Apple Design Challenge

Apple Design Challenge Push-Button IPod Battery Latch Mechanism By: Marcel Okoli 01.07.2016

Scope The goal of this project is to design a robust and aesthetically pleasing push-button latch mechanism for a battery door on an iPod Classic.

Wants and Needs     

Simplicity: The design solution needs to be easy to use as well as easy to install.

Durability: It must be able to last or outlast the life time of the iPod. Aesthetics: Just like every other Apple product out there, it must have a cool aura about it. It should look and feel good.

Repairs: In the event of damage, the components used in the design should be easy to replace.

Low Cost: This is perhaps the most important need as the iPod is sold in the consumer market, and it is imperative that production costs are kept low so as not to drive up the selling price and run the risk of losing interested customers.

Constraints   

Space/Size: The size of the device cannot be drastically increased, therefore the mechanism needs to fit into a small space.

Weight: Just like size, the weight should not be increased beyond reason, bearing in mind that it is a modern day hand held device.

Material: Some materials are already used on the current iPod Classics, therefore it is important to conform to that material selection to maintain aesthetics (e.g. stainless steel used at the back should also be used for the cover).

Benchmarks To get an idea of how a push button latch mechanism works, some research was done, and a few good designs were found. They are listed below along with the links to their pages.  

Harley Davidson Push-Button Fuel Console release1.

SD Card Slot Mechanism2.

These two reference points were good starting points to get thinking of how a push-button mechanism works, however, they were too complex for this application. Once the general idea was understood, a

1 2

http://www.harley-davidson.com/store/smooth-push-button-console-door-release https://www.youtube.com/watch?v=QVO042whQ_4

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number of simplified designs were made, designs that would meet the wants and needs outlined above while staying within the constraints.

Design Concepts Assumptions  

  

Batteries are in the iPod and not in the cover.

The iPod height and width are the same as the 2009 6th generation iPod Classic for all designs.

The depth is changed to accommodate AA batteries. The idea is geared at reducing weight and cost associated with a bigger frame.

All designs make use of hinged covers to avoid loss of battery covers (very common in remote control covers).

Stainless steel material used at the back of the iPod is also used for the battery cover to maintain aesthetics.

All Designs have the button flush with the battery cover.

Note   

All designs are displayed through a transparent iPod to expose the insides of the battery compartment.

Parts inside battery compartment are colored to aid visibility and not indicative of material or material finish.

CAD drawings were made for all Designs.

Concept A – Friction Fit Design This design utilizes a friction fit lock as the locking mechanism, and uses a spring loaded hammer arm as the unlocking mechanism. Below, an illustration of how the mechanism works can be seen.

This transparent view exposes the insides of the iPod with all the contents in its battery compartment. It also shows the insides of the push button mechanism used here. The figures following this would reveal the internal components of this mechanism and show how it works.

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Figure 2 Figure 2 shows an exploded view of the internal components of the lock mechanism. The following figures would show the mechanism in its locked position

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Figure 3 Figure 3 shows a section of the push-button mechanism in its closed position. The figure below shows a more exposed view.

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When the mechanism is in its locked position there are forces acting on the cover and the other parts of the lock mechanism. Figure 5 highlights the forces, however it is important to note that most of these forces are due to the weight of the part, which, in this case is negligible.

đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x;

đ??šđ??ľ,đ??ť đ??šđ??ť,đ?‘† đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘”

