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In order to identify modifications to the design of the stroller to meet a need better, an in-depth study of a baby stroller made by Greco was conducted. During this study, we looked into the major stakeholders and their needs, the use of the product as well as how the system functions, individual parts of the stroller assembly, investigated the manufacturing and assembly process, the failure modes, the environmental impact of the product, as well as a mechanical analysis of the forces the front tray can have before breakage.
We concluded that the stroller was very well designed to suit the manufacturing and assembly process within the manufacturing facility as well as the assembly process for the customer by incorporating necessary tolerances into the design and choosing the right material for its purposes. Furthermore, the needs of the major stakeholders were met by the stroller. It was found that the most environmental impact was contributed during the manufacturing phase of the products life cycle and the use phase was negligible because of how the product will be used during that phase. Overall, the stroller design was well thought through with its purpose and intended stakeholders kept in mind during its design. Nevertheless, there will be potential improvements to resolve some failure modes, to make the product life cycle more environmentally friendly, to improve the manufacturing and assembly process, to make the product easier to use, or to add additional functions.
The user study showed many different ways in which the stroller was utilized to reach specific function goals, some of which were not intended. This study helped create the Failure Modes and Effects Analysis (FMEA) which which addresses the misuse and potential failure modes found in the user study.
The FMEA revealed that we could minimize failures by changing the connection between the fabric and the frame, as well as increasing the structural stability of attached plastic trays. This will increase the safety of the stroller, as well as the user experience. The mechanical analysis confirmed these results by demonstrating that under maximum loading conditions, the use of the tray as a structural component causes up to 17cm of deflection, which does not instill the user that this is safe. Through future changes to this part, we will reduce that deflection.
Bill of Materials
After disassembling the stroller and breaking it down into components by material, we have concluded that the stroller has been efficiently streamlined for ease of manufacture. The three main materials that the stroller utilizes are nylon (woven, as fabric), plastic (injection molded, for plastic parts), and steel (tubing, for frame). The stroller manufacturer also utilized some steel for fasteners such as screws and rivets, but these are insignificant (on a mass percentage basis) compared with the larger parts of the stroller. Figure BoM-2, below, shows the mass percentage breakdown of each material in the overall stroller assembly.
(1): Rear axle subassembly was left assembled due to use of rivets by manufacturer. It was deemed infeasible by group and professor to deconstruct this subassembly at this time, although future considerations will be given to locking mechanism should group decide that it is a point of interest.
(2): Frame subassembly was left assembled for same reason as rear axle subassembly (1).
(3): Seat belt subassembly was left assembled due to use of stitching by manufacturer. It was deemed infeasible by group and professor to deconstruct this subassembly at this time, although future considerations will be given to seat belt system should group decide that it is a point of interest.
Figure BoM-1: Axle sub-assembly with parts labeled. This sub-assembly was not disassembled further.
Figure BoM-2: Baby stroller with main components labeled.
Figure BoM-3: Mass Breakdown of Stroller Components by Material
This study looks at the different functions of the stroller and how the user achieves their goals. It will be broken down into different functions, and give a step by step analysis of different ways the user achieves the goals.
Putting a baby in the stroller/Removing baby from stroller
In order to insert a child into the stroller, the user first (1) ensures the stroller is stationary. Then the user can put the child by (A.2) unlatching the harness (S-3), then (A.3) putting the child into the seat and then (A.4) latching the harness around the child.
Alternatively the user can insert a car set into the stroller. This requires the user to (B.2) put the car seat into the stroller seat and then (B.3) latch the car seat into the stroller by wrapping the car seat latch around the car seat, and fastening it.
In order to remove the baby, the step are simply performed in reverse, where you first (5) unlatch the harness or car seat latch and then (6) remove the child from the seat.
Folding the stroller
In order to fold the stroller, the user must first (1) ensure that the stroller is empty. Once that is done, the user can (A.2) pull on the white push knob (P-9) on the handle assembly. The user then (A.3) twists the handle assembly forward approximately 180 degrees. This unlocks the middle bar by separating the locking mechanism. Then the user (A.4) pushes the handle bar down towards the floor. This often causes the stroller to fold at the locking mechanism and the wheels to slide towards the user quickly, so to combat this affect, the user can either (A.5.1) put their foot on the brake bar (S-1) or (A.5.2) engage the braking mechanism before using this function. Alternatively, the user can (B.2) lift up on the handles of the locking mechanism (P-9) in order to disengage the lock. This method brought about a problem in that these handles are within reach of the child who is in the stroller, and could be used by the child to their danger. We will discuss this point later in the report.
