Babyproof door knob lock

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Image:knobcover.jpg Image:doorknob.jpg


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.
Image:knobnspindle.jpg Image:camlatchslidingplate.jpg

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

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 Image:doorknob_part01.jpg
2 pop rivot latch assembly <0.1 brass molded holds latch assembly together Image:doorknob_part02.jpg
3 turning shaft outer doorknob 0.7 steel molded connects outer doorknob, inner doorknob, and latch assembly to retract latch Image:doorknob_part03.jpg
4 screws x2 outer doorknob 0.1 brass plated cold heading, thread wooling connects outer doorknob rose to inner doorknob rose Image:doorknob_part04.jpg
5 latch plate latch assembly 0.5 brass stamped covers area of the door where the latch actually protrudes Image:doorknob_part05.jpg
6 strike plate latch assembly 0.7 brass stamped and bent covers inside of door frame with hole for latch Image:doorknob_part06.jpg
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 Image:doorknob_part07.jpg
8 lock shaft inner doorknob 0.1 plastic and brass two molded parts, press fitted turns shaft into "lock" or "unlock" position Image:doorknob_part08.jpg
9 ring inner doorknob 0.1 aluminum stamped serves as a buffer between knob spring and spring housing Image:doorknob_part09.jpg
10 inner doorknob torsional spring inner doorknob <0.1 steel molded and wrapped creates resistance when turning doorknob, prevents free spinning Image:doorknob_part10.jpg
11 spring housing inner doorknob 0.2 brass stamped and bent houses and secures torsional spring Image:doorknob_part11.jpg
12 latch assembly outer plate latch assembly 0.5 brass stamped is attached to latch; the latch plate is placed on top of it Image:doorknob_part12.jpg
13 latch assembly inner plate latch assembly 0.2 brass stamped is the closest plate to the actual latch assembly; wedges between the latch assembly outer plate and the door Image:doorknob_part13.jpg
14 latch latch assembly 0.6 steel molded latches door to door jam Image:doorknob_part14.jpg
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 face for outer doorknob outer doorknob 0.1 brass stamped covers outer doorknob hiding the cone shape connector; aesthetically pleasing Image:doorknob_part16.jpg
17 hook latch assembly <0.1 brass stamped hooks to latch or lock attachment Image:doorknob_part17.jpg
18 large hooks x2 latch assembly 0.1 steel stamped pulls back cam which pulls back latch Image:doorknob_part18.jpg
19 hook mechanism plate latch assembly <0.1 plastic molded/injection molding large hooks reach through it to latch; also contain small hook from lock Image:doorknob_part19.jpg
20 cam latch assembly <0.1 brass molded when hooks (#18) are pulled back, it pulls back latch Image:doorknob_part20.jpg
21 pin latch assembly <0.1 brass stamped and bent adjusts locking mechanism between two standard lengths that the hole in the side of the door that contains the latch assembly can be. Image:doorknob_part21.jpg
22 black plastic inner doorknob <0.1 plastic molded (unknown use) Image:doorknob_part22.jpg
23 locking mechanism housing latch 0.6 brass molded and hole punched houses small pieces in latch Image:doorknob_part23.jpg
24 short pin latch <0.1 brass molded slides back and forth when size is adjusted Image:doorknob_part24.jpg
25 adjusters latch <0.1 Delrin plastic molded are usually connected together, holds assembly together so that it slides back and forth when size is adjusted, is guided by pin (21) Image:doorknob_part25.jpg
26 latch housing latch 0.6 brass molded and hole punched houses mechanism controlling hooks for latch Image:doorknob_part26.jpg
27 sliding plate latch 0.2 brass molded and hole punched slides back as knob and spindle is turned, may also align holes to those in the door or second housing piece Image:doorknob_part27.jpg
28 two lock springs latch <0.1 steel/nickel alloy wound pushes latch and auxillary bolt into extended position Image:doorknob_part28.jpg
29 inner doorknob ring inner doorknob <0.1 aluminum stamped and molded locks doorknob into rose, locks doorknob's cylindrical connector Image:doorknob_part29.jpg
30 inner doorknob back plate inner doorknob 0.4 brass stamped and bent attaches doorknob to door, acts as faceplate Image:doorknob_part30.jpg
31 inner doorknob rose inner doorknob 1.3 brass stamped and bent connects door to doorknob, attaches and connects inner and outer doorknobs Image:doorknob_part31.jpg
32 inner doorknob inner doorknob 1.7 brass molded/pressed/clamped it is gripped by the user to engage latch from inside the door Image:doorknob_part32.jpg
33 spring housing outer doorknob 0.4 brass stamped and bent houses spring mechanism on the outer doorknob Image:doorknob_part33.jpg
34 bottom spring plate outer doorknob <0.1 steel stamped spring pushes against it to create torque Image:doorknob_part34.jpg
35 top spring plate outer doorknob <0.1 steel stamped spring pushes against it to create torque Image:doorknob_part35.jpg
36 outer knob torsional spring outer doorknob <0.1 steel molded/wrapped creates resistance when turning doorknob to prevent free spinning of the knob Image:doorknob_part36.jpg
37 outer doorknob back plate outer doorknob 0.4 brass stamped and bent attaches doorknob to door, acts as a faceplate Image:doorknob_part37.jpg
38 outer doorknob rose outer doorknob 3.0 steel molded and tapped houses screws and connects inner doorknob to outer doorknob Image:doorknob_part38.jpg
39 outer doorknob ring outer doorknob <0.1 aluminum stamped and molded secures doorknob connector into rose Image:doorknob_part39.jpg
40 cylindrical connector inner doorknob 0.8 steel molded connects doorknob to rose and secured with ring Image:doorknob_part40.jpg
41 spindle outer doorknob 1.0 brass plated steel molded semi-cylinder that wraps around the turning shaft Image:doorknob_part41.jpg
42 outer doorknob outer doorknob 1.6 brass molded/pressed/clamped is gripped by user to engage latch from outside the door Image:doorknob_part42.jpg
43 cone-shaped connector outer doorknob 1.5 steel molded secures outer doorknob to rose and houses locking mechanism, spindle, and locking shaft Image:doorknob_part43.jpg
44 lock mechanism outer doorknob 2.4 steel/aluminum/other stamped/sealed/molded/assorted mechanism engages lock Image:doorknob_part44.jpg

Bill of Materials (Doorknob Protector)
# Name Weight Material Manufacturing Technique Function photo
45 shell#1 -- plastic molded covers half of the doorknob Image:Doorknob_protector01.jpg
46 shell#2 -- plastic molded covers half of the doorknob Image:Doorknob_protector02.jpg
47 contact pads (4x) -- rubber chemically synthesized/injected to plastic creates friction between doorknob and shell Image:doorknob_cover_rubber.jpg


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 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
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
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"
1. Walk to a door
2. Lift hand
3. Grasp door knob cover with thumb and fore finger on tabs 4. Squeeze tabs until they rest firmly against the door knob
5. Twist the knob away from door jam
6. Push or pull the door open/close

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.

Image:doorknob_cover.jpg Image:doorknob_diagram.jpg
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.


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

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