đ??šđ?‘†,đ??ť

Figure 5 Where: đ??šđ??ľ,đ??ť = đ??šđ?‘œđ?‘&#x;đ?‘?đ?‘’ (đ?‘¤đ?‘’đ?‘–đ?‘”â„Žđ?‘Ą) đ?‘œđ?‘“ đ?‘?đ?‘˘đ?‘Ąđ?‘Ąđ?‘œđ?‘› đ?‘œđ?‘› â„Žđ?‘Žđ?‘šđ?‘šđ?‘’đ?‘&#x; đ??šđ??ť,đ?‘† = đ??šđ?‘œđ?‘&#x;đ?‘?đ?‘’ (đ?‘¤đ?‘’đ?‘–đ?‘”â„Žđ?‘Ą)đ?‘œđ?‘“ â„Žđ?‘Žđ?‘šđ?‘šđ?‘’đ?‘&#x; đ?‘œđ?‘› đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” đ??šđ?‘†,đ??ť = đ?‘…đ?‘’đ?‘Žđ?‘?đ?‘Ąđ?‘–đ?‘œđ?‘› đ?‘“đ?‘œđ?‘&#x;đ?‘?đ?‘’ đ?‘œđ?‘“ đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” đ?‘œđ?‘› đ?‘Ąâ„Žđ?‘’ â„Žđ?‘Žđ?‘šđ?‘šđ?‘’đ?‘&#x; đ?‘Žđ?‘›đ?‘‘ đ?‘?đ?‘˘đ?‘Ąđ?‘Ąđ?‘œđ?‘› đ?‘?đ?‘œđ?‘šđ?‘?đ?‘–đ?‘›đ?‘Žđ?‘Ąđ?‘–đ?‘œđ?‘› đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x; = đ??šđ?‘œđ?‘&#x;đ?‘?đ?‘’ (đ?‘¤đ?‘’đ?‘–đ?‘”â„Žđ?‘Ą)đ?‘œđ?‘“ đ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x; đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” = đ??šđ?‘œđ?‘&#x;đ?‘?đ?‘’ đ?‘œđ?‘“ đ?‘’đ?‘Žđ?‘?â„Ž đ?‘?đ?‘˘đ?‘ â„Ž đ?‘œđ?‘“đ?‘“ đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” While đ??šđ??ľ,đ??ť , đ??šđ??ť,đ?‘† and đ??šđ?‘†,đ??ť are negligible before the button is pushed, đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x; and đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” are important because this mechanism only works if the lock force of the friction fit is greater than the force the push-

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off springs exert on the cover. The next couple of figures show how this mechanism opens and closes the iPod battery compartment.

Force

Hammer Swings Up

Figure 6

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Springs Compress

Springs Eject Cover

Springs Eject Cover

Figure 7 Figure 6 and 7 above show how the mechanism reacts when the button is pushed down. The step-bystep process is outlined below.     

Button is pushed down with a force from the finger The button hits the hammer and pushes it against the spring which pivots about the inside walls of the battery compartment and then hits the cover.

Once the hammer hits the cover, it disengages the friction lock and frees the cover.

The push off springs then release their stored energy and gives the cover an extra pop.

Once the finger is lifted from the button, the spring pushes the hammer which pushes the button back into place (flush with the cover).

After the batteries are changed, the battery compartment is then closed. The figure below shows how the locking mechanism works.

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Expands when contact is made with stub

Stub allows friction lock

Figure 8 The locking mechanism quite simply locks in place when it is pushed down. It latches onto the stub below and locks the battery compartment. Figure 9 shows a cross-sectional view in its locked position.

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Figure 9 The color of the iPod is changed in figure 9 to aid in visibility of the lock stub. Detail 0 above shows it in its locked set up.

Concept B - Hook Latch This design makes use of a highly functional but simple hook latch mechanism. It uses a spring loaded hook latch and a profile on the cover that allows it to be re-locked. Below, an illustration of how the mechanism works can be seen.

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Just like the Concept A above, the exterior looks the same in the locked position.

The figures below show the parts that make up this mechanism and how they work.

Figure 10

Figure 11 10 | P a g e

Figure 11 above shows the exploded view of the internal components of this design. The following figures would show all the parts in its locked configuration.

Figure 12 Detail B in figure 13 below shows a more exposed view. Some components are hidden to emphasize on the locking mechanism.

Figure 13

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In figure 12 and 13 above the hook latch engages the cover hook to keep it securely fastened. Figure 14 below shows a cross-sectional cut through the middle of the iPod.

Figure 14 It is important to know the forces acting on the device when the button is not being pushed. The figure below illustrates that.

