Paper towel dispenser innovation
From DDL Wiki
Executive Summary
We have found our new design to be simple and effective at reducing the occurrences of empty or jammed paper towel dispensers. Dispensers often become jammed and unusable when the sheet of paper towel rolls back into the housing, out of reach to the users. Even with a turning knob on the side, the problem is not often fixed. With our design innovations, we are able to reduce the jamming of the dispenser using a ratchet and pawl that essentially eliminates any recoil caused by the springs inside the housing and we reduce the time when a paper towel dispenser is empty with our electronic sensor that is capable of notifying someone when the dispenser is running low on paper.
Findings from Market Research
Our market research has shown that there is a need for our new product design. The reason is that there is often the problem of finding paper towel dispensers not in the "ready" position; with the paper towel ready to be pulled. This results from the spindle spinning back into the housing, which is what our new design prevents. Our research has also shown that another problem is simply the lack of paper inside of the paper towel dispenser once the roll is used. Our electronic sensor detects when the paper towel roll reaches a certain diameter. After this point it sends a signal through a wireless network to a centralized location, so someone can replace the roll.
Features and Advantages
The product's main features are the prevention of the spindle rolling back into the housing and the notification of an empty paper towel roll. As previously stated, these two features are facilitated from our new design. Some of the advantages over the competition are that the new design will be much more reliable. The second feature will provide less of a chance of going to a paper towel dispenser and finding it empty. The target market are places similar to Carnegie Mellon University. This is because there is a campus-wide wireless network as well as multiple locations of paper towel dispensers. The wireless network is necessary for the electronic sensor to work, and thus makes it easier to monitor multiple paper towel dispensers.
Justification
Our findings from our product analysis are that both new features of our product are fairly simply to implement and the fact that there is a need for them. We feel that with our new design, the process of using and replacing paper towels will become more efficient and therefore, there will be less wasted time checking and replacing rolls unnecessarily.
Market Analysis
The paper towel dispenser is a common product used every day by millions of people from around the world. Therefore, it is very important that this product functions very well without requiring too much maintenance. However, this is not the case with the current paper towel dispensers in the market. There are many different types of papers towels in the market and we have analyzed the major ones below.
Current Major Models and Designs
Our team conducted product research on five existing paper towel dispensers and listed the advantages and disadvantages of each design.
Kimberly-Clark In-Sight Sanitouch Roll Towel Dispenser
The Kimberly-Clark In-Sight Sanitouch paper towel dispenser requires the user to use two hands on either side of the paper to advance the roll. It perforates a 12" sheet with each pull and allows a new sheet to fall into a "ready position" after each use. It is fairly hygienic because the user only has to touch the paper. It can jam or fail, thus causing the user to use the knob on the right to advance the paper.
Pros:
- Generally works relatively well with no jams
- Hygienic, user only touches paper towel
- Quick and easy to use.
Cons:
- Can jam or paper can recede into the machine
- Knob on right does not always work. Must use two hands
- Paper waste
Price: $75
Cost per use (2 sheets at 8" x 12" apiece): $0.01667 per use. Using $40 per carton of 12-8" x 400' rolls
Kimberly-Clark EZ Load Lev-R-Matic Restroom Paper Towel Dispenser
The Kimberly-Clark EZ Load Lev-R-Matic paper towel dispenser requires the user to pull down on the lever to advance the paper. To increase the length of the paper, the user must use the lever more than once. There are some variations on this design, where some of the levers are along the bottom of the dispenser. This can allow the user to use their arm or elbow to advance the towel, instead of using their hands. Overall, these are dispensers are fairly reliable and do not jam.
Pros:
- Translucent, smoked plastic makes it easier to see when paper towel is almost empty
- Adjustable sheet length
- Works relatively well with no jams
Cons:
- Must touch lever to advance paper (not hygienic)
- Paper costs.
Price: $53
Cost per use (2 sheets at 8" x 12" apiece): $0.01667 per use.
Georgia Pacific enMotion Automated Touchless Towel Dispenser
The Geogia Pacific enMotion dispenser is an electric dispenser that releases paper from a sensor to detect hand motion. It is powered by four D-sized batteries. The advantage of using a sensor, is that the user does not need to touch anything on the machine to advance the paper, and only has to touch the towel that he/she is using. The dispenser also has a variety of advanced features.
Pros:
- Hand sensor (hygienic)
- Paper length and delay adjustment
- Replaceable drive module
- Downloadable data usage to PDA. Relatively few jams
- Break-resistant plastic.
Cons:
- Electric sensor may not "see" hand at first
- Battery needs to be replaced
- May take long to get multiple sheets
- Paper costs
Price: $126
Cost per use (2 sheets at 8" x 12" apiece): $0.01667 per use.
XLerator Hand Dryer
The XLerator hand dryer is a heated hand dryer that uses an infrared optical sensor to detect hand motion. When the user wants to use it, the user puts his/her hands under the dryer and the XLerator blows air at a speed of 14,000 LFM (linear feet per minute) at the hands. The dryer uses a 900W heating element to produce a temperature up to 135°F at the hands. Excel Dryer claims you can dry hands completely in 10-15 seconds.
