Babyproof door knob lock
From DDL Wiki
(→FMEA (doorknob protector)) |
Current revision (01:26, 12 October 2007) (view source) (→Quantitative Mechanical Analysis) |
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+ | [[Image:knobcover.jpg]] | ||
+ | [[Image:doorknob.jpg]] | ||
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== Executive Summary == | == 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 == | == 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 == | == 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. <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 == |
+ | |||
+ | {| border="1" style="text-align:center" | ||
+ | |+ '''<big>Bill of Materials (Doorknob)</big>''' | ||
+ | ! # !! Name !! Origin !! Weight (oz) !! Material !! Manufacturing Process !! Function || photo | ||
|- | |- | ||
! 1 | ! 1 | ||
- | | circular wedge || latch || | + | | 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 | ! 2 | ||
- | | pop rivot || latch || | + | | pop rivot || latch assembly || <0.1 || brass || molded || holds latch assembly together || [[Image:doorknob_part02.jpg]] |
|- | |- | ||
! 3 | ! 3 | ||
- | | turning shaft || outer doorknob || | + | | turning shaft || outer doorknob || 0.7 || steel || molded || connects outer doorknob, inner doorknob, and latch assembly to retract latch || [[Image:doorknob_part03.jpg]] |
|- | |- | ||
! 4 | ! 4 | ||
- | | screws x2 || outer doorknob || | + | | 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 | ! 5 | ||
- | | latch plate || latch || | + | | latch plate || latch assembly || 0.5 || brass || stamped || covers area of the door where the latch actually protrudes || [[Image:doorknob_part05.jpg]] |
|- | |- | ||
! 6 | ! 6 | ||
- | | strike plate || latch || | + | | strike plate || latch assembly || 0.7 || brass || stamped and bent || covers inside of door frame with hole for latch |
+ | || [[Image:doorknob_part06.jpg]] | ||
|- | |- | ||
! 7 | ! 7 | ||
- | | spring || inner doorknob || | + | | 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 | ! 8 | ||
- | | lock shaft || inner doorknob || | + | | 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 | ! 9 | ||
- | | ring || inner doorknob || | + | | ring || inner doorknob || 0.1 || aluminum || stamped || serves as a buffer between knob spring and spring housing || [[Image:doorknob_part09.jpg]] |
|- | |- | ||
! 10 | ! 10 | ||
- | | inner doorknob torsional spring || inner doorknob || | + | | 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 | ! 11 | ||
- | | spring housing || inner doorknob || | + | | spring housing || inner doorknob || 0.2 || brass || stamped and bent || houses and secures torsional spring || [[Image:doorknob_part11.jpg]] |
|- | |- | ||
! 12 | ! 12 | ||
- | | outer plate | + | | 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 | ! 13 | ||
- | | inner plate | + | | 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 | ! 14 | ||
- | | latch || latch || | + | | latch || latch assembly || 0.6 || steel || molded || latches door to door jam || [[Image:doorknob_part14.jpg]] |
|- | |- | ||
! 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 | ||
- | | face for outer doorknob || outer doorknob || | + | | 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 | ! 17 | ||
- | | hook || latch || | + | | hook || latch assembly || <0.1 || brass || stamped || hooks to latch or lock attachment || [[Image:doorknob_part17.jpg]] |
|- | |- | ||
! 18 | ! 18 | ||
- | | large hooks x2 || latch || | + | | large hooks x2 || latch assembly || 0.1 || steel || stamped || pulls back cam which pulls back latch || [[Image:doorknob_part18.jpg]] |
|- | |- | ||
! 19 | ! 19 | ||
- | | hook mechanism plate || latch || | + | | 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 | ! 20 | ||
- | | | + | | cam || latch assembly || <0.1 || brass || molded || when hooks (#18) are pulled back, it pulls back latch || [[Image:doorknob_part20.jpg]] |
|- | |- | ||
! 21 | ! 21 | ||
- | | pin || latch || | + | | 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 | ! 22 | ||
- | | black plastic || inner doorknob || | + | | black plastic || inner doorknob || <0.1 || plastic || molded || (unknown use) || [[Image:doorknob_part22.jpg]] |
|- | |- | ||
! 23 | ! 23 | ||
