proe mechanism les16

8/7/2019 Proe Mechanism Les16

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Lesson Objective:In this lesson, we will learn about Cam Followers.

A Cam-Follower is used to simulate an object moving along the surface of another object, or when the

motion of an object must be stopped at the surface boundary of another object. A cam-follower can beused when other mechanism connection types fail to capture the way in which the objects touch each other.

Lesson

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

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For example, a planar connection always assumes two planar surfaces will remain co-planar. This will notallow the components to separate from each other, or allow one surface to rotate away from the other while

still touching.

You will still need to create other mechanism connections, such as a Planar connection to control the motion

of the component that is free to move, and to create any servo motors.

To demonstrate a traditional cam-follower application, we will look at a simplified version of a camshaft that

you would find in an automotive engine. Therefore, open up the Camshaft.asm assembly. It will look like

the following.

This assembly consists of four components. There is one shaft that contains three green extrudedfeatures. These three green extruded features will (as the shaft turns around) contact the magenta levers,

causing them to rock up and down.

Go to Applications, Mechanism, and you will see that we already set up some initial connections and

servo motors, as shown in the next figure.


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The main shaft has a pin connection, and a servo motor on this pin connection. The servo motor will rotatethe shaft 1440 degrees (4 complete rotations) in 10 seconds. Each magenta lever has a slider connection

allowing it to move up and down.We will now need to add three cam-followers to get the magenta levers to touch the three green extrusions.

Cam-Follower 1

To create a cam-follower, you click on the following icon in the Mechanism toolbar.

This brings up a similar window to what we have seen before.

Click on New and when the next window appears, give this a name of Lever1_Cam. The window will look

like the following:


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When you define a cam, you must pick surfaces or curves/edges on two different components. In thiswindow, there are three tabs Cam1, Cam2 and Properties. Cam1 is used to pick all of the surfaces for

the first component. Cam2 is used to pick all of the surfaces for the second component, and the Properties

tab is used to define additional properties of the entire cam.It does not matter which component is cam1 or cam2, therefore, we will pick all of the surfaces that go

around the green protrusion closest to the servo motor end. You could hold down the Ctrl key and select

each surface, or you can click in the checkbox called Autoselect, and then pick one of the tangent

surfaces. All other tangent surfaces to that surface should get selected, as shown below.

Once all of the surfaces have been selected, click on the middle mouse button to accept them. A light blue

outline of the cam 1 geometry will be shown on the model, as we can see in the next figure.


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A purple arrow points towards the outside surface of the cam geometry. You can flip it if it is not going in

the right direction.

Once you have this outline, the window looks like the following.

We can see the surfaces on the CAM_SHAFTcomponent listed, but the depth display setting is currently

set to Automatic. Whenever you have a completely closed boundary of tangent surfaces, like we do in this

case, it is always better to let the depth of the cam be generated automatically. We will see how to set

other depth options in the next tab.

Pick on the Cam2 tab. We will now select the two curve segments that form a half-circle on the top of the

magenta cam that sits underneath the first set of surfaces that we used for Cam1. Both curve segments

should highlight in a bold red when selected, as shown in the next figure.


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Once we have both curve segments selected, click with the middle mouse button to create the second cam.A cam symbol appears between the two components, as we can see in the following figure.

When we click on OK, the two components will snap together, as shown in the next figure.

Cam-Follower 2 & 3

Repeat this same process to create the second and third cam-followers (called Lever2_Cam and

Lever3_Cam). When done, our model will look like the following.


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We will now click on the analysis icon, select the Cam_Motion analysis that was already created, and then

hit the Run button. You should see the rod rotate that contains the three cams. As each cam goes

around, the magenta part beneath it will move up and down to maintain contact with the cam at all times.

Watch your playback and save your results.

In our first example, we forced the two cams to always touch each other. This is not always realistic, or

even possible. In cases where the cams must stop when they encounter each other, but not have to alwaystouch, we will enable liftoff for our cams.

To demonstrate this, we will open up a model called Lock_Cam.asm, which initially looks like the following.

In this model, the green key will slide into the lock (half-model) and then exit again. As the key moves into

the lock, the 10 individual little gold bits should slide up their respective tracks just enough to stay in contactwith the key surface as it moves under them.

Therefore, we need to set up cams for each of the bits so that when the green key comes into contact with

it, the bit will move up. But, as you can easily see, we can not force them to touch at all times, because thefarthest bit from the key wont come in contact with the key until it is all the way in the lock.

To start, go to Applications, Mechanism. We should see that we already have some slider connections

set up, as well as a servo motor. The servo motor is using a table-driven profile to move the key in and outin 6 seconds. An analysis is also set up to save some time. Now it is time to create the cam followers.


