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Sit’n’Stroll

-Vol.1-

May 5, 2013

Team: Selena Boltik

Yongbin Han

Stephanie Hornung

Scott Veenstra

Hanyi Xie

Ye Zhao

Advisors: Bobby Gold

Lilly Gold

Troy Pongratz

Sponsor: Lilly Gold LLC

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1. Team Contributions

Selena Boltik

1) Participated in the creation of initial Product Design Specification, Statement of Work and Work Breakdown

2) Participated in brainstorming, concept generation & selection 3) Created CAD models of front fork design iterations, seat base, and miscellaneous

parts for assemblies 4) Assisted in creation of other CAD models 5) Performed static structural ANSYS analyses on 4 front fork models and manual stress

analysis of rear axle 6) Created concept selection decision matrices for each sub-design 7) Created Part Drawings with Hanyi Xie 8) Created exploded sub-assembly drawings for manufacturing procedure section 9) Wrote Executive Summary, Design Description Chapter, Stress Analysis Evaluation

Results & Report sections, Concept Alternatives section & Concept Selection Section 10) Edited final draft with Stephanie Hornung

Yongbin Han

1)Participated in the creation of initial Product Design Specification, Statement of Work and Work Breakdown 2)Participated in brainstorming and concept generation and selection 3)Participated in creation of the Wheel CAD model and some other part drawings in

Creo 4)Assisted making slides for the Mid-project review and site visits. 5)Wrote the technical review section in problem definition 6)Wrote the evaluation plan and results outline for key design requirements 7)Came up with the evaluation plan and implemented the evaluation for the center of gravity 8)Wrote the evaluation results for weight measurement and center of gravity 9)Worked in the ME shop and participated in the mocking up prototypes 10)Generated the final presentation poster with Ye Zhao and Hanyi Xie

Hanyi Xie 1) Participated in writing PDS, Statement of Work and Work Breakdown 2) Wrote the Brief Summary, Block Diagram, Functional Description 3) Worked in the ME workshop to disassembling the stroller with Scott Veenstra and

Yongbin Han 4) Took measurements for the dimensions used in CAD models 5) Created the 6 inch Wheel CAD model and part drawings in Creo with Yongbin Han 6) Created the Brake attached on the wheel assembling CAD model 7) Created the Front_base_cover CAD model 8) Created the Rear_wheel_cover CAD model,assisted by Selena Boltik, suggested by

Yongbin Han 9) Created the Connecting_rod CAD model and part drawing 10) Helped to add title blocks and dimension annotations on other CAD model

drawings 11) Generated the final presentation poster with Ye Zhao and Yongbin Han 12) Took part in all team meetings and participated in discussions

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Stephanie Hornung 1) Led the creation of the Product Design Specification, Statement of Work, Work

Breakdown, and Gantt Chart 2) Led group meetings and meetings with our advisers 3) Handled communication between team, advisers, and course staff 4) Created documents for Site Visits and led Site Visit meetings 5) Led creation of presentation and slides for Mid-Project Review 6) Created team Google Site 7) Coordinated editing and submission of Design Report Assignments #2, #3, and #4 8) Created smooth transition test plan and rough road test plan with Ye Zhao 9) Performed the smooth transition test and rough road test with Ye Zhao 10) Participated in team brainstorming and collaborative idea generation 11) Researched and purchased components for prototype construction 12) Conducted user needs research 13) Conducted patent search 14) Wrote Detailed Description, Line Drawing, Additional Uses, Regulatory and Safety

Considerations, Annotated Bibliography, Patent Search, User Needs Research, Rough Road Performance Testing, Problem Scope, Technical Review, and Design Requirements sections.

15) Edited final drafts with Selena Boltik. Scott Veenstra

1) Participated in the creation of initial Product Design Specification, Statement of Work and Work Breakdown

2) Created the prototype Sit’n’Stroll for the design show 3) Came up with different design options and mocked up prototypes 4) Took measurements for the dimensions used in CAD models 5) Wrote the evaluation plan, weight results, manufacturing procedure, manufacturing

overview, bill of materials and environmental impact statements. 6) Helped with editing 7) Purchased material for prototyping 8) Made trips to Menards for materials 9) Took part in all team meetings and participated in discussions

Ye Zhao

1)Participated in the creation of initial Product Design Specification, Statement of Work and created Work Breakdown

2)Generated short term tasks for each group member and set up deadlines and in charge of making tasks division.

