Scotch-Yoke: the RPP Dyad
Here, we change the kinematic-chain to a Scotch-Yoke. The purpose of this tutorial is to practice how to delete and add different joints.
This tutorial also helps you to add a Sliding-Joint that is not along the CAD-Line (the centre-line of an Added-Part), but along a Line you add to a Part with the Part-Editor
See http://youtu.be/MrvAKlePm3k?t=33s
To assemble simple, four-bar kinematic-chains,
We need:
| • | One : A Motion-Dimension FB identifies the Part that is the Motion-Part. A Motion-Part is a or a . |
| • | One : All Dyads ALWAYS has: |
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What is a Scotch-Yoke?
Below, there are two typical images of a Scotch-Yoke as they are often found in many kinematics books.
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The Motion-Part is a:
A Dyad uses:
| • | Three Joints: a horizontal Slide-Joint, a vertical Slide-Joint, a Pin-Joint in the Slot |
| • | Two Parts: one horizontal sliding-part – coloured Red – and the other Part is... where? |
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Where is the second Part in the Dyad?
In the image above, the second Part of the Dyad does not seem to exist.
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The image to the left shows the other Part in the Dyad. It is a:
| • | Vertical sliding-part – it is 'sliding-block' - coloured Green. |
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.
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It is a:
R (Pin-Joint) at the end of the Crank
P (vertical Slide-Joint)
P (horizontal Slide-Joint) – between a Part and the Base-Part
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Video:
Step by Step
The Scotch-Yoke looks like the images above.
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| STEP 1: | Delete the Motion-Dimension FB |
| 1. | Mechanism-Editor: Click the Motion-Dimension FB |
| 2. | Mechanism-Editor: Press the Delete key on your keyboard. |
The Part-Outlines in the kinematic-chain are Blue again.
Blue Part-Outlines indicate the Parts are NOT kinematically-defined.
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| 1. | Double-click the Green rectangular Part-Outline to edit the Base-Part |
| 2. | Part-Editor: Delete the dimensions that constrain the Line we used for the Slide-Joint. |
| 4. | Part-Editor: Click the Line - so it becomes Horizontal |
| 5. | Part-Editor: Click the Point on the Y-axis and the Point at the start-Point of Line (see image) |
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We need a Crank as the input to the Scotch-Yoke.
| STEP 3: | Add a Motion-Dimension FB to the Pin-Joint |
| 1. | Mechanism-Editor: Click | Click the Pin-Joint the Line in the Base-Part, and Line along the Part. |
| 2. | Mechanism-Editor: Connect the Linear-Motion FB to the Motion-Dimension FB |
In the Kinematics-Tree, there is a Rocker and R-R-P Dyad.
However, we need an R-P-P Dyad.
This is one Pin-Joint and two Slide-Joints.
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We will replace the middle R joint ( Pin-Joint), with a P (Slide-Joint).
| STEP 4: | 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:
| 2. | Right-click the Pin-Joint |
| 3. | Click Delete in the menu. |
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I have moved the Blue Parts apart slightly to show they not joined.
Blue Parts are not kinematically-defined.
We need to add a Vertical Line in the Sliding-Part.
Refer to the schematic of the Scotch-Yoke at the top of this topic.
| STEP 5: | Add a vertical line to the Sliding-Part. |
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The vertical line is in the Sliding-Part. It is not important where you add the vertical Line
| 1. | Edit the Sliding-Part |
| 2. | Part-Editor: , | Drag to add a Line |
| • | If you drag upwards to add the Line, a Part that you join to it with a Slide-Joint will point upwards. |
| • | If you drag downwards to add the Line, a Part that you join to it with a Slide-Joint will point downwards. |
| 3. | Part-Editor: | Click the Line |
| 4. | Part-Editor: Add dimension as needed so that the Line becomesFully Defined |
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| 5. | Exit the Part-Editor. | Double-click the Line |
| STEP 6: | Add the Slide-Joint between the two Parts to complete the Dyad. |
| 2. | Click the CAD-Line in the Part |
| 3. | Click the vertical Line along the centre of the Sliding-Part |
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The two Lines snap together to add the Slide-Joint.
| STEP 7: | Cycle the Kinematic-chain |
Use the shortcut: Alt+C
The crank rotates continuously. The output slides back-and-forth with Simple Harmonic Motion.
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The Kinematics Tree
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The Rocker has three :
| c. | Mot-Dim Rocker – a Motion-Dimension FB for a Rocker |
The RP-P Dyad has five elements, as do ALL Dyads.
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Video of this Step