Tutorial KinematicAssembly

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Freecad.svg Tutorial
Topic
Assembly3, a simple mechanism
Level
Basic knowledge of Assembly3 tools is helpful
Time to complete
30 minutes
Authors
FBXL5
FreeCAD version
0.20 and later
Example files
None
See also
None

Introduction

This tutorial is about how to set up a simple mechanism, mainly with the tools from the external Assembly3 workbench icon.svg Assembly3 Workbench.

The kinematic assembly we will create consists of four parts: a Base, a Slider, a Crank, and a connecting Rod. They are connected with four joints.

Assembly3 KinematicExample-01.png

Assembled parts: Base (amber), Slider (light blue), Crank (red), connecting Rod (green)

Assembly

Parts

The Base is an object with two main geometries, a hole and a pin. Both are cylindrical. The rest of the shape is not relevant for this tutorial unless it causes clashes. The same goes for the other parts.

Assembly3 KinematicExample-02.png

The Slider consists of a shaft with a pin on one end. Both are cylindrical.

Assembly3 KinematicExample-03.png

The Crank has a hole and a pin. Again both are cylindrical.

Assembly3 KinematicExample-04.png

The Rod has two cylindrical holes.

Assembly3 KinematicExample-05.png

Joints

Locked Base

To keep the assembly at the desired position, the base part should be locked.

(If the Assembly LockMover.svg Lock mover command is activated, motion tools are deactivated as long as a locked part is selected.)
  1. Select one face of the Base.
  2. Press the button Assembly ConstraintLock.svg Create "Locked" constraint to keep the Base in place permanently.

Assembly3 KinematicExample-08.png Button right.svg Assembly3 KinematicExample-09.png

Selected face → Resulting Element

Then all four parts are connected with four joints. The kinematic chain starts at the base.

Base-to-Slider joint

The Base-to-Slider joint is a cylindrical joint. It enables the Slider to slide along and spin around the Base hole's Z axis while keeping both elements' Z axes aligned (colinear).

The matching constraint is the "AxialAlignment" constraint. It works with elements that represent cylindrical geometry such as cylindrical faces, circular faces and circular edges.

  1. Select the cylindrical faces of the Base hole and the Slider shaft.
  2. Press the button Assembly ConstraintAxial.svg Create "AxialAlignment" constraint.
  3. Optionally relabel the created elements (edit their DataLabel property).

Assembly3 KinematicExample-10.png Button right.svg Assembly3 KinematicExample-11.png

Selected faces → Aligned objects

Base-to-Crank joint

The Base-to-Crank joint is a hinge joint. It enables the Crank to spin around the Base pin's Z axis while keeping both elements' Z axes aligned (colinear) and the offset between their XY planes constant.

The matching constraint is the "PlaneCoincident" constraint. It works with elements that represent planar geometry such as circular faces and circular edges (in this case).

  1. Select the circular face or the outer circular edge of the Base pin, and the outer circular edge of the Crank hole.
  2. Press the button Assembly ConstraintCoincidence.svg Create "PlaneCoincident" constraint.
  3. Optionally relabel the created elements.

Assembly3 KinematicExample-12.png Button right.svg Assembly3 KinematicExample-13.png

Selected face and edge → Aligned objects

Slider-to-Rod joint

The Slider-to-Rod joint is a hinge joint. It enables the Rod to spin around the Slider pin's Z axis while keeping both elements' Z axes aligned (colinear) and the offset between their XY planes constant.

The matching constraint is the "PlaneCoincident" constraint (see above).

  1. Select the circular face or the outer circular edge of the Slider pin, and the outer circular edge of the Rod hole.
  2. Press the button Assembly ConstraintCoincidence.svg Create "PlaneCoincident" constraint.
  3. Optionally relabel the created elements.

Assembly3 KinematicExample-14.png Button right.svg Assembly3 KinematicExample-15.png

Selected face and edge → Aligned objects

Crank-to-Rod joint

The Crank-to-Rod joint is a cylindrical joint. It enables the Rod to spin around and slide along the Crank pin's Z axis while keeping both elements' Z axes aligned (colinear). But only spinning will be possible as the sliding movement is restricted through the combination of the Base-to-Crank joint and the Slider-to-Rod joint.

The matching constraint is the "AxialAlignment" constraint (see above).

  1. Select the cylindrical faces of the Crank pin and the Rod hole.
  2. Press the button Assembly ConstraintAxial.svg Create "AxialAlignment" constraint.
  3. Optionally relabel the created elements.

Assembly3 KinematicExample-16.png Button right.svg Assembly3 KinematicExample-01.png

Selected faces → Aligned objects

Redundant Constraints

When the Base is fixed and all four joints are constrained two messages appear in the Report view:

  • A warning (orange): "...redundant constraints".
  • A simple message (black): "...dof remaining: 0".

This combination of messages occurs when parts of an assembly are over-constrained but the solver is still able to find a valid solution. But what causes the redundacy?

It is the Z direction of the pins. If we take a look at the Slider pin for example we will notice that the Z axis of its element object is constrained parallel to the Base pin's Z axis through the assembly chain Base-Crank-Rod-Slider. This means that the Slider pin is prevented from rotating around its X and Y axes.

Assembly3 KinematicExample-06.png

On the other hand the rotation around the X axis (red) is already prevented by the Base-to-Crank joint; and so the corresponding degree of freedom (dof) is constrained twice (= redundant) and causes the warning.

To avoid this redundancy an auxilliary object and corresponding constraints could be inserted, but that is for some other tutorial.
To avoid double constraining the offset between base and Rod, different constraints were used, with only one of them fixing the motion along the Z axis.

Actuator

Now it is still a static assembly. To turn it into a kinematic assembly one constraint has to be used as an actuator. To use the "PlaneCoincident" constraint of the Base-to-Crank joint as an actuator, we need to control the angle between Base pin and Crank. This can be done by setting the property DataLock Angle to true. And for later use the label is marked with the suffix .Driver.

The DataAngle property can now be used to spin the Crank.

Assembly3 KinematicExample-07.gif

Controller

To have a dialog window to change property values without typing and with automatic recomputation would be nice.

Have a look at the Kinematic Controller tutorial.