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  kinematic-chain is:

1.One Motion-Part – a Rocker / Crank

2.One dyad – an R-P-P dyad.

Closure - Review the Change Dyad Closure tool - you may need to use it after the dyad builds the first time).

Note: When the crank has constant angular velocity, the motion of output 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 for a period of its rotation.

* Modulate is to change the speed of the Crank as it rotates.

The video is of 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 graphics-area. Joint the FBs with a wire. You have a CrankRed-14-1b

GST-10-1-SY-001

GST-10-1-SY-002

STEP 2: Add R-P-P DYAD

We must add 2 Parts - we will name the Parts as Part-A and Part-B

We must also add 3 Joints - we will name the Joints as Joint-A, Joint-B, and Joint-C

DYAD: Part-BRed-14-2 & Joint-CRed-14-3

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

1. Mechanism-Editor: Click Kinematic elements- > Add Part : Drag to add the PartRed-14-2 (Part-B)

2.Mechanism-Editor: Click Kinematic elements- > Add Slide-Joint : Add the Slide-Joint:Red-14-3 (Joint-C) Click the CAD-Line in PartRed-14-2 (Part-B) and the horizontal Line in the Base-Part

GST-10-1-SY-003

DYAD: Part-ARed-14-4 & Joint-ARed-14-5

1.Mechanism-Editor: Click Kinematic elements- > Add Part : Drag to add the new PartRed-14-4 (Part-A)

2.Mechanism-Editor: Click Kinematic elements- > Add Pin-Joint:  Add the Pin-Joint: Red-14-5 (Joint-A) : Click the start-Point of the new PartRed-14-4 and the end-Point of the Crank

The two Parts are not solved.

We must add one more joint.

GST-10-1-SY-004

Prepare to add Slide-Joint (Joint-B)

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

1.Part-Editor: Edit PartRed-14-2 (Part-B)

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

3.Close the Part-Editor

This image has the vertical LineRed-14-6 in the Part-Editor.

GST-10-1-SY-005

Add Joint 3: the Slide-Joint (Joint-B)

This image has 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 in PartRed-14-2 (Part-B)

b.Click the Cad-LineRed-14-7 in PartRed-14-4 (Part-A)

MechDesigner completes the dyad to add the Slide-JointRed-14-8 (Joint-B)

The Parts are now 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 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

Press the ALT+C key combination on your keyboard

GST-10-1-SY-006

GST-10-1-SY-007

These are the element in the kinematics-tree for a Scotch-Yoke.

There is a:

•Rocker (the kinematic term we use for a Crank)

•R-P-P dyad: one Pin-Joint and two Slide-Joints.