3)Communicated with Kunlun Plastic Company and get free samples for rear wheels. 4)Participated in brainstorming and concepts generation. 5)Created smooth transition test plan and rough road test plan with Stephanie Hornung. 6)Performed the smooth transition test and rough road test with Stephanie Hornung. 7)Performed fatigue ANSYS analysis on 4 front fork models. 8)Generated the final presentation poster with Yongbin Han. 9)Assisted making slides for the Mid-project review and site visits. 10) Wrote Design Requirements, Strength and Weakness section, Fatigue Analysis

Evaluation and Report sections, Smooth Transition Report Sections.

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2. Executive Summary

The Sit’n’Stroll is a 5-in-1 child safety device that can be used as a stroller, a rear-facing car seat, a forward-facing car seat, a booster seat and a FAA-certified flight seat which is currently available with 4-inch wheels. The current model’s wheels have a tendency to break and do not travel well over rough surfaces such as brick or gravel paths. The goal of this project was to design an ‘all-terrain’ Sit’n’Stroll with larger 6-inch wheels that will retract into the existing seat frame, reusing as many of the existing parts as possible. This was accomplished by redesigning the front and rear retraction mechanisms and determining what modifications would need to be made to the surrounding plastic base parts. The design requirements were ease of mechanism use, the static strength of the design, the fatigue life of the design, the change in center of gravity, overall weight, and improved rough surface performance. Therefore, a new front fork was designed to allow the front wheels to retract and be used in steering without interfering with the surrounding components. The angled rear link and double rear wheels were replaced with a straight link and single wheels utilizing the original brake components. The seat base was made taller with notches added to the front wheel opening and with no side shields, the rear wheel covers were elongated and had side shields added, and the front wheel cover flap was elongated to allow it to operate during wheel retraction.

Figure 1: Redesigned Sit’n’Stroll with 6-inch wheels

The ease of mechanism use requirement was tested by timing a number of people as

they retracted and extended the wheels. The new design met the requirement as the average

person was able to retract or extend the wheels in under 5 seconds. The strength, both static

and fatigue, of the new front fork was analyzed through ANSYS simulations and was found to

have similar stress and fatigue performance to the original 4-inch model. The rear mechanism

was analyzed under static loading using manual calculations and under fatigue loading using

ANSYS. The design met the requirements but has a weak point in the rear wheel axle that

should be analyzed further before continuing with the design. The change in center of gravity

and weight were measured experimentally and were found to be minimal. The rough road

performance was tested using an accelerometer in the stroller to measure the g-forces felt by a

stroller occupant in the original and redesigned strollers for several surfaces. It was found that

the average g-force felt in the redesigned stroller was less than in the original, though the

change was not significant. Overall, the final solution and prototype met the initial requirements.

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Volume I Table of Contents 1 Team Contributions...............................................................................................................2 2 Executive Summary..............................................................................................................4 3 Problem Definition.................................................................................................................6 3.1 Problem Scope.......................................................................................................6 3.2 Technical Review...................................................................................................6 3.3 Design Requirements.............................................................................................9 4 Design Description...............................................................................................................10 4.1 Summary of the Design.........................................................................................10 4.2 Detailed Description..............................................................................................10 4.2.1 Functional Block Diagram.......................................................................10 4.2.2 Functional Description............................................................................11 4.2.3 Overview Drawing...................................................................................13 4.3 Additional Uses......................................................................................................13 5 Evaluation.............................................................................................................................14 5.1 Evaluation Plan......................................................................................................14 5.2 Evaluation Results.................................................................................................15 5.3 Discussion..............................................................................................................20 5.3.1 Strengths and Weaknesses.....................................................................20 5.3.2 Next Steps...............................................................................................21

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3. Problem Definition

3.1 Problem Scope The Sit’n’Stroll is a 5-in-1 combination car seat/stroller designed to make travel easier for parents with young children. It is unique because the stroller wheels retract inside to transform it into a car seat, so parents don’t have to deal with carrying multiple pieces of equipment. The current Sit’n’Stroll has 4-inch wheels, which are compact, allowing them to easily retract into the base. These wheels do not perform well on rough road conditions, so our client would like us to install 6-inch wheels. In order to keep the stroller’s 5-in-1 function, the primary task is to redesign the mechanism so that the 6-inch wheels retract into the car seat frame while modifying the surrounding stroller components as little as possible. 3.2 Technical Review

The car seat and stroller market offers a wide variety of travel systems for parents with young children. Companies such as Graco [1] make strollers and car seats that fit together, but instead of the wheels retracting into the chair, the car seat is removed from a frame with wheels, and the wheeled component must be stored separately. For example, the Graco Snugrider series includes a baby carrier, car seat frame, and stroller frame with wheels. The carrier can fit into either frame.

Figure 2. Graco Snugrider Elite baby carrier and stroller frame [1]

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Having the seat and frame separate can be especially difficult when traveling by air, as the frame must be checked before flying. Another self-contained combination car seat and stroller on the market is called the Strolex 2 [2]. The wheels on this stroller also retract up into the base. The main difference is that this stroller is made for infants 5-22 lbs while the Sit’n’Stroll is made for children 5-40 lbs.

Figure 3. The Strolex 2 [2]

Other products that perform similar functions are a folding hand truck [3] that can be

attached to a car seat for transport through the airport, and a strap that hooks a car seat onto a rolling luggage bag [4]. These two designs are much more unstable and the child has to be pulled behind the parent as he makes his way through the airport.

Figure 4. Brica Roll’n Go Car Seat Transporter [3]

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Figure 5. Go-Go Babyz Travelmate Car Seat Luggage Strap [4]

The Sit’n’Stroll retracts its wheels into the chassis using two separated four bar slider-

crank mechanisms, with the wheels on the ends of the coupler links (see Figure 1). One mechanism is used to retract the front wheels and the other one is used to retract the rear wheels. The two mechanisms share a common slider, which is connected to the handle. Therefore, the four wheels are retracted and extended simultaneously by moving the handle back and forth. [5]

Figure 6. Side view of the wheel retraction mechanism [5]

Figure 7. Back view of wheel retraction mechanism [5]

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When the stroller is lifted before operating the wheel retraction mechanism, an angled washer on the top of each wheel fork lines up the front wheels so they fit inside the plastic base. As the wheels are pulled inside, the flap on the wheel covers catches and pulls them to a closed position. The Sit’n’Stroll is unique because the wheels are covered when retracted, protecting the seat of the car from dirt, water, or snow. The four-bar linkage, described above, is also novel, because it allows all four wheels to be retracted with one motion.

The Sit’n’Stroll is protected under patent number 5,104,134, “Child’s combination car seat and stroller,” and our updated design will also be protected [5]. During our patent search, we found many other designs for stroller/car seat combinations, but none of them are on the market (other than the Strolex 2), giving the Sit’n’Stroll a competitive advantage. 3.3 Design Requirements 1. Installing 6-inch wheels that are easily retracted

As directed by our client, the new wheels must be 6 inches in diameter and must fit into the frame of the car seat when retracted. The wheels must be able to be extended and retracted in under 10 seconds.

2. Change in footprint and center of gravity when folded in the car seat position must be minimal The footprint of the current design is the maximum size that will fit down the aisle and into the seat of an aircraft. Therefore, the width and length of the base cannot exceed the current dimensions of 19 inches. For safety considerations, the change in the center gravity must be minimized, otherwise the stroller will be more likely to tip over.

3. Weight of the stroller must not exceed 20 lbs. It is intended for the parent to be able to convert the Sit’n’Stroll from car seat to stroller form and back without removing the child from the seat. In our user needs research, we learned that many parents have trouble lifting the stroller and operating the wheel retraction mechanism as their child grows. The weight of the stroller must be limited so that a parent can lift it up with a 40 pound child sitting in it without a risk of injury. The current design weighs 16.5 pounds. See Volume II for more information about our user needs research.

4. The useful life of the Sit’n’Stroll must be at least 6 years Per the safety manual, the Sit’n’Stroll has an ‘expiration date’ of six years from the date of manufacture. In order to achieve this, the new design must have similar fatigue test performance as the original 4-inch wheel design.

5. The new design should be at least as strong as the original design. To ensure the new design is able to safetly support the weight of a child, it should have similar static loading performance to the original 4-in wheel design. Therefore, the new design must have comparable or better stress test results than the original one.

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4. Design Description

4.1 Summary of the Design

This project is a re-design of the Sit’n’Stroll to have retracting 6-inch wheels in order to create a more robust stroller which is able to ride smoothly over rough terrain. The re-design focused on three main areas: front wheel mechanism, back wheel mechanism and the plastic base. The front wheels were mounted using a fork design based on those used in the original Sit’n’Stroll but modified to provide additional strength and to accommodate the larger wheel. The rear wheel pairs were replaced by single 6-inch wheels and the connecting link was modified to prevent part interference while in motion. The rear wheel system uses the same brake system as the original. The seat base and rear wheel covers were modified to cover the larger wheels when retracted and to have larger openings for the wheels to retract through.

The final design will make the stroller easier to control on rough surfaces when compared to the original stroller. Also, the overall width and length were not changed so that the design meets the FAA requirements for in-flight use.This will allow a traveling family to exit the airplane, navigate through the crowded airport, take a taxi and walk through a park over brick and gravel paths without having to bring multiple pieces of equipment and transfer a child between them. 4.2 Detailed Description 4.2.1 Block Diagram

Figure 8. Functional Block Diagram

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4.2.2 Functional Description Retracting 6-inch wheels for rough terrain

The purpose of using the 6-inch wheels is to make the stroller travel smoothly, even over rough surfaces such as cobblestone and gravel. The original wheels were small so that they retracted easily into the base and increased maneuverability in crowded areas such as airports and the aisle of an airplane. The new larger wheels will help to reduce shock and the tendency to crack when on rough terrain, but must still retract into the base without increasing either the length or width of the Sit’n’Stroll when in car seat form. Also, the large wheels change the center of gravity of the Sit’n’Stroll in both stroller and car seat form, potentially increasing the likelihood of the system tipping over during use. Front wheels easily retracted

For the front wheels, we chose to use a fork design based on the geometry of the fork used in the original Sit’n’Stroll so that it would be compatible with the existing structure. The new fork was compatible with the angled washer and screw used to align the wheel during retraction, shown in the green box in Figure 9, and prevented the wheel from contacting the bar, shown in the red shape, at any point in its range of motion. This allowed these 3 components to be reused from the original design, reducing the number of parts that must be purchased or manufactured specifically for this design which reduced cost.

Figure 9. Left:Original 4-inch Front wheel Mechanism, Right: Redesigned mechanism

This design also had the minimum vertical distance between the wheel center and the

connection to the stroller which reduced the amount of additional space needed to retract the wheel and fork into the base. This effectively minimized the height added to the base to accommodate the larger wheels, resulting in a smaller change in the center of gravity location. However, as the new fork must be wider than the original to fit around the larger wheel but must connect to the same bar as the original, there may be a point of weakness at the shaft of the fork. Back wheels easily retracted

For the back wheel mechanism, we chose to replace the rear wheel pairs with single wheels to prevent interference with the seat base during wheel retraction. The rear wheel used in this prototype included a brake structure compatible with the original brake mechanism,

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reducing the number of parts that were purchased or made specifically for this design. However, reducing the number of wheels also reduced the number of load supports which could result in increased stresses in the wheel axle and connecting link. Finally, the connecting link which contained a bend in the original Sit’n’Stroll was replaced with a straight link as shown in Figure 10. This provides the additional space needed to accommodate the larger wheel without exceeding the length and width requirements.

Figure 10. Left:Original 4-inch rear wheel Mechanism, Right: Redesigned mechanism

Plastic base pieces cover and support retracted wheels

In order for the complete mechanism to retract smoothly and have the wheels covered when in stroller form, the plastic base components were modified as shown in Figure 11. The opening in the seat base was enlarged to allow the front wheels and forks to retract by removing the material shown in green. As the rear wheels in this design are wider than in the original the side shields, shown in red, were removed. Finally, the base was made 2.5 inches taller to accommodate the larger wheel radius and forks.

