MEEG 401: Senior Design

Final Report University of Delaware – Department of Mechanical Engineering

Team 3:

John Artes

LaMont Cannon

Mark Dilullo

John Gangloff

Joseph Walther

Advisor: Nate Cloud

Sponsor: Schiller – Pfeiffer Inc.

December 16, 2008

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Introduction

Schiller – Pfeiffer Inc. (SPI) is a company that specializes in the production of lawn and

garden equipment for personal and commercial usage. SPI has contracted UDME’s Senior

Design Team 3 to assist with the improvement of their premier product – the Mantis tiller.

Figure 1 displays the Mantis tiller and how it is operated. The Mantis tiller is a low-cost, low-

weight, easy to use personal garden tiller that has been in production for 30+ years. It serves as

a leading personal lawn and garden tiller on the market and must be kept up-to-date with its

engineering to remain competitive.

Figure 1: Mantis Tiller

Problem Definition

SPI has a well-established consumer base within the US. It has expressed interest with

improving its Mantis tiller product line in Europe. Currently within Europe there are strict noise

regulations in place for power equipment operated outside. The regulations have required SPI

to remove the 2-stroke US motor from its product due to its high noise levels and replace it

with a quieter 4-stroke Euro motor. The Euro motor is consequently at lower power versus the

US motor. When the motor reselection was made, no other parts were changed over for the

Euro tiller from the US tiller. SPI has found that the Euro has poorer tilling when compared to

the US tiller. It is believed that this is in part due to the Euro motor being mismatched with the

adapted US transmission. Figure 2 displays the tiller transmission assembly.

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Figure 2: Tiller Transmission Assembly

The US transmission is set at a gear ratio optimized for the US motor and it is believed

that this ratio is not optimized for the Euro motor. SPI would like the team to improve the

performance of its tiller line for Europe and/or the US through the redesign of its transmissions.

Project Needs, Wants, and Constraints

To ensure successful completion of the project, the project’s needs, wants, and

constraints are to be identified and understood by the team. After completing research into the

customer’s needs, wants, and constraints, they are to be ranked in a way that will facilitate

intelligent engineering decision-making. The team chose to adopt the Pairwise Comparison

matrix from Dym and Little’s Engineering Design book for its customer wants ranking system.

Table 1 and Table 2 show the results of the team’s technical and marketing wants and rankings.

Initial discussion with the sponsor was used as to quantify the relative rank of each item.

The Pairwise Comparison of wants chart is a clever tool that the team used to rank the

wants of the customer. In the table, an entry of “1” indicates that the objective in that row is

more important than that of the column in which it is entered. The score column represents the

sum of all of the “1’s” in each row, which allows you to easily rank the wants based on the

information that we were given from our customer. Our application of the chart allows for the

straightforward comparison of the technical and marketing wants of the customer. The chart

shows that having a reverse option is the customer’s most important technical goal while safety

is the most important marketing goal. This chart is very helpful because it allow the team to

focus on the wants that are most important during the design process.

Wants/

Metrics

(target

values)

Adjustabl

e/

Variable

Speed

Compatibl

e with

Different

Engines

Tine

Speed

Between

(230-

240)

Revers

e

Option

5-6k

RPM

on

Engin

e

Fits in

Existing

Space

Outpu

t

Torqu

e (31

ft-lbs) Score

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Adjustable

/ Variable

Speed 0 0 0 0 1 0 1

Compatibl

e with

Different

Engines 1 0 0 1 1 0 3

Tine

Speed

Between

(230-240) 1 1 0 1 1 0 4

Reverse

Option 1 1 1 1 1 1 6

5-6k RPM

on Engine 1 0 0 0 1 0 2

Fits in

Existing

Space 0 0 0 0 0 0 0

Output

Torque

(31 ft-lbs) 1 1 1 0 1 1 5

Table 1: Pairwise Comparison of Technical Wants

Wants/Metrics

(target values)

Cost

Effective Weight Size Durability Safety Score

Cost Effective

($40) 1 1 0 0 2

Weight 0 0 0 0 0

Size 0 1 0 0 1

Durability 1 1 1 0 3

Safety 1 1 1 1 4

Table 2: Pairwise Comparison of Marketing Wants

Key Performance and Cost Metrics

The primary performance and cost metrics that will define the project’s success are if

the tiller transmission design can operate at different input speeds, operate in reverse, fit into

geometric constraints, and maintain a low cost of approximately $40 for the tiller transmission

system. Achievement of these metrics will be used as the baseline for meeting sponsor

expectations. If a successful tiller transmission design cannot be achieved, the sponsor has

specified multiple contingencies that can be pursued for a successful project. One contingency

is to prepare an Excel spreadsheet that can take an input motor and size and output the

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required gearing data needed to retrofit current transmissions for optimum performance.

Another contingency is to create two different transmissions for the US and Europe engines and

uses them for testing purposes by the sponsor. A third contingency is to design a testing

apparatus to test tiller motors for reliability and engineering data. Ideally a “one-size-fits-all”

transmission design can be crafted, but contingencies exist for the “just-in-case” scenario.

Benchmarking

To facilitate initial concept designs, the team turned to a variety of benchmarking

resources. The team decided to research continuously-variable transmission (CVT) designs for

their ability to provide variable output speeds and torques, which adheres to the project wants.

One particularly useful resource was found on HowStuffWorks at the following web address:

http://auto.howstuffworks.com/cvt5.htm, which explains the basics of CVTs and their design.

The team was particularly intrigued at the various diagrams of the different CVT designs. For

example, Figure 2 displays the basic pulley – based CVT design. The system works by adjusting

pulley widths to adjust belt distances between two pulleys. The adjusting of belt distances

provides seamless changes in gear ratios, rather than the discrete amount of ratios found with

conventional manual transmissions. This design is very common amongst CVT systems;

however belt slippage issues and pulley distance issues may affect its implementation into the

tiller transmission project.

Figure 2: Pulley – Based CVT Design

Figure 3 shows another CVT design called the Toroidal CVT. The Toroidal CVT uses discs

and power rollers to provide seamless gear ratio changes. The transmission works by adjusting

the tilt of the power rollers, which affects the relatives speeds of the input and output discs

connected to driveshafts. This design allows for more compact geometries, but can lead to

potential material wear issues for the rollers.

