Step 2.6: Change to a Scotch-Yoke Kinematic-Chain

Below, there are two typical images of a Scotch-Yoke as they are often found in many kinematics books.

The Motion-Part is a:

A Dyad uses:

Where is the second Part in the Dyad?

In the image above, the second Part of the Dyad does not seem to exist.

The image to the left shows the other Part in the Dyad. It is a:

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.

The Scotch-Yoke looks like the images above.

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)]

Remember: The short-cut: Double-click the green rectangular Part-Outline to open the Part-Editor and edit the Base-Part.

Select the Horizontal constraint tool.

•	Make Line horizontal. (Click the Line)

•	Make a Point at the end of Line horizontal to the Point in the sketch that you have used for the Pin-Joint of the Crank. (To do this, select the two Points).

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

STEP 5:

Add a Motion-Dimension FB to the Pin-Joint

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 7:

Delete the Pin-Joint between the horizontal Sliding-Part and the long Part

To do this:

1.	Click the Pin-Joint in the graphic-area. It shows in the Selection-Window.

In the Selection-Window:

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-Part.

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-Part.]

STEP 8:

Edit the Sliding-Part - see image above

STEP 9:

Add a vertical Line

It does not matter where you add the vertical Line to the Sliding-Part.

Note: However, it may be important if you want to add a DXF or an STL Solid to the Part that:

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

STEP 12:

Click the new Line in the output Sliding-Part

STEP 13:

Click the CAD-Line along the centre of the Part

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

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

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

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

The Rocker is one of the two Motion-Parts.

It has three elements:

The RPP Dyad is one of five Planar Dyads.

It has five elements: