Autodesk's VEX® Robotics Curriculum Unit 8: Friction and ... · 6 Autodesk's VEX Robotics Unit 8: Friction and Traction Phase Standard Technology (ITEA) 5.8: The Attributes of Design

1

Autodesk's VEX® Robotics Curriculum

Unit 8: Friction and Traction

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

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 designprocess to solve a given problem. The physics concepts of friction and traction must be considered incountless real-world applications. In STEM Connections, one scenario is presented involving frictionand traction in the design of a snowmobile. After completing the Think Phase and Build Phase in Unit8: Friction and Traction, you will see how concepts regarding friction and traction come into play inthe 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, andangle.

Coefficient ofFriction

The ratio of maximum frictional force between two surfaces to the force holdingthem together. Term used to describe the "grippyness" of two surfaces meshingtogether. Slippery objects have a very low coefficient of friction.

Overview ■ 3

Term

Definition

Component A part or subassembly placed into another assembly. Assembly componentsmay 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, thesketch resizes. Dimensional constraints may be expressed as numeric constants, asvariables in equations, or in parameter files.

Fillet A placed feature applied to edges and corners of a 3D model. A fillet feature isdefined 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 aredesignated as iFeatures and saved in a file with an IDE extension.

KineticFriction

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

Mirrorsketches

Sketch geometry that is copied across a centerline.

Normal Force The amount of force holding two surfaces together. For an object sitting on a levelsurface, 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 offeatures, and spacing between features.

Plane A two-dimensional (flat) part face.

Profile A closed loop defined by sketched or reference geometry that represents a crosssection of a feature. An open profile defined by sketched segments, arcs, or splinesmay define a surface shape or extend to boundaries to close a region. A profilemay enclose islands.

ProjectedGeometry

Geometry (model edges, vertices, work axes, work points, or other sketchgeometry) projected onto the active sketch plane as reference geometry. Mayinclude edges of a selected assembly component that intersects the sketch planewhen it was cut in an assembly cross section.

Properties A characteristic of a Microsoft Windows file that can be manipulated from anapplication or Microsoft Windows Explorer. Properties include author or designerand creation date and may also be unique properties assigned by applications orusers. Specifying properties can be useful when searching for part or assemblyfiles.

Revolve A solid feature created by revolving a profile around an axis.


Term

Definition

Section View In an assembly, a view of the model defined by temporarily hiding portions ofcomponents 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. Tocreate 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 includevisible default reference planes, customized grid settings, color scheme, draftingstandards, and so on.

Traction The friction between a drive member, wheel, and the surface it moves upon. Theamount 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 theUnit 7: Advanced Gears > Build Phase

One modified and assembled differentialtricycle 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 orequivalent rope/string

Overview ■ 5

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


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.

Overview ■ 7

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.


Think Phase

Overview

This phase discusses the physical concepts of friction and traction and their applications to robotdesign.

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


The following supplies are used in this phase:

Supplies

Notebook and pen

Work surface

Think Phase ■ 9

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, theobject 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 onit.

In the above diagram, you can see the opposing relationship between applied force and friction. Asthe 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 massbegins to move; after the mass begins moving, kinetic friction acts upon it. Static friction is greaterthan 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 eachother. Start to move them in a sliding motion. The motion is resisted by the texture of your skin andthe magnitude of the applied force. This is static friction. Now that they are moving relative to eachother, kinetic friction comes into play.

There are two factors which determine the maximum frictional force that can occur between twosurfaces: coefficient of friction and normal force.

The maximum force of friction (Ff) between two surfaces is equal to the coefficient of friction (Cf) ofthose 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 againstone another. Slippery objects have a very low coefficient of friction, while sticky objects have a veryhigh coefficient of friction. This constant is determined for a pair of surfaces, not a single surface, andranges from near zero to greater than one. Each pair of materials has a coefficient of static friction anda coefficient of kinetic friction.

Do not confuse this with actual sticky surfaces like tape or high friction coatings that bind to the othersurface. 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 (pullup) 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.


