Babyproof door knob lock

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Revision as of 04:07, 24 September 2007

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.

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
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	is attached to latch; the latch plate is placed on top of it
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
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	covers outer doorknob hiding the cone shape connector; aesthetically pleasing
17	hook	latch assembly	<0.1	brass	stamped	hooks to latch or lock attachment
18	large hooks x2	latch assembly	0.1	steel	stamped	pulls back latch
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
20	small hook from lock	latch assembly	<0.1	brass	molded	(unknown use)
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.
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

Bill of Materials (Doorknob Protector)

#	Name	Weight	Material	Manufacturing Technique	Function	photo
45	shell#1	--	plastic	molded	covers half of the doorknob
46	shell#2	--	plastic	molded	covers half of the doorknob
47	contact pad	--	rubber	chemically synthesized/injected to plastic	creates friction between doorknob and shell	Image:Doorknob protector03.jpg

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.
• 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

Quantitative Mechanical Analysis

Friction 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.

F1 = minimum force required to turn doorknob with cover F2 = minimum force required to turn doorknob without cover

F1 - F2 = additional force required to open door when cover is used

A hand grip exercise instrument is used to measure the minimum amount of force required to turn a covered doorknob. By squeezing the doorknob until it turns, we will apply the same amount of force on the gripper. A displacement between the grip handles, or x, will be measured to estimate the amount of force applied. Given the spring constant, k, we can find F.

F = kx, where x is the total displacement between the grip handles.

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