Figure 11. Sit’n’Stroll base modifications

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The rear wheel covers were made longer, wider and with a larger radius of curvature to compensate for the changes made to the mechanism. To ensure that the wheels were inaccessible to the child when the Sit’n’Stroll is in car seat form, side shields were added to the wheel covers as well. To ensure that the same part could be used on both sides of the stroller, the side shields were added to both sides of the wheel cover, resulting in a symmetric part. This reduces the cost of manufacturing as the right and left rear wheel covers can be made using the same mold.

On the front wheel cover, the flap used to close the cover when the wheel retracts, shown in blue in Figure 11, was extended 1 inch to ensure that the cover would close as expected with the new mechanism motion.

4.2.3 Line Drawing

Figure 12. Annotated line drawing of Sit’n’Stroll

4.3 Additional Uses As mentioned previously, the Sit’n’Stroll is protected under patent number 5,104,134, so our design improvements cannot be used in any other child safety device application. The design improvements that our team has created are very specific to the Sit’n’Stroll, but portions can be adapted for other uses. For example, a similar mechanism could be used for other travel solutions, such as a golf bag or hand truck, or for creating portable fixtures, such as a kitchen island or medical stretcher.

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5. Evaluation

5.1 Evaluation Plan

The new design was evaluated to ensure it met six key design requirements. The six key design requirements for this project were:

1) Integrating the 6-inch wheels into the stroller and ensuring the smooth transition between car seat form and stroller form 2) Ensuring the new design is at least as strong as the original one 3) Limiting the weight of the Sit’n’Stroll to a maximum of 20 lbs 4) Minimizing the change of center of gravity between the old and new designs 5) Ensuring useful life of at least 6 years 6) Verifying that the addition of 6-inch wheels gives the child a smoother ride on bumpy surfaces

All the key design requirements need to be tested to ensure the final prototype is a fully functional Sit’n’Stroll with 6-inch wheels. The testing plan for each of these design requirements is described as follows: 1) Integrating the 6-inch wheels into the stroller and ensuring the smooth transition between car seat form and stroller form

The user feedback is the most important criteria to judge the ease of use of the new design. To determine if the Sit’n’Stroll can transition smoothly between forms, we will have classmates read through instructions and attempt to use the wheel retraction mechanism. We will see if the transition can be completed in 10 seconds or less on the first attempt. The results and feedback from our classmates will be collected and analyzed to evaluate this design requirement. 2) New design will be at least as strong as the original design

In this test, we will employ ANSYS modeling to simulate the stress in the new front forks, and the results will be compared with the stress analysis results from the original design. The stress results for the 6-inch forks should be equal to or less than those obtained for the 4-inch forks. As the final wheel retraction mechanism for the front wheels use the same components, other than the fork, as the original design, these components do not require additional analysis. It was decided to use an ANSYS model rather than a physical test as the 6-inch fork prototype and the original 4-inch fork were created using different methods and materials. The prototype was created by 3D printing the model with ABS plastic while the original was injection molded from polypropylene. Analyzing the models in ANSYS allows us to eliminate variations due to the material and manufacturing process methods as well as see where the worst stresses occur. It was determined that of the changes to the rear wheel mechanism, the change from a double rear wheel to a single wheel would have the largest effect on the stresses that occur in the mechanism during use as a stroller. Therefore, to analyze performance of the new single rear wheel design in comparison to the previous double wheel design, the maximum stresses in the wheel axle for each design will be calculated manually and compared. 3) Limiting the weight of whole Sit’n’Stroll to a maximum of 20 lbs.

The total weight of the new Sit’n’Stroll is another crucial factor that will affect the stroller’s portability and user experience. Our design requirement for weight is a maximum of 20 lbs. To make sure that we meet the weight requirement the Sit’n’Stroll will be weighed after the prototype is complete. As long as the prototype is under the limit, the commercialized model

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should be as well. The prototype is expected to weigh more than the final product as it will have added weight due to fasteners and extra material that will not be present in the final product. 4) Minimizing the change of center of gravity.

