Six-bar Kinematic-Chains

Objectives of this Tutorial

To learn more about dyads

To learn how to add dyads to any kinematically-defined chain. You will see that you can, in principle, add any number of dyads to each kinematic-chain.

In this tutorial, we will:

1.Add RRR and RRP dyads to build slightly more complex kinematic-chains.
2.Show that a:
a.Slide-Joint does not need to use the Part's CAD-Line that is from its Origin to its end-Point.
b.Pin-Joint does not necessarily need to use the start-Point or end-Point of the CAD-Line along the Part's centre.
3.Learn a little about six-bar* kinematic-chains.

* The term 'bar' is used more frequently than 'Part' when applied to mechanisms. For example four-bar, six-bar, eight-bar...


Terminology: Degrees-of-Freedom of Dyads and Motion-Parts

Dyad :

See also Kinematics-Tree.

A new dyad does not change the number of degrees-of-freedom (DOF) of a kinematic-chain.

Why? Each dyad has two Parts and three Joints, and:

Each Part adds 3 DOF*. Thus, two new Parts add to the model:  +2Parts × +3DOF = +6DOF
Each Joint removes 2 DOF. Thus, three new Joints remove from the model: +3Joints × – 2DOF = – 6DOF

Thus, the change to the total Degrees-of-Freedom = +6 + (– 6) = 0.

* Parts that are on the Mechanism-Plane

Pin-Joint or Slide-Joints are on the Mechanism-Plane.

Motion-Parts :

See the Rockers or Slider in the Kinematics-Tree

A Motion-Part does not change the degrees-of-freedom of a kinematic-chain.

Why? Because a Motion-Part has one Part, one Joint, and one Motion-Dimension (a defined coordinate).

Each Part adds 3 DOF. Thus one new Part in a Motion-Part: 1Part × +3DOF = +3DOF
Each Joint removes 2 DOF. Thus, one new Joint removes from the model: +1Joint × –2DOF = –2DOF
Each Motion-Dimension removes 1 DOF. Thus, one new Motion Dimension: +1MD × –1DOF = –1 DOF

Thus, the change to the total Degrees-of-Freedom = +3 +(–2) + (–1) = 0(Zero).

Kinematically-Defined Parts

 

Green-Parts

'If all Parts have Green Part-Outlines before you add a Dyad, the Parts will also be Green after you add a Dyad'.

'If all Parts have Green Part-Outlines before you add a Motion-Part, the Parts will also be Green after you add a Motion-Part.'

Step by Step Instructions

Add more Dyads

6-Bar Kinematic-Chains: Crank + RRR + RRP - Pin-Joints and one Slide-Joint.

Experiment with different possibilities of the three joints in a Dyad: RRR, RRP, RPR, RPP, PRP

Also, change the driven Part from a Rocker to a Slider.

Tutorial and Reference Help Files for MechDesigner and MotionDesigner 13.2 + © Machine, Mechanism, Motion and Cam Design Software by PSMotion Ltd