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đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x;

đ??šđ??ľ,đ??ť đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘”

đ??šđ??ť,đ?‘†

đ??šđ?‘†,đ??ť

Figure 15 Where: đ??šđ??ľ,đ??ť = đ??šđ?‘œđ?‘&#x;đ?‘?đ?‘’ (đ?‘¤đ?‘’đ?‘–đ?‘”â„Žđ?‘Ą) đ?‘œđ?‘“ đ?‘?đ?‘˘đ?‘Ąđ?‘Ąđ?‘œđ?‘› đ?‘œđ?‘› â„Žđ?‘Žđ?‘šđ?‘šđ?‘’đ?‘&#x; đ??šđ??ť,đ?‘† = đ??šđ?‘œđ?‘&#x;đ?‘?đ?‘’ (đ?‘¤đ?‘’đ?‘–đ?‘”â„Žđ?‘Ą)đ?‘œđ?‘“ â„Žđ?‘œđ?‘œđ?‘˜ đ?‘™đ?‘Žđ?‘Ąđ?‘?â„Ž đ?‘œđ?‘› đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” đ??šđ?‘†,đ??ť = đ?‘…đ?‘’đ?‘Žđ?‘?đ?‘Ąđ?‘–đ?‘œđ?‘› đ?‘“đ?‘œđ?‘&#x;đ?‘?đ?‘’ đ?‘œđ?‘“ đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” đ?‘œđ?‘› đ?‘Ąâ„Žđ?‘’ â„Žđ?‘œđ?‘œđ?‘˜ đ?‘™đ?‘Žđ?‘Ąđ?‘?â„Ž đ?‘Žđ?‘›đ?‘‘ đ?‘?đ?‘˘đ?‘Ąđ?‘Ąđ?‘œđ?‘› đ?‘?đ?‘œđ?‘šđ?‘?đ?‘–đ?‘›đ?‘Žđ?‘Ąđ?‘–đ?‘œđ?‘› đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x; = đ??šđ?‘œđ?‘&#x;đ?‘?đ?‘’ (đ?‘¤đ?‘’đ?‘–đ?‘”â„Žđ?‘Ą)đ?‘œđ?‘“ đ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x; đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” = đ??šđ?‘œđ?‘&#x;đ?‘?đ?‘’ đ?‘œđ?‘“ đ?‘’đ?‘Žđ?‘?â„Ž đ?‘?đ?‘˘đ?‘ â„Ž đ?‘œđ?‘“đ?‘“ đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” While đ??šđ??ľ,đ??ť , đ??šđ??ť,đ?‘† and đ??šđ?‘†,đ??ť are negligible before the button is pushed, đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x; and đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” are important because the force exerted on the cover by the push-off springs travel through the cover hook, onto the hook latch, which in-turn compresses the spring. More details of this can be seen in the calculation section of this report.

To get an idea of how the mechanism works when the button is pushed, the illustrations in figure 16 and 17 show what happens.

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Force

Hook Latch Pivots

Figure 16

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Reaction Spring Compresses

Push off springs push cover upwards

Figure 17 Below, a step-by-step process of how the mechanism works is outlined.    

When a force is applied downward by a finger on the button, the button hits the hook latch shown in figure 16.

The hook latch compresses the reaction spring and pivots about the insides of the battery compartment.

This pivoting motion disengages the cover hook as shown above.

The push off spring then pushes the over upwards (as shown in detail D in figure 17).

After the batteries are changed, the battery compartment is then closed. The figure 18 below shows how the locking mechanism works.

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Figure 18 To close this compartment, a force is applied by the user to the cover. Once the cover hook makes contact with the hook latch, the profile allows the hook latch to slide to the right (as shown by the green arrow in figure 18 above), and as the cover hook comes down, the hook latch would also pivot backwards, thus compressing the spring and allowing the hook latch to re-engage the cover hook and lock the battery compartment.

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Decision Tree Both Concept A & B provide the user with superior aesthetics (with the Apple Logo as the button). They are also safe to use (they have no sharp edges), and most of all, highly intuitive. The decision tree below shows how the better design is chosen and why.

Tree Friction Fit

Hook Latch

Pros Simple Design

Cons Fastening mechanism may weaken after a while

Few Parts

May open with impact Few Parts with the ground Unlocking hammer Very Secure locking may deform cover after mechanism a while Easy Operation Easy Maintenance Hook latch prevents possibility of opening when dropped Saves space (low profile)

Good Locking mechanism Easy Operation Easy Maintenance Saves Space (low profile and maintains h & w of iPod classic)

Pros Simple Design

Cons Hook latch may require advanced manufacturing processes

Best Design (CAD - Render) Based on the Decision tree above, the Hook Latch design concept is a more competent and secure method of opening and closing the battery compartment. Furthermore, more than the pros and cons seen on the decision tree, it is also important to note that this design has been made such that its parts can be easily replaced (even by the user) in the event of any damage. The rendered view of the final design choice is shown below.