Coeficients of Static and Kinetic Friction for Common Materials

Materials in Contact

Coefficient of StaticFriction

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 normalforce is always perpendicular to the two surfaces. Often the normal force acting on a system is theweight 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 tothe sliding surfaces. In this case, only a portion of the object’s weight acts as normal force.

Think Phase ■ 11

Traction

Traction is defined as friction between a drive member (wheel) and the surface it moves upon. It is theamount of force a wheel can apply to a surface before it slips. A rolling wheel is in static contact withthe 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, itresults in an applied force along the ground. If there is friction between the wheel and the ground, anequal and opposite force called the tractive force pushes back against the wheel. The applied force isthe force of the wheel on the surface. The tractive force is the force of the surface on the wheel. Thisis 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 object2 is always equal in magnitude and opposite in direction to the force of object 2 on object 1. So, thegreater the "applied force" of the wheel on the ground, the greater the force of the ground on thewheel (and thus the robot)!

The tractive force is equal to the frictional force between the wheel and the ground. If the wheelis rolling and not slipping, the tractive force is equal to the static friction force. If the applied forceexceeds the maximum static friction, then the wheel will start to slip and the tractive force will equalthe maximum kinetic friction force.

Increasing Traction

Since traction is dependent on the friction of the wheel and the surface, you must maximize thisfriction. It is known that friction is dependent on coefficient of friction (between the wheel and thesurface), and the normal force (the weight of the robot pressing the wheel to the surface). To increasetraction, 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 tractionincluding 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. Frictionalso acts as a brake on the rotating components of the robot. The VEX Robotics Kit has several partsdesigned 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. Metalagainst metal contact is not desirable in moving systems (see steel on steel values in the table above).


Create Phase

Overview

In this phase, you learn how to create a tire for a VEX medium wheel. The workflow uses the basicpart creation techniques such as drawing a sketch and extruding the profile. In addition, you import asketch and use the profile to engrave the tire tread.

Objectives


■ 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.

Create Phase ■ 13

Technical Overview

The following Autodesk Inventor tools are used in this phase. Icon

Name

Description

Half SectionView

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

CreateComponent

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

BrowseTemplates

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

TransparencyOff

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

ProjectGeometry

Projects geometry (model edges, vertices, work axes, work points, or othersketch geometry) onto the active sketch plane as reference geometry.

Line Straight curve bounded by two endpoints. The line tool on the Sketchtoolbar chains line segments together and creates arcs tangent orperpendicular to existing curves.

Centerline Use to manually apply four types of centerlines and center marks toindividual 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. Theycan be expressed as numeric constants, as variables in an equation, or inparameter files.

Mirror Use to mirror sketch geometry across a centerline.

Return Use Return to quit in-place editing and quickly return to the desiredenvironment. The destination depends on which modeling environmentyou are working in.

Revolve Revolved features are created by sweeping one or more sketched profilesaround an axis. If the revolved feature is the first feature in a part file, it isthe base feature.

Project CutEdges

Use the Project Cut Edges tool when creating or editing a sketched featureto model edges onto the active sketch plane from a component cut by asection plane. A projected cut edge is placed in the browser under theSketch symbol.


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 2DSketch

A sketch consists of the sketch plane, a coordinate system, 2D curves, andthe dimensions and constraints applied to the curves.

iFeature An iFeature is one or more features that can be saved and reused in otherdesigns. You can create an iFeature from any sketched feature that youdetermine to be useful for other designs. Features dependent on thesketched feature are included in the iFeature. After you create an iFeatureand store it in a catalog, you can place it in a part by dragging it fromWindows Explorer and dropping it in the part file or by using the InsertiFeature tool.

HorizontalConstraint

The horizontal constraint causes lines, ellipse axes, or pairs of points to lieparallel to the X axis of the sketch coordinate system.

VerticalConstraint

A geometric constraint that causes selected arcs and circles to have thesame 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, andthen 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 andangle 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 orfeatures of a part.

CircularPattern

Part, surface, and assembly features can be arranged in a pattern torepresent hole patterns or textures, slots, notches, or other symmetricalarrangements.

End SectionView

Returns the assembly display to no section view.

