-Part. The joints in the dyad break. In the image, the Motion-Part is a Crank. Length of the Crank + Length of the Line in the Base-Part > Length of the two Parts in the R-R-R Dyad.   The Pin-Joint that is at the end of Part breaks.. Please review the Grashof Criterion to help you understand the inequalities of link-lengths that break a four-bar mechanism.