Autodesk's VEX® Robotics Curriculum
Unit 8: Friction and Traction
1
Overview
In Unit 8: Friction and Traction, you modify the differential tricycle to participate in a tractor pull.
You learn about the concepts of friction and traction while applying your knowledge of the design
process to solve a given problem. The physics concepts of friction and traction must be considered
in countless real-world applications. In STEM Connections, we present one scenario involving friction
and traction in the design of a snowmobile. After completing the Think Phase and Build Phase in Unit
8: Friction and Traction, you will see how concepts regarding friction and traction come into play in
the real world.
Objectives
After completing Unit 8: Friction and Traction, you will be able to:
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Explain the difference between static and kinetic friction and list the factors which determine
traction.
Create a VEX tire in Autodesk Inventor Professional.
Have a robot ready to compete in a “tractor pull” and be proficient in making simple modifications
to VEX robots.
Take advantage of the principles of friction and traction to modify a robot to pull a greater amount
of weight.
Prerequisites and Resources
Related resources for Unit 8: Friction and Traction are:
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Unit 1: Introduction to VEX and Robotics.
Unit 2: Introduction to Autodesk Inventor.
Unit 4: Microcontroller and Transmitter Overview.
Unit 5: Speed, Power, Torque, and DC Motors.
Unit 6: Gears, Chains, and Sprockets.
Unit 7: Advanced Gears.
Key Terms and Definitions
The following key terms are used in Unit 8: Friction and Traction:
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Term
Definition
Chamfer
A placed feature that bevels a part edge and is defined by its placement, size, and
angle.
Coefficient of
Friction
The ratio of maximum frictional force between two surfaces to the force holding
them together. Term used to describe the "grippyness" of two surfaces meshing
together. Slippery objects have a very low coefficient of friction.
Autodesk's VEX Robotics Unit 8: Friction and Traction
Term
Definition
Component
A part or subassembly placed into another assembly. Assembly components
may be single parts or parts combined that operate as a unit (or subassembly).
Components may be treated as parts within other assemblies.
Dimension
Parametric dimensions that control sketch size. When dimensions are changed, the
sketch resizes. Dimensional constraints may be expressed as numeric constants, as
variables in equations, or in parameter files.
Fillet
A placed feature applied to edges and corners of a 3D model. A fillet feature is
defined by its type, radius, and placement.
Friction
The resistance that one surface or object encounters when sliding against another.
iFeature
Features, sketches, or subassemblies that can be used in more than one design are
designated as iFeatures and saved in a file with an IDE extension.
Kinetic
Friction
The frictional force which opposes the motion of an object while it is moving.
Mirror
sketches
Sketch geometry that is copied across a centerline.
Normal Force
The amount of force holding two surfaces together. For an object sitting on a level
surface, this value is equivalent to the objects weight as caused by gravity.
Opacity
Is the measure of how opaque or see-through an assembly component is.
Pattern
Multiple instances of a placed or sketched feature arrayed in a specified pattern.
Patterns are defined by type (rectangular or circular), orientation, number of
features, and spacing between features.
Plane
A two-dimensional (flat) part face.
Profile
A closed loop defined by sketched or reference geometry that represents a cross
section of a feature. An open profile defined by sketched segments, arcs, or splines
may define a surface shape or extend to boundaries to close a region. A profile
may enclose islands.
Projected
Geometry
Geometry (model edges, vertices, work axes, work points, or other sketch
geometry) projected onto the active sketch plane as reference geometry. May
include edges of a selected assembly component that intersects the sketch plane
when it was cut in an assembly cross section.
Properties
A characteristic of a Microsoft Windows file that can be manipulated from an
application or Microsoft Windows Explorer. Properties include author or designer
and creation date and may also be unique properties assigned by applications or
users. Specifying properties can be useful when searching for part or assembly
files.
Revolve
A solid feature created by revolving a profile around an axis.
Overview
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3
Term
Definition
Section View
In an assembly, a view of the model defined by temporarily hiding portions of
components or features on one side of a specified cutting plane.
Static Friction
The frictional force that opposes the motion of an object before it starts moving.
Template
An assembly, part, or drawing file that contains predefined file properties. To
create a file based on a template, you open a template file, create the content,
and then save it with a unique file name. Predefined properties can include
visible default reference planes, customized grid settings, color scheme, drafting
standards, and so on.
Traction
The friction between a drive member, wheel, and the surface it moves upon. The
amount of force a wheel can apply to a surface before it slips.
Tread
The pattern on the surface of a tire.
