﻿ Kinematics of an RPP Dyad

# Kinematics of an RPP Dyad

Dyads do not change the number of Degrees-of-Freedom of a kinematic-chain. If the kinematic-chain has Green Part-Outlines before you add the Dyad, it should have Green Part-Outlines after

To show you how to add this Dyad, we have two design cases.

 • In Case 1, we join the two Parts in the Dyad to the Base-Part and the Rocker [this usually forms a Mechanism called a 'Scotch Yoke', or an Oldham Coupling]
 • In Case 2, we join the two Parts in the Dyad to the Base-Part and the Slider

I follow a consistent three-step procedure.

#### General Three-Step Procedure to add a Dyad

 STEP 1: Add the two Parts
 STEP 2: Add the two 'outer' Joints: Join each Part to two separate Kinematically Defined Parts
 STEP 3: Add the 'inner' Joint: Join the two Parts together
 Note: Occasionally, a Dyad does not operate correctly. The Pin-Joints 'break' when you cycle the machine, even though you cannot see why they break. It is possible that MechDesigner has assembled the Dyad in a Closure where the Parts cannot reach the Joints - even though to you, they should assemble. To correct the problem, you may need to change the Dyad closure until the assembly operates as you expect it to.

CASE 1:RPP across a Rocker and Base-Part

CASE 2:RPP across a Slider with Base-Part

Examples...

 ◄ Videos

Preparation:

Two parts that are Kinematically Defined:

 • Case 1: Base-Part and a Rocker
 • Case 2: Base-Part and a Slider

Step 1:

Add two more parts to the Mechanism.

Note:

Add a line to one Part

Step 2:

Join the two new parts to the two Kinematically Defined Parts. Here, in both examples, the:

 • R (Pin-Joint) is connected to a Point in the Motion Part: Rocker or Slider
 • P (Slide-Joint) is connected to a line in Base-Part.

Step 3:

Finally, connect the two new parts together with P (Slide-Joint) Joint.

The Mechanism becomes Kinematically Defined again. The Part-Outlines are Green

(RPP)

If you look at the Kinematics-Tree, it will shows the

 • Original Rocker or Slider,
 • Plus the new RPP dyad

Expand the RPP dyad. There is

 • Two Parts
 • One Pin-Joint, Two Slide-Joints

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