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Cam Follower 1

We will start by clicking on the cam tool in the feature toolbar, and then click on New to create our first

cam. Accept the default name to save some time. For Cam1, we will select the wavy surface of the key

and the smaller rectangular one to its right, as shown in the next figure.

Note: Using AutoSelect will not work in this case, as the surface is not a completely closed, tangent chain ofsurfaces. Once selected, click on the middle mouse button, and you will see the surface mesh in blue with a

purple arrow pointing up from it.

Now, click on the Cam2 tab, and select the half-circle curve at the bottom of the first bit, as shown in the

next figure.

Click on the middle mouse button to complete this second cam reference. Now, click on the Properties

tab. At the top, there is a check box to enable liftoff. Click in this box, and the window will look like the

following.


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We will not enter a coefficient of restitution at this time, so leave it at zero. Click on OK to complete our first

cam follower. What you will notice this time is that it does not automatically snap the two cam partstogether, like it did in the camshaft. This is due to the liftoff, and we can see our final cam in the next figure.

Cam Followers 2-10

Repeat this same process for the remainder of the cams that we need to define. For each cam, use the

same surfaces on the key, and the corresponding half-circle curve on the bit. Our final cam definition for thisassembly is shown in the next figure (from a FRONT, No Hidden view).

There are actually 10 cam symbols, but they are on top of each other. You should be able to see the 10individual dashed lines going to the individual bits. Now, click on the analysis tool, and edit the existing

Key_Insertionanalysis. It will look like the following figure.


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You might notice that Im using a Dynamic analysis instead of a Kinematic, as we have all along. This is

not necessary for cams, but will be in a later lesson. Turn on shading of your model, and click on Run to

watch what happens.

What do you notice? Pretty weird, isnt it? It actually makes sense if you understand what is going on.First of all, the cams are doing what they are supposed to do as the key comes in contact, the bit gets

pushed upwards. Unfortunately, because we are running a dynamic analysis, impact forces are causing thebits to react to this upwards motion. This wouldnt necessarily be a problem except that we have nothing to

bring the bit back to the key. The key continues upwards until it runs out of energy, and then hangs, asshown in the next figure.

Now, if we had enabled gravity (which we will learn in a later lesson), the bits would have come back down,

but would have passed through the bottom of the lock never to be seen again. If the key were in the way,the bits would come back down, bounce off the key, to the best of their ability based on the coefficient of

restitution, and continue this process until all of the dynamic energy was spent.


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Suffice it to say, that we are not done with our model, but we will have to wait until a future lesson. Fornow, change the analysis type to Kinematic, and then use Mechanism, Connect to bring the parts back to

their starting position. Re-run the analysis, and what do you see?

This time, the bits were pushed up, out of the way of the key as it moves, and then stopped at their highest

location. The Kinematic analysis does not force a collision reaction with the bits and the key, and gravity (orlack thereof) is not even factored in the bits simply move using the cams. We will start to see dynamicanalyses a little more in the upcoming lessons. For now, set the analysis type back to Dynamic, and then

save and close this assembly for later.

In addition to coefficient of restitution, we have the ability to specify any frictional coefficients that may existbased on the two cam materials. You would specify the coefficient of frict ion based on the two materials

that are in contact. For example, if one part is brass and one is steel, you would have one coefficient. Ifanother part were plastic and it hit the same steel part later on, you would define a second cam with a

different coefficient of friction.Any dynamic analysis performed on this cam would take these coefficients into account to give the most

realistic animation possible.

We could have actually used this for our rolling tire down the slide (in Lesson 15), instead of defining arotational servo to simulate the tire rolling. Friction is accessed on the Properties tab in the cam definition,

as shown in the next figure.

You can define a static (s) coefficient, as well as a dynamic (k) coefficient. The static coefficient must be

larger than the dynamic which is based off of valid empirical data.


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In mechanism, you are only allowed to pick on flat or cylindrical-like surfaces for cams. Spherical orsurfaces that bend in two or more directions are not permitted.

Similarly, you can pick straight curves, or rounded curves, as long as they line in a single plane.

When you select a flat surface or a straight edge/curve for the cam, the system does not automaticallycalculate the depth of the cam. Therefore, you must select a point/vertex that represents the front of the

cam surface/curve, and one that represents the back of the surface/curve. You can also specify a depthother than the distance between the two points.

Cams are used when two components must collide or remain touching each other during the motion. Youmust be careful picking references for cams, as they are a bit touchy. In addition to specifying liftoff, you

can also enable friction and coefficient of restitution for the collision of the bodies.

Open up the Maze.asm file. It should look like the following.

The obvious objective for this exercise is to capture a realistic set of constraints that will allow you to move


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your game piece around the maze without cutting through any internal walls. Test your cam(s) with the dragfunctionality. We will not be creating an analysis for this assembly.

Save and close the assembly when done.


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