The location of center of gravity is closely related to performance in the crash test, and has an impact on the customer’s daily use. Given that we do not have access to the full CAD model of the original Sit’n’Stroll, it is impossible for us to compute and compare the center of gravity by using software. We designed an experiment to compare the center of gravity of new design with that of original design to evaluate whether the change is acceptable. In this test, we will determine the locations of center of gravity experimentally by the method of suspension. Since we are unable to simulate the impact situation, the performance of Sit’n’Stroll in car seat form when the car is crashed is hard to evaluated. In order to get a sense of how the change of center of gravity in our new design will affect the performance in crash test, in our evaluation we will keep applying a node force on Sit’n’Stroll in order to rotate it until the Sit’n’Stroll is tipped over. With the determined center of gravity, we are able to calculate the minimum node force needed to rotate the stroller at different rotation angles. The plot of minimum node force vs. rotation angle will be presented. The test will be implemented on both new design and original design and the results will be compared. 5) Ensuring an useful life of at least 6 years

In order to simulate the long term use of the Sit’n’Stroll, we will perform the fatigue test on the front wheel forks using ANSYS. The Sit’n’Stroll user manual states that the stroller has a maximum useful life of 6 years, and the test results from ANSYS will indicate whether the new design will fulfill this key design requirement. 6) Verifying that the addition of 6-inch wheels gives the child a smoother ride on bumpy surfaces

The goal of adding 6-inch wheels is to provide a smoother ride for the child on bumpy surfaces, such as cobblestone or gravel. To measure the bumpiness of the ride for the child, we will use an accelerometer application for Android smartphones. The app measures the G-forces experienced in the x-, y-, and z-directions. By placing the phone in the seat, we can measure the z-direction G-forces, which translate to the bumps felt by the child. 5.2 Evaluation Results 1) Smooth transition between car seat form and stroller form

The original Sit’n’Stroll can be transformed from a car seat to a stroller in a matter of seconds. Being able to transform the stroller easily and quickly is what makes this product so valuable for consumers. The original Sit’n’Stroll is very easy to transform and has set the standard for the new six inch model. The new 6-inch model needed to be able to be transformed in 10 seconds or less for first time users.

We set up a time study to see how long it took a person to transform the 6-inch model of the Sit’n’Stroll. Each of the participants were first given a set of written instruction and a video demonstration on how to transform the stroller. After each person felt they knew how to work the stroller, they were timed to see how long it took to complete the transformation. The time for extending and retracting the stroller wheels were recorded separately for each person. 10 males and 10 females between 20 and 30 years old participated in this experiment. The experiment results are shown in Table 1.

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Table 1. Sit’n’Stroll Transition Time

Male Female

Extension

(s)

Retraction

(s)

Extension

(s)

Retraction

(s)

Original

Design 2.39 2.35 3.68 3.86

New Design 2.47 2.52 3.63 4.03

As shown in the table, the new design has a slight difference on the time spending to

finish the transformation, but all of the times were still much faster than the 10 second requirement. Some of this difference may be due to the quality of the prototype mechanism when compared to the production quality original Sit’n’Stroll. Also, the retraction time tends to be longer than the extension time for both designs. That is likely because the stroller needs to be hold in a nearly perfect vertical orientation in order for the front wheel to retract smoothly.Overall, the new design performed well in this transition test. The results are surpassed our expectations, as all transitions could be done within 5 seconds. 2) New design will be at least as strong as the original design

In order to test the strength of the final front fork design, a three dimensional model was created and analyzed using a static structural test in ANSYS. The results obtained from this test were compared to those obtained for the 4-inch model to ensure that the stresses that occur in the new 6-inch fork are comparable to or less than those that occur in the original 4-inch fork. The forks were modeled using the material properties of polypropylene and tested under three loading conditions. In the first test, only the weight of the stroller and child was applied to the fork. The second test included a horizontal force applied at the shaft of the fork, as well as the weight applied in the first test, to simulate the user accelerating the stroller by pushing on the handle. The final test included a horizontal force applied at the wheel axle, as well as the weight applied in the first test, to simulate the effect of hitting an obstacle such as a curb. The test calculated the equivalent (von-Mises) stresses throughout the part. The figures, meshes and three dimensional stress results are shown in the supporting documentation in Volume II. The maximum equivalent stresses from these tests are summarized in Table 2.