Moving the baby around
In order to move the baby around while in the stroller the user has many options. To push the stroller forward, the user simply (1) pushes forward on the handles.
In order to move the stroller around a corner, the user can either (2) supply a moment to the handle or (3) push down on the handle, lifting the front wheels of the stroller off of the ground, and then supply a moment, allowing the stroller to make sharper or more maneuverable turns.
A note is that the user should not push the stroller anywhere except the handle in order to control the stroller, nor should the user push the handle with excessive force or speed, as doing so results in the stroller becoming unstable and potentially tipping over.
Keeping the stroller stationary
In order to engage the locks of the stroller, the user (1) presses down on either the left or the right break (Part Number). Most users push this break with their foot, as it located on the break bar/rear wheel assembly.
How this assembly works is that a solid bar engages with a gear located on the wheel, which keep the wheel from rotating.
Also in order to lock the front wheels from turning, the user (1) pushes the front wheel locks (P-5) upwards until it clicks. This stops the front wheels from turning, but not rotating.
Transporting stroller without baby
In order to transport the stroller without a child in it, the user first (1) folds the stroller. Then the user can (A1) carry it by one of the structural bars (Part Number), (B1) carry it by the plastic tray (P-3), or (C1) by the handle (P-9). Each method has been observed in user studies.
There are four major stakeholders in the baby stroller product: baby, parents, manufacturers, and stores selling the baby stroller. The major needs for the users (baby and parents) are the safety and comfort of the child. The major need for the manufacturers and stores are the cost of the baby stroller and its ability to sell. Listed below are the major stakeholders and their respective needs.
- a. Safety - The baby's first need is to be safe inside the stroller. This means that the stroller should be able to hold the baby securely with minimal chances of the baby falling out and/or hurting itself. The stroller also needs to be sturdy and stable to avoid tipping or otherwise causing an unsafe situation.
- b. Comfort - Another need for the baby is to be comfortable inside the stroller. This is important because the baby might be inside the stroller for extended periods of time and will be unhappy/cranky if it was uncomfortable. Bumpy rides and uncomfortable seats/restraints will upset the baby. Another consideration is temperature and shielding from the elements.
- c. Pleasure‐ The baby is most happy when enjoying a smooth ride, a nice view (why many strollers have front‐facing baby seats), and also likes to be engaged with activities or other stimulating forms of fun.
- d. Room to grow‐ During the first few years of the baby’s life, the baby grows very quickly, in terms of size and developmental level. A stroller that can adapt to its growth mentally and physically is desirable.
- a. Safety of the child - This is the most important need of the parents because the baby is very important to them and will not utilize a product that they thought would cause harm to their baby. In terms of baby care, safety takes on many forms. Seat belts and wheel locks are standard features on strollers, and so parents come to expect those features. From research we’ve done, parents “assume” that all strollers are safe, and that the difference in safety between strollers is minimal. Therefore, there is a large disconnect between the amount of time parents spend searching for safe strollers, and how important safety is for them.
- b. Comfort – Ease of use because they will be using the stroller frequently and will not use it if it was not comfortable to use. Also, assembling the stroller needs to be relatively easy and intuitive or else the parents will not be satisfied with the stroller. Lastly, the comfort of the child is an important need for the parents because if the baby is uncomfortable then it will make the parents uncomfortable (probably by crying).
- c. Cost ‐ In most cases parents do not want the stroller to be extravagantly priced, but rather be affordable. Additional features increase the price of strollers, so parents tend to find strollers that fit their individual needs, rather than jack‐of‐all‐trades strollers.
- d. Space constraints ‐ There is usually space constraints that parents have to deal with and thus need a stroller that does not take too much space.
- e. Convenience‐ When a parent is juggling multiple actions at once (from running errands with stroller to storing and removing stroller from car), the easier a stroller is to maneuver and manipulate, the more convenient it is for the parents. Additionally, creature comforts like cup holders and storage bins enable the stroller to interface seamlessly with the parents wide range of activities that they perform with the baby on a daily basis.
- f. Quality‐ Our research has shown that parents are much more comfortable with a stroller that gives the feeling of stability and sturdiness (not necessarily safety, but simply the “feeling” of safety). Strollers that have audible “snapping” and “locking” mechanisms reassure the parent that the stroller is performing its desired action, even if the parent is juggling multiple activities at once.