Required Supplies and Software
The following supplies and software are used in Unit 8: Friction and Traction:
Supplies
Software
VEX Classroom Lab Kit
Autodesk® Inventor® Professional 2010
One assembled differential tricycle built in the
Unit 7: Advanced Gears > Build Phase
One modified and assembled differential
tricycle from the Unit 8: Friction and Traction >
Build Phase
Notebook and pen
Work surface
Small storage container for loose parts
6’x12’ of open floor space
Masking tape
Measuring tape
36” of 1/8” Braided nylon and polyester cord or
equivalent rope/string
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Autodesk's VEX Robotics Unit 8: Friction and Traction
VEX Parts
The following VEX parts are used in Unit 8: Friction and Traction > Build Phase:
Quantity
Part Number
Abbreviations
1
BEAM-2000
B2
2
SCREW-832-0250
S2
1
SCREW-832-0750
S6
1
SPACER-THIN
SP1
1
VEX-12-TOOTH-GEAR
G12
4
WASHER-DELRIN
WP
Academic Standards
The following national academic standards are supported in Unit 8: Friction and Traction:
Phase
Standard
Think
Science (NSES)
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Unifying Concepts and Processes: Form and Function; Change, Constancy, and
Measurement
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Physical Science: Motions and Forces
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Science and Technology: Abilities of Technological Design
Technology (ITEA)
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5.8: The Attributes of Design
Mathematics (NCTM)
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Alegbra: Analyze change in various contexts.
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Measurement: Understand measurable attributes of objects and the units, systems,
and processes of measurement.
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Problem Solving: Apply and adapt a variety of appropriate strategies to solve
problems.
■
Communication: Communicate mathematical thinking coherently and clearly to
peers, teachers, and others.
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Connections: Recognize and apply mathematics in contexts outside of mathematics.
Create
Science (NSES)
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Unifying Concepts and Processes: Form and Function
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Physical Science: Motions and Forces
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Science and Technology: Abilities of Technological Design
Overview
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5
Phase
Standard
Technology (ITEA)
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5.8: The Attributes of Design
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5.9: Engineering Design
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6.12: Use and Maintain Technological Products and Systems
Mathematics (NCTM)
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Numbers and Operations: Understand numbers, ways of representing numbers,
relationships among numbers, and number systems.
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Algebra Standard: Understand patterns, relations, and functions.
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Geometry Standard: Use visualization, spatial reasoning, and geometric modeling to
solve problems.
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Measurement Standard: Understand measurable attributes of objects and the units,
systems, and processes of measurement.
Build
Science (NSES)
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Unifying Concepts and Processes: Form and Function; Change, Constancy, and
Measurement; Evidence, Models, and Explanation
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Physical Science: Motions and Forces
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Science and Technology: Abilities of Technological Design
Technology (ITEA)
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5.8: The Attributes of Design
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5.9: Engineering Design
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6.10: Troubleshooting, Research, and Development, Invention and Innovation, and
Experimentation in Problem Solving
Mathematics (NCTM)
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Numbers and Operations: Compute fluently and make reasonable estimates.
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Algebra: Analyze change in various contexts.
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Geometry: Use vizualization, spatial reasoning, and geometric modeling to solve
problems.
■
Measurement: Understand measurable attributes of objects and the units, systems,
and processes of measurement.
■
Measurement: Apply appropriate techniques, tools, and formulas to determine
measurements.
■
Problem Solving: Build new mathematical knowledge through problem solving.
■
Problem Solving: Solve problems that arise in mathematics and in other contexts.
■
Problem Solving: Apply and adapt a variety of appropriate strategies to solve
problems.
■
Connections: Recognize and apply mathematics in contexts outside of mathematics.
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Autodesk's VEX Robotics Unit 8: Friction and Traction
Phase
Standard
Amaze
Science (NSES)
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Unifying Concepts and Processes: Form and Function; Change, Constancy, and
Measurement; Evidence, Models, and Explanation
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Physical Science: Motions and Forces
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Science and Technology: Abilities of Technological Design
Technology (ITEA)
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5.8: The Attributes of Design
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5.9: Engineering Design
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6.10: Troubleshooting, Research, and Development, Invention and Innovation, and
Experimentation in Problem Solving
Mathematics (NCTM)
■
Numbers and Operations: Compute fluently and make reasonable estimates.
■
Alegbra: Analyze change in various contexts.
■
Geometry: Use vizualization, spatial reasoning, and geometric modeling to solve
problems.
■
Measurement: Understand measurable attributes of objects and the units, systems,
and processes of measurement.
■
Measurement: Apply appropriate techniques, tools, and formulas to determine
measurements.
■
Problem Solving: Build new mathematical knowledge through problem solving.
■
Problem Solving: Solve problems that arise in mathematics and in other contexts.
■
Problem Solving: Apply and adapt a variety of appropriate strategies to solve
problems.
■
Communication: Communicate mathematical thinking coherently and clearly to
peers, teachers, and others.
■
Connections: Recognize and apply mathematics in contexts outside of mathematics.
Overview
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7
Think Phase
Overview
This phase discusses the physical concepts of friction and traction and their applications to robot
design.
Phase Objectives
After completing this phase, you will be able to:
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Explain the difference between static and kinetic friction.
List the factors which determine traction:
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Normal force
❏
Coefficient of friction
Prerequisites
Related phase resources are:
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Unit 5: Speed, Power, Torque, and DC Motors.
Required Supplies and Software
The following supplies are used in this phase:
Supplies
Notebook and pen
Work surface
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Autodesk's VEX Robotics Unit 8: Friction and Traction
Research and Activity
Friction is a force that opposes motion.