Pros:
- Cleans hands without touching machine (hygienic)
- Relatively quick. Warm air helps drying
- No paper towel costs
- No "empty" paper roll or maintenance needed
Cons:
- Loud when operating
- Electricity costs
Price: $440
Cost per dry: Using average electricity cost per kWh in United States, $0.115/kwH <ref name="Electricity Cost">http://www.eia.doe.gov/cneaf/electricity/epm/table5_6_b.html</ref>
Using a drying time of 15 seconds and 0.9kW for the dryer, we have an average of 0.00375 kWh per use. This results in $4.3125e-4 per use.
Dyson Airblade
The Dyson Airblade uses a unique design that is similar to a windshield scraping water off glass. The Airblade uses ambient room temperature air that it cleans through a HEPA filter. It uses less energy since it is not heating up the air and is hygienic since it filters the air. To use it, the user places his/her hands into the device where an infrared sensor activates the device. Air blows on the hands and the user pulls them out, allowing the water to be scraped down into the dryer. Dyson claims that the airspeed at the nozzle is 400mph and you can dry your hands in 12 seconds.
Pros:
- Quick drying
- No heat use
- No paper towel use
- Hygienic due to infrared sensor
Cons:
- Costly
- Slightly noisy
- Uses electricity
- Mounting height may be more difficult
- Can only dry hands (not face)
Price: $1,300
Cost per dry: Electricity cost per kWh in United States, $0.0115/kwH. And 0.00468 kWh used per dry at 1.4kW and a 12 second drying time. This results in $5.382e-4 per use.
Conclusion
These paper towel dispensers can be found on the Carnegie Mellon Campus, however the most prevalent is the Kimberly-Clark In-Sight Sanitouch Roll Towel Dispenser. Our team decided to chose this dispenser among the other ones in the market because according to our user studies, it seemed like it caused the most problems. Friends and students that we had surveyed claimed that a jammed roll was the main problem they encountered 36 percent of the time when using this type of dispenser. Our group has outlined these details shown in our usability studies.
Market Opportunity
Usability Studies of Original Design
Our team conducted two studies on the original Kimberly-Clark In-Sight Sanitouch design. First, we tested the model ourselves to report any significant flaws or problems. Then our team went around Carnegie Mellon public restrooms to observe the paper towel dispenser in an actual environment.
First Study – Personal Testing
- Paper towel gets jammed in the dispenser which requires to turn the red knob
- Paper towel portion is cut too short
- Pulling with one hand sometimes did not work
- Pulling too soft did not cut the paper towel
Second Study – Observation in Public Restrooms
In order to understand behaviors of users and causes of inconveniences, we observed 60 users and collected data from restrooms in Doherty hall first floor, Porter hall first floor, University Center first floor and Wean hall fifth floor.
- Note: the observations could be biased because data are only collected from men's restrooms.
Doherty Hall First Floor
The paper towel dispenser in Doherty Hall first floor was brand-new and seems to have no broken component. No unusual feature of machine has been observed. User #5 used turning wheel first without pulling, then tore paper. As a result the paper was not cut properly and not ready for the next user.
Porter Hall First Floor
There were 2 paper towel dispensers analyzed separately.
i. The first paper towel dispenser(PTD1)
While pulling PTD ourselves, we experienced a lot of friction. The spindle did not rotate smoothly. Users had to pull harder than usual while using the PTD1. Most of users used PTD1.
ii. The second paper towel dispenser(PTD2)
Only user # 9 and 14 used PTD2. PTD2 was further away from the faucets which was why there were less users. The only case that we observed was that a roll was not hanging out and the user attempted to use the red knob but ultimately gave up and left. When we used PTD2 ourselves, we noticed that the PTD did not cut. We opened the unit up and realized that there were springs missing that held the white tube guide in place. As the spindle rotates the cutter comes out and cuts the paper towel. The white tube guide provided a force on the paper towel which made the paper towel touch the spindle. This force is necessary so that the cutter was in close contact to the paper towel.
University Center First Floor
The paper towel dispenser in University Center first floor was brand-new and seems to have no broken component. No unusual feature of machine has been observed.
Wean Hall fifth Floor
The paper towel dispenser was missing one of the even hooked spring(component #11) but still worked without failure.
Summary of User Studies
Our observations opened up a window of opportunities for us to improve the existing design and we have listed the problems below
- Most users pulled with one hand
- Users were frustrated when the dispenser got jammed
- One of the PTD's did not cut properly
- A few users did not like using the red knob
- Some of the PTD's had worn out springs which made the spindle difficult to rotate
Conclusion: Improve design so that it does NOT jam when pulling the roll with one hand
Consultation with Maintenance Workers
We interviewed two maintenance workers employed by Carnegie Mellon’s ISS, who are responsible for the cleaning and maintenance of facilities on campus. The importance of these interviews was to find out what types of issues or problems they had when replacing a paper towel roll. Also, we wanted to know if they used two rolls in one machine as listed on the Kimberly-Clark website. As we did not discuss that their identities would be disclosed online, we will refer to them as Respondent 1 and Respondent 2.
Respondent 1
- No problem replacing the roll
- Looked through front of the housing to identify empty roll
- Never used stub roll method
- Used old leftover roll for other cleaning purposes
Respondent 2
- No problem replacing roll
- Could have an easier way to detect low supply of paper towels
- Kept old leftover roll in public restroom for other usage
Summary of Interviews
We noted that neither worker had any significant problems with replacing the paper towel roll. One aspect that we could improve upon could be the ability to see if the paper towel roll is empty or close to empty. Our team addressed this problem with our redesign of the existing Kimberly-Clark In-Sight Sanitouch design. An LED was integrated in our model and was switched on automatically when the roll was running low which triggers a message to be sent via Wi-Fi to a network. More details can be found our analysis below.