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Figure 3: Toroidal CVT

Another benchmark the team found was the NuVinci CVT design (Figure 4), licensed by

Fallbrook Technologies Inc. The design uses a set of ball bearings set similarly to a planetary

gear system with rockers that adjust to different torque/velocity inputs. The rockers adjust the

pitch of the ball, which affects the gearing ratio produced. The system is contained within a hub

that can be retrofitted to a variety of mechanical devices, including lawn and garden

equipment. Pricing of system implementation has yet to be determined.

Figure 4: NuVinci CVT

Benchmarking is critical to concept generation, because it provides a strong foundation

for the team to brainstorm and facilitate design. Also, a solution to the design problem may

exist in industry already for which benchmarking would reveal to the team. Much time and

money in R&D can be saved if a transmission solution is able to be purchased and retrofitted for

the tiller application rather than designing one from scratch. All possibilities will be considered

at this stage of the project.

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Initial Sketches

Figure 5 highlights one concept for the transmission design using inverted cone-shaped

gears and a reinforced belt. The mechanism works by moving the reinforced belt up and down

the inverted cones during rotation to provide a changing set of gear ratios. The belt could be

moved using a screw-based mechanism or lever that is either user controlled or factory set. The

transmission has a reduced number of parts compared to a traditional transmission system

with finite gear ratios. The inverted cone-shaped gears provide a large amount of gear ratios for

smooth transitions, which is important for the low-end torque required for initial tilling. The

reverse mechanism involves a sliding set of gears that changes the direction of the driveshaft

for tine reversing. If the torque upon reversal is too high for the gear teeth, then additional

reverse assemblies can be added to reduce stresses on the teeth. The inverted cone + belt and

reverse assembly concepts would be placed below the motor and above the current

transmission assembly in a custom housing.

Figure 5: Inverted Cone + Belt and Reverse Assembly Concepts

Figure 6 and 7 shows a reverse planetary gear system design. This design aims to replace

the current worm gear with a planetary gear system, which will allow for a high torque reverse

drive. In forward drive mode the carrier ring will lock the center sun and outer ring gears

together creating a rigid hub. In reverse mode the carrier ring will disengage from the center

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hub allowing the planet gears to reverse the directions of rotation of the outer ring. This

concept will fit in the current housing and requires only a small hole drilled for the shift rod. The

current center tine shaft threads will need to be replaced as shown to prevent the threads from

“backing out” when reverse is engaged.

Figure 6: Reverse Planetary Concept

Figure 7: Reverse Planetary Concept – Exploded View

Figure 8 shows an additional reverse gearbox concept. This concept is aimed to add a

reverse gear to the Mantis Tiller. To do this, this design will add a gearbox between the current

transmission and the motor mounted above. While the transmission is selected for forward,

the input shaft from the motor will be in line with the drive shaft of the transmission. There is a

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lever that can be moved upward that will slide a shaft that connects the input shaft of the

motor with the output shaft in the transmission. Mounted on this moveable shaft will be a gear

that spins freely in forward, but when in reverse, it makes contact with another spur gear

mounted on a shaft to the side. Below this spur gear to the side is another gear that makes

contact with two more gears, the main drive shaft as the final contact that reverses the

direction of motion of the tines.

Figure 8: Reverse Gearbox Concept

Figure 9 depicts a compact reverse gearing design derived from ancient Egyptian style

mechanisms. The input drives a worm that drives the splined drive shaft. Forward and reverse

can be selected by driving the left of right plate, along with an option for variable speeds. Speed

1 or speed 2 for forward or reverse can be selected by engaging the inner or outer ring of the

plate with the output cog. The design is very compact and relatively simple to execute in tight

volume constraints.

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Figure 9: Egyptian Reverse Concept

Figure 10 displays a CVT concept. The concept for this Continuously Variable

Transmission came from a video that the team found when we were benchmarking. The idea is

that if you vary the height of the input gear you the two shafts on the outside will rotate and

the gear ratio will change. The shafts are able to rotate due to the addition of a Universal Joint

that over the range of speeds of the shafts is 1:1. The CVT uses friction to operate but choosing

an appropriate spring constant can easily control that.

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Figure 10: CVT Concept

Figure 11 shows another transmission design that uses a gearbox that can be placed on

the top of the existing transmission, just below the engine. The design uses a small belt driven

continuously variable transmission that would allow the user to adjust the speeds by changing

the distance between the belt gears. The user will also be able to shift to reverse by moving the

gear in the reverse section out which will then engage the two other gears and allow the tines

to change direction.

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Figure 11: Transmission Concept

Project Scope

The initial project scope given to Team 3 from SPI was to design an improved tiller

transmission for production”. This scope left the team with many different options involving the

redesign of the transmission. The team brainstormed many ideas regarding variable

transmission designs and reversing feature mechanisms to improve the transmission.Essential

to identifying the project scope is isolating the design’s subsystems. Identification of

subsystems permits for the team to follow necessary design pathways that will ultimately lead

to successful project completion. The following is a description of the project’s subsystems as

concluded by the team:

Subsystem Identification

1) Reverse Feature: To provide the consumer with an easy method of switching the tiller

to operate in reverse for instances where the tiller gets stuck during use.

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2) Variable Speed: To provide the consumer with the ability to adjust the output speed of

the drive shaft. This will allow for other attachments to be installed and used on the

tiller by the consumer. Proposed attachments include a sweeping assembly and a snow

sweeping assembly, though the attachments are not in the scope of this product.

3) Changeable Gearing Option with Excel: To provide the company with an Excel

spreadsheet that can be used to find the proper gearing configuration for different

engines while maintaining the desired tine speed. The subsystem will also include plans

for the construction of the transmission. This subsystem is a contingency plan in the

event that the other avenues do not produce the results we desire.

4) Multiple Transmissions: Should the other transmission subsystems all fail, we may

design two separate transmissions for the two engines that are currently used.

(European and American)

5) Engine Testing: We would gather information on the engines including torque and

power curves as well as temperature information and its effects on performance. These

may be found from the manufacturers or elsewhere, and can be tested by our team if

the data cannot be found anywhere.

Connections

Ideal Result: To include the reverse and CVT subsystems into a single transmission that supplies

a tine speed between 230 and 240 rpm + 31 lb-ft of torque.

Reverse Feature: This feature is optimally desired with subsystems 1-4.

Variable Speed: This feature is optimally desired with subsystems 1-4.

