From DDL Wiki

Contents 1 Executive Summary 1.1 Consumer Information 1.1.1 Consumer Needs 1.1.2 Conventional Use 1.2 Difficulties 1.3 Inputs and Outputs 1.4 Rotating Tie Rack Use Scenarios 1.5 Other Stakeholders 2 Dissection and Analysis 2.1 Mechanical Functioning 3 DFMA 3.1 DFA 3.1.1 Design for ease of part handling 3.1.2 Design for ease of fastening and insertion 3.1.3 Improvements 3.2 DFM (Injection Molding) 3.2.1 Improvements 3.3 FMEA Analysis 4 DFE 4.1 Life Cycle Analysis 5 Other Improvements

Executive Summary

The product we chose to examine was a rotating tie rack. This was designed as a space saving device. It attaches to the closet rod and it can hold up to 64 ties and 8 belts. The tie rack consists of a rotating belt, driven by a battery operated motor. The motor is enclosed by the top and bottom cover. The motor attaches to a gear train, which rotates the belt. The belt has hooks for the ties, while the belts hang in the middle. The energy input is provided by 2 sets of “C” type batteries in series, and the two sets are connected in parallel, such that the voltage input is 3 V. The rack is operated by moving the switch from its neutral position to either the left or right. The rack rotates in the corresponding direction and a light bulb lights up simultaneously.

We dissected the tie rack and analyzed the function and manufacturing processes employed in making all 26 of its components. We concluded that all the plastic parts that formed the body of the tie rack were injection molded in-house, whilst the rest of the parts were purchased. We analyzed the design for DFMA, FMEA and DFE to determine how closely the design of the tie rack followed design guidelines and to identify where there was room for improvement.

The various components of the tie rack were manually assembled. The design for the rack adhered to the design guidelines for DFA fairly closely. A detailed analysis can be found in section 3.1. The assembly started using a large base and proceeded vertically. The assembly needed to be re-oriented only once, and all parts fit together fairly easily. The designers kept all components symmetric where possible, and made sure that the parts could not be inserted the wrong way when symmetry was not possible. One way to improve the design would be to use a single large gear instead of a gear train as this would reduce the total number of parts. In addition, it was difficult to attach the rotating belt to the assembly. We think that increasing the height of the ledge that keeps the belt in place would make this process easier.

When analyzing the DFM of the rotating tie rack, we noted that standard plastic injection molding principles were followed. A table showing guidelines for all of the features that are exhibited by the tie rack’s plastic components, and whether they were followed or not, can be found in 3.4 DFM (Injection Molding). All of those which were followed aid toward cheap manufacture of components, in addition to offering structural and cosmetic benefits. The main ways in which the plastic injection molding process was optimized for low-cost manufacturing are using parts that have simple geomerty, which decreases the mold cost and combining features where possible, which decreases the number of molds necessary. The tie rack has only 13 unique parts. The same material has been used for various components, which decreases the variety of stock needed to be purchased. The final product is formed straight from molding. Only one finishing step added to Plastic (2)and no additional holes or machining required after molding)

We performed FMEA to identify the parts most likely to fail. (Refer to secion 4) The chance of deformation or cracking of belt hooks and the rotating belt can be reduced by using a more durable material. Improper functioning of the motor and gear train would stop the rotation mechanism from functioning properly, and this can be eliminated by using a motor with a higher torque output. The light bulb may short circuit and to avoid this can be replaced by a light bulb with a longer life span. The gears in the gear train may deform, and the chance of this happening can be reduced by using more lubrication. The top and bottom covers are brittle and may fracture. The likelihood of fracture can be reduced by using a stronger,less brittle material and using rubber or some other form of cushioning to reduce the force of impact.

We also analyzed the tie rack for DFE. A detailed anlaysis can be found in section 5. The main concern with this product deals with the material it is made of. We are not sure the exact type of plastic it is, but it can be assumed that it is not recyclable and that it probably emits harmful gases when burned. The main way to improve this product would be to use a recyclable plastic if at all possible. Another minor point is the large amount of plastic was used when packaging the product, which is unnecessary.