Conclusion: Implement low supply notification
Profitability Analysis of Improved Design
Cost of Current Product
Kimberly-Clark In-Sight Sanitouch
We purchased our product from Carnegie Mellon Facilities Management for: $75
After surfing through the internet, the least expensive price we found from wholesalers was about: $30
Cost of Our New Redesign
Estimated Cost of Required Materials
Note: These numbers correspond to the part number in the list of Bill of Materials
Profitability Analysis
P = q(p − v) − f
P: profitability
q: production quantity
p: sale price
v: variable cost
f: fixed cost
- Our team is assuming a production of approximately 100,000 units/year
- Sale price of $75
- Variable cost is $45
- Fixed cost is $500,000
P = 100,000(75 − 45) − 500,000
Profitability = $2.5 million
Design Documentation
CAD model
Whole assembly
Ratchet Mechanism
Quantity Sensor Network System
Bill of Materials
| Part No. | Name | Function | QTY | Weight | Material | Manufacturing Process | Image |
|---|---|---|---|---|---|---|---|
| 01 | Front Housing | Protects device from exterior | 1 | 935g | Plastic | Injection Molding | 
|
| 02 | Front Housing Clip | Holds housings together | 1 | 13g | Steel | Bending, Punching | 
|
| 03 | Front Housing Tab | Secures housing clip | 1 | <1g | Plastic | Injection Molding | 
|
| 04 | Rear Housing | Holds mechanical components | 1 | 986g | Plastic | Injection Molding | 
|
| 05 | Paper Towel Holding Arm | Holds paper towel roll | 2 | 40g each | Plastic | Injection Molding | 
|
| 06 | Phillips Head Screw (1/8 x 1/4 long) | Secures device to rear housing | 1 | 1g | Steel | Machined | 
|
| 07 | Wall Mounting Screw (1/8 x 1.25 long) | Secures housing to wall | 4 | 3.5g each | Steel | Machined | 
|
| 08 | Screw Anchor | Holds screw in wall | 4 | ~0.25g | Plastic | Injection molding | 
|
| 09 | Paper Guide Tube | Guides paper towel to cutting cylinder | 1 | 55g | Plastic | Injection molding | 
|
| 10 | Spring (uneven hooks) | Holds 09 in place | 2 | 2g each | Metal | Extrusion | 
|
| 11 | Spring (even hooks) | Holds lever to spindle housing | 2 | 1.5g each | Metal | Extrusion | 
|
| 12 | Screw (3/32 x 5/8 long) | holds turning wheel and lever to spindle housing | 2 | ~1g each | Metal | Extrusion | 
|
| 13 | Red Screw Cap | Hides screw in turning wheel | 1 | <1g | Plastic | Injection molding | 
|
| 14 | Locking Spring | Ensures correct wheel spin direction | 1 | 4g | Steel | Extrusion | 
|
| 15 | Red Turning Wheel | Turns main spindle | 1 | 30g | Plastic | Injection molding | 
|
| 16 | Guide clips for 09 | Guides paper on 09 | 2 | 2.5g each | Plastic | Injection molding | 
|
| 17 | Screw (3/32 x 3/8 long) | Holds lever assembly together and spindle housing | 2 | <1g each | Metal | Machined | 
|
| 18 | Lever Arm | Connects spring to spindle | 1 | 1g | Plastic | Injection molding | 
|
| 19 | Spring Connector | Connects two springs to lever arm | 1 | <1g | Plastic | Injection molding | 
|
| 20 | Rounded Spring Guide | Angles spring to desired measurement | 1 | <1g | Plastic | Injection molding | 
|
| 21 | Thin Paper Guide | Guides paper out of device | 1 | 24g | Plastic | Injection molding | 
|
| 22 | Spindle Housing Front | Final housing before paper leaves device | 1 | 132g | Plastic | Injection molding | 
|
| 23 | Spindle Housing Back | Back of spindle housing | 1 | 162g | Plastic | Injection molding | 
|
| 24 | Spindle Housing (L) | Left of spindle housing | 1 | 105g | Plastic | Injection molding | 
|
| 25 | Spindle Housing (R) | Right of spindle housing | 1 | 105g | Plastic | Injection molding | 
|
| 26 | White Spindle Guide | Holds spindle in line | 3 | 2.3g each | Plastic | Injection molding | 
|
| 27 | Screw (3/32 x 3/8 long) | Connects spindle halves | 2 | <1g | Metal | Machined | 
|
| 28 | Square nut | For use with 27 | 2 | <1g | Steel | Cast and drilled | 
|
| 29 | White Teeth Guide/Arm | Guides cutting teeth in spindle | 2 | 3g each | Plastic | Injection molding | 
|
| 30 | Cutting Teeth | Cuts paper towel | 1 | 75g | Steel | Cast, punched and bent | 
|
| 31 | Spindle Top Half | Holds cutting tool | 1 | 125g | Plastic | Injection molding | 
|
| 32 | Spindle Bottom Half | Holds cutting tool | 1 | 135g | Plastic | Injection molding | 
|
| 33 | Metal Key | Helps open housing | 1 | 5g | Steel | Punched | 
|
| 34 | Ratchet | Prevents spindle from rotating backwards | 1 | 10g | Steel | Punched | 
|
| 35 | Pawl | Works with ratchet, stops backward motion | 1 | 8g | Steel | Punched | 
|
| 36 | Pawl Spring (purchased) | Pushes pawl into ratchet | 1 | 4g | Steel | Extruded | 
|
| 37 | Sensor Arm | Detects low paper towel | 1 | 25g | Steel | Extruded | 
|
| 38 | Sensor Arm Spring | Keeps Sensor arm to contact paper towel | 1 | 2g | Steel | Extruded | 
|
| 39 | Sensor Assembly | Sends signal to notify person of low paper towel | 1 | 150g | Circuit board | Assembled | 
|
Custom Components
This section includes all of the computer aided design (CAD) drawings for the parts that need to be fabricated.
