Door latch

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Executive Summary

Doorknobs are ubiquitous to households; most doors have doorknobs to allow people to either open or close them as a way to offer a level of security and privacy. In an effort to brainstorm improvements to doorknobs, a common household doorknob was purchased from a hardware store and studied.

For the example doorknob, a use study showed that the product has two main functions: (1) turning the knob retracts a latch that unhinges a door from its closed position, and (2) activating the lock prevents the knob from turning. It is important to note that activating the lock does not disable the door latch. In other words, the user can lock the door before closing it. Dissection showed that there were many small components that each had a single function, suggesting that the doorknob may be overly complicated for its intended function. As an example, the door hinge itself had a complicated spring mechanism simply to retract the hinge and restore it to the original position.

A Design for Manufacture and Assembly (DFMA), a Failure Modes and Effects Analysis (FMEA), and a Design for Environment (DFE) were performed to learn more about the doorknob. Results of the DFMA led us to believe that some parts could be consolidated to come from the same stock material under one manufacturing process (like stamping or casting). The dissection showed that the two doorknobs that come with the product are not identical, which is not ideal for manufacturing.

The FMEA study showed that with a Risk Priority Number (RPN) of 105, the user could break the lock on the doorknob, causing the knob to be disabled. As a result, the user could lock his self in the room.

The DFE analysis showed the packaging could be improved to maximize the number of products that could be delivered in one trip. It also showed that material reduction could be possible, but further studies like a Finite Element Analysis (FEA) may be necessary. Running the Environmental Input Output Life Cycle Assessment (EIO-LCA) showed that most of the environmental impact came from steel mills and power generation and supply, likely from processing the metal and running the plant. Now with some understanding of the product, we can begin to perform market research to find out what other users think of the doorknob in its current state.

Major Stakeholders

Major stakeholders for the doorknob include customers, manufacturers, distributors, and retailers. Their requirements are listed below.

Customers

Cheap to purchase
Reliable
Reasonably secure/offers some sense of privacy
Simple to operate
Ergonomic
Aesthetically pleasing
Easy to assemble and install
Easy to unlock from outside in case of emergency

Manufacturers

Use limited number of parts
Use limited types of parts
Use few manufacturing and assembling processes
Limit the number of parts for the consumer

Distributors

Lightweight to reduce energy consumption during transport
Packaging must be optimized to maximize number of products per load
Product must be strong enough to survive the delivery

Retailers

Packaging must be optimized for easy display
Products must be readily available

Customers clearly have the most requirements for the doorknob, simply because they are in contact with the product for the longest time. However, we should not undermine the importance of the requirements of the other major stakeholders of this product.

Product Use

Door latch in natural state

Door latch in retracted state

For the type of doorknob that the group is assessing, there are two components to the product: (1) the door latch and (2) the lock. The purpose of the door latch is to keep the door in the closed position. It is designed to be closed easily one way by simply pushing on the door. Moreover, the doorknob is designed to be opened in one way only. The lock is designed to prevent the knob from turning. It should be noted that the lock does not stop the latch itself from moving. This allows the user to lock the door before closing it. The lock gives the user a level of privacy and security, for example, while using the bathroom.

Lock

In order to prevent the doorknob from turning, the user can rotate the locking mechanism at any point. This does not stop the door latch from moving, so the door can be closed after locking the door. Once closed, the locking mechanism prevents the doorknob from turning; this locks the door to let outsiders know that access to the room is prohibited.

Latch

Rotating the doorknob turns a mechanism that converts rotational motion to a translational motion, which is connected to the door latch. The door latch makes contact with a metal plate to guide the door latch into place, as well as improve durability after extended operation. Springs are attached to the doorknob as well as the door latch to facilitate the knob reset.

Assembly

The figure below shows a fully disassembled view of the doorknob.