Static friction is the frictional force between two objects that are NOT moving relative to each other.
It is the initial force that must be overcome in order for objects to move. If an object is stationary,
and the force trying to move the object is less than the maximum possible force of static friction, the
object will not move.
Kinetic friction is the frictional force between two surfaces that ARE moving relative to each other
(sliding along each other). Once an object has overcome static friction, it has kinetic friction acting on
it.
In the above diagram, you can see the opposing relationship between applied force and friction. As
the applied force increases, the opposing frictional force also increases until the mass starts moving.
This is a static frictional force. When the applied force reaches the maximum static friction, the mass
begins to move; after the mass begins moving, kinetic friction acts upon it. Static friction is greater
than kinetic friction, so once the mass begins sliding, it takes less force to keep it sliding.
You can duplicate both types of friction by placing your hands together and pushing them against each
other. Start to move them in a sliding motion. The motion is resisted by the texture of your skin and
the magnitude of the applied force. This is static friction. Now that they are moving relative to each
other, kinetic friction comes into play.
There are two factors which determine the maximum frictional force that can occur between two
surfaces: coefficient of friction and normal force.
The maximum force of friction (Ff) between two surfaces is equal to the coefficient of friction (Cf) of
those two surfaces multiplied by the normal force (N) holding those surfaces together.
Ff = Cf x N
Coefficient of Friction
A coefficient of friction is a constant which describes the "grippyness" of two surfaces sliding against
one another. Slippery objects have a very low coefficient of friction, while sticky objects have a very
high coefficient of friction. This constant is determined for a pair of surfaces, not a single surface, and
ranges from near zero to greater than one. Each pair of materials has a coefficient of static friction and
a coefficient of kinetic friction.
Do not confuse this with actual sticky surfaces like tape or high friction coatings that bind to the other
surface. These surfaces almost need to be looked at as being joined together as one. For instance,
tapes resist sliding even when there is no normal force (push down), or a negative normal force (pull
up) when they are clearly not part of the other object.
Here is a table showing the coefficients of friction for some common pairs of materials.
Think Phase
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Coeficients of Static and Kinetic Friction for Common Materials
Materials in Contact
Coefficient of Static
Friction
Coefficient of Kinetic Friction
Steel-Steel
0.78
0.42
Aluminum-Aluminum
1.05-1.35
1.4
Rubber-Asphalt (dry)
0.5-0.8
Rubber-Asphalt (wet)
0.25-0.75
Rubber-Concrete (dry)
0.6-0.85
Rubber-Concrete (wet)
0.45-0.75
Steel-Brass
0.51
0.44
These values are experimentally determined; they cannot be derived.
Normal Force
The force that presses the two sliding surfaces together is referred to as normal force. This normal
force is always perpendicular to the two surfaces. Often the normal force acting on a system is the
weight of one object resting on the other; this is caused by gravity.
As shown in the following diagram, when an object is on a ramp, gravity is not acting perpendicular to
the sliding surfaces. In this case, only a portion of the object’s weight acts as normal force.
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Autodesk's VEX Robotics Unit 8: Friction and Traction
Traction
Traction is defined as friction between a drive member (wheel) and the surface it moves upon. It is the
amount of force a wheel can apply to a surface before it slips. A rolling wheel is in static contact with
the ground if it is not slipping.
As seen in the diagram above (and as discussed in Unit 5), when a torque is applied to a wheel, it
results in an applied force along the ground. If there is friction between the wheel and the ground, an
equal and opposite force called the tractive force pushes back against the wheel. The applied force is
the force of the wheel on the surface. The tractive force is the force of the surface on the wheel. This
is a perfect example of Newton's Third Law of Motion: Forces are interactions between two objects;
they always come in pairs of equal magnitude and opposite direction. The force of object 1 on object
2 is always equal in magnitude and opposite in direction to the force of object 2 on object 1. So, the
greater the "applied force" of the wheel on the ground, the greater the force of the ground on the
wheel (and thus the robot)!
The tractive force is equal to the frictional force between the wheel and the ground. If the wheel
is rolling and not slipping, the tractive force is equal to the static friction force. If the applied force
exceeds the maximum static friction, then the wheel will start to slip and the tractive force will equal
the maximum kinetic friction force.
Increasing Traction
Since traction is dependent on the friction of the wheel and the surface, you must maximize this
friction. It is known that friction is dependent on coefficient of friction (between the wheel and the
surface), and the normal force (the weight of the robot pressing the wheel to the surface). To increase
traction, you must either increase the coefficient of friction or increase the normal force on the wheel.
Building a Pushing Robot
In order to build a robot capable of pushing or pulling with great force, the robot requires two things:
high traction and significant torque applied to the wheels.
Friction in VEX
There are a variety of components in the VEX Robotics Design System that can be used to gain traction
including several types of wheels. Each of these has different characteristics on different surfaces;
experiment to determine which wheel is best for a given application.