Sensor Assembly
The sensor assembly will be a custom component designed by an electrical engineer. We did not fully explore this hardware because of our lack of knowledge with electronics. The sensor would have to be battery powered, and send a wireless signal to a wi-fi network. We feel that this should be feasible especially since it is only sending one signal: nothing if the roll is not empty, and a signal if the roll is almost empty. We plan to have the sensor send the signal when 10% of the roll is left.
Full Assembly
Note: All units are in inches.
Ratchet
Pawl
Sensor Arm
Purchased Components
This section contains the information necessary for the purchased components that are not made in-house.
Pawl Spring
The pawl spring pushes the pawl up towards the ratchet. When the pawl is against the ratchet's teeth, it prevents rotation in the direction that causes the paper towel to retract into the dispenser.
McMaster-Carr
Part Number: 1692K38
Type 302 SS Conical Compression Spring 1-1/2" L, .85" L OD, .343" Small OD,.045" Wire Dia
Type: Conical Compression Springs
Material: Stainless Steel
Stainless Steel Type: Type 302 Stainless Steel
System of Measurement: Inch
Large Outside Diameter: .85"
Small Outside Diameter: .343"
Wire Size: .045"
Overall Length: 1-1/2"
Compressed Length: .09"
Ends: Closed
Wire Type: Round Wire
Load: 4.19 lbs.
Deflection at Load: .75"
Rate: 5.59 lbs./inch
Specifications Met: Not Rated
Sensor Arm Spring
The sensor arm spring keeps sensor arm to be in contact with paper towel
McMaster-Carr
Part Number: 9271K39
Material: Steel Music Wire
Deflection: Angle 90°
Wind Direction: Clockwise (Left-Hand) Wound
Spring Outside: Diameter .357"
Wire Diameter: .045"
Leg Length: 1.250"
Maximum Rod Outside Diameter: .203"
Spring Length at Torque: .259"
Number of Active Coils: 4.25
Torque" 2.143 in.-lbs.
House of Quality
The first thing that is noted from the above house of quality (HoQ) is that Our Design, the improved design based off of the Kimberly Clark Sanitouch, is ranked equal or higher in every category analyzed. Another important note is that in the categories that are listed highest (5) for Customer Importance, the improved design had the highest rankings. These are also all categories where it is improved from the original design.
If we use the Customer Importance to develop weighted Customer Ratings, our improved design score is 155, narrowly ahead of the Dyson Airblade's score of 150 and the XLerator Hand Dryer's score of 148. Additionally, the Airblade is over 18 times the price at $1,300 and the XLerator Hand Dryer is over 6 times the price at $440. The improved design also scored 23 points higher than the original Kimberly Clark Sanitouch model's 132. While using a HoQ may not be the most accurate way of determining the magnitude of improvement, in this case it can be a trusted indicator in determining whether one product is better than another.
After analysis of the house of quality, we can confidently say that our new design is an improvement on the original Kimberly-Clark design. Further, we believe that our new design is better on a per cost basis than the rest of the dispensers we compared it to. While this is a subjective statement, our team believes that almost all users will be in concurrence.
The HoQ also shows that the improved design is between the Target(delighted) - Target(disgusted) range for every engineering specification category chosen. These targets are set by determining the minimum satisfactory values for the categories, the Target (disgusted), and values we believe that would make it the superior dispenser in the market for that particular category. (Note: While we said that our Target (delighted) for Uses Before Reload is 
, we of course do not expect a paper towel dispenser to have infinite paper towels)
Design Analysis
DFMA
The parts that need to be manufactured include the ratchet, pawl, sensor arm, and the circuit board. All other parts can are the same as the Kimberly-Clark Sanitouch dispenser that we based it off of. All of our new parts fit conveniently into Kimberly-Clark's current design. The design of the ratchet, pawl, and sensor arm are relatively simple and should not be difficult. If for whatever reason the ratchet gear is unable to be built as designed, a similar design can be made with the back side of the teeth (opposite the curved side) straight and perpendicular to the tangent of the circumference. This change will make it easier to machine. However, since we intend on using a stamping technique to fabricate the part for high volume, we do not believe it would be necessary. We tried to figure out a way to integrate this into the current design to reduce parts, but we feel that it would be easier to assemble as separate pieces and would not cause much delay in the assembly process. However, for future work, we would want to combine the ratchet gear and spindle as one component so that it is easy to be assembled. The circuit board would have to be assembled separately and installed with the sensor arm.
