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 joints 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.

Note: If the lengths of the Parts and kinematic-chains in the images and videos below are different to your model, then I have edited their lengths to show a kinematic configuration.

Please edit the length of the Parts to be the same approximate length as the Parts in the images when they are clearly different to your model.


Terminology: Reminder from Tutorial 1

Elements :

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 the element's parameters in the element's dialog-box.

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

Plane :

the flat surface that is at a precise angle and position relative to the XYZ axes and XYZ coordinates. When we add a Mechanism-Editor to a Plane, and that editor is active, then the 'Front-View' looks directly at the Mechanism-Plane.

Mechanism-Editor :

the workspace that includes the Mechanism-Plane , the graphic-area, the contextual -toolbars, and the Mechanism-Editor name tab.

Part :

an element that becomes the moving, or fixed, rigid-bodies in the machine.

Joint :

Kinematic-Joints constrain the relative movement between two Parts.

Pin-Joint :

a Kinematic-Joint that constrains together two Points, with one Point in two different Parts.

Kinematic-Chain :

two or more Parts joined together with one or more Joints.

Kinematically-Defined :

Part or Kinematic-Chain that moves in a manner as specified by at least one Motion-Part. Parts that are kinematically-defined have Green Part-Outlines.

When a Part has a:

Green Part-Outline: the Part is a kinematically-defined Part. We also call it a 'Solved Part'.
Blue Part-Outline: the Part is not a kinematically-defined Part. We also call it an 'Unsolved Part'.

Our target is to make sure we have Green Part-Outlines, ALWAYS.

Motion-Part :

a Part, that is kinematically-defined, whose motion is precisely specified with a Motion-Dimension FB

Motion-Dimension FB :

a Function-Block[FB] that specifies the angular or linear position, velocity and acceleration of a Part as a function of the Master-Machine-Angle [there are exceptions]

Linear-Motion FB :

a Function-Block[FB] whose output is 0 – 360 [default]. You can edit the initial value parameter to delay or advance the timing of a kinematic-chain.

Gearing FB :

a Function-Block[FB] with which you can apply parameters to the input motion-values in the form of a linear equation to give a different output.

Master-Machine-Angle :

the 'clock' that, when you cycle the machine, moves at a constant rate from 0 to 360, again and a again.

The positions of all kinematically-defined Parts are a function of Master-Machine-Angle. The Master-Machine-Angle [MMA] 'beats the drum' of the machine.

Rocker :

a Motion-Part that rotates with a specific motion relative to a different Part. It may index progressively, rotate continuously [as a Crank], or oscillate back-and-forth.

Terminology: Tutorial 2

Slide-Joint :

a Kinematic-Joint that constrains together two Lines, with one Line in two different Parts.

Slider :

a Motion-Part that slides with a specific motion relative to a different Part. It may index progressively, slide continuously [as a 'conveyor'], or oscillate back-and-forth.

Dyad :

a Dyad is a kinematic construction of:

2 Parts
3 Joints

Each of the two Parts in the Dyad is joined to two different Parts and then they are joined together, to make three Joints in total.

Dyad Closure :

a different way to assemble the same Dyad.

Motion FB :

a Function-Block[FB] that links a Motion-name and motion-values in MotionDesigner to a kinematic-chain.

Crank-Rocker :

a kinematic-chain with one Motion-Part [the Crank] and one Dyad that has three Pin-Joints, and the output oscillate ['rocking'] back-and-forth.

Crank-Slider :

a kinematic-chain with one Motion-Part [the Crank] and one Dyad that has two Pin-Joint and one Sliding-Joint, with the output reciprocating ['sliding'] back and forth.

Mechanism names :

Scotch-Yoke, Whitworth Quick-Return are common names for some 'standard' mechanisms you may come across at different times in a machine.

 

Icon-AddMechanismEdit 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 http://youtu.be/o3krPdVZW2g.

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