Friction between the wheels and the floor is not the only friction present in VEX robots. Friction
also acts as a brake on the rotating components of the robot. The VEX Robotics Kit has several parts
designed to reduce friction in a robot design. The plastic parts such as the bearing blocks, spacers,
and washers allow other parts to be separated with a material providing a lower friction value. Metal
against metal contact is not desirable in moving systems (see steel on steel values in the table above).
Think Phase
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Create Phase
Overview
In this phase, you learn how to create a tire for a VEX medium wheel. The workflow uses the basic
part creation techniques such as drawing a sketch and extruding the profile. In addition, you import a
sketch and use the profile to engrave the tire tread.
Objectives
After completing this phase, you will be able to:
Create a VEX tire.
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Prerequisites
Before starting this phase, you must have:
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A working knowledge of the Windows operating system.
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Completed Unit 1: Introduction to Vex and Robotics > Getting Started with Autodesk Inventor.
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Completed Unit 2: Introduction to Autodesk Inventor > Quick Start for Autodesk Inventor.
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Autodesk's VEX Robotics Unit 8: Friction and Traction
Technical Overview
The following Autodesk® Inventor® tools are used in this phase.
Icon
Name
Description
Half Section
View
Use a plane or work plane to temporarily slice away a portion of a model.
Create
Component
Use to create a part or assembly while working in an existing assembly.
Browse
Templates
In dialog boxes, it provides access to file listings in Windows Explorer.
Transparency
Off
Displays inactive components as opaque during an in-edit operation.
Project
Geometry
Projects geometry (model edges, vertices, work axes, work points, or other
sketch geometry) onto the active sketch plane as reference geometry.
Line
Straight curve bounded by two endpoints. The line tool on the Sketch
toolbar chains line segments together and creates arcs tangent or
perpendicular to existing curves.
Centerline
Use to manually apply four types of centerlines and center marks to
individual features or parts in a drawing view: Center Mark, Centerline,
Centerline Bisector, and Centered Pattern.
General
Dimension
Adds dimensions to a sketch. Dimensions control the size of a part. They
can be expressed as numeric constants, as variables in an equation, or in
parameter files.
Mirror
Use to mirror sketch geometry across a centerline.
Return
Use Return to quit in-place editing and quickly return to the desired
environment. The destination depends on which modeling environment
you are working in.
Revolve
Revolved features are created by sweeping one or more sketched profiles
around an axis. If the revolved feature is the first feature in a part file, it is
the base feature.
Project Cut
Edges
Use the Project Cut Edges tool when creating or editing a sketched feature
to model edges onto the active sketch plane from a component cut by a
section plane. A projected cut edge is placed in the browser under the
Sketch symbol.
Create Phase
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Icon
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Name
Description
Center Point
Circle
Creates a circle from a center point and radius.
Work Plane
Use work planes when creating axes, sketch planes, or termination planes,
or to position cross-sectional views or cutting planes.
Create 2D
Sketch
A sketch consists of the sketch plane, a coordinate system, 2D curves, and
the dimensions and constraints applied to the curves.
iFeature
An iFeature is one or more features that can be saved and reused in other
designs. You can create an iFeature from any sketched feature that you
determine to be useful for other designs. Features dependent on the
sketched feature are included in the iFeature. After you create an iFeature
and store it in a catalog, you can place it in a part by dragging it from
Windows Explorer and dropping it in the part file or by using the Insert
iFeature tool.
Horizontal
Constraint
The horizontal constraint causes lines, ellipse axes, or pairs of points to lie
parallel to the X axis of the sketch coordinate system.
Vertical
Constraint
A geometric constraint that causes selected arcs and circles to have the
same radius or selected lines to have the same length.
Emboss
Use to represent an area on a model face that is embossed or engraved.
You create the profile as sketch text or sketch geometry in a sketch, and
then select the profile to project or wrap onto the model.
Chamfer
Chamfers bevel part edges in both the part and assembly environments.
Chamfers may be equal distance from the edge, a specified distance and
angle from an edge, or a different distance from the edge for each face.
Fillet
Placed features that round off or cap interior or exterior corners or
features of a part.
Circular
Pattern
Part, surface, and assembly features can be arranged in a pattern to
represent hole patterns or textures, slots, notches, or other symmetrical
arrangements.
End Section
View
Returns the assembly display to no section view.
Autodesk's VEX Robotics Unit 8: Friction and Traction
Required Supplies and Software
The following software is used in this phase.
Software
Autodesk Inventor Professional 2010
Create Phase
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Exercise: Create a VEX Tire
In this exercise, you learn how to create a tire for a
VEX medium wheel. The workflow uses the basic part
creation techniques such as drawing a sketch and
extruding the profile. In addition, you import a sketch
and use the profile to engrave the tire tread.
4.
In the browser, expand the Origin folder. Click
YZ Plane.
5.
On the Assemble tab, Component panel, click
Create.
6.
For New Component name, enter
Medium_Wheel_Tire.
Click Browse Templates.
The completed exercise
Create a VEX Tire
1.
2.
7.