FMEA
Failure mode and effects analysis (FMEA) identifies potential problems of every component in the machinery, lists their effects on the system, and finally provides recommended actions to prevent such failures. The methodology for problem identification depends on ratings of Severity of a failure (S), Occurrence of Failure (O), and Detection of Failure (D). These scales are between 1 and 10 and they are rated in a way that less number of ratings indicates less significant problem and conversely higher rating means more serious failure or effect. The results of a FMEA Analysis (RPN) are calculated by multiplying S, O and D to signify seriousness of the problem.
Conclusion
Our FMEA is very similar to the previous one, with the addition of our parts, which include the ratchet assembly and sensor assembly. These parts are noted in the bottom of the list and are parts that have low potential for failure.
Since our machine is a simple and easy mechanism, it does not require a significant amount of force to operate. Thus, it does not involve any component that undergoes serious stress or force, and under proper usage the mechanism is rigid enough: most parts have low Occurrence of Failure rating. Moreover, it is found that severity of failure of most components is low, but in cases of components with a high severity of failure, the detections are made easy (low Detection of Failure rating), keeping RPN rating to a small number. In conclusion our FMEA analysis, not many parts with high RPN, indicates the mechanism overall is reliable and we found only a few issues.
One issue we did find was the if the cutter was not perforating the paper. We found the culprit to be Part 10, the uneven springs. In the case we find, these springs were missing, which did not allow the paper towel guide (Part 09) to push down on the paper. This, in turn, did not provide enough force to keep the paper towel taught, and ultimately to perforate the paper.
Another issue we found, was a case where one of the Spring 11 were missing. In fact, the paper towel dispenser worked fine even with only one spring, meaning that the second spring may provide more force on a different part of the rotation of the spindle, but the single spring provides enough force to make it complete its rotation. Another consideration is if the spring is stretched quickly (i.e. a user pulls to fast) it often occurs that the next sheet of paper recedes back into the machine. We figured this is why the knob is needed on the side, which is a reliable solution to that problem. Also, pulling at different angles does not affect the way the paper leaves the spindle or gets cut. This is because the thin paper guide is there to make sure the paper is coming straight down off the spindle and not at an angle.
One issue was that the Locking Spring (Part 14) would not prevent the spindle from spinning back into the paper towel dispenser. Although it seems that this may not be its primary function, but rather to prevent the user from turning the spindle using the red knob. Our new design uses a ratchet and pawl to prevent the spindle from turning back into the paper towel dispenser.
The other addition to the assembly is the paper towel sensor to detect the amount of paper left on the roll. This has a possiblity of failure if a wire becomes loose or if the person replacing the paper towel damages it.
We found these occurrences to be very low in our observations of campus paper towel dispensers. Also, the detections are relatively high for the failure of Part 10.
For full FMEA table, please see FMEA data
DFE
In our Design for Environment, we feel that the improvements we have made will not have that strong of an effect on the environment. We are essentially making a new part for the ratchet out of injection molded plastic. The sensor assembly will be electrically powered, but we feel that this is an insignificant amount of power use.
In comparison to our competitors, they are similar in paper towel use or electricity use. For the air-powered hand dryers, they are solely electrically powered and use more energy than their paper counterparts. We feel that we will use a similar amount of energy and paper towel as some of the dispensers that use infrared sensors to advance the paper.
For DFE data, please see DFE data
Mechanical Analysis
In our new paper towel dispenser, we incorporated a ratchet mechanism to prevent spindle from turning more than needed due to its inertia and causing paper towel to be rolled back in. For engineering analysis, we calculated and investigated to compare the effect of the additional inertia of the ratchet as well as the new resistance moment it will provide. First, we varied rotation angle of spindle and performed static analysis to measure force required for users to pull a paper towel with the original paper towel dispenser and with our new design. Then we assumed the average time it takes for a person to pull a paper towel to be 1 sec, and performed dynamic analysis of systems to find out required pulling force with and without ratchet mechanism.
Left view (left) and right view of spindle(right) with ratchet mechanism indicated by red box
Free Body Diagram
Simplied Free body diagram
Where:
D = diameter of spindle = 0.1m
Θ = angle displacement of spindle from initial state
W = weight of the spindle and ratchet gear
Fbase,y = force applied by axle of the base to the spindle in y-axis
Fapplied = force applied by user by pulling paper towel
Mresistant = Mspring + Mratchet
Mspring = moment from lever arm springs on the left side of spindle(varies with θ, angle of rotation of spindle)
Mratchet = moment from ratchet mechanism (moment due to friction force between ratchet gear and pawl, normal force being ratchet spring force and reaction force from axle of the base)
Determination of Mspring and Mratchet
Our group realized that determining Mspring and Mratchet from calculations of actual spring forces, dimensions, and friction forces is inefficient, and also inaccurate. Thus, we tested the actual model at different angles(incremented theta by 45 degrees), and reported forces to obtain Mspring and Mratchet values. To find force, we attached a reference spring to the end of sheet of paper towel and pulled until it reaches certain angle and stays at equilibrium. Then we measure the stretch of spring to find out the force. We applied the same method is used for both of Mspring and Mratchet, but we isolated the two values from each other and tested them separately.