Bill of Materials

Door Accessories
New Part #	Part Name	QTY	Weight (g)	Function	Manufacturing Process	Material	Image
1	Latch Holder (Front)	1	13	Secures Door-Latch to Door	Stamping	Steel (Coated)	Latch Holder (Front)
2	Latch Holder (Back)	1	4	Secures Latch-Housing to Door Holder front	Stamping	Steel (Coated)	Latch Holder (Back)
3	Door Frame Plate	1	20	Attaches to door frame over hole, guides latch into position. Flat face holds latch in position	Stamped & Bent	Steel	Door Frame Plate
4	Frame Circular Latch Plate	1	6	Alternate Door holder front & back	Stamped & Bent	Steel	Frame Circular Latch Plate
Latch Mechanism
New Part #	Part Name	QTY	Weight (g)	Function	Manufacturing Process	Material	Image
5	Latch (Metal)	1	19	Keep door closed (retracted to allow opening)	Cast	Steel	Latch (Metal)
6	Latch (Plastic)	1	0.4	Low friction surface facilitates latch retraction	Injection Molded	Plastic (white)	center	Latch (Plastic)
7	Spring Latch	2	0.1	Returns latch to closed position	Purchased	Steel	Spring Latch
8	Spring Holder	1	0.6	Provides surface for latch springs to press against	Injection Molded	Plastic (black)	Spring Holder
9	Latch Frame Front	1	15	Contains spring holder, houses latch during retraction	Cast	Steel	Latch Frame Front
10	Front Metal Shifters	2	2	Moves along with Back Metal Shifter (#15). Pulls #10 back, causing #10 to rotate about #9	Stamping	Steel	Front Metal Shifters
11	Latch Pin	1	<1	Pins #10 in latch mechanism.	Cast	Steel	Latch Pin
12	Latch Turner	1	<1	Rotates and pulls the Latch (Metal) #3 back	Cast	Steel	Latch Turner
13	Latch Frame Back	1	0.5	Holds the inner components at an ajustable distance from the latch frame front	Cast	Steel	Image:13 center	Latch Frame Back
14	Plastic Shifters	2	<1	Guides the Black Metal Shifter back and forth	Injection Molded	Plastic	Plastic Shifters
15	Shifter Pin	1	<1	Allows the user to change the length of the latch mechanism	Extruded	Steel	Shifter Pin
16	Back Pin	1	<1	Connects the Plastic and Back Metal Shifters together	Extruded	Steel	Back Pin
17	Back Metal Shifter	1	6	Translates rotation movement to linear movement	Stamping	Steel	Image:17 center	Back Metal Shifter
Outside Door Knob
New Part #	Part Name	QTY	Weight (g)	Function	Manufacturing Process	Material	Image
18	Long through bolts	2	3	Connects two door knobs together	Thread Rolling	Steel	Long through bolts
19	Wood Screws	4	1.5	Attaches Latch Holder to Door	Purchased	Steel	Wood Screws
20	Emergency Key	1	3	Key used to unlock door when locked from the other side	Extruded, Bent, Stamped	Steel	Emergency Key
21	Outside Knob	1	34	User Handle (outside). Small hole in middle for Emergency Key	Cast or Stamped	Steel	Outside Knob
22	Outside Knob Rosette	1	12	Finished cover for appearance. Holes on each side for the Long Through Bolts	Stamped	Steel	Image:21 Outside Knob Rosette.jpg Outside Knob Rosette
23	Outside Knob Frame	1	53	Structural Frame for the outside knob. Connects to Inside Knob Frame	Cast	Steel	Outside Knob Frame
Inside Door Knob
New Part #	Part Name	QTY	Weight (g)	Function	Manufacturing Process	Material	Image
24	Knob Turning Sleeve	2	14	???	Cast	Steel	centerKnob Turning Sleeve
25	Quarter Shaft	1	17	Rotates with Door Knob, causing the latch mechanism to turn	Stamped and Bent	Steel	Latch Holder (Front)
26	Locking Shaft	1	4	Controls the locking mechanism	Injection Molded	Plastic	Latch Holder (Front)
27	Spring Housing	1	11	Encases	Stamped and Bent	Steel	Latch Holder (Front)
28	Door Knob Spring	1	1	Returns Knob to original position after turning	Coiled	Steel	Latch Holder (Front)
29	Locking Teeth	1	3	When locked, the locking teeth prevents the door knob from turning	Stamped	Steel	Latch Holder (Front)
30	Spring Shim	1	2	Keeps spring in housing	Stamped and Bent	Steel	Latch Holder (Front)
31	Handle Retention Ring	2	0.9	Connects Door Knobs to Knob Frame	Stamped	Steel	Latch Holder (Front)
32	Inside Knob	1	35	User Handle (inside). Hold in center for the Lock Switch	Cast or Stamped	Steel	Latch Holder (Front)
33	Inside Knob Rosette	1	12	Finished cover for appearance.	Stamped	Steel	Latch Holder (Front)
34	Inside Knob Frame	1	36	Structural Frame for the inside knob. Conects to Outside Knob Frame	Cast	Steel	Latch Holder (Front)
35	Lock Switch	1	4	User turns to lock/unlock door	Cast	Plastic	Latch Holder (Front)

Design for Manufacturing and Assembly (DFMA)

Performing a Design for Manufacturing and Assembly (DFMA) analysis on our doorknob allows us to determine whether or not designs of the product could be changed in such a way that the cost, ease, or quality of manufacturing and assembly can be improved. In order to analyze the product, we had to dissect the doorknob, so we could at least consider the manufacturing process that went into producing the doorknob.

