Six-bar Kinematic-Chains

Objectives of this Tutorial

To learn more about dyads [Also called Assur Groups]

To learn how to add dyads to a kinematically-defined chain.

To learn that a Dyad does not change the degrees-of-freedom [DOF] of a kinematic-chain.

IMPORTANT concept about dyads

'If all Parts are kinematically-defined [Green Part-Outlines] before you add a dyad, the Parts will also be kinematically-defined and Green after you add a dyad'.

In this tutorial, we will:

1.Add R-R-R and R-R-P dyads to build slightly more complex kinematic-chains.
o'R' Joints are Revolute Joints, which are identical to Pin-Joints.
o'P' Joints are Prismatic Joints, which are identical to Slide-Joints.
2.Demonstrate that a:
a.Slide-Joint is not always along the CAD-Line of Added-Parts.
b.Pin-Joint is not always at the start-Point or end-Point of the CAD-Line of Added-Parts.
3.Learn a little about six-bar* kinematic-chains.

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

Terminology: Degrees-of-Freedom[DOF] : Dyads & Motion-Parts

Dyad :

See also Kinematics-Tree.

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

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

•Each Part adds 3 DOF*. Two Parts:  2.Parts × +3.DOF = +6.DOF
•Each Joint removes 2 DOF. Three Joints remove from the model: 3.Joints × – 2.DOF = – 6.DOF

Thus, the change to the total DOF = +6 + (– 6) = 0.

*Parts 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 (DOF) of a kinematic-chain.

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

•Each Part adds 3 DOF. One 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 :

IMPORTANT

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