Make IFI_Unit8.ipj the active project.
Open Medium_Wheel_Hub.iam.
8.
9.
On the English tab, click Standard (in).ipt. Click
OK twice.
In the browser, click YZ Plane. A new part is
created and the sketch is active.
Create a Sketch Profile
In this section of the exercise, you create a sketch
profile of the tire.
1. On the View tab, Appearance panel, click
the arrow beside Transparency On. Click
Transparency Off.
3.
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On the View tab, Appearance panel, click the
arrow beside Quarter Section View. Click Half
Section View.
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2.
3.
4.
On the ViewCube, click Right.
Zoom into the top section of the hub.
On the Sketch tab, Draw panel, click Project
Geometry.
Autodesk's VEX Robotics Unit 8: Friction and Traction
5.
Select the top edge (1) and the lower edge (2).
9.
Click a second point to draw a short vertical
line. Make sure the perpendicular icons are
displayed on the short centerline and line 1.
Note: Take care when selecting the top edge
(1). There is another line just below it. Make
sure you select the top edge as shown.
10. On the Constrain panel, click Dimension.
11. Add a 0.25 dimension to the centerline.
6.
On the Draw panel, click Line.
7.
On the Format panel, click Centerline.
8.
Select the midpoint of the top projected edge.
The midpoint is indicated by a large dot.
12. On the Format panel, click Centerline to turn it
off.
13. On the Draw panel, click Line.
Create Phase
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14. Create the first section of the profile as follows:
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Select the right end of the top line (1).
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Drag the cursor to the right until it is
halfway above the lower projected edge.
Click to place the next point (2).
■
Drag the cursor down until it is below the
top of the notch. Click to place the next
point (3).
■
Drag the cursor to the left until it is close to
the vertical edge of the notch. Click to place
the next point (4).
■
Drag the cursor down until it is coincident
with the lower projected edge (not the
endpoint). Click to place the next point (5).
15. Create the next section of the profile as
follows:
■
Select the endpoint of the previous line (1).
A coincident icon is displayed.
■
Drag the cursor to the right and slightly
upward until it is close to the end of the
hub (2). Click to place the next point.
■
Drag the cursor to the right and upward.
Click to place the next point (3).
■
Drag the cursor up until it is level with the
top of the centerline. Click to place the next
point (4). A dotted line is displayed when
the point is level with the top of the line.
■
Drag the cursor left until it is coincident
with the endpoint of the centerline. Click to
place the last point (5).
16. Right-click in the graphics window. Click Done.
Dimension the Sketch
In this section of the exercise, you add dimensions to
the sketch.
1. On the Constrain panel, click Dimension.
2.
3.
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Add a 0.4375 dimension to the top line. It is
displayed as 0.438.
Add a 1 degree dimension between the short
sloping line, and the projected edge.
Autodesk's VEX Robotics Unit 8: Friction and Traction
4.
Add a 0.193 dimension to the right edge.
Revolve the Sketch
In this section of the exercise, you revolve the sketch
profile to create the 3D part.
1.
2.
On the ViewCube, click Home.
On the Create panel, click Revolve.
3.
4.
5.
Select both sections of the profile.
Under Shape, click Axis.
In the browser, expand the
Medium_Wheel_Tire:1 > Origin folder.
Click Y Axis. A preview of the tire is displayed.
Click OK.
6.
7.
Mirror the Sketch
In this section of the exercise, you mirror the sketch.
1. On the Pattern panel, click Mirror.
2.
3.
4.
5.
6.
Select the lines you created in the previous
steps. There are eight in total. Do not select the
projected edges or centerline.
In the Mirror dialog box, click Mirror Line.
In the graphics window, select the centerline.
Click Apply.
Click Done. The profile of the tire is displayed.
Create the Sketch for the Teeth
In this section of the exercise, you create the sketch
for the teeth that locate the tire on the hub.
1.
2.
3.
4.
7.
In the browser, under Medium_Wheel_Tire:1,
right-click XZ Plane. Click New Sketch.
Right-click in the graphics window. Click Slice
Graphics.
On the Draw panel, click the arrow beside
Project Geometry. Click Project Cut Edges.
Zoom into the top of the assembly.
On the Quick Access toolbar, click Return.
Create Phase
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5.
Select the face of a tooth.
Extrude the Teeth
In this section of the exercise, you extrude the teeth
on the tire using the Cut option.
6.
7.
8.
9.
Press ESC to exit the tool.
In the browser, right-click the part
MEDIUM_WHEEL_HUB:1. Click Visibility to turn
off the visibilty of the part.
On the ViewCube, click Front.
On the Draw panel, click Circle.
10. Draw a circle centered on the tire (1) and
coincident with the lower edge of a tooth
profile (2).
1.
2.
3.
On the ViewCube, click Home.
Press E to start the Extrude tool.
Select inside a tooth profile.
4.
5.
6.
For Distance, enter 0.5.
Under Operation, click Cut.
Click Midplane.
7.
Click OK.
