Autodesk's VEX® Robotics Curriculum Unit 8: Friction and Traction

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, one scenario is presented 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:

■ Explain the difference between static and kinetic friction and list the factors that 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:

■ 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:

Term Definition

Chamfer A placed feature that bevels a part edge and is defined by its placement, size, and angle.

Coefficient of The ratio of maximum frictional force between two surfaces to the force holding Friction them together. Term used to describe the "grippyness" of two surfaces meshing together. Slippery objects have a very low coefficient of friction.

2 ■ 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 The frictional force which opposes the motion of an object while it is moving. Friction

Mirror Sketch geometry that is copied across a centerline. sketches

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 (model edges, vertices, work axes, work points, or other sketch Geometry 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 ■ 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 2011

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

4 ■ 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) ■ Unifying Concepts and Processes: Form and Function; Change, Constancy, and Measurement ■ Physical Science: Motions and Forces ■ Science and Technology: Abilities of Technological Design

Technology (ITEA) ■ 5.8: The Attributes of Design

Mathematics (NCTM) ■ Alegbra: Analyze change in various contexts. ■ Measurement: Understand measurable attributes of objects and the units, systems, and processes of measurement. ■ 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.

Create Science (NSES) ■ Unifying Concepts and Processes: Form and Function ■ Physical Science: Motions and Forces ■ Science and Technology: Abilities of Technological Design

Overview ■ 5

Phase Standard

Technology (ITEA) ■ 5.8: The Attributes of Design ■ 5.9: Engineering Design ■ 6.12: Use and Maintain Technological Products and Systems

Mathematics (NCTM) ■ Numbers and Operations: Understand numbers, ways of representing numbers, relationships among numbers, and number systems. ■ Algebra Standard: Understand patterns, relations, and functions. ■ Geometry Standard: Use visualization, spatial reasoning, and geometric modeling to solve problems. ■ Measurement Standard: Understand measurable attributes of objects and the units, systems, and processes of measurement.

Build Science (NSES) ■ Unifying Concepts and Processes: Form and Function; Change, Constancy, and Measurement; Evidence, Models, and Explanation ■ Physical Science: Motions and Forces ■ Science and Technology: Abilities of Technological Design

Technology (ITEA) ■ 5.8: The Attributes of Design ■ 5.9: Engineering Design ■ 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. ■ Algebra: 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. ■ Connections: Recognize and apply mathematics in contexts outside of mathematics.

6 ■ Autodesk's VEX Robotics Unit 8: Friction and Traction

Phase Standard

Amaze Science (NSES) ■ Unifying Concepts and Processes: Form and Function; Change, Constancy, and Measurement; Evidence, Models, and Explanation ■ Physical Science: Motions and Forces ■ Science and Technology: Abilities of Technological Design

Technology (ITEA) ■ 5.8: The Attributes of Design ■ 5.9: Engineering Design ■ 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 ■ 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:

■ Explain the difference between static and kinetic friction. ■ List the factors that determine traction: ❏ Normal force ❏ Coefficient of friction

Prerequisites

Related phase resources are:

■ 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

8 ■ 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 ■ 9

Coeficients of Static and Kinetic Friction for Common Materials

Materials in Contact Coefficient of Static Coefficient of Kinetic Friction 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.

10 ■ 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 ■ 11 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.

Prerequisites

Before starting this phase, you must have:

■ A working knowledge of the Windows operating system. ■ Completed Unit 1: Introduction to Vex and Robotics > Getting Started with Autodesk Inventor. ■ Completed Unit 2: Introduction to Autodesk Inventor > Quick Start for Autodesk Inventor.

12 ■ 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 Use a plane or work plane to temporarily slice away a portion of a model. View

Create Use to create a part or assembly while working in an existing assembly. Component

Browse In dialog boxes, it provides access to file listings in Windows Explorer. Templates

Transparency Displays inactive components as opaque during an in-edit operation. Off

Project Projects geometry (model edges, vertices, work axes, work points, or other Geometry 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.

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 Use the Project Cut Edges tool when creating or editing a sketched feature Edges 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 ■ 13

Icon Name Description

Circle Creates a circle from a center point and radius.

Plane Use work planes when creating axes, sketch planes, or termination planes, or to position cross-sectional views or cutting planes.

Create 2D A sketch consists of the sketch plane, a coordinate system, 2D curves, and Sketch 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 The horizontal constraint causes lines, ellipse axes, or pairs of points to lie Constraint parallel to the X axis of the sketch coordinate system.

Vertical A geometric constraint that causes selected arcs and circles to have the Constraint 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 Part, surface, and assembly features can be arranged in a pattern to Pattern represent hole patterns or textures, slots, notches, or other symmetrical arrangements.

