Scotch-Yoke: the RPP Dyad

Here, we change the kinematic-chain to a Scotch-Yoke. The purpose of this tutorial is to practice how to delete and add different joints.

This tutorial also helps you to add a Sliding-Joint that is not along the CAD-Line (the centre-line of an Added-Part), but along a Line you add to a Part with the Part-Editor


To assemble simple, four-bar kinematic-chains,

We need:

One Motion-Part : A Motion-Dimension FB identifies the Part that is the Motion-Part. A Motion-Part is a Rocker or a Slider.
One Dyad : All Dyads ALWAYS has:
oTwo Parts
oThree Joints

What is a Scotch-Yoke?

Below, there are two typical images of a Scotch-Yoke as they are often found in many kinematics books.

Scotch-Yoke Mechanism - False!

The Motion-Part is a:

Crank – coloured Black.

A Dyad uses:

Three Joints: a horizontal Slide-Joint, a vertical Slide-Joint, a Pin-Joint in the Slot
Two Parts: one horizontal sliding-part – coloured Red – and the other Part is... where?

Where is the second Part in the Dyad?

In the image above, the second Part of the Dyad does not seem to exist.

Scotch-Yoke - Kinematic-chain

The image to the left shows the other Part in the Dyad. It is a:

Vertical sliding-part – it is 'sliding-block' - coloured Green.

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

One top image shows a pin that is in a slot. The bottom image shows a pin that is in a sliding-block, and the sliding-block is in a slot.

Kinematically they are the same, and you must model them in MechDesigner in the same way.

It is a:

Rocker (for the Crank)
RPP Dyad

R (Pin-Joint) at the end of the Crank

P (vertical Slide-Joint)

P (horizontal Slide-Joint) – between a Part and the Base-Part



tog_minusVideo of this Step

Tutorial 2; Step 2.6

Edit model to design a simple Scotch-Yoke mechanism


Step by Step

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

The Scotch-Yoke looks like the images above.


STEP 1: Delete the Motion-Dimension FB
1.Mechanism-Editor: Click the Motion-Dimension FB
2.Mechanism-Editor: 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.Double-click the Green rectangular Part-Outline to edit 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 LineRed-14-1b - so it becomes Horizontal
5.Part-Editor: Click the Point on the Y-axisRed-14-2 and the Point at the start-Point of LineRed-14-3 (see image)
6.Exit the Part-Editor


We need a Crank as the input to the Scotch-Yoke.

STEP 3: Add a Motion-Dimension FB to the Pin-Joint
1.Mechanism-Editor: Click Kinematic-FB > Add Motion-Dimension FB | Click the Pin-JointRed-14-1b the Line in the Base-Part, and Line along the Part.
2.Mechanism-Editor: Connect the Linear-Motion FB to the Motion-Dimension FB

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

However, we need an R-P-P Dyad.

This is one Pin-Joint and two Slide-Joints.


We will replace the middle R joint ( Pin-Joint), with a P (Slide-Joint).

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

To do this:

1.Click the Pin-JointRed-14-2 in the graphic-area. It shows in the Selection-Window.

In the Selection-Window:

2.Right-click the Pin-Joint
3.Click Delete in the menu.


I have moved the Blue Parts apart slightly to show they not joined.

Blue Parts are not kinematically-defined.

We need to add a Vertical Line in the Sliding-Part.Red-14-3

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

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


The vertical line is in the Sliding-Part. It is not important where you add the vertical Line

1.Edit the Sliding-PartRed-14-3
2.Part-Editor: Geometry toolbar > Add Line, | Drag to add a LineRed-14-2
If you drag upwards to add the Line, a Part that you join to it with a Slide-Joint will point upwards.
If you drag downwards to add the Line, a 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. | Double-click the Line
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 PartRed-14-1b
3.Click the vertical LineRed-14-2 along the centre of the Sliding-Part


The two Lines snap together to add the Slide-JointRed-14-1b.

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


The Rocker has three elements:

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

The RP-P Dyad has five elements, as do ALL Dyads.


Tutorials and Reference Help Files for MechDesigner and MotionDesigner 14.2 + © Machine, Cam, Mechanism, and Motion Design Software by PSMotion Ltd