Changeable Gearing Option with Excel: This subsystem could involve 1 and 2, but will focus on

the calculations and programming in Excel.

Multiple Transmissions: This could also involve 1 and 2, though the focus will be on developing

two transmissions that will likely closely resemble the current transmission.

Engine Testing: This subsystem, alone, is the final fallback if all other subsystems fail. This can

also be done to supplement the work in other subsystems to provide us with more complete

information.

In addition, the team determined a series of “critical” and “non-critical” wants to drive

the decision making of the design. The “critical-wants” or wants that must be met by the design

to generate a profitable and functional tiller are the following: competitive cost, constant

power, and unchanged transmission housing geometry. The “non-critical wants” or wants that

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are desired but not required for the design are the following: variable engine support and

reversing feature. These wants were tabulated and used to systematically compare the team’s

concepts and determine the best of those concepts. Table 1 displays the results of this

comparison.

Critical Wants

Satisfies

Want

Does Not

Satisfy

Want

Non-Critical

Wants x -

Concepts Cost Space

Constraint

Constant

Power

Variable

Engine

Support

Reversing

Feature

Satisfied

Critical

Want

Total

Satisfied

Non-

Critical

Want

Total

CVT+ Reverse

Mechanism in

Modified Housing

- x x x x 2 2

Reverse

Mechanism in

Current Housing

- x x - x 2 1

CVT + Reverse

Mechanism in

New External

Housing

- - x x x 1 2

Change Worm

and Worm Gear

Ratio

x x x - - 3 1

Ta

ble 1: Concept Selection

After the concept selection process, it was determined that most of the brainstorming

ideas the team generated would exceed cost constraints for production. The concept the team

generated that fit the cost and additional constraints was to simply changing the worm and

worm gear assembly within the current US transmission to a different gear ration optimized

for the Euro motor. This concept selection narrowed and focused the project scope. Instead of

designing an “all-in-one” transmission with many features, the team will focus on optimizing

the older US transmission for the Euro tiller. Although the Euro tiller will be of lower

performance versus the US tiller because of it having a lower power engine versus the US

version, the team would like to make the Euro tiller perform to its best capability with better

gearing. This leaves the primary objective of the team to be how to find the optimal gear

ratio.

Project Path Deleted: Concepts

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At the end of Phase 1, the team and the sponsor isolated two project paths that would

be suitable in meeting requirements. The project paths address different aspects of the overall

problem, but provide concrete engineering solutions. The first project path is a reverse

planetary gear feature that would be designed for the direct tiller product. The second project

path is a variable speed transmission test setup that would be designed for research and

development purposes for future tillers and other Schiller – Pfeiffer products. Each project path

provides critical performance and/or engineering data to the sponsor that would ultimately

lead to the design and sales of high performing tillers. The team systematically analyzed both

project paths to determine which could be best executed by the team in the timing allotted for

Senior Design.

Reverse Planetary Gear Feature Project Path

After researching the options, we have concluded that designing a planetary gear reverse

feature for production-line tillers does not seem feasible given the restraints on cost. This original

concept is shown in Figures 13 and 14.

Figure 13: Reverse Planetary Concept

Figure 14: Reverse Planetary Concept – Exploded View

Deleted: concepts

Deleted: project

Deleted: concepts

Deleted: concept

Deleted: concept

Deleted: experiment

Deleted: concept

Deleted: concepts

Deleted: Concept

Deleted: 2

Deleted: 3

Deleted: 2

Deleted: 3

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We have, instead, decided that building a test transmission featuring a variable speed

transmission is a more reasonable project, and will benefit the sponsor by finding optimal gearing

configurations for any compatible combination of engine and tines. The planetary reverse design,

chosen for further study due to its simplicity and compactness, would require the parts listed below. The

prices marked “R” indicate the retail cost from a supplier, and the prices marked “P” indicate the

estimated cost when purchased in quantity. To estimate the difference between these prices, we

considered the current worm gear used in the transmission. The cost for one unit was approximately

twice that of the part when purchased in quantity, providing a 50% price decrease when purchased in

quantity. The 3” brass gear used currently costs $7.84, and the planetary system seeks to replace it.

List of Parts Required for Planetary Reverse

1 4’’-6’’inch internal gear R $130 [4] P $65

4 1’’ to 2’’ planetary spur gears R $41.4 [1] P $20.7

1 2’’-3” center sun gear R $80* P $8 [2]

1 gear selector ring R $.87 [1] P $.43

1 gear selector rode R $.17 [1] P $.085

4 housing closing screws R $.75 [1] P $0.5

4 small roller bearing capable of 12000rpm R $46.28 [1] P $ 23

1 custom planet carrier, R $130 P $65

1 two piece custom housing R ? P $7 [3]

Retail total: $429 plus housing Production total:$189

These costs demonstrate that with current concepts, including a planetary gear reverse feature

dramatically exceeds our maximum cost of $14. As a result of the information listed above, we are

proposing the development of an experimental gear ratio testing unit as our main focus for the design

process.

CVT Experiment Setup Project Path

From our meeting on September 23, 2008 the project path of developing an experimental

transmission for testing the performance of different gear ratios was discussed. This variable speed-

testing box is not subject to the restrictive cost guidelines that prevented the reverse feature from being

a viable solution. What follows is a description of the plan for the continuously variable speed

transmission box.

We plan to design a CVT transmission that will mount between the engine and the transmission

of the Mantis Tiller. Figure 15 displays conceptual sketches for the experimental CVT and housing. The

setup will serve as a test instrument for Schiller-Pfeiffer and will help to find the optimum gear ratio for

Deleted: Concept

Deleted: concept

Deleted: 4

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any model in the Mantis Tiller product line. Use of this device will improve the functionality of the

European model, which is severely restricted, and optimize the quality of the American model. In

addition, the experimental setup can be used to find the optimal gear ratios for a variety of attachments

to the tiller, such as a snowbrush or other similar add-ons. This adds developmental value to the

experimental setup because it will be useful in testing tiller add-ons and other engine applications that

can benefit from gear testing in the future. The fixture will be able used for test tilling so does not have

to be suitable for production. It will have an ideal maximum volume of 6”x6”x6”. It should feature a

wide enough selection of gears so that any reasonable gear ratio can be obtained. Figure 16 displays

conceptual sketches for the proposed CVT cones and a keyway plate for mounting the cones.