Table 2. Front fork ANSYS stress test results

Model Original 4-inch Fork Max

Stress 6-inch Front Fork

Max Stress

Test 1: Weight only 3236 psi 1487 psi

Test 2: Weight and accelerating

2066 psi 1487 psi

Test 3: Weight and wheel impact

3625 psi 1647 psi

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As the maximum von-Mises stresses that occur in the new 6-inch fork are significantly lower than those that occur in the 4-inch model, an injection molded version of the new part would perform at least as well as the original.

Finally, to determine the effect of changing from a double rear wheel to a single wheel, the stresses in the wheel axle were compared through hand calculations as shown in Volume II. The double wheel was modeled as a cylindrical beam with fixed supports on each end with the load distributed over the axle. The single wheel was modeled as a cantilever beam with the same load. The calculated results showed that the stress in the axle of the single wheel design could be as much as six times higher than that of the double wheel design, which could have damaging long term effects. 3) Limiting the weight of whole Sit’n’Stroll to a maximum of 20 lbs

The ability to transform without having to remove the child from the seat, is one of the primary competitive advantages of the original Sit’n’Stroll. In order to maintain this function. the weight of the seat should not exceed 20 lbs. To check this the prototype model will be weighed to get an idea of how much the final product would weigh.

The stroller was brought to the rec center and placed on the scale in the locker room. To ensure that the scale was accurate, a weight from the weight room was placed on the scale to verify that the stated weight and the measured weight matched. To ensure the accuracy of the stroller weight measurement, the stroller was weighed several times to determine the average reading. The prototype stroller weighed 17 lbs. This means that even with the added height of the seat base and the larger wheels that the stroller will meet the weight requirement of not exceeding 20 lbs. It is safe to say that the commercialized model will meet the requirement as well as it should weigh less than the prototype. The prototype has added weight due to the additional rivets and material which will not be present in the final model. 4) Minimizing the change of center of gravity

As the complete CAD file is not available, we must determine the location of center of gravity through the suspension method. The Sit’n’Stroll was suspended from different points and the center of gravity was determined by finding the intersection of different suspension lines. The suspension method is described by Figure 24 in Volume II. After employing the suspension method for both the new and original designs, the location of the center of gravity was located and its coordinates in x-y plane are shown below. The results show that the center of gravity was raised by 0.62 inch in our prototype for new design.

Original Design: (-4.00, 8.13) in

New Design: (-4.75,8.75) in

With the exact location of center of gravity determined, the minimum node force required to rotate the Sit’n’Stroll at each rotation angle could be calculated. The plots of the minimum node force as a function of rotation angle for both the new and original design are shown below.

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Figure 13. Plot of Minimum Node Force vs. Rotation Angle

From the figure, we can see that the change of center of gravity is acceptable in our new design. The plot of minimum force vs. rotation angle shows that when a force is applied at the same spot, our prototype for new design is harder to rotate than the original design, which is a desired results. This results is caused by a combination of the forward movement of center of gravity, the raised height and the increased mass of our prototype. 5) Ensuring an useful life of at least 6 years

In order to make sure the new design could endure a useful life of at least 6 years, fatigue analysis was performed using ANSYS on the front wheel fork. Fatigue analysis was also performed on the equivalent components from the original Sit’n’Stroll, so the results could be compared.

Two tests were performed to analyze the front wheel fork. The first test simulated a horizontal force and a vertical force applied at the shaft of the fork. The second test simulated not only the horizontal force and the vertical force from the first test, but also an impact force on the wheel. The two tests were designed to correspond to real life situations: using the stroller on a smooth road, or using it under a rough road condition. When performing the fatigue analysis, the material of the fork was modeled as polypropylene, and the S-N curve was approximated according to these properties. Detailed ANSYS setup is shown in Volume II.