In addition, we came up with other improvements not highlighted by the above analyses. The quality of the wires used was very poor and we recommend using higher quality wires, and ensuring the soldered connections are stronger than in the present design. Introducing variable speed control and having a separate switch for the light bulb to increase the life of the batteries would also improve the design.

Consumer Information

Consumer Needs

• Principle needs:
  • Place for storing ties
  • Compact-- space saver
  • Display and access ties easily
  • See ties in closet when dark

• Added feature:
  • Belt hanger
  • Easy to fix if broken
  • Affordable

Conventional Use

The product is fairly simple to use. The user begins by removing the item from its packaging. The user first inserts four C batteries into the battery compartment, and attaches the battery cover (004). The user then takes the top assembly (003) and places it over the closet rod, then places the rest of the assembly beneath it, and fastens the two together. For some people, this is a difficult process. The top cover (011) is difficult to attach to the top assembly, and can get stuck, leaving the product partially assembled, and difficult to pull back apart. Once the product is assembled on the rod, the user must use the locking screws (002) to lock the tie rack in place on the rod. The tie rack can be attached to any rod that is smaller than the hole created when the top cover is attached to the top assembly. The locking screws can even hold a rod that is much smaller than the hole firmly in place.

Finally, once the product is assembled and attached to the closet rod, it is ready to be used. Ties are hung on the hangers that are part of the rotating belt (007), which is done easiest by letting the rack rotate while hanging a tie on each hanger. Once the ties are all hung, one can also hang belts on the belt hangers (003) hanging from the bottom of the assembly. These are difficult to acces however, because the ties are in the line of sight and the line of access to the belt hooks. To remove a tie that is not at the front of the rack, one must push one of two buttons, left or right, to rotate the belt. When the user pushes one of these buttons, a light turns on, spotlighting the ties at the front of the rack. This is a good feature, since most closets tend to be dark. However the light itself does not have its own switch, meaning that the user cannot turn it off if it is not required. The light turns on whenever the belt is in motion. This drains the batteries faster, since energy is consumed both by the rotating belt and the light.

Difficulties

• Assembly is difficult: top assembly and top cover are difficult to attach.
• Hanging ties is difficult: rack rotates too quickly-- must be constantly stopped while hanging ties.
• Access to belts is difficult: ties are in the way.
• Light turns on automatically when tie rack is in use, and cannot be turned off by user. This drains the battery more quickly.
• User must push the opposite button as already pushed to stop the rack. For example, if the user is pushing the right button to rotate the rack, he or she must push the left button to stop it, BUT, if the user pushes the left button too hard, the rack will begin spinning in the opposite direction.
• The speed of rotation cannot be controlled. There is no way to speed up the rotation if you already no which tie you want to wear and where on the rack it is hanging.

Inputs and Outputs

There are many factors affecting the use and production of this product.

The inputs are:

• Energy input : 3 V (from 2 C type batteries in series. There are two sets of batteries in series, and the two sets are connected in parallel)
• Information input: User presses switch and selects direction of rotation

The outputs are:

• Energy output : Rotation of rack, light from bulb

Rotating Tie Rack Use Scenarios

The way the rack is used may vary from customer to customer. The conventional use is described in a section above, but other uses are also possible:

• Use by a man to hang ties
• Use by a woman to hang ties
• Use by a man to hang ties and belts
• Use by a woman to hang ties and belts
• Use by a man to hang belts only
• Use by a woman to hang belts only
• All of the above could be placed inside or outside of closet

Age or disability is most likely not a problem for consumers, because if hung in the proper place in closet it can be used by consumres of all heights, weight ratios, ages, and abilities.

Other Stakeholders

Aside from the consumers, there are others who are affected by the engineering and manufacturing of the product. Those individuals include, but are not limited to, the workers in the assembly line, the supervising engineers, and the people designing the manufacturing equipment.