Figure 15: The original CVT concept envisioned by the team (left).Proposed housing (right).

Calculations find that a box with ratios varying from 1:2 to 2:1 will provide a suitable range of

output torque vs. tine rpm. These can vary from 71rpm, 62 ft-lb to 285rpm, 16 ft-lb on the 6000rpm

engine and 119rpm, 62ft-lb to 476rpm, 16ft-lb on the 10,000rpm engine. The high- and low-end values

for each engine are above and below, respectively, the ranges that would actually be feasible for the

tillers.

Deleted: 5

Deleted: 4

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Figure 16: Close-up of CVT gear cone without gripping inserts (left).Bolt-on keyway plate to secure

gear to rotating shaft, 1 of 2 per gear (right).

Price Sources and Footnotes

[1] McMaster-Carr

[2] Planet gears

-Steel Plain Bore 14-1/2 Deg Spur Gear 32 Pitch, 18 Teeth, 0.562" Pitch Dia,

3/16".$10.35 Each.

-Ultra-Precision Mini SS Ball Bearing - ABEC-7 Open for .1875" Shaft Diameter, .5" OD,

.1562".$11.57 Each.

- Round Head Slotted Machine Screw with Nut Zinc-Plated Steel, 1/4"-20 Thread,

3" Length. $9.39 per Pack of 50, $.18 each

- Wide-Rim Plain Steel Shim .134" Thick, 2" ID, 3" OD. $8.75 per Pack of 10, $.87

each

[3] The price for this gear is assumed to be similar to the cost of the current drive gear based on its

similar dimensions and requirements.

[4] The current transmission housing costs $6, and we estimate that an additional housing would

cost approximately the same.

[5] This information was found through Boston Gear, a leading gear manufacturer.

[6] No sources used, price assumed from known real production price.

Deleted: 5

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As a result of this analysis, the team’s project scope is to design a variable speed

transmission test setup to find the optimized gear ratio for tilling in terms of tine torque and

RPM. The ideal ratio will be used in the final project design concept by installation of a new

worm gear into the current transmission housing or by providing the modified drawing plans

needed to build the new parts. The team will provide an outline for the new worm gear design

and installation, based on data from the variable speed transmissiontest setup. Please refer to

Figure 1 for project scope clarification.

Figure 1: Project Scope Diagram

Concept Selection

In the design process, concept selection can often prove to be an arduous task. A

weighted benchmark chart is a very useful tool that can be implemented to aid in the selection

process. Figure 2 shows a concept design selection chart adopted from Dym and Little’s

Engineering Design, that was used to determine the type of transmission to implement within

the variable speed transmission test setup

Type of

Transmission

Gearing

Range Cost Weight

Ease of

Mounting Drivetrain Size Durability Safety Total

NuVinci CVT 4 3 1 2 2 3 4 4 23

Belt and Pulley CVT 2 1 4 1 1 4 1 3 17

Belt and Cone CVT 3 2 0 0 0 0 0 2 7

9 Speed 0 4 2 4 4 2 2 1 19

14 Speed 1 0 3 3 3 1 3 0 14

Figure 2: Transmission Selection Weighted Benchmark Chart

The structure of the chart is such that the top row consists of a list of constraints

pertinent to the application, and the column on the left contains possible design concepts. The

numbers in the chart are a ranking of each design concept relative to the constraint. The

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Bold, Font color: Auto

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Bold, Font color: Auto

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Deleted: n experimental

Deleted: implemented

Deleted: experimental

Deleted: 1

Deleted: .

Deleted: 1

Deleted: Concept Design

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concept that meets the constraint the best gets the highest score (4) and the concept that least

satisfies the constraint get the lowest score (0). Summing the numbers in each row

corresponding to the design concept will yield a total score. The total score is directly

proportional to how well each concept meets the concepts overall. The concept with the

highest total score is the one that meets all of the constraints the best. From the chart the

NuVinci CVT system is the concept is best suited for the tiller test feature application. Figure 3

displays a picture of the transmission hub with a cut away to display its interior.

Figure 3: NuVinci CVT Hub

The NuVinci CVT works through a mechanism described in Figure 4, which involves sets

of ball bearings that are attached to rockers and provide a mechanical connection between two

rotating input / output discs for torque and velocity. After correspondence with the

manufacturer, the team has learned that the NuVinci system has been tested with positive

results at an input power of 41 horsepower, which is far higher than our application requires.

The cost, size, and weight of the unit are also acceptable. It is uncertain at this point, if the

NuVinci transmission can handle the required rotational speed of the engine, 10,000rpm.

Action has been taken to retrieve this answer.

Design Overview

The team has determined with SPI that in order to execute the process of improving its

tiller transmissions for the European market, a systematic approach will be necessary to find

the optimal gear ratio. The team has decided to find the ratio by creating an experiment setup

that will vary the gearing of the current motor and transmission assemblies. Tiller performance

will be evaluated at various ratios and the best performing ratio will be utilized in the redesign

of the worm and worm gear assembly for future transmissions. Figure 3 displays a schematic

and a three-dimensional sketch of the purposed experimental setup design.A detailed view of

the experimental setup design is provided in Figure 4. The system works by taking power input

from the motor that sits on top of the setup and runs it into a continuously variable

Deleted: 2

Deleted: 2

Deleted: 3

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transmission (CVT). The CVT permits the user to vary the gear ratio experienced by the system,

which will permit the user to select different gear ratios for optimal tilling. The power is

transmitted from the CVT to the setup output. From the output, power is transmitted to the

current transmission and ultimately the tines. The system is such that the power output from

the motor is assumed constant and equivalent to the power output of the tines. Efficiency

losses throughout the mechanism are negligible, based on sponsor discussion and motor &

transmission operation data.

Figure 3: Experimental Setup Schematic and Three-Dimensional Sketch

Figure4: Experimental Setup – Detailed View

Design Subsystems

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The design can be broken up into six subsystems: Motor Input, Step Down Mechanism,

NuVinci CVT, Step Up Mechanism, Transmission Output, and Enclosure. Figure 5 displays a side

view of the experiment setup with labeled subsystems. Section views of the design can be

inspected in Appendix Afor shaft and bearing locations and types.

