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6-bar Kinematic-chains - R-R-R+R-R-R

Contents

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

	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.
	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.
	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.