Step 10.1: The Scotch-Yoke Kinematic-Chain

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Step 10.1: The Scotch-Yoke Kinematic-Chain

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

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STEP 2: Add R-P-P DYAD

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

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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


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Preparation for Vertical Slide-Joint 3

Add a vertical lineRed-14-6 to give the direction of the Slide-Joint 3

1.Start Part-Editor: Edit Horizontal Sliding PartRed-14-2 (see above)

2.Part-Editor: Geometry Tree: Add LineRed-14-6, Add Vertical constraint, Add Dimensions

3.Close the Part-Editor

This image shows the Vertical LineRed-14-6 in the Part-Editor.

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Add Joint 3: the Slide-Joint

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

1.Mechanism-Editor: Click Kinematic elements- > Add Slide-Joint:

a.Click the vertical LineRed-14-6

b.Click the Cad-LineRed-14-7 in PartRed-14-4

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

The Part-Outlines are green to indicate the Parts are kinematically-defined.


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

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


Cycle the mechanism

Do ALT+C on your keyboard

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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).