11. Right-click in the graphics window. Click Done.
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Autodesk's VEX Robotics Unit 8: Friction and Traction
Create a Tangent Work Plane
In this section of the exercise, you create a tangent
work plane.
1. On the Work Features panel, click Plane.
2.
3.
10. Click Next twice.
11. Click Activate Sketch Edit Immediately.
12. Click Finish.
In the browser, click YZ Plane.
Select the top face of the tire.
Locate the Tread Profile
In this section of the exercise, you locate the tire
tread profile.
Insert the Tread Profile
1.
2.
On the ViewCube, click Top.
On the Draw panel, click the arrow beside
Project Cut Edges. Click Project Geometry.
3.
Select edges (1) and (2).
4.
On the Constrain panel, click Horizontal
Constraint.
5.
Select the midpoint of the tread profile (1) and
the endpoint of the projected edge (2).
In this section of the exercise, you insert the profile of
the tire tread.
1.
2.
3.
4.
5.
6.
7.
8.
9.
On the ViewCube, click Home.
On the Manage tab, Insert panel, click Insert
iFeature.
Click Browse.
Click Workspace.
Select Tread_Profile.ide.
Click Open.
Select the work plane.
In the Insert iFeature dialog box, click the 0.00
deg angle value.
Enter 90.
Create Phase
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6.
7.
Zoom into the top edge of the tread profile.
On the Constrain panel, click Vertical
Constraint.
8.
Select the midpoint of the tread profile (1) and
the endpoint of the construction line (2).
9.
4.
Select the five tread profiles.
5.
6.
7.
Select the Wrap to Face check box.
Select the outside face of the tire.
Click OK.
8.
Turn off the visibility of the work plane and the
sketch.
On the ViewCube, click Home.
Create the Tread
In this section of the exercise, you create the tread.
1. On the Quick Access toolbar, click Return.
2.
On the Create panel, click Emboss.
Add Chamfers and Fillets to the Tread Profile
3.
In the Emboss dialog box, click Engrave from
Face.
In this section of the exercise, you add chamfers
to the sides of the tread profile to increase the
width of the walls. You also add fillets to break the
sharp edges. These features also make it easier to
manufacture the tire.
1.
2.
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Zoom into the tread.
On the Modify panel, click Chamfer.
Autodesk's VEX Robotics Unit 8: Friction and Traction
3.
In the Chamfer dialog box, select Distance and
Angle.
10. Click Cancel.
11. On the Modify panel, click Fillet.
4.
Select an inside face of the tread.
12. For Radius, enter 0.01.
13. Under Select Mode, click Loop.
14. Select the loop when the three edges are
displayed as shown.
5.
Select the lower edge of the tread.
15. Repeat for the remaining four loops.
6.
7.
8.
9.
For Distance, enter 0.1.
For Angle, enter 20.
Click Apply.
Repeat this workflow for all the inside faces of
the tread. There are seventeen in total.
16. Click OK.
Create Phase
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Complete the Tread Pattern
9.
Turn on the visibility of
MEDIUM_WHEEL_HUB:1.
In this section of the exercise, you create a circular
pattern of the single tread.
1. On the Pattern panel, click Circular Pattern.
2.
3.
4.
5.
6.
In the browser, select the emboss, chamfer,
and fillet features.
In the Circular Pattern dialog box, click Rotation
Axis.
Select the outside face of the tire.
For Placement, enter 8.
Click OK.
10. Save the file.
Change the Properties of the Tire
In this section of the exercise, you change the
material of the tire to rubber.
1.
2.
3.
4.
5.
6.
7.
8.
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In the browser, right-click
Medium_Wheel_Tire:1. Click iProperties.
Click the Physical tab.
Select Rubber from the Material list.
Click Apply. The values are updated. For
example, the mass of the tire is 0.069 pounds.
Click Close.
On the Quick Access toolbar, click Return.
On the ViewCube, click Home.
On the View tab, Appearance panel, click the
arrow beside Half Section View. Click End
Section View.
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Autodesk's VEX Robotics Unit 8: Friction and Traction
Build Phase
Overview
In this phase, you modify the gearing on a previously built robot.
Phase Objectives
After completing this phase, you will be able to:
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Have a robot ready to compete in a “tractor pull,” where you can apply the lessons on friction and
traction from the Unit 8 > Think Phase.
Make simple modifications to VEX robots.
Build Phase
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25
Prerequisites and Resources
Before starting this phase, you must have:
Completed the Unit 8: Friction and Traction > Think Phase.
Have an assembled differential tricycle from the Unit 7: Advanced Gears > Build Phase.
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Related phase resources are:
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Unit 1: Introduction to VEX and Robotics.
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Unit 4: Microcontroller and Transmitter Overview.
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Unit 5: Speed, Power, Torque, and DC Motors.
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Unit 6: Gears, Chains, and Sprockets.
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Unit 7: Advanced Gears.
