In this Tutorial

The four-bar is a Crank plus an R-R-R Dyad. In this tutorial we will add a new R-R-R Dyad to the four-bar we built in Tutorial 2A - or a new one if you want.

The names of the five Classic six-bar kinematic-chains are Watt 1 & 2, and Stephenson 1, 2 & 3.  You do not need to be concerned with their names.

Objective of the Tutorial

To build the classic six-bar kinematic-chains. They have Pin-Joints only. Why Pin-Joints only?

Why are they classic and why are they important?

In the past, Pin-Joints were much easier to manufacture, reproduce and less costly than Slide-Joints - they still are! Hence, there is a motivation to design useful kinematic-chains with only pin-joints.

In the 21st Century, you can purchase slide-ways from manufacturers (for example, from SKF, THK) that have incredible accuracy. It might seem, therefore, you can put slide-ways in your designs whenever you want a tool or end-effector to move along a straight-line. However, even now, there are advantages with Pin-Joints. If you can find a mechanism that has only Pin-Joints, the mechanism might perform better, be cheaper, easy to seal against contamination, be faster, and have more convenient locations of machine components.

The six-bar kinematic-chains with Pin-Joints are usually called:

Watt 1
Watt 2
Stephenson 1
Stephenson 2 - this is not easy to model in MechDesigner.
Stephenson 3

The six-bar kinematic-chains are very simple to do (with the exception of the Stephenson 2).

1.Start with a four-bar mechanism: a Crank and R-R-R dyad - all Pin-Joints -  see Step 2A
2.Add a new R-R-R Dyad. It has two Parts and three Pin-Joints

T2B-6bar-001

We have a four-bar similar to this model - see image to the left.

The four-bar has a Rocker (1) which is the Motion-Part, and an R-R-R Dyad.

The Motion-Part(1) has a motion relative to 4 - the Base-Part.

The R-R-R Dyad consists of Parts 2 and 3, and Pin-Joints, R1, R2, R3.


Remember, a Pin-Joint is equivalent to a Revolute Joint, hence the letter R for Revolute in the R-R-R Dyad.

The Classic Six-Bar Mechanisms

Watt 1

Watt1

Watt 1

The standard four-bar mechanism plus one more R-R-R Dyad:

The Four-Bar Mechanism: Parts '1','2','3','4'.

The new R-R-R Dyad: Parts '5' & '6'.

Part 5:
Join to the Coupler (Part 2), at P1.
Part 6:
Join to the output 'Rocker' (Part 3), at P2.
Join the Part 5 and Part 6 together.

Standard-Watt1Prep

Add Points P1 and P2:

Edit Parts 2 and 3 to add sketch-elements.

You can add a Point, or two Lines sketch-elements to give a Point.

For example, add two Lines to the Coupler and two Lines to the Rocker, to construct a triangle in each Part.

Each triangle gives the Points: P1 and P2.

Standard-Watt1Final

Add the R-R-R Dyad

Add two Parts: 5 & 6.

Join one Part to P1, the other to P2, then join the two Parts, 5&6, together.


6-Bar Mechanism Watt 1

Example Watt 1 Mechanism: Cabinet Hinge Mechanism

Watt 2

Watt2

Watt 2

The 'standard' four-bar kinematic-chain is Parts - 1,2,3,4.

The extra Dyad, Parts 5 & 6, is joined to the Rocker, Part 3, and the Ground, Part 4.

Standard-Watt2Prep

Edit Parts to add Points P1 and P2.

You can add a Point, or a sketch-element with a Point.

For example: add two lines to the Frame and two Lines to the Rocker to give a triangle in each Part.

The apex of each triangle give the Points: P1 and P2.

Standard-Watt2Final

Add the R-R-R Dyad

Add two Parts: 5 & 6.

Join one Part to P1, the other to P2, then join the two Parts, 5&6, together.


Stephenson 1

Stephenson1

Stephenson 1

The 'standard' four-bar kinematic-chain -  Parts 1,2,3,4.

The extra Dyad, Parts 5 & 6, is joined to the Crank, Part 1, and the Rocker, Part 3.

Standard-Stephensson1Prep

Edit Parts to add Points P1 and P2.

Add two lines to the Rocker and two Lines to the Crank to give a triangle in each Part.

Each triangle gives the Point: P1 and P2.

Standard-Stephenson1final

Add the R-R-R Dyad

Add two Parts: 5 & 6.

Join one Part to P1, the other to P2, then join the two Parts, 5&6, together.

Stephenson 1 Mechanism: Car Hood.

Stephenson 2

What does a Stephenson 2 look like?

Stephenson2

It is not possible to make this a 'kinematically-defined chain'.

You can connect the Parts, but they will not become Green!

Stephenson2-1

Only the Motion-Part is Green.

The Stephenson 2 cannot be created in MechDesigner. Can you see why?

The Parts and Joints move correctly, but not as quickly as when the Parts have Green Part-Outlines.

Stephenson 3

Stephenson3

Stephenson 3

Very similar to the Watt 1. The Watt 1 has a Dyad across the(Output)Rocker and the Coupler.

The Stephenson 3 has a Dyad across the (Input)Crank and the Coupler

The 'standard' four-bar mechanism with four Parts - 1,2,3,4..

+ the R-R-R Dyad, the two Parts 5 & 6.

Part 5:
To the Frame, Part 1, at Pin-Joint P1.

Part 6:
To the output Coupler, Part 3, at Pin-Joint, P2.


Edit Parts 2 and 4 to add Points P1 and P2.

Add two lines to the Coupler and two Lines to the Frame to give a triangle in each Part.

Each triangle gives the Point: P1 and P2.

Add the R-R-R Dyad

Add two Parts: 5 & 6.

Join Part 5 at Pin-Joint, P1,

Joint Part 6 at Pin-Joint, P2,

Parts 5 & Part 6, to each other, with Pin-Joint.

Stephenson-3a

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