From DDL Wiki
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.
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.
Product Use
An uncovered, unlocked doorknob is simply turned to open or close a door. 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.
System Function
(include pictures)
Bill of Materials (Doorknob)
# | Name | Origin | Weight (oz) | Material | Manufacturing Process | Function | photo
|
1
| circular wedge | latch assembly | 0.2 | Aluminum | stamped | acts as a wedge to keep latch assembly firmly in place inside door |
|
2
| pop rivot | latch assembly | <0.1 | brass | molded | holds latch assembly together |
|
3
| turning shaft | outer doorknob | 0.7 | steel | molded | connects outer doorknob, inner doorknob, and latch assembly to retract latch |
|
4
| screws x2 | outer doorknob | 0.1 | brass plated | cold heading, thread wooling | connects outer doorknob rose to inner doorknob rose |
|
5
| latch plate | latch assembly | 0.5 | brass | stamped | covers area of the door where the latch actually protrudes |
|
6
| strike plate | latch assembly | 0.7 | brass | stamped and bent | covers inside of door frame with hole for latch
|
|
7
| spring | inner doorknob | <0.1 | harden steel, nickel based alloy | wound, prehardened or hardened during fabrication | guides the lock by creating resistance in a locking shaft and keeping it aligned in a "locked" or "unlocked" position |
|
8
| lock shaft | inner doorknob | 0.1 | plastic and brass | two molded parts, press fitted | turns shaft into "lock" or "unlock" position |
|
9
| ring | inner doorknob | 0.1 | aluminum | stamped | serves as a buffer between knob spring and spring housing |
|
10
| inner doorknob torsional spring | inner doorknob | <0.1 | steel | molded and wrapped | creates resistance when turning doorknob, prevents free spinning |
|
11
| spring housing | inner doorknob | 0.2 | brass | stamped and bent | houses and secures torsional spring |
|
12
| latch assembly outer plate | latch assembly | 0.5 | brass | stamped | attaches to latch plate |
|
13
| latch assembly inner plate | latch assembly | 0.2 | brass | stamped | is the closest plate to the actual lock assembly; wedges between #12 and the door |
|
14
| latch | latch assembly | 0.6 | steel | molded | latches door to door jam |
|
15
| lock attachment | latch assembly | <0.1 | steel | molded | attached to latch; when locked, it does not slide preventing latch from sliding |
|
16
| face for outer doorknob | outer doorknob | 0.1 | brass | stamped | aesthetically pleasing |
|
17
| hook | latch | <0.1 | brass | stamped | hooks to latch or lock attachment |
|
18
| large hooks x2 | latch | 0.1 | steel | stamped | pulls back latch |
|
19
| hook mechanism plate | latch | <0.1 | plastic | molded/injection molding | hooks #18 to latch |
|
20
| small hook from lock | latch | <0.1 | brass | molded | (unknown use) |
|
21
| pin | latch | <0.1 | brass | stamped and bent | adjusts locking mechanism between two different lengths |
|
22
| black plastic | inner doorknob | <0.1 | plastic | molded | (unknown use) |
|
23
| locking mechanism housing | latch | 0.6 | brass | molded and hole punched | houses small pieces in latch |
|
24
| short pin | latch | <0.1 | brass | molded | slides back and forth when size is adjusted |
|
25
| plastic hooks | latch | <0.1 | plastic | molded | slides back and forth when size is adjusted, may pull back when hook is tilted to latch |
|
26
| latch housing | latch | 0.6 | brass | molded and hole punched | houses mechanism controlling hooks for latch |
|
27
| sliding plate | latch | 0.2 | brass | molded and hole punched | slides back and forth, may align holes to those in the door or second housing piece |
|
28
| two lock springs | latch | <0.1 | steel/nickel alloy | wound | impacts latch mechanism |
|
29
| inner doorknob ring | inner doorknob | <0.1 | aluminum | stamped and molded | locks doorknob into rose, locks doorknob's cylindrical connector |
|
30
| inner doorknob back plate | inner doorknob | 0.4 | brass | stamped and bent | attaches doorknob to door, acts as faceplate |
|
31
| inner doorknob rose | inner doorknob | 1.3 | brass | stamped and bent | connects door to doorknob, attaches and connects inner and outer doorknobs |
|
32
| inner doorknob | inner doorknob | 1.7 | brass | molded/pressed/clamped | it is gripped by the user to engage latch from inside the door |
|
33
| spring housing | outer doorknob | 0.4 | brass | stamped and bent | houses spring mechanism on the outer doorknob |
|
34
| bottom spring plate | outer doorknob | <0.1 | steel | stamped | spring pushes against it to create torque |
|
35
| top spring plate | outer doorknob | <0.1 | steel | stamped | spring pushes against it to create torque |
|
36
| outer knob torsional spring | outer doorknob | <0.1 | steel | molded/wrapped | creates resistance when turning doorknob to prevent free spinning of the knob |
|
37
| outer doorknob back plate | outer doorknob | 0.4 | brass | stamped and bent | attaches doorknob to door, acts as a faceplate |
|
38
| outer doorknob rose | outer doorknob | 3.0 | steel | molded and tapped | houses screws and connects inner doorknob to outer doorknob |
|
39
| outer doorknob ring | outer doorknob | <0.1 | aluminum | stamped and molded | secures doorknob connector into rose |
|
40
| cylindrical connector | inner doorknob | 0.8 | steel | molded | connects doorknob to rose and secured with ring |
|
41
| spindle | outer doorknob | 1.0 | brass plated steel | molded | semi-cylinder that wraps around the turning shaft |
|
42
| outer doorknob | outer doorknob | 1.6 | brass | molded/pressed/clamped | is gripped by user to engage latch from outside the door |
|
43
| cone-shaped connector | outer doorknob | 1.5 | steel | molded | secures outer doorknob to rose and houses locking mechanism, spindle, and locking shaft |
|
44
| lock mechanism | outer doorknob | 2.4 | steel/aluminum/other | stamped/sealed/molded/assorted | mechanism engages lock |
|
DFMA
FMEA
Material/Analysis | Doorknob | Doorknob Protector
|
Who Are the Stake Holders?