[insert correct picture here and rewrite subsystems]

Figure 5: Experiment Setup – Subsystems

Motor Input

The motor input is a custom machined part that is designed to conform to the motor

clutch assembly output of the motor. The motor input houses the driveshaft of the experiment

setup and contains a roller bearing to maintain gear and driveshaft alignment.

Step Down Mechanism

The step down mechanism is a series of two spur gears that reduces the power input to

the CVT by a 10:1 ratio. This mechanism is necessary for meeting the nominal input RPM

specifications of the purchased CVT. The pinion gear is at 14 teeth and the larger gear is at 144

teeth. The larger gear will be custom machined to mount properly to the CVT. Lubrication is

provided by initial gear greasing during design manufacturing and periodic greasing after

experiments via a grease gun.

NuVinci CVT

The NuVinci CVT, as viewed in Figure 6, is a purchased system from Fallbrook

Technologies designed for bicycles that will be adapted for the experimental setup. It is the

“critical” subsystem of the design, because it houses the mechanisms that provide the gearing

variability and thus the primary function of the experiment setup. The decision to use the

system was very important and was assisted through the use of a weighted benchmark chart

adopted from Dym and Little’s Engineering Design. Table 2 displays the weighted benchmark

chart with results.

Table 2: Weighted Benchmark Chart

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The structure of the chart is such that the top row consists of a list of constraints

pertinent to the application, and the column on the left contains possible design concepts. The

numbers in the chart are a ranking of each design concept relative to the constraint. The

concept that meets the constraint the best gets the highest score (4) and the concept that least

satisfies the constraint get the lowest score (0). Summing the numbers in each row

corresponding to the design concept will yield a total score. The total score is directly

proportional to how well each concept meets the concepts overall. The concept with the

highest total score is the one that meets all of the constraints the best. From the chart the

NuVinci CVT system is the concept is best suited for the tiller test feature application.

The NuVinci CVT works through a mechanism described in Figure 7, which involves sets

of ball bearings that are attached to rockers and provide a mechanical connection between two

rotating input & output discs for torque and velocity. The system allows for continuously

variable gearing from ratios of 1:0.5 to 1:1.75, which is within the ideal experimental gear ratio

range for the project. In addition, the system provides a wide variety of velocity and torque

output at the tines, ranging from 280 RPM/18 ft-lb to 80 RPM/ 54 ft-lb. These values are also

within the ideal experimental ranges after sponsor discussion. Following correspondence with

the manufacturer, the team has learned that the NuVinci system has been tested with positive

results at an input of 41 horsepower, which is far higher than project requirements. The cost,

size, and weight of the unit are also acceptable.

Step Up Mechanism

The step up mechanism is a series of two spur gears that increases the power

outputfrom the CVT by a 10:1 ratio. It is of a similar design to the step down mechanism and is

necessary in providing the high RPM for input into the transmission output for tilling. The

pinion gear is at 14 teeth and the larger gear is at 144 teeth. The larger gear will be custom

machined to mount properly to the CVT.

Transmission Output

The transmission output is designed to conform to the geometry of the current

transmission housing and mate with the current transmission shaft. It is a part adapted from

the current motor geometry that contains a second driveshaft that turns the worm gear set

connected to the tines. The output contains two ball bearings to maintain gear alignment and

ensure reliability. Lubrication is provided by initial gear greasing during design manufacturing

and periodic greasing after experiments via a grease gun.

Enclosure

The enclosure subassembly contains a variety of parts designed to secure and protect

the other subassemblies. The enclosure top and bottom consist of two machined aluminum

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alloy plates that hold the step down/up mechanism gear shafts, the NuVinci shafts, the motor

input, and the transmission output securely in place. One of the enclosure sides is a machined

aluminum alloy plate that is designed to hold two L-brackets in place. The plate also provides

structural support for the experiment setup. The L-brackets house the bearings and shafts

connected to the pinions of the step down and step up mechanisms. The remaining three sides

of the enclosure are made of fiberglass to safely contain the moving parts of the design. The

corners of the enclosure consist of two support struts on fiberglass side and four support corner

blocks on the machined aluminum plate side.

Design Performance

The primary design driver for the experiment setup was motor power management at

the tines. For this application, power is assumed to be the torque multiplied by the RPM either

at the input or output of the experiment setup. It is also assumed that the input power of the

motor is equal to the output power of the tines. Efficiency losses can be assumed to be

negligible at nominal tilling gear ratios. If it is assumed that there is a constant input torque and

RPM from the motor, there is a trade off between the output torque and RPM. The output

torque and RPM distribution are controlled by the gear ratio.

Calculations were performed to determine if the NuVinci hub would provide the desired

output range of tine RPM and torque. The generated numbers were important for design by

establishing performance limits for the design. Table 3 displays the calculations.In addition, a

Microsoft Excel spreadsheet was created to precisely predict specific output ratios for any given

input. Figure 8 displays a screenshot of the spreadsheet with generated data.

Table 3: Tine Output RPM and Torque Calculations

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Figure 8: Design Spreadsheet Screenshot

Design Analysis

The following is a finite-element analysis of a plain carbon steel input pinion that drives

the experimental test setup. The inputpinion receives load from a driveshaft that is connected

to the input pinion with a roll pin. The input piniondrives an idler gear, which then drives a

larger gear fixed to the NuVinci CVT. A reverse geartrain at theoutput of the NuVinci is used to

transmit power to the output of the experimental test setup and ultimately thecurrent tiller

transmission. FEA was performed on the input pinion, because the team was concerned that

thepinion would possibly not withstand power transmission at the .74 lb-ft torque input.

Figure ???: Finite-Element Analysis – Plain Carbon Steel Drive Pinion

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According to the FEA, the gear will endure the given loading. The von-Mises stress value

is approximately .5times lower than the yield strength of the material, which indicates that the

gear will be able to performnominally. Also, the results show that the pinion would withstand

the 0.74 lb-ft, 6000 RPM input with a theoretical factor of safety at 2.02. Further testing will be

performed to validate initial FEA results.

Drawing Package

A drawing package of the complete design has been provided in Appendix D. The

drawing package contains a 3D isometric view, 3D exploded view + bill of materials, and 2D

orthographic views of each part of each subsystem. The drawings are to scale and are ready for

use in design manufacturing. Note: Fasteners were not shown so to improve drawing clarity.

Design Assembly

The assembly process of the experiment setup can be broken down into a series of

instructions. The team and/or the sponsor to put the design together for testing will use these

instructions. Assembly instructions are provided in Appendix C.

