Four-bar Kinematic-Chains

The Four-Part Kinematic-chain - which you may call a 'four-bar mechanism', 'four-bar linkage', or even 'four-bar' - is the basis of the most frequently used mechanism in packaging machines.

This tutorial uses the Pin-Joint and Slide-Joint elements. We combine the joint with the Parts in different ways to configure the mechanism in different ways.

Objective of this Tutorial

To build kinematic-chains that have a total of four Parts and four Joints.
To learn about Dyads.

Terminology reminder from Tutorial 1


Elements are the objects we add to the model. They represent the objects in a machine.

Nearly always: the sequence of steps we use:

1.Add the element...
2....Open the element's dialog-box...
3....Edit its parameters.

In addition to elements that you can see in the graphic-area [Parts, Pin-Joints, ...], there are elements you cannot see [Constraints, ...]

The elements in Tutorial 1:


Parts are the elements that move relative to each other. They are 'rigid-bodies'. They remain coplanar to the Mechanism-Plane. [The Mechanism-Plane' is the Plane you select to add the Mechanism-Editor].

Base-Part: the fixed element on the Mechanism-Plane. Think of it as the machine frame.

Added-Parts [a different name for a Part]: the moving elements. They can move relative to the Base-Part.


Part-Outline: the 'symbol' for the element that represents a Part.

The Base-Part has a rectangular Part-Outline. A Part, or Added-Part, has an oval Part-Outline.

You cannot edit the shape of a Part-Outline. You select it when you want to edit a Part.

Part-Editor, Mechanism-Editor, Model-Editor.

Use the Part-Editor to edit an Added-Part and the Base-Part. You can add sketch-elements to Parts. You can edit the length of a Part. You can add constraints between sketch-elements.

Sketch-elements can define the position of Joints.

Use the Mechanism-Editor to build kinematic-chains, with Parts and Joints.


The kinematic-chain in Tutorial 1 has one Joint:

Pin-Joint: joins the Added-Part to the Base-Part.


Kinematic-Chains are identified in the Kinematics-Tree.

A kinematic-chain has two or more Parts and one or more Joints.

The kinematic-chain in Tutorial 1 has two Parts:

Base-Part: is the Part that is fixed in the mechanism-Plane [or the mechanism frame].
Added-Part: is the Part that, in the case of Tutorial 1, rotates, with a specified motion, relative to the Base-Part.

The Added-Part became the 'Rocker', as defined in the Kinematic-Tree.


A Part that rotates, or oscillates.

The Rocker is made up of three elements:

A Part
A Pin-Joint: Pin-Joints join two Points, with each Point in a different Part.
A Motion-Dimension FB: specifies the start-angle of the Part relative to the Base-Part. This FB also identifies the Part we wish to move with a specified motion.


Function-Blocks specify the motion for Rockers and Sliders.

A Linear-Motion FB specifies a motion that changes at the same rate as the Master Machine Angle.

Master Machine Angle

Master Machine Angle: the master-clock in the machine - it beats the drum for all kinematic-chains. It has a range of 0 to 360.

Mobility, Degrees-of-Freedom, kinematically-defined Parts

Parts have Green Part-Outlines or Blue Part-Outlines.

When a Part has a:

Green Part-Outline: the Part is a kinematically-defined Part in a kinematically-defined chain. We also call it a 'Solved Part'.
Green Part-Outlines: are your target.
Blue Part-Outline: the Part is not a kinematically-defined Part. We also call it an 'Unsolved Part'.

Kinematically-defined chains have a Mobility equal to Zero. Their position is completely defined by one or more Motion-Dimensions.

Kinematically-defined chains have an equal number of Motion-Dimensions as the Kinematic-Chain has degrees-of-freedom.

Icon-AddMechanism Edit Kinematic-Chains

Optionally, review this video of a Bernays Steam Engine – Kinematics Only

Kinematics of Beranys Steam Engine

This Bernays Engine has a number of Dyads, with only one Motion-Dimension.

At the end of this tutorial, you will be able to model kinematic-chains such as this.

Look at the YouTube video to see the solid model

I apologise for the poor video. Make sure your speakers are off if you are in an open-plan office!

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