Babyproof door knob lock
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Current revision (01:26, 12 October 2007) (view source) (→Quantitative Mechanical Analysis) |
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== Product Use == | == Product Use == | ||
- | An uncovered, unlocked doorknob is simply turned to open or close a door. | + | An uncovered, unlocked doorknob is simply turned to open or close a door. When the user grasps and turns the doorknob, the spindle also rotates, pushing forward the sliding plate.When the plate is pushed forward, the cam is rotated and pulls the latch back. <br /> |
+ | [[Image:knobnspindle.jpg]] [[Image:camlatchslidingplate.jpg]]<br /> | ||
+ | To deter a small child from doing so, there are two different types of covers we're exploring. The first is specifically for round doorknobs. It covers the knob loosely so that it spins freely unless tabs are pressed on its side that creates friction between the cover and knob, causing the knob to turn when the cover is turned. The second is for levers. There is a lever cover and a "lock". This lock is a latch that is attached to the door above the knob with a tab that fits into a notch in the cover. The tab prevents the cover from moving unless the tab is lifted. | ||
+ | The effectiveness of the product depends on children's strength and their ability to learn from observation. The round protector requires a large force to be exerted on the tabs in order to operate the door, whereas the lever doorknob protector can be operated by simply lifting the gray latch and turning the lever arm. Most children are probably able to operate the lever doorknob successfully after seeing their parents or guardian use it, even as young as a 2-year-old baby. It is true that adults with physical disabilities such as arthritis may be unable to operate the doorknob protector. Also, since the lever protector requires two hands, it may be an obstacle for adults if they are opening the door with one hand. Other possible difficulties include installing and removing the protector. The lever protector consists of two different plastic parts, in which one of the two parts is actually drilled into the door. The round doorknob protector requires lining of the tabs when installing. Frequent installation and removal of both doorknob protectors may decrease the durability and life expectancy of product. | ||
== Product Dissection == | == Product Dissection == | ||
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|- | |- | ||
! 15 | ! 15 | ||
- | | | + | | auxillary bolt || latch assembly || <0.1 || steel || molded || attached to latch; when locked, it does not slide preventing latch from sliding || [[Image:doorknob_part15.jpg]] |
|- | |- | ||
! 16 | ! 16 | ||
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|- | |- | ||
! 28 | ! 28 | ||
- | | two lock springs || latch || <0.1 || steel/nickel alloy || wound || | + | | two lock springs || latch || <0.1 || steel/nickel alloy || wound || pushes latch and auxillary bolt into extended position || [[Image:doorknob_part28.jpg]] |
|- | |- | ||
! 29 | ! 29 | ||
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<big>'''Design for Assembly'''</big><br /><br /> | <big>'''Design for Assembly'''</big><br /><br /> | ||
'''Door Knob:'''<br /> | '''Door Knob:'''<br /> | ||
- | • The latch assembly was not well designed for assembly. There are several parts that must be put into place deep inside of the latch housing, but since the housing is a tube-like structure, that appears very difficult. The pieces inside the latch assembly do not appear to fit firmly together outside of the housing, so it is doubtful that the mechanism was placed together and then placed wholly into the housing.<br /> | + | • The latch assembly was not well designed for assembly. There are several parts that must be put into place deep inside of the latch housing, but since the housing is a tube-like structure, that appears very difficult. The pieces inside the latch assembly do not appear to fit firmly together outside of the housing, so it is doubtful that the mechanism was placed together and then placed wholly into the housing. To improve the latch assembly, rather than having a hollow tube being in once piece, they should be separated into two halves, the assembly can then be placed into one half, the second half can be placed on top and fastened together. <br /> |
• Inner and outer doorknob are simpler, having smaller assemblies placed into larger assemblies such as the torsional spring placed into the two halves of the spring housing which in turn was placed into the rose. Many of these pars are press fitted to each other.<br /> | • Inner and outer doorknob are simpler, having smaller assemblies placed into larger assemblies such as the torsional spring placed into the two halves of the spring housing which in turn was placed into the rose. Many of these pars are press fitted to each other.<br /> | ||