Create Phase ■ 15


The following software is used in this phase.

Software

Autodesk Inventor Professional 2011


Exercise: Create a VEX Tire In this exercise, you learn how to create a tire for aVEX medium wheel. The workflow uses the basic partcreation techniques such as drawing a sketch andextruding the profile. In addition, you import a sketchand use the profile to engrave the tire tread.

The completed exercise

Create a VEX Tire 1. Make IFI_Unit8.ipj the active project. 2.

Open Medium_Wheel_Hub.iam.

3.

On the View tab, Appearance panel, click thearrow next to Quarter Section View. Click HalfSection View.

4.

In the browser, expand the Origin folder. ClickYZ Plane.

5.

On the Assemble tab, Component panel, clickCreate.

6. For New Component name, enter

Medium_Wheel_Tire. 7.

Click Browse Templates.

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.

Create Phase ■ 17

Create a Sketch Profile In this section of the exercise, you create a sketchprofile of the tire. 1.

On the View tab, Appearance panel, click thearrow next to Appearance. Click the arrow nextto Transparency On. Click Transparency Off.

2. On the ViewCube, click Right. 3. On the QuickAccess toolbar, click Open. 4. Open Tread Profile Profile.ipt.

5. In the browser, right-click Sketch1. Click Copy. 6. Return to Medium_Wheel_Hub.iam. 7.

Right-click in the graphics window. Click Paste.The sketch profile of the tire is placed in thecorrect location on the wheel.

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


Revolve the Sketch In this section of the exercise, you revolve the sketchprofile 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.

Create the Sketch for the Teeth In this section of the exercise, you create the sketchfor 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 toProject Geometry. Click Project Cut Edges.

4. Zoom into the top of the assembly.

5.

Select the face of a tooth.

6. Press ESC to exit the tool. 7. In the browser, right-click the part

MEDIUM_WHEEL_HUB:1. Click Visibility to turnoff 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) andcoincident with the lower edge of a toothprofile (2).

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

Create Phase ■ 19

Extrude the Teeth In this section of the exercise, you extrude the teethon the tire using the Cut option. 1. On the ViewCube, click Home. 2. Press E to start the Extrude tool. 3.

Select inside a tooth profile.

4. For Distance, enter 0.5. (1) 5. Under Operation, click Cut. (2) 6.

Click Midplane. (3)

7.

Click OK.

Create a Tangent Work Plane In this section of the exercise, you create a tangentwork plane. 1.

On the Work Features panel, click Plane.

2. In the browser, click YZ Plane. 3.

Select the top face of the tire.

Insert the Tread Profile In this section of the exercise, you insert the profile ofthe tire tread. 1. On the ViewCube, click Home. 2.

On the Manage tab, Insert panel, click InsertiFeature.

3. Click Browse. 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.


10. Click Next twice. 11. Click Activate Sketch Edit Immediately. 12.

Click Finish.

Locate the Tread Profile In this section of the exercise, you locate the tiretread profile. 1. On the ViewCube, click Top. 2.

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

3.

Select edges (1) and (2).

4.

On the Constrain panel, click HorizontalConstraint.

5.

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

6. Zoom into the top edge of the tread profile. 7.

On the Constrain panel, click VerticalConstraint.

8.

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

9. 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 Model tab, Create panel, click Emboss.

3.

In the Emboss dialog box, click Engrave fromFace.

Create Phase ■ 21

4.

Select the five tread profiles.

5. Select the Wrap to Face check box. 6. Select the outside face of the tire. 7.

Click OK.

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 chamfersto the sides of the tread profile to increase thewidth of the walls. You also add fillets to break thesharp edges. These features also make it easier tomanufacture the tire. 1. Zoom into the tread. 2.

On the Modify panel, click Chamfer.

3.

In the Chamfer dialog box, select Distance andAngle.

4.

Select an inside face of the tread.

5.

Select the lower edge of the tread.

6. For Distance, enter 0.1. 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.


10.

Click Cancel.

11.

On the Modify panel, click Fillet.