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Figure 19

Figure 20 18 | P a g e

As an added functionality, a gasket can be added on the inner lining of the battery compartment to prevent water from sipping through the clearance hole made for the cover door and potentially getting to the batteries. It is not done in this rendered view because it goes beyond the scope of this project.

Calculations and Testing (FEA) Assumptions  Friction on cover hinge is negligible.  Force (weight) on spring by hook latch part is negligible Note  These calculations are done only for Concept B Before diving into the calculations, the following terms used throughout the calculation are defined below for clear understanding. đ??šđ??ľđ?‘˘đ?‘Ąđ?‘Ąđ?‘œđ?‘› = đ??šđ?‘œđ?‘&#x;đ?‘?đ?‘’ đ?‘œđ?‘“ đ?‘Ąâ„Žđ?‘’ đ?‘?đ?‘˘đ?‘Ąđ?‘Ąđ?‘œđ?‘› = đ??šđ?‘œđ?‘&#x;đ?‘?đ?‘’ đ?‘’đ?‘Ľđ?‘’đ?‘&#x;đ?‘Ąđ?‘’đ?‘‘ đ?‘?đ?‘Ś đ?‘“đ?‘–đ?‘›đ?‘”đ?‘’đ?‘&#x; đ??šđ?‘&#x;−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” = đ??šđ?‘œđ?‘&#x;đ?‘?đ?‘’ đ?‘œđ?‘“ đ?‘Ąâ„Žđ?‘’ đ?‘&#x;đ?‘’đ?‘Žđ?‘?đ?‘Ąđ?‘–đ?‘œđ?‘› đ?‘†đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” = đ??šđ?‘œđ?‘&#x;đ?‘?đ?‘’ đ?‘œđ?‘“ đ?‘’đ?‘Žđ?‘?â„Ž đ?‘?đ?‘˘đ?‘ â„Ž đ?‘œđ?‘“đ?‘“(đ?‘’đ?‘—đ?‘’đ?‘?đ?‘Ąđ?‘–đ?‘œđ?‘›) đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x; = đ??šđ?‘œđ?‘&#x;đ?‘?đ?‘’ (đ?‘¤đ?‘’đ?‘–đ?‘”â„Žđ?‘Ą)đ?‘œđ?‘“ đ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x; đ?‘Ľ = đ?‘Ľ − đ?‘Ľâ€˛ = đ??šđ?‘˘đ?‘™đ?‘™ đ??żđ?‘’đ?‘›đ?‘”đ?‘Ąâ„Ž − đ??śđ?‘œđ?‘šđ?‘?đ?‘&#x;đ?‘’đ?‘ đ?‘ đ?‘’đ?‘‘ đ??żđ?‘’đ?‘›đ?‘”đ?‘Ąâ„Ž. đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x;−đ?‘›đ?‘’đ?‘Ą = 2đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” − đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x; đ?‘˜1 = đ?‘†đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” đ??śđ?‘œđ?‘›đ?‘ đ?‘Ąđ?‘Žđ?‘›đ?‘Ą đ?‘œđ?‘“ đ?‘ƒđ?‘˘đ?‘ â„Ž đ?‘œđ?‘“đ?‘“ đ?‘†đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘”đ?‘ đ?‘˜2 = đ?‘†đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” đ??śđ?‘œđ?‘›đ?‘ đ?‘Ąđ?‘Žđ?‘›đ?‘Ą đ?‘œđ?‘“ đ?‘…đ?‘’đ?‘Žđ?‘?đ?‘Ąđ?‘–đ?‘œđ?‘› đ?‘†đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘”đ?‘

In its locked position, đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” is exerting an upward force on the cover. The cover on the other hand is also exerting a downward force (weight) on the springs (see figure below). For the springs to be able to pop the cover upwards đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” > đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x; . đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x;

Figure 21 19 | P a g e

đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘”

đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘”

