Getting Started Tutorials - MechDesigner > Tutorial 2A: Four-bar Kinematic-Chains > Step 2.6: Reconfigure to a 4-bar Scotch-Yoke

Scotch-Yoke: the R-P-P dyad

In this step, you will change the kinematic-chain to a Scotch-Yoke.

I do not know where the name Scotch-Yoke has come from. Do you?

However, it is a kinematic-chain that is frequently used in packaging, assembly, and textile machines.

You will add a Slide-Joint. The Slide-Joint is not along the CAD-Line that is from the start-Point to the end-Point of all a Parts you add to the model.

In this step, we dd a new Line to the Part, and then use it for the Slide-Joint.

See http://youtu.be/MrvAKlePm3k?t=33s

To assemble simple, four-bar kinematic-chains.

We need:

•One Motion-Part : A Motion-Dimension FB controls the motion of a Motion-Part. A Motion-Part is a Rocker or a Slider.

•One dyad : All dyads have:

oTwo Parts

oThree Joints

What is a Scotch-Yoke?

Below, there are two schematic images of a Scotch-Yoke.

Scotch-Yoke Mechanism - False!

The Motion-Part is a:

•Crank – colored Black.

A dyad uses:

•Three Joints: a horizontal Slide-Joint, a vertical Slide-Joint, a Pin-Joint

•Two Parts: one horizontal sliding-Part – colored Red – and the other Part is... where?

In the image above, where is the second Part in the dyad? The second Part of the dyad does not seem to exist.

Scotch-Yoke - Kinematic-chain

The other Part in the dyad is now in the image to the left. It is a:

•Vertical sliding-Part – the Part is a sliding-block - colored Green.

The kinematic-chains in the images to the left are kinematically equal, but they are slightly different physical designs of a scotch-yoke.

Can you guess the kinematic tree?

Video:

Change the Kinematic-chain from a Slider-Crank to a Scotch-Yoke

The Scotch-Yoke looks like the images above.

GST-2.6-01

STEP 1: Delete the Motion-Dimension FB

1.Click the Motion-Dimension FB

2.Press the Delete key on your keyboard.

The Part-Outlines in the kinematic-chain are Blue again.

Blue Part-Outlines indicate the Parts are NOT kinematically-defined.

STEP 2: Edit the Base-Part

1.Edit the Base-Part: Double-click the Green rectangle that is the Part-Outline of the Base-Part

2.Part-Editor: Delete the dimensions that constrain the Line we used for the Slide-Joint.

3.Part-Editor: Click Constraints toolbar > Add Horizontal

4.Part-Editor: Click the Line - so it becomes Horizontal

5.Part-Editor: and also Horizontal: click the Point on the Y-axis and the Point at the start-Point of Line (see image)

6.Exit the Part-Editor

You are in the Mechanism-Editor again.

STEP 3: Add a Motion-Dimension FB to the Pin-Joint

The input to a Scotch-Yoke is usually a Crank (Rocker that rotates with a uniform angular velocity).

1.Click Kinematic-FB > Add Motion-Dimension FB | Click

a. the Pin-Joint

b.the Line in the Base-Part, and

c.the Line along the Part.

2.Connect the Linear-Motion FB to the Motion-Dimension FB

In the Kinematics-Tree, there is a Rocker and R-R-P dyad.

We need an R-P-P dyad - one Pin-Joint and two Slide-Joints.

GST-2.6-02

GST-2.6-03

GST-2.6-04

We will change the middle joint from an R joint ( Pin-Joint) to a P joint (Slide-Joint).

STEP 4: Delete the Pin-Joint between the horizontal Sliding-Part and the long Part

1.Click the Pin-Joint in the graphic-area. It is in the Selection-Window.

In the Selection-Window:

2.Right-click the Pin-Joint

3.Click Delete in the menu.

GST-2.6-05

Note: I have moved the Blue Parts slightly.

Refer to the schematic of the Scotch-Yoke at the top of this topic.

The Block slides vertically in a slot that is in the Part that slides horizontally along the Base-Part (the machine-frame).

Thus, we need a vertical Line in the Sliding-Part.

STEP 5: Add a vertical line to the Sliding-Part.

1.Edit the Sliding-Part

2.Part-Editor: Geometry toolbar > Add Line | Drag to add a Line

•If you drag upwards to add the Line, the Part that you join to it with a Slide-Joint will point upwards.

•If you drag downwards to add the Line, the Part that you join to it with a Slide-Joint will point downwards.

3.Part-Editor: Constraints toolbar > Add Vertical Line | Click the Line

4.Part-Editor: Geometry toolbar > Add Dimensions | Add dimension as needed so that the Line becomes Fully Defined.

5.Exit the Part-Editor. | E.g. Double-click the Line

You are in the Mechanism-Editor again.

GST-2.6-06

GST-2.6-07

STEP 6: Add the Slide-Joint between the two Parts to complete the dyad.

1.Click Kinematic-elements toolbar > Add Slide-Joint

2.Click the CAD-Line in the Part

3.Click the vertical Line along the center of the Sliding-Part

The two Lines snap together to add the Slide-Joint

GST-2.6-08

STEP 7: Cycle the Kinematic-chain

Use the shortcut: Alt+C

The crank rotates continuously. The output slides back-and-forth with Simple Harmonic Motion.

The Kinematics Tree for a Scotch-Yoke

GST-2.6-09

The Rocker has three elements:

a)Pin-Joint

b)Part

c)Mot-Dim Rocker – a Motion-Dimension FB for a Rocker

The R-P-P dyad has five elements, as do all dyads.

a)Pin-Joint1

b)Part3

c)Slide-Joint2

d)Part2

e)Slide-Joint