12. For Radius, enter 0.01. 13. Under Select Mode, click Loop. 14.

Select the loop when the three edges aredisplayed as shown.

15.

Repeat for the remaining four loops.

16. Click OK.

Complete the Tread Pattern In this section of the exercise, you create a circularpattern 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

Axis. 4. Select the outside face of the tire. 5. For Placement, enter 8. 6. Click OK.

Change the Properties of the Tire In this section of the exercise, you change thematerial 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. 6.

On the Quick Access toolbar, click Return.

7. On the ViewCube, click Home. 8.

On the View tab, Appearance panel, click thearrow beside Half Section View. Click EndSection View.

Create Phase ■ 23

9.

Turn on the visibility ofMEDIUM_WHEEL_HUB:1.

10. Save the file.


Build Phase

Overview

In this phase, you modify the gearing on a previously built robot.

Phase Objectives


■ 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 ■ 25


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.


■ 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



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


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 givethe robot more torque for the upcoming tractor pull.

As you work on building this project, have some of your team members focus on expandingtheir expertise using Autodesk Inventor software. Later in the curriculum, you will be challenged tocome up with your own creative solutions for robot design. You will save time and maximize yourability to create winning solutions if your team understands how to leverage the power of digitalprototypes using Inventor.

Note: Team members can download a free version of Autodesk Inventor Professional software to useat home, so you can come to class prepared to build and test your best ideas! To do this, simply jointhe 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.

Build Phase ■ 27

The completed model is as shown:


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 ■ 29




Build Phase ■ 31

■ 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.




Build Phase ■ 33

■ 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 PlasticWashers, 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 andtighten. 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.



5. To complete the final step:

■ Bolt one Thin Spacer [SP1], one Thick Spacer [SP2], and one 2” Beam [B2] between thetwo 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 ■ 35


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


Note: Battery and electrical connections not shown.

Amaze Phase ■ 37

Amaze Phase

Overview

In this phase, you compete in a tractor pull against another robot.

Phase Objectives


■ Take advantage of the principles of friction and traction to modify a robot to pull a greater amountof weight.

■ Make decisions based on theoretical physics, and apply them to a dynamic situation.


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.


■ 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




Supplies

One modified and assembled differential tricycle from the Unit 8: Friction and Traction > BuildPhase

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’sdifferential tricycle. The two robots are attached by a length of nylon rope, and the goal is to pull youropponents robot 3’ from its starting position.

Since all robots are starting with the same configuration, in theory each tractor pull should be astalemate. To do well in this competition, you will need to modify your robots, and apply some of thelessons 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 thischallenge, 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 weightTherefore, an increase in the coefficient of friction between the robot and the ground, or anincrease 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.

Amaze Phase ■ 39

■ Be creative! There are many ways to modify your robot to perform better in this challenge. Do notbe 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 thismark. 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 thechange and what effect it had on your robot in the tractor pull. Explain why certain changes were moreeffective 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.


STEM Connections

Background

A snowmobile uses many of the concepts from this unit to function. Because snowmobiles operate insnowy and icy areas, they have a special track mechanism resembling a tank tread, which drives thevehicle 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 aharder 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 increasethe 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?

STEM Connections ■ 41

Math

The kinetic coefficient of friction for rubber on dry asphalt is roughly 0.67, while the static coefficientof 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 goesinto a skid, then the rubber tire surface is sliding along the asphalt surface. You multiply the kineticcoefficient 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 staticcoefficient of friction (0.85) instead. Why? In this case, friction can apply at most 85 lbs. of force toslow the bike.

Back to the snow: Your snowmobile broke down in the middle of a blizzard and you had to go the restof the way on skis. Which do you think is harder to do with skis on level snow: to start moving fromrest, 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 skion snow is about 0.05, while the static coefficient of friction is about 0.14. Which of these numbersshould you use when talking about a stationary skier, and which should you use when talking about askier 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?

Documents

Autodesk's VEX® Robotics Curriculum Unit 8: Friction and ... · 6 Autodesk's VEX Robotics Unit 8: Friction and Traction Phase Standard Technology (ITEA) 5.8: The Attributes of Design