• The whole door knob assembly is sold to the consumer in three main parts not including the latch plate and strike plate. This is because of the standard way the door knobs and latch assembly are placed into the door relevant to each other. The latch assembly slides into a hole in the side of the door that faces the door frame with the latch protruding from the hole. The outer and inner doorknobs are then placed on the outer and inner side of the door relatively. To connect to each other and make the latch function, the inner and outer doorknobs must reach through the latch assembly.<br /> | • The whole door knob assembly is sold to the consumer in three main parts not including the latch plate and strike plate. This is because of the standard way the door knobs and latch assembly are placed into the door relevant to each other. The latch assembly slides into a hole in the side of the door that faces the door frame with the latch protruding from the hole. The outer and inner doorknobs are then placed on the outer and inner side of the door relatively. To connect to each other and make the latch function, the inner and outer doorknobs must reach through the latch assembly.<br /> | ||
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FMEA is an analysis tool used to assess the possible failure modes of a product. Each possible failure is rated for its ''Severity'' (S), ''Occurrence Rate'' (O) and ''Detectability'' (D). These numbers are then multiplied together to define the ''Risk Priority Number'' (RPN). This overall number is used to rank the priority in which failure modes are to be addressed. | FMEA is an analysis tool used to assess the possible failure modes of a product. Each possible failure is rated for its ''Severity'' (S), ''Occurrence Rate'' (O) and ''Detectability'' (D). These numbers are then multiplied together to define the ''Risk Priority Number'' (RPN). This overall number is used to rank the priority in which failure modes are to be addressed. | ||
+ | |||
+ | Following FMEA the failure with the highest RPN was surprisingly inability for an adult to easily use a baby door knob cover. Difficulty could be encountered in exerting enough normal force via the 'friction tabs' on the door knob cover to create enough friction to rotate the doorknob. The recommended solution for this problem is to increase the size of the pads to make area for exerting normal force or change the locking function. | ||
+ | |||
+ | The items with the second highest RPN numbers were the flame resistance of the door knob cover and inability of the cover to adjust for non-standard sized door knobs. Upon testing, if the plastic used to manufacture the doorknob covers is found to not be flame retardant a comparable plastic can be found that has fire resistant properties. For the problem of nonstandard doorknobs redesigns will be necessary to accommodate nonstandard doorknobs while also possibly incorporating lever functionality. | ||
+ | |||
{| class="wikitable" border="1" | {| class="wikitable" border="1" | ||
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! Item and Function !! Failure Mode !! Effects of Failure !! S !! Causes of Failure !! O !! Design Controls !! D !! RPN !! Recommended Actions !! | ! Item and Function !! Failure Mode !! Effects of Failure !! S !! Causes of Failure !! O !! Design Controls !! D !! RPN !! Recommended Actions !! | ||
|- | |- | ||
- | ! Door Knob Cover Tabs<br /> • Is squeezed by user to create friction between the tabs and the door knob. | + | ! Door Knob Cover Tabs<br /> • Is squeezed by adult user to create friction between the tabs and the door knob. |
| User is not able to squeeze tabs and generate enough friction to turn the knob || User cannot open door || 10 || Contact area of tabs is too small || 5 || Test the amount of force needed to squeeze tabs and turn knob || 3 || 150 || based on test, increase area of tab or change lock function | | User is not able to squeeze tabs and generate enough friction to turn the knob || User cannot open door || 10 || Contact area of tabs is too small || 5 || Test the amount of force needed to squeeze tabs and turn knob || 3 || 150 || based on test, increase area of tab or change lock function | ||
+ | |- | ||
+ | ! Door Knob Cover Design<br /> • Is unusable by child but usable by adult. | ||
+ | | Child manages to actually engage the doorknob despite the cover || Child can open door || 10 || Design is too simple to deter child || 2 || Test the amount of force needed to squeeze tabs and turn knob || 3 || 60 || based on test, increase area of tab or change lock function | ||
