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Lesson Objective:In this lesson, we will be introduced to the Mechanism functionality inPro/ENGINEER.

There are very few assemblies that we design that do not have moving parts. Even if the moving part is assimple as a lid that rotates about a pivot point, or as complex as a pin that rides in a slot, the ability to

capture this dynamic behavior is something we want to be able to do.

Lesson

http://sharptechdesign.com/Tutorials/Mechanism_WF2/MDX_WF

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Mechanism is the tool in Pro/ENGINEER that provides this behavior. Without using Mechanism, you wouldhave to assemble components using datum planes or other references to capture offsets or angular

placements. Then, by changing any dimensions associated with these placements, you are able toregenerate the assembly and see a static condition that represents a different state.

This is still not capturing the dynamic behavior of the assembly, nor does it provide an adequate method for

checking interference along the entire path of motion the component can go through.

Mechanism allows you to assemble a component into an assembly, drag its open degrees of freedom (seesection below), create servo motors to move the components based on certain conditions, run animationsthat capture the motion, and then perform interference and other measurements on the assembly as the

motion is being carried out.

Degrees of Freedom

Every component you assemble initially has six degrees of freedom three translations (along the X, Y andZ axes) and three rotations (about the X, Y, and Z axes). When we use placement constraints such asmate, align, parallel, offset or through, we are start ing to fix some or all of these degrees of freedom (DOF).

When a component is fully assembled using traditional Pro/ENGINEER placement constraints, you have

successfully fixed all six DOF. Even if you build in a rotation by using a datum plane through an axis at anangle to some other plane, you are still fixing the model to this plane.

Packaged Components

If you recall from your Pro/ENGINEER Fundamentals training, you can leave a component partiallyconstrained, and it is considered Packaged. In the model tree, you would see a white square next to the

component, indicating this condition exists.

The problem with partially constraining a component using traditional placement constraints, is that no othercomponent can be assembled to a packaged component. Therefore, if you needed (for example) to create

a drawer that pulls out from a cabinet, and then assemble a knob onto that drawer, you could not do it if thedrawer itself were not fully constrained.

Connections

Connections are placement constraints in the Mechanism mode that fix certain constraints but leave others

free to move. The component is still considered packaged because it is only partially constrained, but therules are a little different. With the connections in place, you can dynamically move the open DOF, and youcan also assemble other components to the packaged part.

This gives you the complete flexibility to capture the motion that exists in your assembly, and still be able to

create a realistic assembly in terms of order of assembly and connections between bodies.

There are several locations where you will encounter Mechanism tools. The first is available when you go toplace a component in your assembly. When you click on the add component icon, and then select the

component you are going to add, you get the following window.


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As we will learn in the next lesson, there are many different connection types: Pin, Slider, Planar, Bearing,etc. Each of these connection types has constraints that must be satisfied for them to work. For example,a pin connection requires two axes or cylindrical surfaces to be aligned, and two planes, points or vertices to

fix the translation through the selected axes.

The rotation about the axis is all that is permitted. With a single component, you can add several differentconnection types until the placement is considered complete. Each of these connections will need to be fully

defined.The second way to access mechanism is to go to Applications, Mechanism from the menu bar, as shown

in the next figure.

This will bring up the following Mechanism menu and corresponding icons in the feature toolbar.


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Step 2 Modify Joint Axis Settings

Control your connections by modifying the joint axes created by the connection. This is explained in greaterdetail in the connection lessons individually.

Step 3 Create Slots, Cams or Gear Pairs

Slots, Cams and Gear Pairs are special tools in mechanism that capture complex interactions between

components. These will each have their own lesson.

Step 4 - Drag Components and Create Snapshots

Dynamically pull or push on components that have open DOF to see them move in the assembly. Take

snapshots of your assembly at different states of motion to use in drawings or to come back to forreference.

Step 5 Create Servo Motors or Force Motors

Servo motors and Force Motors are used to drive analysis and move your assembly on their own withoutusing drag tools. Each of these topics will be covered in great detail in their own lessons.

Step 6 Create and Run AnalysesStart your animations to calculate the results you are looking for. With servo motors in your assembly, you

will be able to produce motion animations. With Force motors, you will be able to calculate resultant forcesand other measures while the animation is running.

Step 7 View Results and Take Measurements

Run the animation to create MPEG movies, or to calculate interference along the path of the movingobjects. Create and view graphs that measure certain factors over time, such as position or force.

The table at the top of the next page lists the different connections available through the componentplacement window at the time you assemble in a component. In addition, the number of translational and/or

rotational degrees of freedom are shown for each connection type.

Connection Type Translational DOF Rotational DOF References Needed

Pin 0 1 2 Axes or Edges & 2Planes, Planar Surfaces,

Datum Points or Vertices

Slider 1 0 2 Axes or Edges & 2Planes or Planar Surfaces

Cylinder 1 1 2 Axes or Edges

Planar 2 1 2 Planes or PlanarSurfaces

Ball 0 3 2 Datum Points or Vertices

Bearing 1 3 1 Datum Point and 1 Axisor Edge

Weld 0 0 2 Coordinate Systems

There are two additional connection types in the list, but they dont fit into the categories above. These are


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the Rigid and General connections, and they allow you to use standard assembly placement constraints,

such as Align, Mate, Insert, Tangent, etc. We will see its usage in Mechanism later.

Every connection that you build creates a joint axis symbol that appears on the model when you are inmechanism mode. The joint axes contains information about the connection that you can control. For

example, a pin constraint is free to rotate about an axis or edge. The joint axis for a pin constraint containsthe angle for the rotation.

By modifying the joint axis settings, you can control the motion of the open DOF. When we get into each

connection type in more detail, we will also look at the joint axis settings in more detail

Mechanism is the tool in Pro/ENGINEER assembly mode that allows you to capture realistic degrees offreedom in your assembly.

You access mechanism in the component placement window, or through the Applications menu in the menu

bar.

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proe mechanism les1

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