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 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' kinematic-chain.


STEP 1: Add the 'Input' Part – a Crank

The 'Crank' is the 'input' Part to the Scotch-Yoke. See Tutorial 1.

1.Add a Part in a new Mechanism-Editor
2.Add a Line to the Base-Part – make the Line horizontal
3.Join the Part to the Base-Part with Pin-Joint
4.Add a Motion-Dimension FB to give the Angle of the Part
5.Add a Linear-Motion FB and connect it to the Motion-Dimension FB.

You have a 'Crank'1s-red


STEP 2: Add the Dyad to complete the Scotch-Yoke

The 'Bill of Materials' is Part 1, Part 2, Joint 1, Joint 2 and Joint 3.

Top Tip: Add the 'Output' Part and Joint first.

Dyad: Part 12s & Joint 13s

The 'Output' Part' of the Scotch-Yoke is a 'Sliding-Part',

1.Add a Part2s (Part 1)
2.Add a Slide-Joint3s

Click the CAD-Line in Part 1 and the horizontal Line in the Base-Part


Dyad: Part 24s & Joint 25s

A Point at the end of the Crank slides in a 'Vertical Slot' in the output Part.

1.Add a Part4s
2.Add a Pin-Joint5s

Click a Point in Part4s and the end of the Crank1s-red

The two new Parts are blue.

You must always add a Part to move along the slot.  I find it easier to imagine the 'Slot' is actually a Sliding-Block on a Slide Rail.


Dyad: Joint 3 between Part 1 & Part 2

The third joint is a vertical slide-joint (the slot) between the two new Parts2s4s.

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

1.Edit Part 1 in the Part-Editor - the slot is machined in Part 1
2.Add a Line6s, with vertical a constraint and the 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 Line6s in the Mechanism-Editor.

1.Click Add Slide-Joint
2.Click the Vertical Line6s in the sliding-part (Part 1)
3.Click the Cad-Line7s in Part 2
4.If necessary, click the graphic-area to complete the command.

MechDesigner completes the Dyad to Add the Slide-Joint8s

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

Note: The Part8s 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 6s[see image above], by 'dragging' it in the opposite direction, from top to bottom, or vice versa.

Cycle the mechanism (C on your keyboard).


Kinematics of a Scotch-Yoke: Crank + RPP Dyad

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).

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