The main goal of the dissection and the analysis was to identify areas of the product where we believe the designer made good decisions, or where we believe improvements could be made. Did the designer:

design the simplest solution?
make good cost-saving decisions?
design for easy manufacturing?
design for easy assembly?

The doorknob is somewhat unique in that some assembly by the customer is required, so design for assembly should take the customer's point of view in consideration in addition to the assembly plant's point of view.

The table below lists guidelines that we followed as well as our findings in this analysis.

Design for Manufacturing Guidelines
Minimize Part Count: Eliminate fasteners, part consolidation	We found there to be a lot of small parts inside the doorknob, especially considering its function (open, close, lock, unlock).
Standardize Components: Take advantage of economies of scale & known component properties	Dissection revealed that the inner and outer doorknobs were in fact not the same part. Instead, it appears that different molds were used to create them. In general, we are looking to make the product as symmetric as possible.
Commonize Product Line: Economies of scale and minimum training and equipment	It appears that most of the components are either stamped or cast. Thanks to this small number of types of manufacturing, training can be minimized, and employees become more flexible in terms of position on the production line.
Standardize Design Features: Common dimensions for fewer tools and setups	Again, small differences in the design of the two doorknobs require different molds to produce each doorknob. We believe improvements can be made to use one mold for both doorknobs.
Multifunctional Parts:	Many of the parts inside the doorknob only serve one unique function. Moreover, each part is unique, requiring multiple manufacturing processes. Design changes should be considered to replace some of the parts with one part that performs multiple functions.
Ease of Fabrication: Choose materials easy to work with	Dissecting the product revealed that the two primarily materials used in production were mostly steel and some plastic. While switching to one material should be considered, some choices were made with a functional purpose. For example, a low-friction plastic piece on the door latch was meant to facilitate the door latch sliding on surfaces.
Design for Assembly Guidelines
Mistake-Proof: unambiguous, unique assembly orientation	Some components of the doorknob, like the latch frame, are not symmetric, which makes insertion difficult during assembly. Moreover, because the asymmetry of the components are not clear enough with markings or indentations, a customer could have trouble or confusion during installation. We also had difficulty connecting the two doorknobs to each other because they were not symmetric, and there was no clear indication for how we should connect them. We believe other customers who are installing a doorknob could run into the same problem.
Minimize Fasteners: snap fits and part consolidation	There were only two sets of fasteners in this doorknob: one set to attach the outer doorknob to its reinforced frame, and another set for the customer to install the door latch holder to the door or the door frame.

In general, we believe there are too many parts inside the doorknob, many of which have only one function. This aspect of the product results in a complicated assembly, which leads to lower productivity. Design changes should be considered to consolidate multiple functions into one part. The end goal is to create the simplest design for the doorknob; focusing on consolidating multiple functions in one part can reduce the number of parts inside the doorknob, standardize components, and make manufacturing more cost-effective.

Secondly, we believe that the customer could face problems while installing the doorknob, because it is difficult to orient the knobs before connecting the two through the door. Making the correct orientation more obvious would reduce or eliminate this problem.

Failure Mode and Effects Analysis (FMEA)

Performing a failure mode and effects analysis (FMEA) allows us to determine potential areas of concern, particularly ways in which our product can fail. The FMEA also allows us to prioritize each failure mode. After determining the severity (S), the probability of occurrence (O), and detectiblity of failure (D), on a scale of 1-10, the three values are multiplied, which yields a risk priority number (RPN). This number is then used to order the failure modes in terms of importance. The table below shows the results of our FMEA analysis on our doorknob.

Item and Function	Failure Mode	Effects of Failure	S	Causes of Failure	O	Design Controls	D	RPN	Recommended Actions
Locking mechanism	Shaft fails	User cannot lock or unlock	7	User turns locking lever too hard	3	None	5	105	Perform FEA on shaft to consider strengthening
Doorknob	Doorknob fails	Door cannot be opened	8	User turns doorknob to hard	1	None	5	40	Perform FEA on doorknob to consider strengthening
Door latch	Latch spring fails	Door inoperable - fails to stay closed	7	Age/wear	1	None	5	35	Unlikely to happen over normal operating life, no action necessary
Door knob frame	The frame detaches from door	Creates an annoyance to user	1	Screw become loose	7	None	2	14	No action necessary - user can screw the screws back on

Design for Environment (DFE)

DFE Worksheet

DFE Guideline

Good Aspects of Competitor Product and Ideas for Improvement

0. New Concept Development

De-materialization (email, voice, email)
Increase Shared Use (carpooling)
Integration of Functions (telephone/fax/scanner/printer, laptops)
Functional optimization (alternative powertrain systems)

Cut doorknobs out of design for weight, and material savings along with durability increase.