Required Supplies and Software
The following supplies are used in this phase:
Supplies
One assembled differential tricycle built in the Unit 7: Advanced Gears > Build Phase
Notebook and pen
Work surface
Small storage container for loose parts
Optional: Autodesk Inventor Professional 2010
VEX Parts
The following VEX parts are used in this phase:
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Quantity
Part Number
Abbreviations
1
BEAM-2000
B2
2
SCREW-832-0250
S2
1
SCREW-832-0750
S6
1
SPACER-THIN
SP1
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Autodesk's VEX Robotics Unit 8: Friction and Traction
Quantity
Part Number
Abbreviations
1
VEX-12-TOOTH-GEAR
G12
4
WASHER-DELRIN
WP
Activity
Modify the Differential Tricycle
In this activity, you modify the differential tricycle from Unit 7 by increasing the gear reduction to give
the robot more torque for the upcoming tractor pull.
As you work on building this project, have some of your team members focus on expanding
their expertise using Autodesk Inventor. Later in the curriculum, you will be challenged to come up
with your own creative solutions for robot design. You will save time and maximize your ability to
create winning solutions if your team understands how to leverage the power of digital prototypes
using Inventor.
Note: Team members can download a free version of Autodesk Inventor Professional to use at home,
so you can come to class prepared to build and test your best ideas! To do this, simply join the
Autodesk Student Engineering and Design Community at www.autodesk.com/edcommunity.
1.
To complete the next step:
■
Loosen the Collars [COL] on both the 3” Shafts [SQ3].
■
Remove the shafts, associated Gears, Collars and Spacers.
Note: to remove the back 3" shaft, you will need to pull the wheels off one side.
Build Phase
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The completed model is as shown:
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Autodesk's VEX Robotics Unit 8: Friction and Traction
2.
To complete the next step:
■
Unscrew and remove the Motor [MOT] and associated Bearing Flat [BF].
■
Unfasten the Bearing Flat across from the Motor.
■
Note: the motor does not need to be unplugged.
Build Phase
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The completed model is as shown:
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Autodesk's VEX Robotics Unit 8: Friction and Traction
3.
To complete the next step:
Build Phase
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31
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Rebolt the Motor and Bearing Flat one hole closer to the wheels than before.
Reinstall the Bearing Flat across from Motor, one hole closer to the wheels than before.
The completed model is as shown:
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Autodesk's VEX Robotics Unit 8: Friction and Traction
4.
To complete the next step:
Build Phase
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Insert the first removed 3” Shaft into the Bearing Flat closest to the Differential Housing.
Slide two Plastic Washers [WP], a 60 Tooth Gear [G60], and two Collars onto the shaft.
Slide a Collar up against the 60 Tooth Gear and another Collar against the Bearing Flat.
Tighten both Collars.
Insert the second removed 3” Shaft into front set of Bearing Flats. Slide two Plastic
Washers, a 12 Tooth Gear [G12] and a Collar onto the shaft.
Insert the 3” Shaft fully into the motor. Slide the Collar up against the 12 Tooth Gear and
tighten. Ensure that the 12 Tooth and 60 Tooth Gears mesh properly.
Note: to remove the back 3" shaft, you will need to pull the wheels off one side.
The completed model is as shown:
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Autodesk's VEX Robotics Unit 8: Friction and Traction
5.
To complete the final step:
■
Bolt one Thin Spacer [SP1], one Thick Spacer [SP2], and one 2” Beam [B2] between the two
5x25 Plate Pieces. This bar is meant to tie the rope to for the tug of war.
■
Note: to remove the back 3" shaft, you will need to pull the wheels off both sides.
Build Phase
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35
The completed model is as shown:
6.
36
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Your differential tricycle is now geared appropriately for the upcoming tractor pull!
Autodesk's VEX Robotics Unit 8: Friction and Traction
Note: Battery and electrical connections not shown.
Build Phase
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37
Amaze Phase
Overview
In this phase, you compete in a tractor pull against another robot.
Phase Objectives
After completing this phase, you will be able to:
Take advantage of the principles of friction and traction to modify a robot to pull a greater amount
of weight.
Make decisions based on theoretical physics, and apply them to a dynamic situation.
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■
Prerequisites and Resources
Before starting this phase, you must have:
■
Completed Unit 8: Friction and Traction > Think Phase.
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Completed Unit 8: Friction and Traction > Build Phase
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A modified and assembled differential tricycle from the Unit 8: Friction and Traction > Build Phase.
Important Note: This challenge will not work properly without making the specified modifications
to the differential tricycle as outlined in the Unit 8: Friction and Traction > Build Phase.
Related phase resources are:
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Unit 1: Introduction to VEX and Robotics.
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Unit 4: Microcontroller and Transmitter Overview.
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Unit 5: Speed, Power, Torque, and DC Motors.
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Unit 6: Gears, Chains, and Sprockets.
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Unit 7: Advanced Gears.
