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 – colored 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_minus        Video 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

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

Blue Part-Outlines indicate they are NOT kinematically-defined.

You can drag the Part-Outlines if you like.

[To Drag: hover above the Part-Outline to make it Red, then Drag.

(Note: Occasionally, you can only hover to make a Part-Outline Red by hovering over the Part-Outline at the arcs, near the start and end Points, at each end of the Part)]


STEP 2: Edit the Base-Part  [Double-click the green rectangular Part-Outline to edit the Base-Part]
STEP 3: Edit the Line in the Base-Part

Select the Horizontal constraint tool.

Make Line1s-red horizontal. (Click the Line)
Make a Point at the end of Line1s-red horizontal to the Point in the sketch that you have used for the Pin-Joint of the Crank2s. (To do this, select the two Points).
STEP 4: Close the Part-Editor


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

STEP 5: Add a Motion-Dimension FB to the Pin-Joint 1s-red
STEP 6: Connect the Linear-Motion FB to the Motion-Dimension FB

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

However, we need an RPP Dyad. This is one Pin-Joint and two Slide-Joints.


Replace the middle 'R' Joint (Pin-Joint), with a P (Slide-Joint).

STEP 5: Delete the Pin-Joint1s-red between the horizontal Sliding-Part and the long Part

To do this:

1.Click the Pin-Joint1s-red 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.

Add a vertical line to the Sliding-Part1s-red.

The vertical line is not in the Base-Part, because the new vertical Slide-Joint is not in the Base-Part.

[In the image in 'What does a Scotch-Yoke look like?' [above], the slot is a child to the red Part, which is a child to the Sliding-Part1s-red.]


STEP 8: Edit the Sliding-Part1s-red - see image above
STEP 9: Add a vertical Line2s
STEP 10: Close the Part-Editor.

It is not important where you add the vertical Line to the Sliding-Part.


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.


We can now add the Slide-Joint between the two Parts to complete the Dyad.


STEP 11: Click Kinematic-elements toolbar > Add Slide-Joint
STEP 12: Click the new Line in the output Sliding-Part1s-red
STEP 13: Click the CAD-Line2s along the centre of the Part


The two Lines snap together to add the Slide-Joint1s-red. See Note, to see why the Part is 'pointing upwards'.

STEP 14: Cycle the Kinematic-chain

Use the shortcut 'Alt+C, [C for R13+]'.

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

The Kinematics Tree


This kinematic-chain is a Rocker and an RPP Dyad.

The Rocker is one of the two Motion-Parts.

It has three elements:

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

The RPP Dyad is one of five Planar Dyads.

It has five elements:


Tutorial and Reference Help Files for MechDesigner and MotionDesigner 13.2 + © Machine, Mechanism, Motion and Cam Design Software by PSMotion Ltd