Based on this diagram we can find what đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” is using the equation below: ∑ đ??šđ?‘Ś = 2đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” − đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x; = 0 The value of 2 in front of the spring force is as a result of two springs (as shown in the figure above). The equation above would give an equal force to the force exerted on the springs by the cover, however, it is required that the cover pops off, therefore, a force greater than the force exerted by the cover is needed. For this calculation, 2X the force of the cover would be used. The modified equation is shown below. ∑ đ??šđ?‘Ś = 2đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” − 2đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x; = 0 ∴ đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” = đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x; To determine the force of the cover, the mass of the cover (using stainless steel as the material) was determined to be 0.046kg using the iProperty tool in Inventor. This number is then converted to force by multiplying it by the gravitational force. đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x; = 0.046 Ă— 9.81 = đ?&#x;Ž. đ?&#x;’đ?&#x;“đ?&#x;?đ?‘ľ = đ?‘­đ?’‘−đ?’”đ?’‘đ?’“đ?’Šđ?’?đ?’ˆ Using the equation for springs below, the spring constant (đ?‘˜1 ) for each of the push-off springs can be found. đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” = đ?‘˜1 đ?‘Ľ, đ?‘¤â„Žđ?‘’đ?‘&#x;đ?‘’ đ?‘Ľ = 0.002848đ?‘š đ?’Œđ?&#x;? =

đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” = đ?&#x;?đ?&#x;“đ?&#x;–. đ?&#x;‘đ?&#x;”đ?‘ľ/đ?’Ž đ?‘Ľ

To determine the stiffness of the reaction spring, the force applied when pushing down the button must be determined. After research, it was found that, according to the Human Systems Engineering Branch at the Georgia Tech Research Institute3, the minimum force exerted on a 10mm button pressed by “bare handsâ€? is 2.8N, while the maximum (probably used in industrial buttons) is 11N. Considering that the user would occasionally grip the device, and it is not desired for the battery compartment to be accidentally opened, the average of the minimum and maximum was taken to give a value of 6.9N. This value would be used the following calculations. đ??šđ??ľđ?‘˘đ?‘Ąđ?‘Ąđ?‘œđ?‘› =

2.8đ?‘ + 11đ?‘ = 6.9đ?‘ 2

The figure below shows the forces that affect the spring stiffness calculation:

3

http://hsimed.gtri.gatech.edu/guidelines/wd_buttons.php

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đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x;

đ??šđ??ľđ?‘˘đ?‘Ąđ?‘Ąđ?‘œđ?‘› đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘”

đ??šđ?‘&#x;−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘”

Figure 22

It is important to note that the forces in the push-off spring affect the reaction spring because the cover is connected to the hook (in grey) which applies a force to the hook latch (in red), therefore a summation of all the forces acting on the cover needs to be taken to determine đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x;−đ?‘›đ?‘’đ?‘Ą before proceeding. đ?‘­đ?’„đ?’?đ?’—đ?’†đ?’“−đ?’?đ?’†đ?’• = 2đ??šđ?‘?−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” − đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x; = đ?&#x;Ž. đ?&#x;’đ?&#x;“đ?&#x;?đ?‘ľ However, since the upward movement of the push-off springs leads to the compression of the reaction spring, đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x;−đ?‘›đ?‘’đ?‘Ą = −0.452đ?‘ . Using these values, đ??šđ?‘&#x;−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” can be determined using the equation below. ∑ đ??šđ?‘Ś = đ??šđ?‘&#x;−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” − đ??šđ??ľđ?‘˘đ?‘Ąđ?‘Ąđ?‘œđ?‘› + đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x;−đ?‘›đ?‘’đ?‘Ą = 0

đ?‘­đ?’“−đ?’”đ?’‘đ?’“đ?’Šđ?’?đ?’ˆ = đ??šđ??ľđ?‘˘đ?‘Ąđ?‘Ąđ?‘œđ?‘› − đ??šđ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x;−đ?‘›đ?‘’đ?‘Ą = đ?&#x;•. đ?&#x;‘đ?&#x;“đ?&#x;?đ?‘ľ Using the equations for springs used above: đ??šđ?‘&#x;−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” = đ?‘˜2 đ?‘Ľ

21 | P a g e

đ??ˇđ?‘–đ?‘ đ?‘?đ?‘™đ?‘Žđ?‘?đ?‘’đ?‘‘ đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” đ?‘™đ?‘’đ?‘›đ?‘”đ?‘Ąâ„Ž = đ?‘Ľ = 2.586đ?‘šđ?‘š = 0.002586 đ?’Œđ?&#x;? =

đ??šđ?‘&#x;−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” = đ?&#x;?đ?&#x;–đ?&#x;’đ?&#x;‘đ?‘ľ/đ?’Ž đ?‘Ľ

Finally, to find the maximum force transferred from the spring to the end of the hook latch (in red), the summation of moments about the pivot point can be set to zero and the forces can be resolved. The diagram below shows how this is done.