|- | |- | ||
! Door Knob Cover Latches<br /> • Connects too halves of cover together | ! Door Knob Cover Latches<br /> • Connects too halves of cover together | ||
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| Latch assembly rusts or fills with debris || Latch fails to retract || 9 || Exposure to environment || 3 || Lubricate assembly || 1 || 27 || n/a | | Latch assembly rusts or fills with debris || Latch fails to retract || 9 || Exposure to environment || 3 || Lubricate assembly || 1 || 27 || n/a | ||
|} | |} | ||
- | |||
- | |||
- | |||
- | |||
== Quantitative Mechanical Analysis == | == Quantitative Mechanical Analysis == | ||
- | + | The amount of perpendicular force applied to the doorknob plays an important role when turning a doorknob covered with a protector. The protector allows free spinning when force is not applied. This is results in selective door entrance/exit as young children are unable to grip the knob. | |
[[Image:doorknob_cover.jpg]] [[Image:doorknob_diagram.jpg]] | [[Image:doorknob_cover.jpg]] [[Image:doorknob_diagram.jpg]] | ||
+ | <br /> | ||
+ | The yellow arrows in the diagram above shows frictional force generated between the contact points, whereas F is the force applied by grip.<br /> | ||
+ | <br /> | ||
+ | ''Friction = u*N'', where u ~= 0.6 <br /> | ||
+ | ''Torque = r * Friction'', where r = 1.063 in <br /> | ||
+ | <br /> | ||
+ | F1 = minimum force per unit area required to turn doorknob with cover <br /> | ||
+ | F2 = minimum force per unit area required to turn doorknob without cover <br /> | ||
+ | F1 - F2 = additional force required to open door when cover is used <br /> | ||
+ | <br /> | ||
+ | A bathroom scale is used to measure the minimum amount of force required to turn a covered doorknob. By squeezing the doorknob until it turns, we applied the same amount of force on the scale. The scale shows force in pounds, and this is our estimated amount of force required to operate the doorknob lock. <br /> | ||
- | + | [[Image:Bathroomscale.jpg]] | |
- | + | ||
- | + | ||
- | + | ||
- | + | ||
- | [[Image: | + | |
- | + | '''Doorknob With Protector'''<br /> | |
+ | F1 = ~9.8 lbs <br /> | ||
+ | A minimum grip of 9.8 lbs is required to operate the doorknob with the protector attached. <br /> | ||
+ | Friction = 0.6*9.8 = 5.88 lbs <br /> | ||
+ | Torque = 1.063*5.88 = 6.25 lb-in | ||
+ | <br /> | ||
- | A | + | '''Doorknob Without Protector'''<br /> |
+ | F2 = ~4.8 lbs <br /> | ||
+ | A minimum grip of 4.8 lbs is required to operate the doorknob when the protector is not attached.<br /> | ||
+ | Friction = 0.6*4.8 = 2.88 lbs<br /> | ||
+ | Torque = 1.063*2.88 = 3.06 lb-in | ||
+ | <br /> | ||
- | + | '''Additional Force/Torque Required''' <br /> | |
+ | F1 - F2 = 5 lbs <br /> | ||
+ | Additional Torque = 6.25 - 3.06 = 3.19 lb-in | ||
+ | <br /> | ||
+ | By adding the doorknob protector, an extra 5 lbs of force is needed to open the door. This extra force requirement successfully deters most children from operating the doors. | ||
== DFE == | == DFE == |
Current revision
Contents |
Executive Summary
Babyproof door knob locks are a common method in many households to restrain children within a certain area. This information page analyzes both the babyproof lock in addition to type of door knob to which it is attached. By fully understanding the consumer needs in addition to the exact function and manufacturing methods of the system under study our team hopes to innovate and improve upon products already available for purchase. By not only analyzing the babyproofing cover but also studying the complex mechanism of the doorknob itself, this study is provided extra depth.
A doorknob's primary purpose is to allow selective entry and exit between connected rooms. This simple of objective has evolved a complex mechanism with many points for possible failure. Locking and unlocking, the doorknob-to-door interface, and even the impact of rotation must be accommodated by the doorknob within a reasonable failure tolerance. Via product dissection, a deeper understanding of each item of the assembly is gained. FMEA allowed possible failure areas to be outlined and ranked while DFMA structured possible improvements in a systematic fashion.
A babyproof door knob lock's initial simplicity is complicated by its defined interaction with the doorknob mentioned previously. Deconstruction and analysis must always take into consideration what effect each component has upon a doorknob. Varying standards in size, quality and complexity of doorknobs themselves complicate the manufacture of the door knob locks. By combining the information from both of these products, a comprehensive improvement plan can be produced that minimizes parts and cost while maximizing effectiveness.
