RPR 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, add the Dyad to a Rocker
In Case 2, add the Dyad to a Slider

There are two Pin-Joints and one Slide-Joint in the RPR Dyad, and two Parts.

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 be able to reach the Joints. To correct the problem, you may need to change the Dyad closure until the assembly operates as you expect it to.

See Kinematic Closure

tog_minus        '3D Sample Video of Case2: Slider connected to an RPR Dyad'.

Slider and an RPR Dyad

1.A Motion is designed to move a gripper in time with the rest of the machine.
2.A Slide-Joint is added to define the direction of motion for the gripper.
3.The gripper slides along a slide rail.
4.A Pin is mounted in the gripper – shown in blue. This is an R Joint of the Dyad
5.A link – in red – is mounted to pivot on the machine frame. This is a different R Joint.
6.The pin is mounted in a slot cut into the red link – the P joint.

A Cam drives the red link.


A Design-Set was used to improve the initial layout of the Mechanism.

Rocker and Base-Part

Slider and Base-Part



'Expand' then 'Play'



'Expand' then 'Play'






Two Kinematically Defined Parts:

Case 1: Part 1:Base-Part, Part 2:Rocker
Case 2: Part 1:Base-Part, Part 2:Slider



Step 1: Two New Parts

Add two Parts.



Step 2: Two 'Outer' Joints

Add the two 'outer' Joints (R-R Joints). In this case, a:

Pin-Joint with a new Part and the Motion PartRed-14-1b
Pin-Joint with the other new Part and the Base-Part Red-14-2

The two Joints are the two 'outer' Joints of the Dyad: R-R

RPR Dyad with Rocker

RPR Dyad with Slider

Step 3: One 'Inner' Joint

Add the 'inner' Joint (- P-). In this case, join the two new parts together with a Slide-JointRed-14-3

The Kinematic-Chain becomes Kinematically Defined

Note: The Parts seem to 'flip' into a different 'assembly' closure. In fact, the orientation of the Parts are a function of the direction of the Lines in each Part.




Shows the original Rocker or Slider, plus the new RPR Dyad.

Tutorials and Reference Help Files for MechDesigner and MotionDesigner 14.2 + © Machine, Cam, Mechanism, and Motion Design Software by PSMotion Ltd