• Common problems for the assembly workers could be:
  • Assembling motor: grease on hands
  • Parts not snapping together properly
  • Soldering circuit board and wires could cause plastic to melt

Dissection and Analysis

The product was dissassembled to analyze each individual part of it. Each part was weighed, measured and described, and its purpose was thoroughly analyzed. The components of the assembly can be found at Tie rack components.

Mechanical Functioning

General: By pressing the external Switch [022] of the tie rack to the left or right, the Rotating belt [007] is driven to turn in the corresponding direction. When the Switch [022] is pressed in either direction, the Light bulb [021] simultaneously lights up with the rotating function.

Circuitry: The rotating tie rack functions with a gear train adjoined to an electrical subassembly. The electrical subassembly consists of a Circuit board (with wires) [020], Light bulb [021], two Battery end plates [025], two Springs [019] and a Magnetic motor [018]. The Circuit board [020] has a lever mechanism which engages with the arms of the Switch [022] such that pressing the Switch [022] to the right or left moves the lever and closes the circuit for functioning; leaving the Switch [022] in the center position puts the circuitry in idler mode with an open circuit. In the case of switching right or left and completing the circuit, current is allowed to pass through the subassembly. Wires from the Circuit board [020] connect to the two Battery end plates [025], thus providing power to the assembly. This power is split into two components of the circuitry, one being to the Light bulb [021] to provide light during rotation. It also travels to the drive train of the tie rack. First, the Magnetic motor [018] receives electrical energy through the wires and converts it into rotational energy. This rotational energy is transferred from the motor to the Gear driver [014] via the Gear belt [012] that connects the two. The Gear driver [014] then engages with the first Gear, clear [015], which in turn activates the second Gear, clear [015]. Both of these function to decrease the speed output of the Magnetic motor [018] and increase torque. The second Gear, clear [015] drives the final Gear, yellow [016] which has the same effect on motor output as the other gears. The Gear, yellow [016] transfers the motor’s now geared down output to the Belt gear [008]. The Belt gear [008] is engaged with the Rotating belt [007], also engaged with a stationary Belt rotator [009], thus allowing the Rotating belt [007] to turn and make ties visible to the user.

An analysis for finding the motor speed and torque can be found at Tie rack calculations, along with a free body diagram, gear ratios and methods of testing.

Bottom cover [023]:

Top cover [011]:

DFMA

Design for Manufacture and Design for Assembly are aimed at producing a product that is of a high quality at a low cost in minimum time. The motivation behind DFMA is that if both manufacture and assembly are taken into account while designing the product the resulting manufacturing process will be considerably more efficient. Design for Assembly refers to putting together the various parts and components that comprise the final product. It is based on the principle that significant improvement can be obtained from simplification of the product by reducing the number of separate parts and from making the parts used easy to handle and install. Design for Manufacturability refers to the actual making of the various parts and components. It is based on the fact that considerable savings can be achieved if manufacturing constraints and costs are considered when designing the product rather than after the product is designed. This includes considering factory capabilities, assembly practices, manufacturing of individual types of parts and testing of the finished product.

The process of manufacturing the tie-rack consisted of two major steps – injection molding the plastic parts, and assembling the tie-rack using the manufactured as well as the bought out components.

DFA

In order to suggest improvements in design of the product to aid assembly, we disassembled the tie rack and then attempted to recreate the stages in which it had been put together. The various steps in the assembly process can be found at Tie rack assembly

The assembly process is neither very labor efficient, nor very precise and so we concluded that the assembly process was carried out manually. The process of manual assembly can be divided into two distinct areas – handling and insertion and fastening. There are design guidelines that ensure easy part handling and easy insertion and fastening. We found that the design of the tie-rack adhered to the DFMA guidelines for both areas fairly closely.