Based on FMEA the major issue encountered with the use of babyproof door covers is difficulty in actual operation by an adult. The small size of the rubber 'friction tabs' used is uncomfortable and ill-suited for casual operation. The recommended solution for this problem is to increase the tab size so that the operator has a larger surface area through which to exert force. The second area of concern during FMEA analysis is the possible fire resistance properties of the plastic used to construct the door knob cover. Should the plastic not be fire retardant, in the case of an emergency door operation would become next to impossible and if achieved cause extreme injury to the user. Selection of a fire proof plastic would solve this possible manufacture issue.
During DFMA analysis it was found while the door knob cover is a relatively simple mechanism to assemble and failure easily identified, the door knob is much more challenging. The most problematic area is the latch housing of the door knob, ironically where failure is very likely to occur. The individual components are housed within a long hollow tube and once removed are unable to retain their assembly order. In addition to making it difficult for casual repair in case of failure it also seems very difficult from an assembly line perspective. However, the overall structure of a doorknob is user friendly. The complex organization of the doorknob's inner workings are housed in such a way to convey a since of simplicity for the user and make installation easy. Overall door knobs cater extremely well to the casual market where consumers will probably replace a door knob when broken rather than attempting repairs themselves.
Finally, DFE was performed to analyze areas for possible environmental improvement. Using EIOLCA software, emissions from both the doorknob and the doorknob covers were compared. The final recommendation that the doorknob cover be incorporated into the doorknob comes from comparison of both toxic and geen house gas emissions associated with plastic manufacture. The magnitude of gas emission sin plastic manufacture are an order of magnitude higher for plastic manufacture than metal. Therefore, by incorporating the functionality of a baby-proof doorknob cover into the doorknob eliminates the need for plastic manufacture, cutting down on overall gas emissions.
Customer Needs
A customer with a toddler needs to safeguard their home while the child is in a stage where they want to explore their surroundings and abilities but are unaware of hidden dangers. To deter a child from opening or closing a door but still make it as accessible to normal adult traffic as possible, a special mechanism is needed, potentially on the doorknob. Customers need to also take into consideration the type of door knob (knob, lever, etc.), it's size, shape, and whether or not the knob has a lock that they also want to easily access in other situations.
This product is made of simple, washable plastic. When applied on doorknob, the cover spins loose to prevent a firm grip. Adults can easily turn the doorknob by squeezing the two tabs on the side. The doorknob protector also includes special tabs to prevent children from accessing the lock. To function the lock, simply open the tab and access the lock.
Product failure is inevitable, but highly detectable. An average doorknob protector has a lifespan of approximately 6-12 months, if used regularly. A replacement of the product is strongly recommended when failure occurs. Small parts, if broken, may cause serious injuries. Older children may be able to grip doorknob cover without assistance.
Product Use
An uncovered, unlocked doorknob is simply turned to open or close a door. When the user grasps and turns the doorknob, the spindle also rotates, pushing forward the sliding plate.When the plate is pushed forward, the cam is rotated and pulls the latch back.
To deter a small child from doing so, there are two different types of covers we're exploring. The first is specifically for round doorknobs. It covers the knob loosely so that it spins freely unless tabs are pressed on its side that creates friction between the cover and knob, causing the knob to turn when the cover is turned. The second is for levers. There is a lever cover and a "lock". This lock is a latch that is attached to the door above the knob with a tab that fits into a notch in the cover. The tab prevents the cover from moving unless the tab is lifted.
The effectiveness of the product depends on children's strength and their ability to learn from observation. The round protector requires a large force to be exerted on the tabs in order to operate the door, whereas the lever doorknob protector can be operated by simply lifting the gray latch and turning the lever arm. Most children are probably able to operate the lever doorknob successfully after seeing their parents or guardian use it, even as young as a 2-year-old baby. It is true that adults with physical disabilities such as arthritis may be unable to operate the doorknob protector. Also, since the lever protector requires two hands, it may be an obstacle for adults if they are opening the door with one hand. Other possible difficulties include installing and removing the protector. The lever protector consists of two different plastic parts, in which one of the two parts is actually drilled into the door. The round doorknob protector requires lining of the tabs when installing. Frequent installation and removal of both doorknob protectors may decrease the durability and life expectancy of product.