đ??šđ??ťđ?‘œđ?‘œđ?‘˜âˆ’đ?‘šđ?‘Žđ?‘Ľ đ?‘ƒđ?‘–đ?‘Łđ?‘œđ?‘Ą

6đ?‘šđ?‘š

4.51đ?‘šđ?‘š

đ??šđ?‘&#x;−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘”

∑ đ?‘€đ?‘ƒđ?‘–đ?‘Łđ?‘œđ?‘Ą = đ??šđ?‘&#x;−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” (6đ?‘šđ?‘š) − đ??šđ??ťđ?‘œđ?‘œđ?‘˜âˆ’đ?‘šđ?‘Žđ?‘Ľ (4.51đ?‘šđ?‘š) = 0 Therefore: đ??šđ??ťđ?‘œđ?‘œđ?‘˜âˆ’đ?‘šđ?‘Žđ?‘Ľ =

(đ??šđ?‘&#x;−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” (6đ?‘šđ?‘š)) = 9.78đ?‘ 4.51đ?‘šđ?‘š

Using these values obtained above, a stress analysis can be done using the stress simulator (FEA) in inventor. This is done for the button, the hook latch and the hook of the cover. The results can be seen in the screenshots below:

Finite Element Analysis It is important to note that the diagrams produced by the software are excessively exaggerated. This is done so that the user can see what part has the most impact. What is most important are the actual values that are displayed. For example, the cover below has a maximum displacement of 0.003238mm.

This value is almost negligible. It is also important to note that Stainless Steel was used for the button and cover, on the other hand, Aluminum 6061 was used for the hook latch. They also all have a factor of safety ranging from 7.87-15 which is >> 1, therefore, no failure is experienced.

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Button To obtain an accurate FEA for the button, the button was fixed at the bottom and a force of 6.9N was applied to the top.

Figure 23

Figure 24

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The Hook Latch To obtain an accurate FEA for this part, the worst case scenario was considered. The front face (grove) which latches onto the cover is fixed, the two rods on the side are pinned (free to rotate), and a force of đ??šđ?‘&#x;−đ?‘ đ?‘?đ?‘&#x;đ?‘–đ?‘›đ?‘” (which is computed above) is applied to the face shown by the yellow arrow.

Figure 25

Figure 26

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Figure 27 Although this part experiences high stresses, it is important to note that minimum factor of safety is 7.87>>1, therefore the hook latch wouldn’t fail under the worst case scenario.

The Cover Hook To obtain an accurate FEA analysis for the cover, the top face is fixed and a force of đ??šđ??ťđ?‘œđ?‘œđ?‘˜âˆ’đ?‘šđ?‘Žđ?‘Ľ (which is computed above) is applied to the grove in the cover hook to account for the worst case scenario.

Figure 28

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Figure 29 The maximum displacement seen by this part under peak stresses is 0.003228mm, which (as highlighted in the introduction) is almost negligible. This part also has a minimum factor of safety of 15 which is >>1, therefore no failure is experienced.

Life Cycle Based on the FEA analysis above, it is a point of concern to figure out if the weakest component will outlast the life cycle of the iPod itself, therefore, a fatigue analysis is done for the hook latch to determine how many cycles of loading and unloading it can go through before failure. Some assumptions were made and they are listed below:

Assumptions   

Time taken to change batteries is negligible Device is used constantly through its life cycle Batteries are changed immediately the battery dies

Based on the assumptions above and the assumption that an average handheld device (phone, iPod or mp3 player) is used for an average of 2 to 5 years, calculations would be made based on constant use over a 5 year (to account for the worst case scenario) period to determine how many cycles the device goes through before experiencing failure.

AA batteries are rated to have a capacity ranging from 1200mAh – 2500mAh, however, since this analysis is considering the worst case scenario, the lower bound would be used. It is also important to note that various factors affect the rate at which a battery discharges, therefore a benchmark is used to determine the lowest possible playback time obtained from a pair of AA batteries.