- g. Versatility‐ This goes along with value. If one stroller could perform every desired action, it would save the parents money, even if it was expensive, over buying multiple strollers. A stroller than can perform equally well in urban and rural environments, and has space for all sorts of features but is still compact, light, and maneuverable is the “holy grail” of stroller design. It is so rare that most parents have given up on finding one.
- h. Room to grow‐ A stroller that fits a parent’s needs from infant to toddler to small child equally well will save the parent money and time otherwise spent shopping for a new stroller when baby outgrows the old one.
- i. Assembly‐ Many strollers require some post‐purchase assembly, which can often times be daunting to the average consumer. It is important to the consumer that the assembly be simple, single‐modal (i.e. you can’t assemble it the “wrong way”), requires few or no additional tools, and wastes little or no extra material.
- a. Individual parts - This includes individual screws and plastic parts used in the stroller.
- i. Standardization‐ A manufacturer would love to design every stroller out of one material with a standard range of components, but today’s consumer demands customization, versatility, and value. The manufacturer must keep the part counts and material selections to a minimum.
- ii. Quality‐ Manufacturers can increase their margins by producing quality components without high costs. In strollers, components that have tight tolerances in assemblies and are lightweight, durable, and easy to clean are more desirable for the consumer, and thus more profitable.
- iii. Range‐ A manufacturer can tap every market segment if it is able to produce strollers in the range from low‐price bargains to high‐price boutique strollers. This is not an uncommon practice in the stroller industry, as every family type is different. A manufacturer must simultaneously balance how to produce low‐ and high‐end strollers with their need for standardization. They can do this by using identical components on strollers that are similar, such as wheels or less‐visible components which do not necessarily tie to customers’ views of quality.
- b. Assembler - This includes putting individual parts together to be packed into box for shipping.
- i. Cost - Machinery or labor for assembly must be kept to a minimum to decrease costs.
- ii. Speed - Faster assembly speeds clear each stroller from the plant quicker, and bring it to market faster.
- iii. Ease of assembly‐ Tying into speed, a stroller that is easy for the assembly line worker will increase the speed at which the stroller is produced, and will also maintain worker health and satisfaction, which keeps them working harder and faster for longer. Many strollers contain a combination of screws and rivets, so standardization of assembly pieces and reduction in number of pieces enables the plant to have less steps in the assembly line. Many strollers also require some post‐purchase assembly, so the manufacturer must take into account what is reasonable for a parent (or store worker for display model construction) to be expected to know how to assemble.
- iv. Portability‐ Many strollers now collapse for ease of use by the consumer, but this also allows them to be shipped in much smaller boxes. Ideally, weight and total volume of the stroller can have a significant impact on how high shipping costs are (particularly in today’s economy).
- a. Individual parts - This includes individual screws and plastic parts used in the stroller.
- a. Cost - The biggest cost considerations for stroller vendors are inventory, space, and labor. To this end, strollers must take up little storage and shelf space and be simple to assemble (display model), replace on shelf, place in shopping cart, and to manipulate by cashier during checkout.
- b. Customer satisfaction ‐ Need to maximize this to ensure minimal returns/exchanges. For this reason, vendors are only interested in selling quality strollers that will reflect well on their profits and reputation.
- c. Visibility‐ A typical stroller section has a display model that corresponds to its respective “box” on the shelf. Consumers can interact with each display model, understand the strengths and weaknesses of each, and then be able to accurately correspond the display model to the box they want to purchase, usually with a model number and corresponding graphic. It is important for a stroller to pass the FMOT (first moment of truth) that draws the consumer to it. Additionally, the boxes should be accessible but out of sight at the same time, so that the consumer can interact with the box when needed, but otherwise is not deterred by something that is very un‐stroller‐like in place of the display model.
- d. Features‐ Usually accompanying a display model, a feature list must simultaneously be descriptive, brief, and influential. Customers will attempt to reconcile the stated features with the actual features they find on the stroller, and then correlate them with the prices. Nothing can frustrate a customer more than inaccurate descriptions or poor correlations, further complicating the already difficult buying decisions they must make, which can result in a no‐sale.
The baby stroller has five main functions: transporting the baby forward and backwards, navigating turns, locking mechanism to reduce possibility of the stroller moving unintentionally, folding capability for ease of use, and most importantly keeping the baby securely fastened inside the stroller. Listed below is a detailed explanation of each of the functions.