- | | locking mechanism housing || latch || | + | | locking mechanism housing || latch || 0.6 || brass || molded and hole punched || houses small pieces in latch || [[Image:doorknob_part23.jpg]] |
|- | |- | ||
! 24 | ! 24 | ||
- | | short pin || latch || | + | | short pin || latch || <0.1 || brass || molded || slides back and forth when size is adjusted || [[Image:doorknob_part24.jpg]] |
|- | |- | ||
! 25 | ! 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 | ! 26 | ||
- | | latch housing || latch || | + | | latch housing || latch || 0.6 || brass || molded and hole punched || houses mechanism controlling hooks for latch || [[Image:doorknob_part26.jpg]] |
|- | |- | ||
! 27 | ! 27 | ||
- | | sliding plate || latch || | + | | 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 | ! 28 | ||
- | | two lock springs || latch || | + | | two lock springs || latch || <0.1 || steel/nickel alloy || wound || pushes latch and auxillary bolt into extended position || [[Image:doorknob_part28.jpg]] |
|- | |- | ||
! 29 | ! 29 | ||
- | | inner doorknob ring || inner doorknob || | + | | 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 | ! 30 | ||
- | | inner doorknob | + | | inner doorknob back plate || inner doorknob || 0.4 || brass || stamped and bent || attaches doorknob to door, acts as faceplate || [[Image:doorknob_part30.jpg]] |
|- | |- | ||
! 31 | ! 31 | ||
- | | inner doorknob rose || inner doorknob || | + | | 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 | ! 32 | ||
- | | inner doorknob || inner doorknob || | + | | 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 | ! 33 | ||
- | | spring housing || outer doorknob || | + | | spring housing || outer doorknob || 0.4 || brass || stamped and bent || houses spring mechanism on the outer doorknob || [[Image:doorknob_part33.jpg]] |
|- | |- | ||
! 34 | ! 34 | ||
- | | bottom spring plate || outer doorknob || | + | | bottom spring plate || outer doorknob || <0.1 || steel || stamped || spring pushes against it to create torque || [[Image:doorknob_part34.jpg]] |
|- | |- | ||
! 35 | ! 35 | ||
- | | top spring plate || outer doorknob || | + | | top spring plate || outer doorknob || <0.1 || steel || stamped || spring pushes against it to create torque || [[Image:doorknob_part35.jpg]] |
|- | |- | ||
! 36 | ! 36 | ||
- | | outer knob torsional spring || outer doorknob || | + | | 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 | ! 37 | ||
- | | outer doorknob | + | | 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 | ! 38 | ||
- | | outer doorknob rose || outer doorknob || | + | | 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 | ! 39 | ||
- | | outer doorknob ring || outer doorknob || | + | | outer doorknob ring || outer doorknob || <0.1 || aluminum || stamped and molded || secures doorknob connector into rose || [[Image:doorknob_part39.jpg]] |
|- | |- | ||
! 40 | ! 40 | ||
- | | cylindrical connector || inner doorknob || | + | | cylindrical connector || inner doorknob || 0.8 || steel || molded || connects doorknob to rose and secured with ring || [[Image:doorknob_part40.jpg]] |
|- | |- | ||
! 41 | ! 41 | ||
- | | spindle || outer doorknob || | + | | spindle || outer doorknob || 1.0 || brass plated steel || molded || semi-cylinder that wraps around the turning shaft || [[Image:doorknob_part41.jpg]] |
|- | |- | ||
! 42 | ! 42 | ||
- | | outer doorknob || outer doorknob || | + | | 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 | ! 43 | ||
- | | cone-shaped connector || outer doorknob || | + | | 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 | ! 44 | ||
- | | lock mechanism || outer doorknob || | + | | lock mechanism || outer doorknob || 2.4 || steel/aluminum/other || stamped/sealed/molded/assorted || mechanism engages lock || [[Image:doorknob_part44.jpg]] |
|} | |} | ||
- | |||
- | |||
- | |||
- | |||
- | |||
- | |||
- | ''' | + | {| border="1" style="text-align:center" |
- | < | + | |+ '''<big>Bill of Materials (Doorknob Protector)</big>''' |
- | < | + | |- |
- | + | ! # !! 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]] | ||
+ | |} | ||
- | + | == DFMA == | |
- | + | ||
- | + | ||
- | + | ||
- | + | ||
- | + | ||
- | ''' | + | In this section we will address the door knob and its baby proof cover's design for manufacture and assembly.<br /><br /> |