Testing Methodology

Design testingis divided into two segments. Testing will be conducted to validate the

functionality of the experiment setup and then preliminary testing will be conducted to begin

the process of determining the ideal gear ratio for tilling. Future testing for recommendation to

SPI after the conclusion of Senior Design is provided.

FunctionalTesting

Rotational Power Transmission and Variability Output RPM Test

1) The test box will be installed on the Mantis tiller.

2) The tiller will be suspended above the ground and the engine started.

3) The transmission of power through the test box will be verified by rotation of the output

tines.

4) Variability of the output will be verified by changing the NuVinci ratio from low to high

and checking for observed variation in tine output RPM.

5) The engine will be stopped and the testing setup will be visually examined for any signs

of damage.

Unaltered and Low/High Ratio Test

1) The tiller will be suspended above the ground.

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2) The engine will be started and the engine RPM will be measured using a tachometer.

3) The NuVinci will be adjusted to a 1:1 ratio.

4) The output tine RPM will be measured using an infrared tachometer. This value will be

compared to the known value of output RPM without the test box installed on the tiller.

5) The engine will be stopped and the testing setup will be visually examined for any signs

of damage.

6) The test will be repeated with the NuVinci set to high and low ratios.

In-Ground Function Test

1) The tiller with installed test box will be started.

2) The NuVinci will be placed in the 1:1 ratio setting.

3) Tilling a portion of untilled ground in SPI’s tiller testing bed will attempted.

4) If the top layer of soil is successful broken and turned over, exposing the dirt below, and

the tiller tines sink fully into the soil without becoming jammed the test will be

considered a success.

5) The engine will be stopped and the testing setup will be visually examined for any signs

of damage.

6) The test will be repeated for ratio gradually varied above and below 1:1 to verify

functionally.

Performance Testing

1) In the typical test soil located at SPI’s garden test bed, untilled ground will be tilled while

varying the gear ratio from high to low in 10 even increments.

2) The tiller will be tested at each increment for 1 minute.

3) Each ratio will be evaluated by ranking the following categories which may quantify the

subjective notion of “good tilling” on a 1 -10 scale with the score of 1

representing“strongly disagree” and the score of 10 representing “strongly agree”:

• The tines can break the soil.

• The tiller easily turns over the top layer of soil.

• The tiller tills without becoming clogged by debris.

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• The tiller is controllable by the user.

• The tilled soil is of an approbated consistency for gardening.

• The tiller successfully handles small debris without stopping.

• The tiller tills better then without the test box.

4) The result will be tabulated and then tests will be repeated for narrower range of

increments to more precisely find the idea ratio range.

Suggested Future Testing

The team suggests the following tests be performed by SPI in the future:

• Repeating the abovementioned tests in different soils typical of different regions of the

world, practically representative of the European countries in which the new tiller is to

be marketed.

• Tilling soil with different ratios and taking samples of the tilled earth to be closely

examined for consistency and permeability to differentiate soils.

• Tilling different parts of a garden with different ratios and then planting crops in the

different regions. Then examining the any difference in plant growth between regions.

Cost Analysis

The cost analysis for the design is divided into three sections. The first section highlights

the estimated costs of engineering research and development. Note that the course does not

additionally charge SPI for engineering services, but rather engineering services are provided

for academic credit. Engineering costs are included as a reference for comparison to other real

design costs. The second section highlights the bill of materials for the design, which includes

part names, amount of parts, cost per part, source of part, and the total costs. The third section

highlights parts that were purchase and in hand with real costs after shipping.

Estimated Engineering Cost

As an estimated cost of the real engineering associated with the research and

development of the project, the team presents the following approximation (Assuming a rate of

$50/hours of R&D):

16 weeks * 5 team members * 20 hours/week * $50/hours of R&D = $80,000

Bill of Materials for Test Gear Box

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Part Amount Cost/Part Source Total Cost

Mounted Bearing 2 $10.26 McMaster $20.52

Thrust Bearing 2 $16.50 McMaster $33.50

Nuts 2 $0.63 McMaster $5.00*

Washers 10 $0.60 McMaster $5.00*

Screws 20 $0.19 McMaster $5.00*

Aluminum Plates 1 $150.52 McMaster $150.52

Aluminum Square 3ft Stock 1 $35.70 McMaster $35.70

Spurs Gears (2 different sizes) 4 N/A SDP/SI $220.00

Threaded Shaft 2 $28.04 McMaster $56.08

Fallbrook NuVinci CVT 1 $409.00 Fallbrook Tech $409.00

Aluminum Round Stock 1 $20 McMaster $20

Aluminum Block 1 $40 McMaster $40

Machining Cost 4 $70/hr $280

Miscellaneous 20% $256.07

Total $1536.39

*Small costs such as nuts and bolts are rounded up to $5 because part availability is only in

quantity.

Purchased Parts

NuVinci CVT $284.00

Gears $145.00

Total $429.00

Proof of Concept

Figures ??? and ??? displays the completed design assembly and the assembly installed

onto the tiller. The system works by taking power form the motor on top of the experimental

setup and inputting it into the NuVinci CVT. The user varies the setup gear ratio at the gear

shifter mounted to the tiller handle in order to select the best ratio for tilling. The NuVinci

permits for variable gearing between the ratios of 1:0.5 to 1:1.75. Power is transferred from the

CVT to the output of the experimental setup, where it is ultimately transmitted to the current

transmission and the tines. The range of RPM and torque at the given gear ratio limits are from

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280 RPM, 18 lb-ft of torque to 80 RPM, 54 lb-ft of torque at the tines. It is assumed that power

output from the motor is assumed constant and equivalent to the power output of the tines.

Figure ???: Completed design assembly

Figure ???: Design Assembly Installed to Tiller

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The proof-of-concept testing is intended to demonstrate that the developed

continuously variable transmission will work as expected to aid Schiller-Pfeiffer in finding a

more optimal gear ratio for the European tiller model. Figure ??? displays in ground tiller

testing with the experimental setup.