Calculation of Mspring and Mratchet
References spring constant, k = 163.3 N/m
Initial length of reference spring constant, Linitial = 0.023 m
Fspring, i = k ∆xi = (163.33)∆xi N where ∆ xi = Lengthspring,i - Linitial = (Lengthspring,i - 0.023) m
Mspring, i = (D/2) Fspring, i = 0.05 Fspring, i Nm , where i = 0, 45°, 90°, …315°, 360 ° (eq.1)
Summary of results
The difference between Mspring and Mratchet is that unlike Mspring, Mratchet does not depend on angle of rotation. We found that Mratceht is relatively small compared to Mspring (approx. 8% of Mspring). Therefore we believe that adding ratchet mechanism would not cause a huge difference.
Table of resistant forces, moments at different angles of rotation
Resistant moments at different angles of rotation
Determination of Dry and Wet Paper Towel Tensile Capacity
Again, we perforemd experiment to find dry and wet paper towel tensile capacity. First we taped one end of paper towel to fix towel and taped weights to the other end. We incremented amount of weights until the paper towel breaks and measured weights to find the capacity. From this we found forces needed to break paper towel when it is dry to be 11.27N (Fdry_towel) and wet to be 9.31N (Fwet_towel). We will use this as an indicators to test validity of our mechanical design.
Static Analysis
In order for the system to be static, sum of forces in all directions and sum of all moments should equal to zero. Solving Force Applied without Ratchet
∑Fy = 0 => Fbase,y = W + Fapplied
∑Fx = 0: The thin paper towel guide (part 21) cancels forces in x-direction, no matter in what angle user pulls the paper towel.
∑M = 0 at the center of the spindle,
- Mspring + Fapplied(D/2) = 0 => Fapplied = (2/D)(Mspring) (eq.2)
Solving Force Applied with Ratchet
Note: Force applied by user = Force required to advance
Simlarly, ∑M = 0 at the center of the spindle,
-Mratchet - Mspring + Fapplied(D/2) = 0 => Fapplied = (2/D)(Mratchet + Mspring) (eq.3)
Summary of results
Using eq.2 and eq.3 we calculated F applied with and without ratchet mechanism. As expected, although system with ratchet mechanism requires more force to be applied, it is not significant enough.
Table of Force applied at different angles of rotation at static state
Force applied at different angles of rotation at static state
Dynamic Analysis
In our dynamic system, sum of forces will equal to zero but the sum of moments would not equal to zero since the spindle will rotate at some angular velocity. In order to analyze this system, we need to figure out inertia of spindle and angular acceleration.
Solving Moment of Inertia
We calculated both the moment of inertia of the spindle and the spindle with the ratchet, assuming spindle as a hollow tube with radius of 5cm, and mass of 0.335 kg and ratchet gear as a disk with radius of 3cm and mass of 0.067 kg,
Jspindle = mspindlerspindle2 = (0.335 kg)(0.10 m/2)2 = 0.0008375 kg-m2
Jspindle, with ratchet = mspindlerspindle2 +1/2 mratchet rratchet2 = (0.335 )(0.05)2 + 1/2(0.067) (0.05)2 = 0.00084504 kg-m2
Solving Angular Acceleration
We first assumed time it takes for user to pull a paper towel is 0.5 s and the angular acceleration of spindle to be some constant value since it
Assume constant θ ̈=a and tpull = 0.5 s
Integrating θ ̈ with respect to t, ∫θ̈dt=∫a dt => θ=at+c1 (c1=0, since θ=0 at t=0) => θ̇=at
Integrating θ ̇ with respect to t, ∫θ̇dt=∫at dt => θ=(at2)/2+ c2 (c2=0, since θ=0 at t=0) => θ=(at2)/2
Since a pull cycle(2π) takes 0.5 s, θ=2π=(a(0.5s)2)/2 => a= (2π×2)/(0.5s)2 => a=50.27 rad/s2
Solving Force Applied without Ratchet for this angular acceleration
∑M=-Jθ̈= Mspring,i - Fapplied,i D/2
=> Fapplied,i = 2/D ( Mspring,i+ J θ ̈ ) (eq.4)
Solving Force Applied with Ratchet for this angular acceleration
∑M=-Jθ ̈= Mratchet,i + Mspring,i + - Fapplied,i D/2
=> Fapplied,i= 2/D (Mratchet,i + Mspring,i + J θ ̈ ) (eq.5)
Summary of result
Using eq.4 and eq.5 we calculated F applied with and without ratchet mechanism. As expected, although system with ratchet mechanism requires more force to be applied, it is not significant enough.
Table of Force applied at different angles of rotation at dynamic state
Force applied at different angles of rotation at dynamic state
Conclusion
Adding ratchet mechanism increases moment of inertia, and force required for user to pull the paper towel in both static and dynamic state. However, this increase is insignificant because average human pulling force with two hands for male ranges from 310-370N and for female ranges from 70-180N. In addition, the required force in the new model does not exceed the amount of forces to break paper dry and wet towels. In both designs, the required forces maximized at when spindle is half way (180 degrees from initial position) of whole turn, and the values were found out to be 7.070N for regular design and 7.572N for our design. With our model, users need to pull dry paper towel less than in 0.219 sec and wet paper less than in 0.294 sec in order to get paper tower get ripped. But we consider these to be extreme cases because we expect average user pulling time to be > 0.5 sec from obeservations and experimentation using paper towel dispenser ourselves. Therefore we believe our design is practical for normal uses.