|
Homeowners and families
anyone who wishes to enter or exit a door
anyone who wants to secure a door
|
Moms, dads, guardians, children, anyone in the house
|
What it Does and How it is Used
|
Secures a door
Allows selective entry or exit
Steps:
1. Walk to a door
2. Lift hand
3. Grasp doorknob firmly
4. Twist the knob away from door jam
5. Push or pull the door open/close
|
"Deter curious children from entering rooms or closets that might contain a danger"
|
Potential Failures
|
If latch disconnects or mis-aligns, door does not open
Loosening of the knob
Material fatigue
Rusting and sticking
Loss of spring constant
|
Potential Failures:
Cracking on tabs/components while putting on or removing product from doorknob
Inadequate contact between doorknob and doorknob cover
Melting at high temperatures
Child may be able to grip the cover
Size differences (too big or too small)
Deformation/fatigue of tabs or repeated usage
Chemical breakdown of plastic
|
How Failure Affects Consumers
|
Doorknob becomes difficult or impossible to operate
Doors may or may not be opened
Repair or replacement
|
Not reparable
Small parts, if broken, may cause serious injuries
|
How Dangerous the Failures Would Be
|
Cannot leave room during an emergency
No longer reserving privacy
Small parts may be swallowed by children
|
Kids swallow small parts, if broken
Kids may be able to grip the cover
Sharp edges may cut people
Emergency operations, could put children in risk
People with prosthesis cannot open door
|
How Likely/Often Failure Would Occur
|
5~60 years
|
Inevitable failure
Depends on usage frequency
Approximately 6 months
|
Which component fails?
|
Knob detaches from door
Shaft jamming
Latch does not function
Rivot failures: rivots hold many necessary pieces together in the latch
- 3 (turning shaft) may fail
Screws that attach latch and doorknob to door (wood-knob interface)
|
Tabs, grab tab buttons
Key hole cover, lock cover tab
Snap locks
If on fire, the plastic melts (possibility)
Cracking of structure when external force exceeds limit
|
Conditions during failure
|
Damage to door jam, wood cracking
Dead-bolting doors to keep it open
Exerting force inappropriately
Aging of doorknob
Material failure
|
Weak grip
Fire
Fracture
|
How detectable failure is
|
Not always visible
Able to feel difference in resistance when operating
If knob detaches, it is detectable
|
Failure is obvious
|
Cost to correct failure
|
Replace of repair (replace when cheap, repair if original was expensive or warranted)
|
New product
Material recycling
|
Maintenance related failure
|
Oil with WD40 on latch
Use appropriately
Re-tighten screws when necessary
Cleaning
|
Temperature range
Reactions to chemicals
Cleaning precautions (eg. resistance to dish washers, detergents, etc.)
|
Quantitative Mechanical Analysis
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.
Production | Transportation | Use | End of Life
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cell 1,1 | cell 1,2 | blank | blank
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cell 2,1 | cell 2,2 | blank | blank
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Bill of Materials (doorknob protector)
# | Name | Origin | Weight | Material | Manufacturing Technique | Function
|
45
| blank | blank | -- | blank | blank | blank
|
46
| blank | blank | -- | blank | blank | blank
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47
| blank | blank | -- | blank | blank | blank
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48
| blank | blank | -- | blank | blank | blank
|
49
| blank | blank | -- | blank | blank | blank
|
50
| blank | blank | -- | blank | blank | blank
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24-441 Engineering Design Course, Fall 2007, Carnegie Mellon University