Configure the 'Power Source' for Payload Analysis

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Configure the 'Power Source' for Payload Analysis


A simple cam-mechanism typically has two kinematic-chains, each with a Motion-Dimension FB.

a kinematic-chain that moves the cam

a kinematic-chain that moves the cam-follower

In the default case, the Power Source of each kinematic-chain is assigned to the joint with the Motion-Dimension FB. However, the Power-Source for the cam-follower is actually the cam, at the contact between the cam and cam-follower.

The power flows from the main drive motor, to the cam-shaft and cam, to the cam-follower, and finally to the tool or payload. Power flows in the other direction when the payload is being decelerated, of course.

Thus, nearly always, the default arrangement of the Power Source is NOT correct.

We must move the Power-Source for the cam-follower from the joint with the Motion-Dimension FB, to the cam, so that the cam becomes the Power Source.

The action: 'move the Power-Source from one element to a different element' is to 'Configure the Power Source'.

Special Cases of 'Configure the Power Source'

In MechDesigner, it is possible that a kinematic-chain has more Parts and Joints than the real-world application. In the real-world application, you would not need some Parts.

For example: we might use Piggyback-Sliders to define the motion and path of a Point with two Motion-Dimension FBs. After we have defined the motions, we usually connect one or more dyads between a Point on the Piggyback-Sliders and the Machine-Frame. The dyads become the real-world kinematic-chain, while the Piggyback-Sliders are in the model only to define the motion and path for the dyad to follow. The Power-Source would be assigned to the dyads and not the Piggyback-Sliders.

When this is the case, you MUST model the mechanism again, but without the Piggyback Sliders. You must then define the motions again for the Parts that will actually be driven, and then give the Power Source directly to these Parts and Joints.  There are some very useful MechDesigner 'tricks' that you can use to create the motions of the Parts in the dyads. Please email for details.

General Case of Configure Power Source to correctly model the Payload for Cam Analysis

Step 1: Set the machine speed

Use Edit menu (or toolbar) > Edit Machine Settings

Step 2: Add the correct masses and inertia to each Part.

See CAD-Line dialog-box | Mass-Properties tab

See CAD-Line dialog-box | SOLIDWORKS tab

See Extrusion dialog-box

Step 3: Power Source

Make sure the kinematic-chain receives its 'power' from the cam.

See Configure the Power Source

Step 4: Extrusion Depths

Make sure the Extrusion Depth of the Profile/Extrusion that is the Cam-Follower is the same as the Cam-Follower you will use in your machine.

See 2D-Cam display dialog-box > Display tab > Contact Force/Contact Stress : Select the Correct Cam-Follower from the drop down list.

Step 5: Direction of Force: towards or away from the cam?

As a cam rotates, the Contact-Force and Contact-Stress values change continuously. It is important to review their values with a Graph FB to find when, and if, in the machine cycle their values become less than 0.0N and 0.0N/mm2 respectively.

If the Cam is one of two conjugate cams that drive the same Cam-Follower Part, then see Machine-Elements toolbar >Conjugate Cam FB

If the values do become less than zero, then if the cam is a:

Form or Body Closed Cam

Conjugate Cam: Use Machine-Elements toolbar >Conjugate Cam

Groove Cam: You must decide on which cam-flank the contact-force is active.

If necessary, use the Cam Display and Properties dialog-box to change the cam from Cam 1 to Cam 2.

Force Closed Cam

Make sure the Contact-Force does not become less than 0.0N

MechDesigner does not show the cam-follower literally lift off the cam.