By comparing the testing results of the smooth road condition tests, the new front fork design has approximately 23657 cycles which is slightly lower than the design of the original front fork which had a minimum fatigue life of 24027 cycles. Although, the new front fork design has a slightly lower fatigue life it is close enough to indicate that the two designs will have a similar useful life.

For the rough road testing results, a 3330 cycle minimum fatigue life was obtained for the original design and a 2380.5 cycle minimum fatigue life was obtained for the new front wheel fork. Although, the number of cycles seems really low in both cases, the stroller won’t be always

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uses under the rough road condition. While the new fork design has a shorter fatigue life than the original design, it is close enough that it is likely that the useful life of the designs will be comparable.

Since the tests were just simulations, the results might not be accurate enough to directly conclude that the fork will have a 6 year useful life. However, combining the two testing results, the newly designed front wheel fork should have a similar durability to the original design. In order to make sure the fork will have a useful life of at least 6 years, further tests should be performed before putting this fork into production. 6) Verifying that the addition of 6-inch wheels gives the child a smoother ride on bumpy surfaces

Using the Physics Toolbar Accelerometer app for Android, we measured the g-forces experienced in the stroller seat as it is rolled over varying surfaces. We compared the 4-inch and 6-inch models on the smooth tile in the Mechanical Engineering building hallways and on the brick portion of Church Street. In Figure 14, the results of the indoor test for the 4-inch and 6-inch models are plotted. It is very clear from this plot that the 6-inch wheels provide a smoother ride for the child.

Figure 14. G-force vs. Time for 4-inch and 6-inch models indoors

Similarly, in Figure 15, the results of the outdoor test are plotted. The results are not as

clear as for the indoor test, but the range of g-forces experienced in the 6-inch model is slightly smaller. Also, due to the unevenness of the chipped bricks, it is possible that one stroller went over a bumpier patch of road than the other.

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Figure 15. G-force vs. Time for 4-inch and 6-inch models outdoors

In addition to the data collected, we physically noticed a difference between the strollers

as we pushed them across the smooth and bumpy surfaces. The model with 6-inch wheels transmitted less bumps through the handle of the Sit’n’Stroll, giving the sensation of higher quality. Although this is not a scientific method of testing how the seat feels for the child, most parents judge products based on their perception of how it feels. Both our empirical testing and our data support the goal of making the ride less bumpy for children. 5.3 Discussion 5.3.1 Strengths and Weaknesses

The re-designed 6-inch wheel Sit’N’Stroll appears to have a better performance than the original Sit’n’Stroll under rough road condition. The new design proved to be more reliable when under the static loading conditions as the front wheel fork had overall lower stresses than the original design. However, the weak point on the new designed fork appears to be the shaft which might cause problems should the impact forces exceed those predicted. Furthermore, the rear wheel design has higher stresses in the axle of the single wheel design compared to the double wheel design. This could cause problems in fatigue and should be analyzed further under more realistic loading conditions. From the useful life perspective, the new design should have similar fatigue life to the original design, which matches the design requirements. The transition of the stroller could be done within a reasonable time, but the front fork occasionally caught on the seat base when retracting and extending the stroller wheels if the stroller was not in a perfectly vertical position. This problem occurred in both the new design and the original design.

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5.3.2 Next Steps Our prototype proves that it is possible to put 6-inch wheels on a Sit’n’Stroll, and that it does make a difference in the bumpiness of the ride for the child. However, the project is far from production-ready. First, we recommend market research to determine whether the 6-inch wheels make the product more appealing for parents, and how much they would be willing to pay for it. If LillyGold determines a business need to move forward with our design, we also would recommend having a professional engineer investigate the strength of the rear wheel axle under realistic loading conditions. The analysis that we performed used a highly simplified model, and further examination should take place before the new design is put into production. The plastic seat base, front wheel cover and rear wheel cover should be analyzed to provide sufficient structural support and for manufacturability through injection molding. The last step before production is to take the mock-up to the NHTSA crash-testing facility to get child safety certification for the design. Since the child will be sitting higher, the center of mass with the child in the seat will be different. Passing the crash-test is mandatory for all child vehicle safety seats, so this test should be performed before the new design can be produced.