- 1. Forward and Backward Motion
- a. Force applied to the handle which is transferred to the four wheels, which makes the stroller go forward or backward. The wheels are made of rubber, which makes it suitable for a variety of surfaces (like sidewalk, grass, tiles, etc…) that it might be used on.
- 2. Turning
- a. Front wheel is free to rotate when a moment is applied to the handle bar (from the centered position).
- 3. Braking
- a. Tab at the rear axle that can be activated which locks the back wheels and thus makes it harder for the stroller to go forward or backwards. Also the front wheels can be locked to limit the range of motion of the stroller (about 15 degrees).
Figure SF 1
Figure SF 2
- 4. Folding
- a. By sliding and rotating the handle grip (shown in figure below) the entire stroller assembly can be folded for ease in transportation and storage.
Figure SF 3
- 5. Keeping baby secure
- a. 5 point safety harness (shown in figure below) which ensures that the baby is securely held in the stroller. If, for some reason, the stroller tips forward or sideways the safety harness will keep the baby inside the stroller. Furthermore, it makes it harder for the baby to let itself out of the stroller.
Figure SF 4
- 6. Car seat integration
- a. A car seat can be placed on the stroller and secured by snapping device shown in figure below. This provides ease of transportation of the baby for the parents.
Figure SF 5
Failure Modes and Effects Analysis (FMEA)
This study will observe different failure modes and effects, and list the recommended changes to the design in order to minimize or remove these failures. The proposed changes will be addressed by the recommended actions. A 10 represents an unacceptable level of damage (S), a low probability that the cause can be detected (O) and that there are no design controls in place to detect the cause (D). Conversely, a 1 represents little to no damage (S), an obvious cause of failure (O), and that there are sufficient controls in place to detect the cause (D).
|Item and Function||Failure mode||Effects of Failure||S||Causes of Failure||O||Design Controls||D||RPN||Recommended Actions|
|Cloth seat - holds weight of child and transfers that weight to the structural frame||Cloth tears at one of the screw locations||Cloth rips, child falls and seat is unusable||6||Screws are driven through the cloth rather than having a stitched hole pre-drilled to fit the screw||2||general durability test, visual inspection for tear||7||84||pre-drill holes in cloth, or make plastic washers clear for visual tests|
|Pins which hold the joints of the frame together||pins shear||the structural frame becomes unstable and holding an unbalanced load||5||overloading the stroller||8||user warning||1||40||Make warning label and weight restrictions more visible from user position|
|Plastic Tray meant to hold food and drink items for the child||tray snaps off easily when loaded||the stroller falls||6||weight is applied to the tray for lifting the stroller||5||user warning and assembly process||4||120||Make the tray a structural component so that it may be safely loaded in a broader manner|
|Locking mechanism is meant to keep the stroller upright, and conditionally allow for folding||handle is pushed too hard breaking internal components||stroller frame is bent, internal components for handle are broken||3||handle rotation is excessive, user pushes on handle before locking mechanism is disengages||3||user feed back and visual feedback of moving locking component||3||27||reduce motion of handle to disengage locking mechanism|
|Plastic Tray meant to hold food and drink items for the adult||tray is knocked off||liquid spills on child or adult||5||The connection between the tray and the structural component is easily removed||5||User and assembly||3||75||make the connection between the tray and the structure more secure|
The majority of failures are caused by improper use by the user. This includes lifting and holding the stroller in ways which were not intended. However, our design should account for these mis-uses to the best of our ability. With this in mind, the recommended improvements include more visible warning labels and reinforced connections between the plastic molded trays to give them structural stability.
Design for Manufacturing and Assembly (DFMA)
To make a stroller, first the individual parts are manufactured. Then, certain parts are assembled together into separate subassemblies. Finally, the consumer brings home a box full of subassemblies along with an instruction book and proceeds to assemble the stroller to its usable state. In this section we will analyze each of these stages separately.
Design for Manufacturing
The stroller has a metal structure for support of other materials. This structure is formed by individually bent hollow tubes with holes intended for later assembling the bars together by riveting. From general inspection, because the part was fairly light, we can be accurately assume the part is made of aluminum. The bars are painted providing a very smooth texture over the rustic aluminum feel and do not have any sharp edges. In addition, it is worthy to note that there were no dents in the metal tubes and they were very circular, creating no safety or quality concerns.