- | <br /> - | + | <big>'''Design For Manufacture'''</big><br /><br /> |
- | <br /> - | + | '''Door Knob:'''<br /> |
- | <br /> | + | • 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<br /> |
+ | • 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.<br /> | ||
+ | • The doorknob does contain a number of individually molded parts such as the latch, turning shaft, and roses.<br /> | ||
+ | '''Door Knob Cover:'''<br /> | ||
+ | • The frame of the cover is injected molded in two halves. | ||
+ | • <br /><br /> | ||
+ | <big>'''Design for Assembly'''</big><br /><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. 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 /> | ||
+ | • 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 /> | ||
+ | '''Door Knob Cover:'''<br /> | ||
+ | • 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. | |
- | + | ||
- | |||
- | + | {| class="wikitable" border="1" | |
- | + | ||
- | + | ||
- | + | ||
- | {| class="wikitable" border="1" | + | |
|- | |- | ||
- | ! | + | ! Material/Analysis !! Doorknob !! Doorknob Protector |
- | + | ||
- | + | ||
|- | |- | ||
- | | | + | ! Who Are the Stake Holders? |
- | + | | | |
+ | Homeowners and families <br /> | ||
+ | anyone who wishes to enter or exit a door <br /> | ||
+ | anyone who wants to secure a door | ||
+ | || | ||
+ | Moms, dads, guardians, children, anyone in the house <br /> | ||
- | + | |- | |
+ | ! What it Does and How it is Used | ||
+ | | | ||
+ | Secures a door <br /> | ||
+ | Allows selective entry or exit <br /> | ||
+ | Steps: <br /> | ||
+ | 1. Walk to a door <br /> | ||
+ | 2. Lift hand <br /> | ||
+ | 3. Grasp doorknob firmly <br /> | ||
+ | 4. Twist the knob away from door jam <br /> | ||
+ | 5. Push or pull the door open/close | ||
+ | || | ||
+ | "Deter curious children from entering rooms or closets that might contain a danger"<br /> | ||
+ | Steps: <br /> | ||
+ | 1. Walk to a door <br /> | ||
+ | 2. Lift hand <br /> | ||
+ | 3. Grasp door knob cover with thumb and fore finger on tabs | ||
+ | 4. Squeeze tabs until they rest firmly against the door knob<br /> | ||
+ | 5. Twist the knob away from door jam <br /> | ||
+ | 6. Push or pull the door open/close | ||
+ | |}<br /><br /> | ||
{| class="wikitable" border="1" | {| class="wikitable" border="1" | ||
|- | |- | ||
- | ! | + | ! 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 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<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 |
- | | | + | | 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 <br /> • 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 <br /> • 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 <br /> • 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]] | ||
+ | <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]] | ||
+ | |||
+ | '''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 /> | ||
+ | |||
+ | '''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 == | ||
+ | |||
+ | 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” | ||
+ | |||
+ | [[Image:Doorknob_DFE_Toxic.JPG]] | ||
+ | |||
+ | Figure 1 EIOLCA data for toxic gas emissions | ||
+ | |||
+ | |||
+ | [[Image:Doorknob_DFE_Greenhouse.JPG]] | ||
+ | |||
+ | Figure 2 EIOLCA data for greenhouse gas emissions | ||
+ | |||
+ | |||
+ | [[Image:Doorknob_DFE_Conventional.JPG]] | ||
+ | |||
+ | Figure 3 EIOLCA data for conventional air pollution | ||
+ | |||
+ | |||
+ | [[Image:Doorknob_DFE_Energy.JPG]] | ||
+ | |||
+ | 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” | ||
+ | |||
+ | [[Image:DoorknobCover_DFE_Toxic.JPG]] | ||
+ | |||
+ | Figure 5 EIOLCA data for toxic gas emissions | ||
+ | |||
+ | |||
+ | [[Image:DoorknobCover_DFE_Greenhouse.JPG]] | ||
+ | |||
+ | Figure 6 EIOLCA data for greenhouse gas emissions | ||
+ | |||
+ | |||
+ | [[Image:DoorknobCover_DFE_Conventional.JPG]] | ||
+ | |||
+ | Figure 7 EIOLCA data for conventional air pollution | ||
+ | |||
+ | |||
+ | [[Image:DoorknobCover_DFE_Energy.JPG]] | ||
+ | |||
+ | 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. | ||
+ | |||
+ | |||
+ | |||
+ | == == | ||
+ | <br />[[24-441 Engineering Design Course]], Fall 2007, [[Carnegie Mellon University]] | ||
+ | [[Category:Design studies]] |
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