Design for ease of part handling

• Where it was possible, parts were symmetric about the axis of insertion. For example, the belt holder (006) was symmetric about the horizontal axis (the axis perpendicular to its axis of insertion). Increasing symmetry reduces handling time as the time required to align the part is reduced.
• Where it was not possible to have symmetrical parts, easily identifiable features were provided to indicate the correct direction of insertion. In addition, obstructions were provided to ensure that parts could not be inserted the wrong way. The belt gear (008) only fit into the hole on the bottom cover (023) that corresponded to where the yellow gear be inserted. This was done by partially filling the other hole to make the two holes different depths. The long projection on the belt gear did not fit into the partially filled hole. The top cover (011) does not fit into the bottom cover (023) when the rectangular opening is not positioned over the battery holder. This is done by making the projections on the top cover different lengths and corresponding holes on the bottom cover different depths.

Design for ease of fastening and insertion

• The assembly begins using the bottom cover (023), which has a large mass relative to the other parts and a low center of gravity. The rest of the assembly process proceeds vertically. Parts are positioned with the aid of gravity.
• The assembly needs to be re-oriented only once (flipped over, i.e rotated through 360 degrees) and then right at the end to snap the battery cover (004) into place. The fixturing required to whole the assembly in place is very simple – it only needs to be clamped at two ends in order to keep it in place.
• The design eliminates the need for complex orientation and assembly movements. All parts need to be aligned only with respect to the horizontal.
• The tie-rack was also designed so that there was little or no resistance to insertion, and all parts fit together fairly easily.
• The wires are soldered onto the circuit board (020), light bulb (021), both battery end plates (025), springs (019) and motor (018) prior to attaching these components to the bottom cover (023). This serves a dual purpose. It prevents the molten metal from dripping onto the body of the tie-rack and damaging the plastic. It also makes it possible for the electric “module” of the tie-rack to be tested separately before final assembly. This is important, since the circuitry is the most sensitive part of the tie rack and requires more thorough testing than the plastic components. This is evidence that the designers attempted to keep the design of the tie rack modular to the extent possible.
• The designers used common parts where possible. The use of common parts enables the purchase of larger volumes and also reduces inventory carrying costs. Though screws (005) fasten 3 different parts of the tie-rack, the screws used are the same. The 2 clear gears (015) are also the same.
• The light cover (010) attaches to the top cover (011) by means of snap fit, an integral fastening method. This eliminates the use of threaded fasteners and so reduces assembly time.

Improvements

• One of the fundamental principles of DFMA is to minimize the number of parts. Instead of using a gear train to reduce the speed and increase the torque of the motor, it would have been easier to use a single larger gear. This would also reduce the number of pins required from 3 to 1. Each part has the possibility of being defective and also of causing an assembly error. Reducing the number of parts reduces the total cost of fabricating and assembling the product and reduces the chance of the product being defective. It also reduces levels of inventory and work-in-progress.
• Another way to reduce the part count would be to eliminate the gear train completely and use a motor with a higher torque output. However after researching the costs of various dc motors, we concluded that the cost of the gear train would probably be less than the cost of a motor with a higher torque output.
• Another DFMA guideline is to avoid the necessity for holding parts down to maintain their orientation during placement of another part. This was not followed in the procedure for attaching the rotating belt (007) to the belt gear (008) and belt rotator (009). First, the belt gear was attached to the bottom cover (023). The rotating belt was placed in a groove to hold it in place and engage the belt gear. Lastly the belt rotator was put into place. This last step was somewhat difficult because the rotating belt kept slipping out of its groove. It was difficult to hold the belt in place and insert the rotator simultaneously. This was because the ledge surrounding the groove was not high enough to keep the belt in place. An easier way to attach the belt would have been to make the ledge higher, to prevent the belt from slipping out of its groove. The belt should be positioned in the groove first and then belt gear and belt rotator should be inserted, such that they engage with the belt.

