Motion-Path: Ram-R and Ram-P

This is one example of how we use a Motion-Path FB. We can use the Motion-Path FB to build two new Dyads:


An application is to model the opening and closing of an engine valve. We can use the Ram-R dyad to control the distance between the centre of the rotating cam and the centre of the roller.

Ram-R Dyad



STEP 1: 1s-red Edit the Base-Part to add a horizontal Line. Make it 150mm long
STEP 2: Close the Part-Editor
STEP 3: 2s Add two Parts
STEP 4: Make the left-hand Part 120mm long and the right-hand Part 140mm long.
STEP 5: 3s Add Pin-Joints between the Origin of each Part and a Point at each end of the Line in the Base-Part.

Step 5 is complete.


Add A Motion-Path FB

STEP 6: Click Add Motion-Path FB in the Kinematics-FB Toolbar

You must select a sketch-path [in this case, the CAD-Line].

STEP 7: Click the CAD-Line of the left-hand Part.
STEP 8: Click OK-tiny-11-15 in the Command-Manager.

MechDesigner adds the Motion-Path FB1s-red. The Motion-Point2s is at the start-Point of the CAD-Line.


You can show a Dimension between the start and active position of the Motion-Point, in mm or in degrees. The 'mm' dimension is the actual distance moved from its start position. The 'degrees' dimension is normalized from 0 to 360º. 360 is complete length of the sketch-path.

See Motion-Point FB > Point List and Data Display separator


Move the Motion-Point

STEP 9: Add a Motion FB and a Linear-Motion FB to the graphic-area.
STEP 10: Double-click the Motion FB to open the Motion FB dialog-box
STEP 11: In the Motion FB dialog-box, make the Output Data-Type to Linear, then close the dialog box.
STEP 12: Connect wires between the FBs in the usual way

Step 12 is Complete

When you connect a wire motion from a Motion FB to a Motion-Path FB, you MUST edit the Output Data Type in the Motion FB dialog-box to Linear or Rotary. In this case, make it Linear.


Add a Pin-Joint

When you add a Motion-Path FB, the Motion-Point is at the start-Point of the sketch-element you select

In this case, it is necessary to move the Motion-Point from the start-Point of the CAD-Line.

STEP 13: Click Cycle - or step the cycle with the 'F' key.

Wait until the Motion-Point moves a short distance along the CAD-Line, so that you can see it.

STEP 14: Click Add Pin-Joint in the Kinematic-elements toolbar
STEP 15: Click the end-Point of the Cad-Line in the right-hand Part and then the Motion-Point that is partly along the CAD-Line

Step 15 is Complete

The Parts are now kinematically-defined Parts. They have Green Part-Outlines.

If it is necessary to change the closure of the Dyad, then use the Kinematic-elements Change-Dyad tool

STEP 16: Click Run menu > Cycle.

You will see the Motion-Point move along the CAD-Line.

However, when the Motion-Point is near to the Origin of the CAD-Line, the Kinematic-Chain breaks.

We need to move the 'Start Position' of the Motion-Point along the CAD-Line so that it does not start its motion at the start-Point of the CAD-Line.


Edit the Motion-Path FB

STEP 17: Double-click the Motion-Path FB in the graphic-area.


The Motion-Path FB dialog-box opens.

STEP 18: In the Point Parameters separator, edit the Base-Value to move the 'Start Position' to 20mm

See image to the left.

The start position of the Motion-Point is now 20mm from the Origin along the CAD-Line.

STEP 18: Click Run menu > Cycle

The Kinematic-chain now operates as intended.

We call this kinematic-chain the Ram-R Dyad

The Ram-R dyad is often used in packaging machines to represent an Air-Cylinder.

Tutorial and Reference Help Files for MechDesigner and MotionDesigner 13.2 + © Machine, Mechanism, Motion and Cam Design Software by PSMotion Ltd