Pro E–Top Down Design With 4 Bar Mechanism

Following traditional top-down design technique for assembly with moving components has been difficult in the past. Usually some variability is built in the skeleton to simulate motion. Unfortunately this does not allow the usage of mechanism/dynamics to study the motion of the assembly.

With the introduction of motion skeletons in Pro/ENGINEER Wildfire 3.0 this issue has been resolved. This short example will give you an idea on how to get started with Motion Skeletons.

The goal is to build Figure 1 with full mechanism constraints, based off a skeleton that will define the length of all the links. At any given point the skeleton can be changed to adjust the lengths, but the mechanism needs to be maintained.

Figure 1

The skeleton for something like this would look like Figure 2. Basically it is a simple sketch that defines the lengths of all the links. The angle in this case was needed to properly constrain the sketch. It is meaningless as far as the mechanism is concerned, although it can be used to define the start position of the mechanism.

Figure 2

Let’s begin:

• Create a new empty assembly
• Create a new component and select the Skeleton Model Type and chooseMotion for Sub-Type (Figure 3)

Figure 3

• Choose a standard start assembly to Copy From (Figure 4)

Figure 4

• Now we need to create the base skeleton. Activate the motion skeleton as shown in Figure 5 and create a new component. In this case chooseStandard as this will be our reference for the bodies. Choose a standard start part as a template for the new skeleton. (Figure 6)

Figure 5

Figure 6

• The model tree will look like Figure 7. Open the just created skeleton as shown.

Figure 7

• Create a sketch as shown in Figure 8

Figure 8

• Return to the main assembly and Activate the MOTION_SKEL_4BAR skeleton. (Figure 9) Create a new component and select Body as the Sub-type. Name it BODY_GROUND. (Figures 10) This represents the fixed link at the bottom.Figure 8 shows it as the blue link.

Figure 9

Figure 10

• This brings up a dialog box. (Figure 11) Select the bottom curve as the Chain reference. This will automatically create a copy of this curve into the BODY_GROUND part. The first component created is always designated as ground. (Figure 12)

Figure 11

Figure 12

• The model tree should look like Figure 13 this at this point

Figure 13

• With the MOTION_SKEL_4BAR Activated create a new Component>Body. Name it BODY1. Ensure that you use a Start Part selected in the next dialog box. (Figure 14)

Figure 14

• Now we reach the point where we define our first moving body. The dialog box shown in Figure 15 asks us to select a chain reference. This will become the skeleton for the first link.

Figure 15

• Once the curve is selected click on the Update button. Notice that it automatically recognized a Pin joint between the ends of the two curves. (Figure 16) Sometimes this assumption is not the desired connection type. In this case select the Invoke component placement dialog radio box. This will bring up the standard assembly dialog dashboard and you can choose any type of constraint at that point. (Figure 17)

Figure 16

Figure 17

• As before, with the MOTION_SKEL_4BAR Activated create a newComponent>Body for the other two bodies. Name the bodies BODY2 & BODY3 respectively. (Figure 18)

Figure 18

• The model tree should look like Figure 19 this at this point. Now we are ready to create the parts.
• Activate the top level assemble (tip: Ctrl-A is a quick short cut for this)

Figure 19

• Create a new component. (Figure 20) Now we are creating a Solid Part. Notice that there is a new selection at the bottom called Attach Component to Body. Select this option and choose the BODY_GROUND part. (Figure 21)

Figure 20

Figure 21

• Open this new part. You will notice that an External merge is added to the model. (Figure 22) This is an automatic copy geometry from theBODY_GROUND.prt. Notice it contains the curve and the two axes that were also automatically created for the pin joints. (Figure 23) This geometry can be used as a reference to create features such Protrusions.

Figure 22

Figure 23

• Create the solid link as shown in Figure 24, using the curve and axes as sketching references
• Create a new component, as above from Figure 20, for the other links

Figure 24

• The resulting assembly should look like Figure 25. Experiment with the motion using the Drag command . (Figure 26)

Figure 25

Figure 26

• Now you can experiment by changing the dimensions of the skeleton. Edit the sketch located in the DESIGN_SKEL.prt. Change the 2” dimension to a 6” and regenerate. (Figures 27 & 28)

Figure 27

Figure 28

• The assembly regenerates and the mechanism is preserved (Figure 29)

Figure 29

This simple example demonstrates how Pro/ENGINEER Wildfire 3.0 automates the creation of mechanism connections using motion-skeletons and Top-Down design techniques.

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