Figure ???: In Ground Tiller Testing

Our first test will prove that the device is capable of engaging the tines and shifting

through the full range of gears while running at full power from the engine. This test proves

that the device is able to withstand the power load required during tilling at all possible RPM

and torques. The first trial of the experiment showed that a large percentage of the power

from the engine was dissipated in the transmission due to minor misalignments. After

correcting these, a second trial demonstrated that the tiller was indeed capable of tilling soil

across its gear range. The device was run continuously for 6 minutes without any failure or

visible wear. The shifting mechanism was also tested and proven to shift gears successfully in

both directions and while the tiller is on and off. It was also found that the weight of the added

test transmission impedes tilling, suggesting to the team that counter-balance measures will

have to be taken to provide accurate quantitative testing. The success of this test suggests that

the transmission can, in fact, prove to be a useful tool in simulating different gear ratios for the

tiller transmission.

A laboratory experiment in which we use a power drill capable of reaching 2400 rpm to

drive the transmission without any load on the tines will quantify the power losses in the

experimental transmission. Figure ??? displays the power drill bench testing setup.

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Figure ???: Power Drill Bench Test Setup

The data recorded from this test has been used to plot a chart of power losses versus

rpm. The resulting chart is then extrapolated to just over the full speed of 6400 rpm to give an

estimate of the inherent power losses at each rpm. (Need real data to compare here) The

losses, mainly due to friction in the NuVinci, gears, and bearings, are converted into

percentages of total engine power, which is based on a supplied power curve for the engine.

This data is vital to relate the performance the tiller when equipped with the experimental

transmission to the production tiller, which does not have to overcome the friction of the

experimental box.

The final experiment is another in-ground test during which we will use the device in a

counterbalanced system to till soil at all possible gear combinations. The team will collect data

from each trial to objectively and subjectively find the area of the possible gear spectrum where

the optimal ratio seems to lie. While the concept of the experimental continuously variable

transmission has already been proven, this test serves as the first experiment towards finding

an optimal gear ratio for the European tiller model. (More once testing is completed)

Experimental Results

Upon complete assembly of the experimental box testing was conducted to verify the

completion of the original design metrics. Specifically these metric tested are the range of

output speeds predicted was physicallyachieved

Experiments conducted on to validated and quantify the performance of the test figure.

To quantify the mechanical loses induced by the test box the following experiment was

performed using a corded power drill, an induction amp meter

Testing

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Bench top input/output test

A 2500rpm electric power drill was used to supply input to the box and two inferred

tachometers to measure input and output speeds. This test was performed in with the NuVinci

transmission in full under drive, 1:1, and full over drive. This test demonstrated that when

provided with an input the test box can successful deliver the target range of gear reduction. It

also demonstrates the physical ability of the fixture to handle high RPMwithout mechanical

failure. The results of the tests shown in the table below demonstrate the predicted range of

reduction is delivery by the fixture. Also the box was examined during and after the testing and

now signs of damage were seen.

ADD CHART OF INPUT OUTPUT RPM from drill)

Quantification of 10 discreet NuVinci ratios’ for the purpose of testing

While the strength of the NuVinci is its ability to provide and infinitely variable ratio with in its

allowable range in order to begin to quantify tiller performance certain discrete points of

reduction need to be measured. This was accomplished using by providing a constant input rpm

with the power drill and measuring the output rpm while the NuVinci was change in 10th

increments of it’s total range. The chart below shows these ranges as well as the reduced

reduction at each range

(Add chart-showingratios for 10 marks and reduced ration at each mark)

Tiller mounted unloaded input/output test

Once the ability of the fixture to successfully produce the desired range of output was

confirmed by the bench top test and 10 discrete positions on the NuVinci were mapped to

there appropriate ratio further testing was conduced using the fixture fully mounted to the

tiller and the gx25 engine. This was preformed using the engine tachometer was well as an

inferred tachometer to measure the tine output rpm. The engine was run a full throttle and the

tines were suspended above the ground to yield zero load output rpm values. At each of the 10

positions the output tine output RPM were tested. As can be seen below the achieve values of

compared to the predicted values. The results of this test show that the tiller performed

successfully up to the two highest rpm testing in which the load applied to the engine by the

box caused a drop in rpm and thus a drop in output

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(Add charts showing engine rpm, tine output rpm, and predicted tine output rpm.

Dynamic frictional loss tests

Form the results of the tiller mounted input/output test was determined that for the

tiller to be used to yield accurate data the dynamic frictional losses from the box needed to be

measured. This was done thought the use of a drill and an electrical current meter. The drill

was run at various constant rpm levels from 0 to 2500 and the current drawn be the drill at

each point was measured. The drill was then connected to the NuVinci mounted to the tiller

transmission and the test was preformed at high, 1:1; and low range. The test was then

repeated with only the current transmission. As see in the table below the torque required to

turn the NuVinci determined by this method.

Preliminary performance test

Because of the losses produced at by the test box, direct testing of high RPM ranges will not

provide accurate data because the output torque at the tines is less then the torque would be

with out the NuVinci. However the minimal using the know losses the minimal torque required

for acceptable can be determined. With the minimal torque know then the optimal gear for

tilling can be found

Fill in tests

Plan for future testing at by SPI

The testing has shown the experimental transmission performs up to the predicted

target values. SPI cannot continue the process of testing different soils to determine which ratio

by provide the most ideal tilling conditions. One this ratio has been determined it will be

implemented into there line through replacements of the current worm gear with a new worm

gear which provides the new ratio.

Path Forward

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Following the conclusion of Senior Design, SPI will continue more detailed performance

testing to determine the ideal gear ratio for the Mantis tiller. Once the ratio is determined,SPI

will order a new custom worm and/or worm gear with the ideal ratio and continue testing. By

2010,SPI will conclude testing and introduce the Mantis tiller with the new worm and/or worm

gear to the European market for sale.

When the team hands the project over to Schiller-Pfeiffer, the sponsor will continue to

run experiments using the test transmission in soil to pinpoint the gear ratio to be used on

European production models. The experiments will be carried out both in the testing garden

outdoors and in a testing box indoors at the sponsor’s location. Outdoor testing will mimic

actual use, where the tiller may encounter grass, twigs, stones, and other natural debris.

During indoor experiments, varying soil types will be used to attempt to replicate European

soils as well as many other common soils. To simulate the weight of a regular tiller, a

counterbalance system will be used for all tests with a weight equal to that of the experimental

transmission exerting an upward force on the tiller. In the same manner that existing tillers are

commonly tested already, trials will not exceed 10 minutes of constant operation.