Prototype Documentation
Our prototype displays the two key features that we have redesigned on the paper towel dispenser: the ratchet and pawl as well as the electronic sensor. One key difference between our prototype and the final design is that the parts will all be internally placed inside the housing, so when the paper towel dispenser is closed, one would not notice any change from the outside. Also, in our final design, the ratchet will be small enough that the red turning knob on the right will be able to fit as well. We are also using a normal compression spring, but plan to use a conical spring to ensure that the force it produces is straight up and because he has less side-to-side motion.
The CAD drawing of the ratchet is the one we have chosen to use in our final design. The ratchet that is currently attached to our prototype has fewer teeth, but still works very well. We believe that fewer teeth will further decrease the rollback distance and further decrease the likelihood of the dispenser becoming jammed. Unfortunately we were unable to use our designed ratchet in our prototype as the CNC machine is unable to machine such an intricate component.
The electronic sensor we have right now lights up a LED when the lever arm reaches a certain level. In our final design, we plan to have this hooked up to a circuit board, so once it is lowered to a certain point, (a smaller diamter of the paper towel roll), the circuit will be closed and a wireless signal will be sent to the centralized database via the wireless internet network. The reason our prototype does not include this is because we feel that the LED would depict how the overall system would initiate the signal and after that point an electronic system could be programmed to send that signal.
Process Flowchart
The flowchart below depicts the steps it takes for a paper towel to be replaced. The sensor detects if there is a smaller diameter and sends a signal to a centralized location. A maintenance worker will see which paper towel dispenser needs to be replaced and in which location. The worker replaces the paper towel dispenser and the sensor is reset.
User Testing and Feedback
Our user testing was conducted by having a user pull a sheet from both the original Kimberly-Clark Sanitouch dispenser and our modified version. For the purposes of our report we are considering a failed test to be one where a paper towel is torn due to something other than the serrated cutting blade. We tested users with both wet and dry hands to simulate different levels of wetness when pulling a new paper towel sheet. When conducting the wet hand testing, we also tested pulling with two wet hands. Neither dispenser failed a two-handed test
After the six tests the user was asked to write down any comments they had on their experience. We ultimately wanted to determine whether the ratchet we added would lead to a change in the number of torn paper towels. Torn paper towels can lead to overuse and waste as the user will likely need to pull out another towel to dry their hands.
User Testing
After testing 62 users and compiling our data, we calculated the following statistics:
Original Kimberly Clark:
- Wet: 4.8% torn (3 users)
- Dry: 1.6% torn (1 user)
New Design:
- Wet: 3.2% torn (2 users)
- Dry: 0% torn (0 users)
Note: we also tested 2 handed pulls, there were no failures recorded
The data above suggests that our new design is an improvement on the original design as it has fewer failed tests. However, given the relatively small sample size of 62, we are unable to firmly stand by that statement. Instead, we believe that the data shows us that the two dispensers will have essentially the same failure rates, which we believe to be acceptable as they are very low.
User Feedback
Our user feedback also confirmed something that we had discovered after testing it ourselves, there is no noticeable differences between using the original and ratcheted versions. We believe this is acceptable as the force required is very little and in no way stressful to the body.
Almost all of the comments stated that the two dispensers worked the same. A few samples of comments are included below:
- "They are the same."
- "No differences between new and old."
- "The ratchet is as smooth as the first paper towel dispenser."
We view all of these comments as positive. One of our goals was to be able to create a paper towel dispenser that would not get jammed and not lose any performance in doing so. We feel that these responses support our goal. Not all of the responses were positive, however. We found two negative comments, both related to the noise caused by the ratchet:
- "The ratchet makes a lot of noise..."
- "The clicking from the ratchet is annoying. Sorry!"
We understand that the added noise is noticeable, but we maintain that it is not a major deterrent. It is barely louder that the other noises from the dispenser, and is still considerably quieter than any air hand-dryer. We also were unable to conduct the testing in a bathroom which has additional noises that would routinely drown out the clicking from the ratchet.
In conclusion, our team believes that the user feedback was overall positive, and that the issue with the ratchet sounds is worth looking into, but fixing the issue is likely not worth increasing the cost of the product.
Design Process
Throughout our design process, we set our goals using the Gantt Chart(shown below) and tried to stick to it. In the beginning stages, we focused on the stakeholders and how to improve their interactions with the paper towel dispenser. Our two improvements affect two different stakeholders: the user and the maintenance worker that replaces the paper towel. We met at least once a week during our days that we did not have team meetings for class to brainstorm as well coordinate our efforts for the next stage of the project.
Once we decided on the best ideas, we would often thin of what parts we would need and acquire them from a home improvement store.