End Section Returns the assembly display to no section view. View

14 ■ 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 2011

Create Phase ■ 15

Exercise: Create a VEX Tire

4. In the browser, expand the Origin folder. Click In this exercise, you learn how to create a tire for a YZ Plane. 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.

5. On the Assemble tab, Component panel, click Create.

The completed exercise

6. For New Component name, enter Medium_Wheel_Tire. Create a VEX Tire 7. Click Browse Templates.

1. Make IFI_Unit8.ipj the active project.

2. Open Medium_Wheel_Hub.iam. 8. On the English tab, click Standard (in).ipt. Click OK twice.

9. In the browser, click YZ Plane. A new part is created and the sketch is active.

3. On the View tab, Appearance panel, click the arrow next to Quarter Section View. Click Half Section View.

16 ■ Autodesk's VEX Robotics Unit 8: Friction and Traction

Create a Sketch Profile 5. In the browser, right-click Sketch1. Click Copy.

6. Return to Medium_Wheel_Hub.iam.

In this section of the exercise, you create a sketch 7. Right-click in the graphics window. Click Paste. profile of the tire. The sketch profile of the tire is placed in the

1. On the View tab, Appearance panel, click the correct location on the wheel. arrow next to Appearance. Click the arrow next to Transparency On. Click Transparency Off.

8. Right-click in the graphics window. Click Done.

2. On the ViewCube, click Right.

3. On the QuickAccess toolbar, click Open.

4. Open Tread Profile Profile.ipt.

Create Phase ■ 17

Revolve the Sketch 5. Select the face of a tooth.

In this section of the exercise, you revolve the sketch profile to create the 3D part.

1. On the Create panel, click Revolve.

2. In the browser, expand the Medium_Wheel_Tire:1 > Origin folder.

3. Click Y Axis. A preview of the tire is displayed.

4. Click OK.

6. Press ESC to exit the tool.

7. In the browser, right-click the part MEDIUM_WHEEL_HUB:1. Click Visibility to turn off the visibility of the part.

8. On the ViewCube, click Front.

9. On the Draw panel, click Circle.

10. Draw a circle centered on the tire (1) and Create the Sketch for the Teeth coincident with the lower edge of a tooth profile (2).

In this section of the exercise, you create the sketch for the teeth that locate the tire on the hub.

1. In the browser, under Medium_Wheel_Tire:1, right-click XZ Plane. Click New Sketch.

2. Right-click in the graphics window. Click Slice Graphics.

3. On the Draw panel, click the arrow next to Project Geometry. Click Project Cut Edges.

4. Zoom into the top of the assembly.

11. Right-click in the graphics window. Click Done.

18 ■ Autodesk's VEX Robotics Unit 8: Friction and Traction

Extrude the Teeth Create a Tangent Work Plane

In this section of the exercise, you extrude the teeth In this section of the exercise, you create a tangent on the tire using the Cut option. work plane.

1. On the ViewCube, click Home. 1. On the Work Features panel, click Plane.

2. Press E to start the Extrude tool.

3. Select inside a tooth profile. 2. In the browser, click YZ Plane.

3. Select the top face of the tire.

4. For Distance, enter 0.5. (1)

5. Under Operation, click Cut. (2)

6. Click Midplane. (3)

Insert the Tread Profile

In this section of the exercise, you insert the profile of the tire tread.

1. On the ViewCube, click Home.

2. On the Manage tab, Insert panel, click Insert iFeature.

3. Click Browse. 7. Click OK. 4. Click Workspace.

5. Select Tread_Profile.ide.

6. Click Open.

7. Select the work plane.

8. In the Insert iFeature dialog box, click the 0.00 deg angle value.

9. Enter 90.

Create Phase ■ 19

10. Click Next twice. 6. Zoom into the top edge of the tread profile.

11. Click Activate Sketch Edit Immediately. 7. On the Constrain panel, click Vertical

12. Click Finish. Constraint.

8. Select the midpoint of the tread profile (1) and the endpoint of the construction line (2).

Locate the Tread Profile

In this section of the exercise, you locate the tire tread profile.

1. On the ViewCube, click Top.

2. On the Draw panel, click the arrow next to 9. On the ViewCube, click Home.

Project Cut Edges. Click Project Geometry.

Create the Tread Select edges (1) and (2). 3. In this section of the exercise, you create the tread.