Another component of the stroller is the fabric that is fastened onto the metal structure. This fabric is rather strong and well stitched together. It can withstand very forceful tugs.
The rest of the stroller is made of plastic components; most likely ABS. ABS is very stiff and can be molded easily into many different shapes. This flexibility along with other desirable properties this material provides, and its cheap price makes it the logical choice of material for the stroller application.
The parts are designed very well to complements the material. The walls of all the components are designed with uniform thickness, thus avoiding sinks in the part and other manufacturing difficulties, resulting in an aesthetically pleasing and touch appealing surface finish. It is also worthy to note that although from a quick glance, the front and back wheels differ. However, when one looks closer, the wheels in the front are essentially composed from two sets of the wheels from the back. This usage helps decrease cost because it effectively puts to use manufacturing for scale.
Another advantage of using plastic is that it can be molded into various shapes that have the ability to be very detailed. This makes it practical to make many safety locks for the parts on the stroller. For instance, the locks for the wheels and the snap on trays are great examples of how the material was molded.
In addition, for the application of the product, the designers were very thoughtful in adding many fillets and rounding all the edges making it safe for interactions with children.
Nevertheless, plastic is not perfect. In making the injection mold for the mass produced part, designers have to keep in mind, shrinkage of the material. As can be seen in the figure below (Figure DFMA 1), when the parts are looked at individually, they seem very well made and little defects can be seen. However, when they are assembled, the molds were not created perfectly to account for the amount of shrinkage and some parts do not mate perfectly. But these defects and tolerances are built into the design. The overhangs have rounded corners, thus they will not be dangerous. The miss alignment gap is enclosed on the edges, thus it is hard to get one’s hangs pinched while in the handle is in its closed position.
Figure DFMA 1
From our analysis, we have concluded that the individually manufactured parts are very well made and are appropriate for the application of interaction with children. The injected or blow molded plastic parts have designs that consider tolerances and the properties of the material.
Design for Assembly
All the metal tubes fit together very well. They are secured together with rivets and will not loosen up easily. These rivets would need to be put in with special equipment that would hold the two pieces being riveted in place, while the rivet is inserted. However, these rivets are necessary for safety reasons, so that the joints do not come loose easily. In addition, some wires are slipped through the tubes for the function of collapsing the stroller. Then the metal is secured with fabric.
Before the fabric is ready to be fastened on the metal frame, different materials are stitched together and metal buttons are added nylon strips. This makes later assembly onto the frame easier when the strip is wrapped around the metal bars and is secured. Another way that the fabric is secured is by directly screwing them into the metal tubes that had pre-drilled holes in them, as can be seen in figure DFMA 2 . In this way, the fabric is very securely attached to the main support and the additional plastic serves as a good way to distribute the pressure onto a larger area adding an additional layer of safety when a child is placed in the stroller. However, for this additional safety feature, it requires the assembler to hold the fabric in place while lining up the screw with the plastic piece as well as the pre-drilled hole in the frame. This also adds variability because there are no pre-drilled holes or any visible indications as to where the screws should be placed. But this fact also makes it easy for assembly.
Figure DFMA 2
After the fabric and padding are added, additional plastic support are placed on. Some parts only add to the aesthetic appeal, such as such as the rubber above the wheel that looks like a suspension system. Others are to hold the subassemblies in place for transport in box to the customers such as a white plastic hook part. While a majority of the plastic adds additional futures for the stroller, such as the trays added to give a place for the parent to place bottles or other things.
All of these parts are designed for easy assembly and have a very logical order (first metal frame assembled, then fabric added, and finally plastic components added). Parts fit together very well and does not require the assembly worker to get into awkward positions or use excessive force repeatedly. This is very important for the high volume production required by stroller manufacturers. It is also worthy to note that the designers tried to keep the amount of varying parts to a minimum. For instance, the front and back wheels use the same part, except the front wheels have an additional part to link two wheels together. This not only saves cost for manufacturing the part by not requiring an additional mold. It also decrease the amount of stock needed to be kept and avoids mistakes during assembly by mixing up the front and back wheels on accident.
After most of the parts are assembled, they are packed up in boxes and shipped off to be sold to customers.
Design for Customer Final Assembly
To conserve the size of the shipping box, because having the plastic trays on the assembly prior to shipment will make the assembly bigger in the closed position, it is required that customers assemble certain parts themselves. When consumers purchase the stroller, they are presented with eight parts, as can be seen in figure DFMA 3; one main assembly, front and back wheels, wheel lock, front tray and foot rest for the child. We noted that there were no screws or small metal parts that needed to be involved in the final assembly process with the exception of two end caps for the back wheels. Also, no tools were needed with the exception of a hammer and a pair of scissors to cut a tie that secured the main part in its closed position.
