Step 13.4: A Rocker Driven by a 2D-Cam

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Step 13.4: A Rocker Driven by a 2D-Cam

Add a 2D-Cam and Configure the Power Source

When we add a 2D-Cam, we must also Configure the Power Source

In this Step 13.4, we will

1.Delete the Spring FB

2.Delete the wire from Linear-Motion FB to the Motion-Dimension FB

3.Do Add Polyline and Add Profile to extrude the 2D-Cam as a Solid (Note this is not needed to  establish the  Contact-Force

4.Configure the Power Source. to make the 2D-Cam the Power Source for the Cam-Follower.

A Stationary Rocker and a 2D-Cam


Add a Cam-Follower Roller to the Rocker.

To add the Cam-Follower Roller: Edit the Rocker, Add a Circle (100mm Radius), Add Coincident to move its center-Point to the end-Point of the Rocker, Close the Part-Editor,

Add a Profile to the Circle.

Edit the Density of the Profile/Extrusion until its mass is ~1kg. The Cam-Follower Profile:

Weighs 10N (a mass ~1kg)

Center-of-mass (the center of the Circle) is 200mm, horizontally, from the Pin-Joint.


The image shows the end of the Rocker.

Note: there are three symbols that identify the:

Red-14-1b Center-of-mass of the original Profile (1kg, @100,0)

Red-14-2 Center-of-mass of the Cam-Follower Roller Profile (1kg,@200,0)

Red-14-3 Center-of-mass (or center-of-Gravity) of all of the Profiles in the Part / Rocker (2kg, @150mm). It has a color that matches the color of the Part-Outline.


Forces and Moments with a Profile (1kg @100mm) and New Cam-Follower Roller Profile (1kg @ 200mm)


Rocker:  Vertical Forces: (1(kg) + 1(kg))* 9.81(m/s/s) = -19.62N

∴ Reaction force from the Base-Part that acts on the Rocker is 19.62N UP.


Rocker has a Clockwise Moment (-ve) of -1(kg)*9.81(m/s^2)*0.1(m) - 1(kg)*9.81(m/s^2)*0.2(m) = -2.943Nm

 Reaction Moment from the Base-Part that act on the Rocker = +2.943Nm Anti-clockwise.


GST-Icon-Add2DCamAdd a 2D-Cam

I have de-selected Show Force Vectors  temporarily, to remove the Force Vectors, while we add the cam..

STEP 1:Add a Part that will carry the 2D-Cam.Red-14-1b

Put the center of the 2D-Cam vertically below the center of the Cam-Follower Roller, at '200,-200mm'.

STEP 2:Add the Motion-Dimension FB with the Linear-Motion FB to rotate the cam-shaft.


STEP 3: Click Machine elements toolbar >Add 2D-Cam

Click the new cam-shaftRed-14-1b (image above) and the Cam-Follower Profile

The 2D-CamRed-14-1b (left image) is circular because the Cam-Follower Roller does not move.

STEP 4: Edit the 2D-Cam  

To remove Outer Cam:

1.Double-click the 2D-Cam in the graphic-area to open the 2D-Cam dialog-box.

2.Clear the check-box for Outer Cam in the Cam Visibility separator.

Give the Cam a Mass

Note: We do not need to Give the Cam a Mass. However, it is a good opportunity to remind you of how you can, if you really want to, do Add Polyline and Add Profile. to extrude the 2D-Cam as a Solid.


STEP 1: Add a Polyline to the 2D-Cam

Before we can add a Profile to a 2D-Cam, we must first add to it a Polyline.

1.Mechanism-Editor: Click the Home (or ALT+H ) key on your keyboard.

2.Mechanism-Editor: Click Solids toolbar > Add Polyline

3.Mechanism-Editor: Click the 2D-Cam shape

4.Click OK-tiny-11-15 in the Command-Manager


STEP 2: The Polyline of a 2D-Cam is a sketch-loop. We can add a Profile to a sketch-loop

1.Mechanism-Editor: Click Solids toolbar >Add Profile

2.Mechanism-Editor: Click the Polyline (sketch-loop)

3.Click OK-tiny-11-15 in the Command-Manager


You can see the force and moment that act on the Rocker are the same as before, 19.61N and 2.942Nm, respectively.

The Problem : Where is the Contact Force between the Cam and Cam-Follower?  


(For convenience, I have made the mass of the 2D-Cam equal 4kg (editing its Density). Therefore, there is a Reaction Force from the Base-Part of 39.227N to support the Cam).

Configure the Power Source


Why is the Cam-Force zero?

In the image to the left, there are two Power Source ( 2 MotorsRed-14-1bRed-14-2)

A Motor to drive the RockerRed-14-1b

A Motor to drive the Cam-shaftRed-14-2

There is usually ONE motor to drive this mechanism! This is because:

MotorRed-14-1b does not really exist. The cam drives the Cam-Follower / Rocker, and not a motor.

MotorRed-14-2 is OK. It rotates the Cam-shaft


We must reconfigure the Power Source to make the 2D-Cam drive the Rocker.

To open the Configure Power Source dialog-box:

In the Kinematics-Tree, do these three clicks

A: Click the Kinematic-Chain that includes the Cam-Follower

B: Right-Click the Kinematic-Chain

C: Click Configure Power Source


STEP 12: Or: A: Click the kinematic-chain with the Cam-Follower
B: Click Force Toolbar >Configure Power Source


The Configure Power Source dialog-box is now open.

Power Source tab > BoxRed-14-1b Motion-Dimensions  | Motor Elements

Select the Motion-Dimension for which you want to configure the Power Source.

STEP 13: In boxRed-14-1b : click the Mot-Dim RockerRed-14-1b

BoxRed-14-2 lists the possible Power-Sources (or Motor Elements) that can drive the Motion-Dimension you select in boxRed-14-1b

Pin-JointGST-icon-MotorElement icon to indicate it is the default Power-Source

2D-Cam - We want to use the 2D-Cam as the Power-Source

STEP 14: In boxRed-14-2 : Click the 2D-Cam

Now, the GST-icon-MotorElement icon is to the left of the 2D-CamRed-14-2 to indicate the 2D-Cam is the Power-Source

Also you should see the 2D-Cam in box Red-14-1b  as the Motor-Element for the Mot-Dim Rocker

STEP 15: Click OK-tiny-11-15 to close the dialog


You should see that:

1.The Motor symbol is not at the Pin-Joint of the Cam-Follower RockerRed-14-1b  

2.There is not a Torque vector at the Cam-Follower Pin-JointRed-14-2.

This 2D-Cam does not need a torque because it does not do useful work. The Cam simply supports the mass of the Cam-Follower and Cam-Follower Roller.

3.Action and reaction forces at the Cam ContactRed-14-3

4.The Motor symbol at the Cam-ShaftRed-14-4

5.The Action and Reaction Forces at the center of the 2D-Cam  are now 53.94N,

The center of the Cam must support the weight (m.g) of the Cam-Follower/Rocker and the Cam-Follower RollerRed-14-5