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Autodesk's VEX Robotics Unit 8: Friction and Traction
Required Supplies and Software
The following supplies are used in this phase:
Supplies
One modified and assembled differential tricycle from the Unit 8: Friction and Traction > Build Phase
Notebook and pen
Work surface
6’x12’ of open floor space
Masking tape
Measuring tape
36” of 1/8” braided nylon and polyester cord or equivalent rope/string
Evaluation
Tractor Pull Challenge
In this challenge, you use your differential tricycle to compete in a “tractor pull” against a classmate’s
differential tricycle. The two robots are attached by a length of nylon rope, and the goal is to pull your
opponents robot 3’ from its starting position.
Since all robots are starting with the same configuration, in theory each tractor pull should be a
stalemate. To do well in this competition, you will need to modify your robots, and apply some of the
lessons from the Think Phase of this unit.
Modification Ideas:
How can you easily gain more pushing/pulling power with your robot? To compete well in this
challenge, this is the question that needs to be answered.
Some suggestions:
■
As learned in the Unit 8: Friction and Traction > Think Phase:
Friction Force = coefficient of friction x weight
Therefore, an increase in the coefficient of friction between the robot and the ground, or an
increase in the mass of the robot, would increase your pushing/pulling power.
■
Consider changing the wheels on your robot to increase friction.
■
Consider adding dead weight to your robot.
■
Be creative! There are many ways to modify your robot to perform better in this challenge. Do not
be afraid to try some “off-the-wall” ideas.
Amaze Phase
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39
Challenge Instructions
1.
2.
3.
4.
5.
6.
Attach the 36” length of rope/string to the 2” beam at the rear of your differential tricycle.
Attach the other end of the rope to the rear beam of your opponent’s differential tricycle.
Place both robots on the floor so they are as far apart as possible, and facing in opposite
directions.
Measure 3’ from the front of each robot. Place a piece of masking tape on the ground at this
mark. The tape marking will serve as the finish line for the tractor pull. See the following figure.
Turn both robots and transmitters on.
Drive towards your finish line. The robot that reaches the finish line is the winner!
Engineering Notebook
For each change you made to your robot, document in your engineering notebook why you made the
change and what effect it had your robot in the tractor pull. Explain why certain changes were more
effective than others.
Try doing a tractor pull between two identical robots. You will notice that it is not always a stalemate.
Explain what factors can be giving one of the seemingly identical robots an advantage.
Presentation
Describe the modification that you made to your robot that had the most impact during the
challenge. Explain why you made this change, and if it had the effect that you expected.
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Autodesk's VEX Robotics Unit 8: Friction and Traction
STEM Connections
Background
A snowmobile uses many of the concepts from this unit to function. Because snowmobiles operate in
snowy and icy areas, they have a special track mechanism resembling a tank tread, which drives the
vehicle instead of wheels. Also, snowmobiles are steered by using handlebars attached to skis.
Science
A snowmobile track is usually made of rubber, whereas a similar track for a tank is usually made of a
harder material like metal.
■
Which material do you think has a greater coefficient of friction with ice, and why is this important
for snowmobile design?
■
Where would you look to determine the values of these coefficients of friction?
Technology
Some snowmobiles can be outfitted with studs attached to the snowmobile track in order to increase
the track’s grip.
■
What are the advantages and disadvantages of adding this cleat-like effect?
■
How would you design a track and cleat system that can be equipped or unequipped depending on
driving conditions?
Engineering
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Why do snowmobiles use a track and skis to drive the vehicle instead of wheels?
What are the forces affecting this design decision?
The track on the snowmobile is moving in the direction of travel. Based on your understanding of
gears and pulleys, how does the rotation of the engine shaft transfer force to the track?
What are the differences in required torque when you drive a snowmobile up a steep hill versus
when you drive the snowmobile across a flat, smooth surface?
STEM Connections
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41
Math
The kinetic coefficient of friction for rubber on dry asphalt is roughly 0.67, while the static coefficient
of friction for rubber on dry asphalt is 0.85. What does this mean?
Suppose a child is riding a bicycle (combined weight: 100 lbs.) and slams on the brakes. If the bike
goes into a skid, then the rubber tire surface is sliding along the asphalt surface. You multiply the
kinetic coefficient of friction (0.67) by the weight (100 lbs.) to find that friction can apply no more
than 67 lbs. of force to slow down the bike. On the other hand; if the bike does not skid, then you use
the static coefficient of friction (0.85) instead. Why? In this case, friction can apply at most 85 lbs. of
force to slow the bike.
Back to the snow: Your snowmobile broke down in the middle of a blizzard and you had to go the rest
of the way on skis. Which do you think is harder to do with skis on level snow: to start moving from
rest, or to keep moving once you have started?
Consider a 120 lb. skier on a level patch of snow. The kinetic coefficient of friction for a waxed ski
on snow is about 0.05, while the static coefficient of friction is about 0.14. Which of these numbers
should you use when talking about a stationary skier, and which should you use when talking about a
skier gliding over the snow? Use these coefficients to calculate answers to the following questions:
■
How much force will a skier at rest need in order to overcome friction and start moving?
■
Once the skier is moving, how much force will it take to maintain speed?
■
Finally, compare your numerical answers. Which is larger, and what does this mean?
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Autodesk's VEX Robotics Unit 8: Friction and Traction