The Scotch-Yoke Kinematic-Chain

A Scotch-Yoke is a mechanism that changes rotary motion at its input to linear motion at its output.

The input rotates with constant angular speed.

The output reciprocates with Simple Harmonic Motion.

The kinematic-chain is:

1.One Motion-Part – a Rocker / Crank
2.One Dyad – an RPP Dyad.

(Closure - Review the Change Dyad Closure tool - you may need to use it when the Dyad builds the first time!).

Note: When the crank has constant angular velocity, the Slider is Simple Harmonic Motion

However, there are two Motion-Laws in MotionDesigner that can modulate* the rotation of the Crank in a useful way.

The Y-Inverse-Sinusoid and the Crank-Constant-Velocity modulate the Crank rotation so that the Slider moves with a Constant Linear Velocity.

* Modulate in this context means change the speed of the Crank but it still continues to rotate, without reversing, one rotation for each machine-cycle.

The video shows a Scotch-Yoke.




STEP 1: Add the Input – a Crank
1.Edit the Base-Part :
2.Part-Editor: Geometry toolbar > Add Line : Drag to add the Line , Add Horizontal Constraints, Add Dimensions, Close the Part-Editor:
3.Mechanism-Editor: Click Kinematic elements- > Add Part : drag to add the new Part.
4.Mechanism-Editor: Click Kinematic elements- > Add Pin-Joint: Click the start-Point of the new Part and the start-Point of the Line in the Base-Part.
5.Mechanism-Editor: Click Kinematic FB toolbar > Add Motion-Dimension FB: Click the Pin-Joint the Line in Base-Part; the CAD-Line in Part.
6.Click OK-tiny-11-15 in Command-Manager
7.Mechanism-Editor: Click Kinematic FB toolbar > Add Linear-Motion FB: and click the Graphic-area. Joint the FBs with a wire. You have a CrankRed-14-1b




We must add 2 Parts and 3 Joints (Top Tip: Add the 'Output' Part and Joint first).

DYAD: PartRed-14-2 & JointRed-14-3

The output of the Scotch-Yoke is a Sliding-Part

1. Mechanism-Editor: Click Kinematic elements- > Add Part : Drag to add the new PartRed-14-2
2.Mechanism-Editor: Click Kinematic elements- > Add Slide-Joint To add the Slide-Joint:Red-14-3 : Click the CAD-Line in PartRed-14-2 and the horizontal Line in the Base-Part


DYAD: PartRed-14-4 & JointRed-14-5

1.Mechanism-Editor: Click Kinematic elements- > Add Part : drag to add the new PartRed-14-4
2.Mechanism-Editor: Click Kinematic elements- > Add Pin-Joint: To add the Pin-Joint: Red-14-5 :Click the start-Point of the new PartRed-14-4 and the end-Point of the Crank

The two Parts are Blue Parts


Dyad: Joint 3 between Part 1 & Part 2

This joint is a vertical slide-joint (the slot) between the two new PartsRed-14-2Red-14-4. We need a Line in the two Parts for a Slide-Joint.

Preparation for Joint 3 – draw a vertical line to give the direction of the 'slot'Red-14-6

1.Edit PartRed-14-2
2.Part-Editor: Geometry Tree: Add LineRed-14-6, Also Add Vertical constraint, Add Dimensions to specify it fully
3.Close the Part-Editor

This image shows the Vertical Line in the Part-Editor.


Add Joint 3: the Slide-Joint

This image shows the vertical LineRed-14-6 in the Mechanism-Editor.

1.Click Add Slide-Joint
2.Click the vertical LineRed-14-6 in the sliding-part (Part 1)
3.Click the Cad-LineRed-14-7 in PartRed-14-4
4.If necessary, click the graphic-area to complete the command.

MechDesigner completes the Dyad to Add the Slide-JointRed-14-8

The Part-Outlines become green to indicate the kinematic-chain is kinematically-defined.

Note: The PartRed-14-8 along the Slide-Joint is either upwards or downwards. This does not change the kinematics.

If you prefer the Part to be in the other direction: Add the Line Red-14-6(see image above), by 'dragging' it in the opposite direction, from top to bottom, or vice versa.

Cycle the mechanism (ALT+C on your keyboard).



This is the kinematics of a 'Scotch-Yoke'.

It has a:

Rocker (as a Crank)
R-P-P Dyad: one Pin-Joint (R - - ) and two Slide-Joints (- P-P).

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