Figure DFMA 3
The final assembly was relatively simple. The parts were mainly snapped onto each other. When alignment of the parts was necessary, there were guides built into the design of the product, such as ribs that suck out, to ensure the right alignment. One example can be seen in figure DFMA 4.
Figure DFMA 4
We did not have any difficulties with the assembly process. However, the foot reset for the child was hard to snap onto the metal frame. Nevertheless, we could fully assembly the stroller within ten minutes. In addition, little material remains after the assembly is complete. As can be seen in figure DFMA 5, only some cardboard pieces, a tie, some caps that were on the ends of the metal rod that stuck out from the back wheel brakes, a plastic piece to hold the main sub-assembly in the closed position while it was in the box, a hammer used to secure the cap onto the back wheel, and the instruction, were left from the assembly.
Figure DFMA 5
Overall, this product was designed very well with many aspects considered. As noted in the detailed analyzes above, there were variations and tolerances that were left. It is noticeable that the designer kept the involved parties for this product in mind during the design. Parts were consolidated for ease of assembly. In addition, the designer included enough tolerances and flexibility in the design to avoid necessary tight precisions that can be tedious for the factory assembler and the customer. The material was well chosen to allow the designs of the parts to be well rounded, no sharp edges, and pleasing to the senses. Nevertheless, the assembly of the stroller can be made so that processes involving the hand to hold too many things in place at once is avoided. Such as the plastic screwed in piece to hold the fabric in place, we might have a screw that has a surface that matches the metal tube radius, thus taking out an unnecessary part and making the assembly process easier. As for manufacturing, stroller designers might consider material use and try to optimize the places where thinner wall thicknesses are permitted while maintaining the safety integrity.
Design for Environment (DFE)
Every man made product has various impacts on the environment that are added up during the manufacturing process, transportation, use and end of life. We have analyzed the impacts that our strollers will have on the environment by using the economic input-output life cycle assessment (EIO-LCA) from the LCA software found at http://www.eiolca.net using data from 1997. This assessment takes looks into the economic activity within an appropriate sector and predicts the environmental impact directly linked to each sector.
Economic Input-Output Life Cycle Assessment (EIO-LCA) of Manufacturing, Transportation, Use, and End-of-Life phase
In order to analyze the impact stroller would have on the environment, we chose to look into the “doll, toy, and game manufacturing” sector (sector #339930), which was the sector that contains child transportation/strollers. We felt this category was particularly appropriate because the other products made in this sector also use the same materials (part plastic, part metal).
In this sector, for one million dollars worth of products in this sector, there is a total of 2.23 million dollars of economic activity. The pie chart below depicts the economic activity in different sectors related to our main sector, with specific depictions of the top five.
The EIO-LCA software also produces an estimation of the air pollutants outputted by each sector. As can be seen in the pie charts below, power generation and supply contributes 71% of the conventional air pollutants estimated, while contributing around 32% of the greenhouse emissions. Next in line for most greenhouse gas impact is the transportation of the product (7.6% of total sector emission). The transportation phase of the product life cycle also stands for a small portion of petroleum industry. Nevertheless, the negative impact of this phase is unavoidable in the life cycle unless the automobile industry make more efficient transportation or the supply chain transportation network is re-routed to become more efficient. We, as designers, might be able to design a more compact stroller so that more strollers can be transported within one transportation device.
Although the plastics sector is not in the top ten sectors for air pollutants and ranked fifth in greenhouse emissions (4.5% of total sectors), it is number one on the charts for toxic releases as can be seen in the table below. It stands for 7.1% of the total releases kg of total for all sectors. This sector is also related to petroleum and oil, which is responsible for a significant portion of negative environmental impact from the stand point of air pollutants and toxic releases. Therefore, there is potential for increasing the efficiency of plastic use in stroller design and manufacturing to decrease the amount needed without compromising safety of the child.