When sufficient testing has been completed and overwhelmingly points to a specific

optimal gear ratio, the sponsor will consult a Microsoft Excel spreadsheet in which they can

enter the numerical reading from the gear shifter and receive the proper gear ratio for the

worm and spur gears in the standard transmission. This reading could potentially become

inaccurate if the cable operated shift mechanism loses tension in the operating cables. The

repair instructions can be found in the Transmission Manual, which is supplied to the

sponsor. To gain a failsafe measure of the true gear ratio, readings from the engine

tachometer can be compared to rpm readings found at the tines by an optical tachometer. This

must be done at the desired gear ratio without any load on the tines, to prevent any slippage

on the engine clutch, which would offset readings.

After positively identifying the desired gear ratio, a worm and spur gear must be

designed and built to order. The design and manufacture of these gears is outsourced to the

local company that produces the current gears used in the tiller transmission. To prevent

spending on an incorrect design, a small quantity of gears should first be ordered and tested in

the tiller. The gears produced in this step will be more expensive than the gears used in the

actual production run, because the quantity needed initially is much smaller than the

production quantity. When the gear is proven to work effectively to improve the operation of

the European tiller, and production quantities for the European model are established, a full

order can be placed with the gear vendor.

The assembly process for the European transmissions will be identical to the assembly

procedure for the existing transmission except that a worm and spur gear set with a different

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pitch will be used in place of the old gears. This helps to make the shift on the assembly line

smoother because the workers do not need to change their work habits and no other parts

need to be changed. The ability of the new design to replace exactly and only the two gears

saves a substantial amount of money because the cast-molded housing is no different than the

regular housing and no other hardware needs to be used. Part ordering for future production is

also simplified because all part ordering, except for the gears, concerns the total number of

tillers produced and requires no differentiation between the European and American models.

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Appendix A: Design Section Views

Design Section View – General

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Design Section View – Motor Input

Design Section View – Transmission Output

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Appendix B: Gantt Chart

Mantis Tiller TransmissionSchiller-Pfeiffer, Inc.

Project Lead: Lana Gendelman

Today's Date: 12/16/2008 (vertical red line)

Start Date: 9/3/2008

Tasks Start End Dur

atio

n (D

ays)

Phase 1 - Design Requirements and Project Scope 9/03/08 9/17/08 14Benchmarking 9/03/08 9/11/08 8Obtain Tiller and Transmissions 9/08/08 9/08/08 1Investigate CVT and Other Transmission Types 9/03/08 9/11/08 8Tiller Transmission Wants 9/08/08 9/11/08 3Tiller Transmission Metrics 9/08/08 9/11/08 3Project Subsystem Identification 9/11/08 9/17/08 6Transmission Ideas and Sketches for Reverse Gear 9/08/08 9/16/08 8Test Mantis Tiller 9/12/08 9/16/08 4Assemble Phase 1 Report and Prepare for Sponsor Meeting 9/16 9/11/08 9/15/08 4

Review Concepts and Sketches with Sponsor for Approval 9/16/08 9/16/08 1

Phase One Report Due 9/03/08 9/17/08 14Phase 1.5 - Concept Selection and Project Plan 9/18/08 9/23/08 5

Decision Made between Reverse Feature or Test Gear Box 9/18/08 9/18/08 1

Prepare Presentation Explaining Subsystem Choice 9/18/08 9/23/08 5

Continue to Benchmark CVTs for Test Gear Box 9/18/08 9/23/08 5

Meeting with Mrs. Gendelman and Phase 1.5 Report Presented 9/23/08 9/23/08 1Phase 2 - Concept Selection and Project Plan 9/23/08 10/07/08 14Decide between NuVinci Hub and 9 Speed Bike Hub 9/23/08 9/25/08 2Research Ways to Implement Bike Hub in Test Box 9/23/08 9/30/08 7

Create First Draft Drawing Package of Concept 9/25/08 9/30/08 5

Prepare Presentation for Ms. Gendelman 9/25/08 10/02/08 7

Research Parts and Lead Times for Bike Hub Concept 9/25/08 10/07/08 12Meeting with Ms. Gendelman at SPI to Review Test Box 10/07/08 10/07/08 1Phase 3 - Detailed Design 10/08/08 11/05/08 28Final Tiller Transmission Drawing Package 10/08/08 11/03/08 26Complete Power Point Presentation and Discuss with Mr. Cloud 10/20/08 10/24/08 4Formulate a Detailed Cost Analysis 10/20/08 11/03/08 15Order Necessary Parts 10/22/08 11/06/08 17Design a Testing Methodology 10/22/08 11/03/08 11Oral Presentation to Engineering Panel 10/30/08 10/30/08 1Meeting with Ms. Gendelman and Phase 3 Report Presented 11/11/08 11/11/08 1Phase 4 - Performance Validation 11/06/08 12/16/08 40First Draft of Poster 11/06/08 11/13/08 7Machining of Gears 11/06/08 11/06/08 1Machining of Frame Components and Brackets of Box 11/11/08 11/11/08 1Machining of Top & Bottom Plates for Shaft Locations 11/13/08 11/13/08 1Machining of Shafts 11/17/08 11/25/08 1Bearings Pressed and Parts Press Fit, Initial Assembly 11/17/08 12/02/08 15Carbon Fiber & Polycarbonate Side Panels Fabricated 11/20/08 12/02/08 12Final Assembly of Experimental Transmission Box 12/02/08 12/02/08 1Tiller Assembled with Experimental Box in Place 12/04/08 12/04/08 1Initial Testing with Experimental Transmission with Mr. Cloud 12/04/08 12/04/08 1Troubleshooting and Modifications to Transmission Box 12/04/08 12/06/08 2Second Round of Testing and Tiller Evaluation 12/08/08 12/08/08 1Final Poster Due 12/08/08 12/10/08 2Design Experiment Procedure for Tiller Testing at SPI 12/06/08 12/09/08 3Test Designed Tiller Experiment Before Final Testing at SPI 12/10/08 12/10/08 1Final In-Ground Testing at SPI & Reverse Feature Presentation 12/11/08 12/11/08 1Final Report and Presentation Due 12/09/08 12/13/08 4Final Presentation to Panel of Engineers 12/15/08 12/15/08 1Final Presentation Given to SPI 12/16/08 12/16/08 1

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Appendix C: Design Assembly

[Insert new assembly instructions]

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Appendix D: Drawing Package