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

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

Configure Power Source

It is important to 'Configure the Power Source' when we use a cam to move a cam-follower.

In the default case, a power-source drives the Part specified by the Motion-Dimension: thus, the cam and the cam-follower kinematic-chains each have a power-source.

We must re-configure the model so that the cam is the power source for cam-follower.


For this tutorial, we will remove the Spring Force FB.

1.Delete the Spring Force FB

2.Delete the wire that connects the Linear-Motion FB to the Motion-Dimension FB

A Stationary Rocker 'driven' by a Cam

MD-QST13-4-0a

Add a Cam-Follower Roller to the end of the Rocker.

To add the Cam-Follower Roller: Edit the Rocker Part, Add a Circle, constraints and dimensions, Close the Part-Editor, Add a Profile to the Circle.

The Cam-Follower Profile:

Weighs 10N (a mass ~1kg)

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

Rocker-Forces-13

The image shows a 'close up' view of the end of the Rocker

Note: there are now three 'dowel symbols' – each identifies the position of the center-of-Mass of each Extrusion. The three dowel Symbols are:

center-of-mass of the original ProfileRed-14-1b (1kg, @100,0)

center-of-mass of the Cam-Follower RollerRed-14-2 Profile (1kg,@200,0)

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

Rocker-Forces-14

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

Forces:

Rocker has a Vertical Forces down: (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.

Moments:

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.

Rocker-Forces-15

Icon-Add2DCamAdd a 2D-Cam

I have de-selected Display Forces  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.

Rocker-Forces-16

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 Cam 1:

1.Double-click the Cam-track in the graphic-area to open the Cam Display and Properties dialog-box.

2.Clear the check-box for Cam 1 in the Cam Visibility separator.

Give the Cam a Mass

We can add a Profile to the Cam to give it a mass.

Icon-AddPolyline

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

STEP 5: Click the 'Home' key on your keyboard.

STEP 6: Click Add Polyline;

STEP 7: Click the 2D-Cam shape

STEP 8: Click OK in the Command-Manager

Icon-AddProfile

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

STEP 8: Click Solids toolbar >Add Profile

STEP 9: Click the Polyline (sketch-loop)

STEP 10: Click OK in the Command-Manager

Rocker-Forces-17

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

 

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

Configure the Power Source

Rocker-Forces-18

The 'Problem'

Where is the Contact Force between the Cam and Cam-Follower? 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 moves the Rocker.

MotorRed-14-2 is OK. It rotates the cam.

MD-QST13-4-5

Reconfigure the Power Source to make the 2D-Cam drive the Rocker.

STEP 11: Either: A: Right-Click the kinematic-chain with Cam-Follower
B: Click the Configure Power Source

Icon-ConfigurePowerSourceFB

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

Rocker-Forces-19

The Configure Power Source dialog-box opens.

 

Motion-Dimensions and Joints tab:

Motion-Dimensions

Here, you select the Motion-Dimension for which you want to configure the Power Source.

STEP 13: Click the Mot-Dim Rocker in the Motion-Dimensions List Red-14-1b  

Possible Motor Elements

After you click a Motion-Dimension, a list of 'Possible Motor Elements' shows in Red-14-2.

You can see there are two 'Possible Motor Elements':

Pin-Joint – selected by default

2D-Cam.

We want to use the 2D-Cam as the 'Motor' for the kinematic-chain

STEP 14: Select 2D-CamRed-14-2  in the dialog in the Possible Motor Elements box

You should see a Green tick, tick16 ,  next to the 2D-CamRed-14-2 element.

STEP 15: Click OK

 

Rocker-Forces-20

You should see:

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

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

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

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

4.The Motor symbol at the cam-shaft AxisRed-14-4

5.The cam-shaft center 'Action and Reaction Forces' have increased to 53.94N, to support the weight (mg) of the Rocker and the Cam-Follower RollerRed-14-5