Product Dissection
DFMA
In this section we will address the door knob and its baby proof cover's design for manufacture and assembly.
Design For Manufacture
Door Knob:
• Parts such as the latch plate and strike plate that are single flat pieces that are stamped and cut most likely come from a single sheet of metal, minimizing waste
• Springs usually come pre-manufactured as one large coil that can be cut to the length desired. The screws, bolts, rivets, and pins also most likely were pre-manufactured by an external provider.
• The doorknob does contain a number of individually molded parts such as the latch, turning shaft, and roses.
Door Knob Cover:
• The frame of the cover is injected molded in two halves.
•
Design for Assembly
Door Knob:
• The latch assembly was not well designed for assembly. There are several parts that must be put into place deep inside of the latch housing, but since the housing is a tube-like structure, that appears very difficult. The pieces inside the latch assembly do not appear to fit firmly together outside of the housing, so it is doubtful that the mechanism was placed together and then placed wholly into the housing. To improve the latch assembly, rather than having a hollow tube being in once piece, they should be separated into two halves, the assembly can then be placed into one half, the second half can be placed on top and fastened together.
• Inner and outer doorknob are simpler, having smaller assemblies placed into larger assemblies such as the torsional spring placed into the two halves of the spring housing which in turn was placed into the rose. Many of these pars are press fitted to each other.
• The whole door knob assembly is sold to the consumer in three main parts not including the latch plate and strike plate. This is because of the standard way the door knobs and latch assembly are placed into the door relevant to each other. The latch assembly slides into a hole in the side of the door that faces the door frame with the latch protruding from the hole. The outer and inner doorknobs are then placed on the outer and inner side of the door relatively. To connect to each other and make the latch function, the inner and outer doorknobs must reach through the latch assembly.
Door Knob Cover:
• The Door knob cover only requires assembly by the user.
FMEA
FMEA is an analysis tool used to assess the possible failure modes of a product. Each possible failure is rated for its Severity (S), Occurrence Rate (O) and Detectability (D). These numbers are then multiplied together to define the Risk Priority Number (RPN). This overall number is used to rank the priority in which failure modes are to be addressed.
Following FMEA the failure with the highest RPN was surprisingly inability for an adult to easily use a baby door knob cover. Difficulty could be encountered in exerting enough normal force via the 'friction tabs' on the door knob cover to create enough friction to rotate the doorknob. The recommended solution for this problem is to increase the size of the pads to make area for exerting normal force or change the locking function.
The items with the second highest RPN numbers were the flame resistance of the door knob cover and inability of the cover to adjust for non-standard sized door knobs. Upon testing, if the plastic used to manufacture the doorknob covers is found to not be flame retardant a comparable plastic can be found that has fire resistant properties. For the problem of nonstandard doorknobs redesigns will be necessary to accommodate nonstandard doorknobs while also possibly incorporating lever functionality.
Material/Analysis | Doorknob | Doorknob Protector |
---|---|---|
Who Are the Stake Holders? |
Homeowners and families |
Moms, dads, guardians, children, anyone in the house |
What it Does and How it is Used |
Secures a door |
"Deter curious children from entering rooms or closets that might contain a danger" |
Item and Function | Failure Mode | Effects of Failure | S | Causes of Failure | O | Design Controls | D | RPN | Recommended Actions | |
---|---|---|---|---|---|---|---|---|---|---|
Door Knob Cover Tabs • Is squeezed by adult user to create friction between the tabs and the door knob. | User is not able to squeeze tabs and generate enough friction to turn the knob | User cannot open door | 10 | Contact area of tabs is too small | 5 | Test the amount of force needed to squeeze tabs and turn knob | 3 | 150 | based on test, increase area of tab or change lock function | |
Door Knob Cover Design • Is unusable by child but usable by adult. | Child manages to actually engage the doorknob despite the cover | Child can open door | 10 | Design is too simple to deter child | 2 | Test the amount of force needed to squeeze tabs and turn knob | 3 | 60 | based on test, increase area of tab or change lock function | |
Door Knob Cover Latches • Connects too halves of cover together | Latches break | Two halves are not able to connect to each other properly | 7 | Material fatige | 2 | Test maximum force latches can endure | 1 | 14 | based on test, redesign tabs | |
Door Knob Cover Material • Is injected molded PC plastic | Plastic may not be fire resistant | Impedes exit in case of fire | 10 | Material failure due to heat | 5 | Test peak temperature resistance | 1 | 50 | based on test, choose material or coating | |
Shape of Door Knob Cover • Cover is round and shaped based on average diameter of door knob | Despite size of cover being based on an average, door knob sizes can vary widely creating an ill fit. Also, is not designed for levers | Cover may not be usable on that particular door knob | 8 | Design limitation | 5 | Test different sized door knobs | 1 | 50 | based on test, consider design changes and compatibility with levers | |
Door knob latch • Secures door against door jam in closed positon, retracts when knob is turned to allow door to open | Latch assembly rusts or fills with debris | Latch fails to retract | 9 | Exposure to environment | 3 | Lubricate assembly | 1 | 27 | n/a |
Quantitative Mechanical Analysis
The amount of perpendicular force applied to the doorknob plays an important role when turning a doorknob covered with a protector. The protector allows free spinning when force is not applied. This is results in selective door entrance/exit as young children are unable to grip the knob.