1. On the Quick Access toolbar, click Return.

2. On the Model tab, Create panel, click Emboss.

4. On the Constrain panel, click Horizontal 3. In the Emboss dialog box, click Engrave from Constraint. Face.

5. Select the midpoint of the tread profile (1) and the endpoint of the projected edge (2).

20 ■ Autodesk's VEX Robotics Unit 8: Friction and Traction

4. Select the five tread profiles. 3. In the Chamfer dialog box, select Distance and Angle.

4. Select an inside face of the tread. 5. Select the Wrap to Face check box.

6. Select the outside face of the tire.

7. Click OK.

5. Select the lower edge of the tread.

8. Turn off the visibility of the work plane and the sketch.

Add Chamfers and Fillets to the Tread Profile

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. Zoom into the tread. On the Modify panel, click Chamfer. 6. For Distance, enter 0.1. 2. 7. For Angle, enter 20.

8. Click Apply.

9. Repeat this workflow for all the inside faces of the tread. There are seventeen in total.

Create Phase ■ 21

10. Click Cancel. Complete the Tread Pattern

In this section of the exercise, you create a circular pattern of the single tread.

1. On the Pattern panel, click Circular Pattern.

2. In the browser, select the emboss, chamfer, and fillet features.

3. In the Circular Pattern dialog box, click Rotation

11. On the Modify panel, click Fillet. Axis.

4. Select the outside face of the tire.

5. For Placement, enter 8. 12. For Radius, enter 0.01. 6. Click OK. 13. Under Select Mode, click Loop.

14. Select the loop when the three edges are displayed as shown. Change the Properties of the Tire

In this section of the exercise, you change the material of the tire to rubber.

1. In the browser, right-click Medium_Wheel_Tire:1. Click iProperties.

2. Click the Physical tab.

3. Select Rubber from the Material list.

4. Click Apply. The values are updated. For example, the mass of the tire is 0.069 pounds.

5. Click Close. 15. Repeat for the remaining four loops. 6. On the Quick Access toolbar, click Return.

7. On the ViewCube, click Home.

8. On the View tab, Appearance panel, click the arrow beside Half Section View. Click End Section View.

16. Click OK.

22 ■ Autodesk's VEX Robotics Unit 8: Friction and Traction

9. Turn on the visibility of MEDIUM_WHEEL_HUB:1.

10. Save the file.

Create Phase ■ 23 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:

■ 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.

24 ■ Autodesk's VEX Robotics Unit 8: Friction and Traction

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.

Related phase resources are:

■ Unit 1: Introduction to VEX and Robotics ■ Unit 4: Microcontroller and Transmitter Overview ■ Unit 5: Speed, Power, Torque, and DC Motors ■ Unit 6: Gears, Chains, and Sprockets ■ 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 2011

VEX Parts

The following VEX parts are used in this phase:

Quantity Part Number Abbreviations

1 BEAM-2000 B2

2 SCREW-832-0250 S2

1 SCREW-832-0750 S6

1 SPACER-THIN SP1

Build Phase ■ 25

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 software. 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 software 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 Education 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.

26 ■ Autodesk's VEX Robotics Unit 8: Friction and Traction The completed model is as shown:

Build Phase ■ 27

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.

28 ■ Autodesk's VEX Robotics Unit 8: Friction and Traction The completed model is as shown:

Build Phase ■ 29

3. To complete the next step:

30 ■ Autodesk's VEX Robotics Unit 8: Friction and Traction ■ 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:

Build Phase ■ 31

4. To complete the next step:

32 ■ Autodesk's VEX Robotics Unit 8: Friction and Traction ■ 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:

Build Phase ■ 33

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.

34 ■ Autodesk's VEX Robotics Unit 8: Friction and Traction The completed model is as shown:

6. Your differential tricycle is now geared appropriately for the upcoming tractor pull!

Build Phase ■ 35 Note: Battery and electrical connections not shown.

36 ■ Autodesk's VEX Robotics Unit 8: Friction and Traction 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.

Prerequisites and Resources

Before starting this phase, you must have: ■ Completed Unit 8: Friction and Traction > Think Phase. ■ Completed Unit 8: Friction and Traction > Build Phase. ■ 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:

Amaze Phase ■ 37

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.

38 ■ Autodesk's VEX Robotics Unit 8: Friction and Traction ■ 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.

Challenge Instructions

1. Attach the 36” length of rope/string to the 2” beam at the rear of your differential tricycle.

2. Attach the other end of the rope to the rear beam of your opponent’s differential tricycle.

3. Place both robots on the floor so they are as far apart as possible, and facing in opposite directions.

4. 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.

5. Turn both robots and transmitters on.

6. 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 on 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.

Amaze Phase ■ 39 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

■ 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?

40 ■ Autodesk's VEX Robotics Unit 8: Friction and Traction

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?

STEM Connections ■ 41