Gantt Chart
For the first design review, the team member roles were:
- Eric Totong - Stakeholders, DFE
- Chang Keun Jung - FMEA, Mechanical Analysis
- James Li-Yang Lee - Usability Study, Assembly pictures, How it works
- Nishan Kulatilaka - DFMA, Executive Summary, Parts list
For the second design review, the team member roles were:
- Eric Totong - Interviews, Stakeholders, Competitors, Existing Patents, Pugh Chart
- Chang Keun Jung - Market Research/Observations, Gantt Chart, Findings
- James Li-Yang Lee - Executive Summary, User Survey, Hypothetical Scenario
- Nishan Kulatilaka - Concept Sketches, Descriptions of opportunities, Recommendations
Team Member Roles
- Eric Totong - Design Analysis, Flowchart, Executive Summary, DFE
- Chang Keun Jung - Mechanical Analysis, CAD modelling, FMEA
- James Li-Yang Lee - Market Analysis
- Nishan Kulatilaka - House of Quality, Design Documentation, ENG Drawings, Prototype Documentation
Appendix
FMEA Data
| Part No. | Name | Function | Failure Mode | Effects of Failure | S | Causes of Failure | O | Design Controls | D | RPN | Recommended Actions | Responsibility |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 01 | Front Housing | Protects device from exterior | Fractures, Warped |
| 7 |
| 4 |
| 2 | 56 |
| Reliability
|
| 02 | Front Housing Clip | Holds housings together | Bends |
| 4 |
| 2 |
| 6 | 48 |
| Assembly and Reliablity
|
| 03 | Front Housing Tab | Secures housing clip | Strips |
| 2 |
| 2 |
| 8 | 32 |
| Assembly
|
| 04 | Rear Housing | Holds mechanical components | Fractures |
| 7 |
| 4 |
| 1 | 28 |
| Reliability |
| 05 | Paper Towel Holding Arm | Holds paper towel roll | Bends |
| 6 |
| 3 |
| 2 | 36 |
| Assembly and Manufacturing |
| 08 | Screw Anchor | Holds screw in wall | Fractures |
| 7 |
| 2 |
| 4 | 56 |
| Reliablity |
| 09 | Paper Guide Tube | Guides paper towel to cutting cylinder | Failed |
| 4 |
| 3 |
| 2 | 24 |
| Manufacturing |
| 10 | Spring (uneven hooks) | Holds 09 in place | Deformed |
| 8 |
| 2 |
| 5 | 80 |
| Assembly and Reliablity |
| 11 | Spring (even hooks) | Holds lever to spindle housing | Deformed |
| 5 |
| 5 |
| 4 | 100 |
| Assembly and Reliablity |
| 14 | Locking Spring | Hides screw in turning wheel | Deformed |
| 3 |
| 2 |
| 4 | 24 |
| Assembly and Reliablity |
| 16 | Guide clips for 09 | Guides paper on 09 | Break |
| 4 |
| 3 |
| 2 | 24 |
| Assembly and Manufacturing |
| 18 | Lever Arm | Connects spring to spindle | Breaks |
| 5 |
| 2 |
| 2 | 20 |
| Manufacturing and Reliablity |
| 19 | Spring Connector | Connects two springs to lever arm | Breaks |
| 5 |
| 2 |
| 3 | 30 |
| Assembly and Reliablity |
| 20 | Rounded Spring Guide | Angles spring to desired measurement | Breaks |
| 2 |
| 2 |
| 7 | 28 |
| Manufacturing |
| 21 | Thin Paper Guide | Guides paper out of device | Breaks |
| 4 |
| 3 |
| 1 | 12 |
| Manufacturing |
| 22 | Front Spindle Housing | Final housing before paper leaves device | Fractures |
| 3 |
| 2 |
| 6 | 36 |
| Assembly and Manufacturing |
| 23 | Spindle Housing Back | Back of spindle housing | Fractures |
| 3 |
| 2 |
| 6 | 36 |
| Assembly and Manufacturing |
| 24,25 | Spindle Housing (L,R) | Sides of spindle housing | Breaks |
| 5 |
| 2 |
| 6 | 60 |
| Assembly and Manufacturing |
| 29 | White Teeth Guide/Arm | Guides cutting teeth in spindle | Breaks |
| 3 |
| 3 |
| 6 | 36 |
| Assembly and Manufacturing |
| 30 | Cutting Teeth | Cuts paper towel | Wears,Bends |
| 4 |
| 2 |
| 2 | 16 |
| Manufacturing |
| 31,32 | Spindle | Guides paper out of dispenser | Warps,Breaks |
| 5 |
| 1 |
| 3 | 15 |
| Assembly and Manufacturing |
| 33 | Metal Key | Opens up front housing | Wears,Breaks |
| 1 |
| 2 |
| 2 | 4 |
| Manufacturing |
| 34 | Ratchet | Prevents spindle from turning backwards | Wears,Breaks |
| 2 |
| 2 |
| 2 | 8 |
| Manufacturing |
| 35 | Pawl | Works with ratchet, prevents backward motion | Wears,Breaks |
| 4 |
| 2 |
| 2 | 16 |
| Manufacturing |
| 36 | Pawl Spring | Pushes pawl into ratchet | Wears,Bends |
| 4 |
| 4 |
| 2 | 32 |
| Manufacturing |
| 37 | Sensor Arm | Detects low paper towel | Wears,Bends |
| 4 |
| 4 |
| 2 | 32 |
| Manufacturing |
| 38 | Sensor Arm Spring | Keeps Sensor Arm in contact with paper towel | Wears,Bends |
| 4 |
| 4 |
| 2 | 32 |
| Manufacturing |
| 39 | Sensor Assembly | Sends Wi-Fi signal info of towel roll size | Wears,Burns out |
| 4 |
| 3 |
| 2 | 24 |
| Manufacturing |
DFE Data
References
1. W. Karwowski, International encyclopedia of ergonomics and human factors, volume 1, NW: CRC Press, 2006, page 500.
2. http://www.mcmaster.com/#9271k39/=4u42dj
3. http://www.mcmaster.com/#1692k38/=4u3mkt
4. http://www.eia.doe.gov/cneaf/electricity/epm/table5_6_b.html