The yellow arrows in the diagram above shows frictional force generated between the contact points, whereas F is the force applied by grip.
Friction = u*N, where u ~= 0.6
Torque = r * Friction, where r = 1.063 in
F1 = minimum force per unit area required to turn doorknob with cover
F2 = minimum force per unit area required to turn doorknob without cover
F1 - F2 = additional force required to open door when cover is used
A bathroom scale is used to measure the minimum amount of force required to turn a covered doorknob. By squeezing the doorknob until it turns, we applied the same amount of force on the scale. The scale shows force in pounds, and this is our estimated amount of force required to operate the doorknob lock.
Doorknob With Protector
F1 = ~9.8 lbs
A minimum grip of 9.8 lbs is required to operate the doorknob with the protector attached.
Friction = 0.6*9.8 = 5.88 lbs
Torque = 1.063*5.88 = 6.25 lb-in
Doorknob Without Protector
F2 = ~4.8 lbs
A minimum grip of 4.8 lbs is required to operate the doorknob when the protector is not attached.
Friction = 0.6*4.8 = 2.88 lbs
Torque = 1.063*2.88 = 3.06 lb-in
Additional Force/Torque Required
F1 - F2 = 5 lbs
Additional Torque = 6.25 - 3.06 = 3.19 lb-in
By adding the doorknob protector, an extra 5 lbs of force is needed to open the door. This extra force requirement successfully deters most children from operating the doors.
DFE
The analysis for this section is broken down into DFE for the doorknob and DFE for the doorknob protector. The first analysis consists of just the doorknob. The following tables show the data obtained using the 1997 purchaser price model in the EIOLCA for $1 million worth of “Metal household furniture manufacturing”
Figure 1 EIOLCA data for toxic gas emissions
Figure 2 EIOLCA data for greenhouse gas emissions
Figure 3 EIOLCA data for conventional air pollution
Figure 4 EIOLCA data for energy consumption
The accuracy of these approximations is fairly rough as doorknobs do not necessarily take part in all sectors that contribute to certain emissions, ie paper mills & copper smelting. However, this data gives a rough idea of sectors that have the largest effect on environmental areas.
The following DFE is for the doorknob protector. The following tables show the data obtained using the 1997 purchaser price model in the EIOLCA for $1 million worth of “Plastic material and resin manufacturing”
Figure 5 EIOLCA data for toxic gas emissions
Figure 6 EIOLCA data for greenhouse gas emissions
Figure 7 EIOLCA data for conventional air pollution
Figure 8 EIOLCA data for energy consumption
These approximations are likely more accurate as the babyproof doorknob covers are made of plastics and likely undergo processes extremely similar to those in the industry area selected. Comparison of the two industries shows that green house and toxic gas emissions are the largest factors during manufacture. If the doorknob cover can be incorporated into the doorknob itself, it will cut down on emissions resulting from manufacture. The use of metal alloys instead of possibly non biodegradable plastics will also eliminate pollution upon end of life.
24-441 Engineering Design Course, Fall 2007, Carnegie Mellon University