1. Select Low Impact Materials

Avoid Toxics (Pb, Hg, etc)
Avoid Ozone Depleters (CFCs)
Avoid Hydrocarbons
Leander Materials
Renewable Materials
Low Energy Content Materials
Recycled Materials
Recyclable Materials

Reduce amount of steel in the part. Use recyclable parts or plastics to reduced inputted energy.

Replace greased surface with plastic slips.

2. Reduce Material Amount

Reduce Weight (rigidity by design, avoid oversizing)
Reduce Transport Volume (smaller size, less packaging, nest able)

Thinner cast parts. Remove outer handle.

Remove signage on product to cut down on box size

3. Eco-Manufacturing

Chose Alternate Production Processes
- Low emission production (bend instead of weld, join instead of solder)
- Use materials efficiently (power coat vs spray)
Eliminate Production Steps
- Net shape casting vs machining
- Integrated surfaces to avoid painting
Reduce Energy Consumption
Use renewable Energy
Fewer/Cleaner Production Consumables

Green the plants.

Use outer handle for inner handle, same molds. Integrate skirt into frame. Employ electroplating for shiny surfaces.

4. Optimize Distribution

Less/Cleaner/Reusable Packaging
Energy Efficient Transport Mode
Energy Efficient Logistics

Reuse packaging when distributed to contractors.

Thinner plastic packaging, stealing doorknobs is infrequent. Compress packaging to allow for better stacking.

5. Reduce Use-Phase Impact

Lower Energy Consumption
- default power down
- programmable clocks
- reduce mass of movable products
- insulation
Cleaner Energy Sources
- wind, solar, renewables, low sulfur coal
- Rechargeable batteries
Reduce Consumables
- Batteries, fluids, coffee filters, etc.
- Minimize leaks
Cleaner Consumables
Reduce Consumable Waste

The Doorknob has very little use-impact.

The only possible consumable is the grease that slicks the handle, which could be replaced with low friction slides.

6. Maximize The First Life

Increase Reliability and Durability (FMEA)
Easy Maintenance and Repair
- Indicate procedure for disassembly, cleaning, maintenance
- Indicate subassemblies that wear and locate for easy access
Design for Modularity (upgrades and style changes)
Avoid Trendy Designs
Promote Product User Relationships

Reduce wobbling between handle and latch.

Screw firmly into door or consider squeeze-fit to remove door failure.

Simplify Compoments: Easier to Repair. Reclaimable components.

Stave off the West Wing Problem: Preventing knobs from impacting walls.

Sell alternate handles for same mechanism.

7. End of Life

Reuse of Product (sylistic and technological obsolescence)
Re-manufacturing / Re-furbishment of Product
Recycling of Product
Safter Incineration
Design for Disassembly
Use Recyclable Materials for which a market exists
Prefer Recycling to "Down Cycling"
Integration Functions into Fewer Parts
Avoid Large Variety of Materials
Avoid Elements that Interfere with Recycling (stickers on plastics)
Mark Part Material

This appears to be a minimal concern due to the infrequency replacement.

Assuming separable vanity handles can be used,

packaging could be greatly reduced for the individual mechanism,

and handles could be sold separately.

Product's Life Cycle

Life Cycle Stage	Description	Inputs	Outputs
Material Extraction	Steel from Iron Ore Plastic from Petroleum Raw materials must be processed.	Iron Ore Petroleum	Steel Plastic Grease
Production	Springs (Acquired) Casting Stamping Assembling	Steel Plastic Grease	Components
Use		Doorknobs, Two Woodscrews	Working Door
End of Life	Retiring/Disposal	Doorknob	Landfill
Transportation	Truck	Fuel, Door Latches	Door Latches

Sector Chart

	Sector 1 (Production)	Sector 2 (Production)
Item Consumed
Sector # and name
Reference Unit
Units consumed per product life
Cost per unit ($₂₀₀₂) (retail or wholesale?)
Lifetime cost ($₂₀₀₂)
Sector mtCO₂e per $1M
Implied mtCO₂e per product life
CO₂tax @ $30/mtCO₂e

Top 5 GHG contributing sectors

Sector	mtCO₂e/product	% of product mtCO₂e