During the end-of-life phase of the product, it is recommended that the stroller be design for a long life cycle and encouragement of society to re-use second hand strollers to decrease the amount of environmental impact resulting from recycling or production of entirely new stroller from raw material. This is suggested because the environmental impact during the use phase of the product is negligible or even no negative impact contribution at all. This can be concluded because the only energy input that is necessary during the use phase is a person’s force. Unless we take into account a person breathing and the manufacturing processes of his or her food, it is very hard assess that environmental impact and the impact is relatively small compare to impact during the manufacturing, transportation, and end-of-life phase of the product. Thus the use phase was neglected in our analysis.
Accuracy of Analysis
It must be kept in mind that this analysis was using data from 1997. There might has been significant process improvement of the manufacturing process such that there is less environmental impact (such as greenhouse gas emission, toxic release, and air pollutant releases) resulting from the processes. Thus the numbers might be an over-estimation of the impact the sectors. In addition, in the recent years, much attention has been paid to the entire supply chain transportation sector such that the transportation of the product might be smaller due to the efficiency of transportation network modifications within the past few years. Nevertheless, raw material use and the relative percentages of the environmental impact between the sectors should be relatively accurate because it can be assumed that all sectors have been improving in the environmental impact area.
From our analysis, we can conclude most environmental impact was contributed during the manufacturing phase of the life cycle resulting from forming the raw material into the stroller form, such as plastic manufacturing, metal manufacturing, and the power and energy used during those processes. During these phases, a significant amount of greenhouse gas, air pollutant and toxic releases are contributed to the environmental impact. There is room for improvement for the designs to use less plastic while not compromising safety. By doing this, the petroleum industry as well as plastic manufacturing industry would have the most decrease in its negative environmental impact. In addition, it is recommended that infrastructure in society be placed to encourage reuse of second hand strollers, while increasing the quality of the stroller at the same time. This would significantly decrease the amount of environmental impact the sectors would have, since it will be decrease the activity in the manufacturing phase of the product which presently contribute the most environmental impact.
In this mechanical analysis we looked at a specific failure mode where loading the plastic molded tray caused deflections. In order to determine where the stresses were located, as well as the maximum deflection. In order to do this, we created a simplified SolidWorks model of the tray. Assuming the maximum load of 40lbs (which is the posted maximum weight the stroller can hold, neglecting the weight of the stroller), distributed between the handle and the tray, the load was then simplified as a 20lb load at the center of the tray. The tray was also modeled as ABS plastic of 0.1 inch thickness which was fixed at the bottom of the tray where it attached to the frame of the stroller. ABS has the following material properties:
|Tensile Yield Strength (MPa)||E: Young's Modulus (GPa)||Poisson's Ratio||Yield strength (MPa)|
Figure Mechanical Analysis 1: Restraints and Loading conditions
The following plots demonstrate the maximum stress and stress distribution, as well as the maximum deflection on the tray under these loading conditions.
Figure Mechanical Analysis 2: Displacement
Figure Mechanical Analysis 3: Stress Distribution
This analysis shows that the maximum displacement is up to 17cm, which is acceptable in that the max stresses do not exceed the yeild stress for the material, however not acceptable in terms of the user. The large amount of deflection does not instill the user with the idea that this method of carrying the stroller is safe, and the user will discontinue this method of transporting the stroller. The implication is that although this is safe, it is not preferred.
Mechanical Analysis Conclusion
By creating a model of the tray and modeling loading conditions using SolidWorks, we were able to determine the maximum displacement of the tray, which was 17cm. This number is close to the observed 6 inch deflection which the tray underwent following a user test. This verifies that the results from the mechanical analysis are fairly accurate. Although this loading does not cause the part to yield, it is unacceptable because it feels unsafe to the user. As a result, we will redesign the connection to displace less under maximum loading conditions.
Our team composes of Brad Hall, Sixiao Joy Liu, Madhur Paharia, Nick Selman.
For this analysis, the following are the roles of each member:
- Brad Hall – Team lead, responsible for User Study, FMEA, and Mechanical Analysis sections
- Sixiao Joy Liu – Responsible for the Design for Manufacturing and Assembly as well as the Design for Environment analysis
- Madhur Paharia – Responsible for identification of major stakeholders and their needs, system functions, and the presentation
- Nick Selman – Responsible for bill of materials, assembly photos, and other documentation
As a team, we met four times. First, we met to decide and gather the product. Then, we conducted the dissection of the product to gain a clear understanding of it as well as conducting a usability tell while the stroller was assembled. Next, we had a meeting to distribute the work, assign roles for members, and agree on the timeline for this project. Finally, we met to put together our work into a Wiki page. During this process, whenever we had questions, we would communicate through email.
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