The 930mAh battery (though not an AA battery) in the iPod touch provides up to 40hrs of music playback, and since the lower bound of 1200mAh is used in this analysis, it’ll take approximately 51hrs 26 | P a g e

to discharge. Using these values, a number can be determined for the number of cycles required to outlast the iPod’s life cycle. đ?‘ đ?‘˘đ?‘šđ?‘?đ?‘’đ?‘&#x; đ?‘œđ?‘“ â„Žđ?‘œđ?‘˘đ?‘&#x;đ?‘ đ?‘–đ?‘› 5 đ?‘Śđ?‘’đ?‘Žđ?‘&#x;đ?‘ = 5 Ă— 365 Ă— 24 = 43800â„Žđ?‘&#x;đ?‘ đ?‘ đ?‘˘đ?‘šđ?‘?đ?‘’đ?‘&#x; đ?‘œđ?‘“ đ?‘?đ?‘Śđ?‘?đ?‘™đ?‘’đ?‘ đ?‘‘đ?‘’đ?‘Łđ?‘–đ?‘?đ?‘’ đ?‘›đ?‘’đ?‘’đ?‘‘đ?‘ đ?‘Ąđ?‘œ đ?‘™đ?‘Žđ?‘ đ?‘Ą đ?‘Ąâ„Žđ?‘&#x;đ?‘œđ?‘˘đ?‘”â„Ž =

43800 = ~859 đ?‘?đ?‘Śđ?‘?đ?‘™đ?‘’đ?‘ 51

Knowing the number of cycles this part needs to last through, a fatigue analysis was done using Simulation Mechanical, and the results are shown in the figure below:

Figure 30 This figure shows that the part has a cycle to failure of đ?&#x;’. đ?&#x;?đ?&#x;• Ă— đ?&#x;?đ?&#x;Žđ?&#x;• under peak loads, which is >>> 859.

This number shows that the part will outlast the device’s life cycle (of 5 years) 952 times even if the batteries were changed every hour.

Material Selection Based on the material used on the back of the iPod (stainless steel), and also keeping in mind that there is a need to maintain the aesthetics of this product, a decision was made to retain the stainless steel used on the back for the cover, and that decision drives the other material selections. While the materials looked at in the table below were chosen for their hardness/stiffness (which is determined by the modulus of elasticity), there may be other materials that may be applicable based on cost, weight, fatigue and other factors. Additional research could be done in this area to narrow down the best materials to use.

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Part

Material

Hook Latch Cover Button Reaction Springs Push-off Springs Button Holder

Aluminum 6061 Stainless Steel Stainless Steel Stainless Steel Stainless Steel Aluminum 6061

Manufacturing /Cost Analysis The parts used in this design concept are relatively small and can be manufactured a number of ways, however, knowing that Apple has one of the largest precision manufacturers in its supply chain would tremendously reduce the cost of manufacturing these parts. Some parts, like the springs are standardized parts and could be bought in high volumes from their manufacturers. With the 100th million iPod sold as far back as 2007, Apple enjoys discounts that comes with being such a volume customer. The cost analysis below are for the raw materials that go into manufacturing these parts.

Part Hook Latch, Button Holder Cover, Button Springs

Price $1.46/KG $10/ 1đ?‘“đ?‘Ą 2 Ă— 1" Plate $0.25 each

Vendor MetalPrices.com Discountedsteel.com Mrspring.com

Having all the parts present, it should take a person approximately 3 minutes to put these simple parts together. Considering Apple’s manufacturing is done in China, and the labor rates there are ~$2/hour, the labor costs required for this assembly is approximately $0.10. However, it should be noted that Apple may have other advanced automated assembly equipment that may drive down this cost further.

Without knowing all the facilities available for the manufacturing of these parts at such volume, a cost approximation would be inaccurate.

Moving Forward‌ The next steps would be to discuss with the manufacturing team to learn about the different manufacturing processes at Apple’s disposal that would cut down the cost of manufacturing. It would also shed some light on design for manufacturability (DFM), and some parts may have to be redesigned to be able to use a more cost effective manufacturing process.

It is also important to discuss with the operations team to get an understanding of how the company’s existing supply chain may be beneficial, and potentially aid in cutting down the cost associated with sourcing new vendors/suppliers.

Following these two steps, and any possible redesigns (based on DFM), prototypes of the mechanism should be made to confirm that the device works as it was designed to (i.e. Testing) before going into mass production.

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