DFM (Injection Molding)

This analysis was useful for suggesting improvements in design of the plastic components for injection molding, to further decrease manufacturing costs aw well as provide better product quality. The table listing plastic injection molding guidelines and thier purpose in design consideration, as well as whether they were utilized in the rotating tie rack's components can be found at Tie rack DFM for injection molding

Improvements

From DFM analysis, the manufacturing process could be improved by adding a tapering feature to all ribs located on plastic components. This could be done without significant addition to mold cost, and would improve product quality by adding strength to the thin walls of the ribs and eliminating a 90 deg angle where the rib meets the wall. Also, numbers could be raised vs. recessed so that they do not experience shrinking, as the ones on current components have, thus making them difficult to discern. This will help with part recycling, reducing costs by providing materials can be safely reused.

The injection molding process can lead to various defects during within plastic parts. A table found in http://en.wikipedia.org/wiki/Injection_moulding#Moulding_defects gives a near-complete explanation of what these may be, but does include all defects possible from processes required to manufacture the rotating tie rack. Defects which were found in the plastic components include flow marks, weld lines and sink marks. However, it is not necessary to change the manufacturing process to eliminate these results. One reason is that the entire outer casing has been finished, leaving no cosmetic concerns. Sink marks, occurring at some wall intersections, are not located anywhere that results in a high possibility of structural failure. Neither do the weld lines, occurring at places experiencing low stress.

FMEA Analysis

The Tie Rack FMEA yielded several points of potential failure. None of these, however, had potential of failure resulting in accidents as severe as death or injury to the user. Most of the problems were already adressed as well as possible, although several had room for improvement, though not always without substantially increasing cost or impeding manufacturing and assembly.

DFE

There are a number of environmental issues with our product:

• Most of the parts are made from plastic
• There is a lot of plastic packaging
• The plastic is assumed to be non-recyclable
• Non-rechargeable batteries
• Reduce the gear train

The improvements that can be made are:

• Use different material
  • Recyclable plastic
  • Use plastic that does not emit harmful gases when incinerated
• Reduce the amount of plastic used
  • There is no need to package everything in a plastic bag
  • The base of the product only needs to be as wide as the belt for the rotating tie rack and the parts inside of it
  • The gear train can be reduced but then the motor output would need to be increased
    • Motor is reusable after the product breaks, the gears aren’t necessarily

Life Cycle Analysis

Scope: The purpose of this LCA is to determine the effects of manufacturing the rotating tie rack on the environment.

Inventory:

• Inputs: energy from 4 C batteries, light bulb
• Outputs: a rotating mechanism

This product does not emit any gas, energy, or substance when it is running. There is, however, a large amount of energy needed to produce, distribute and dispose of this product. Most of our parts are made via injection molding, which requires a large amount of heat and pressure. The distribution of this product requires sending it from the factory to stores nationwide. This is either done via truck, plane, or boat, all of which consume large quantities of gas and add to the pollution and global warming (CO2 emissions) problem. Lastly, at the end of the product’s life, it needs to be disposed of. Since it is not biodegradable nor recyclable, it will just sit in a landfill. The only waste materials generated throughout the products life are batteries and replacement light bulbs, which is not a lot considering some other products currently in use.

Impact Assessment: In the overall scheme of things, this product has a very low impact on the environment. The largest effect it has is during distribution, when gas is needed to transport the product to different parts of the country. That is extremely hard to avoid, however. In an assembled state, the product would take up roughly a 2x1x.5 ft cubed area. In a disassembled state, it would take up slightly more space since all of the parts fit under a top and bottom cover.

Improvement Assessment: As stated above, the improvement with the most impact would be to use recyclable plastic, but it is impossible to tell if that would be cheaper that the plastic already being used. It would save the company money if they used less plastic wrapping during packaging, but we would have to make sure it doesn’t affect the shipping of the product.

Other Improvements

We came up with some other design improvements in addition to the improvements we identified in the areas of DFMA and DFE and after performing FMEA.

• The quality of the wires used was very poor. The inside consisted of only 5 fibers twisted together, and as a result the wires were very thin. The soldered connections were weak and came apart when we disassembled the rack. This could also happen if the tie-rack were handled roughly. It would be better to use wires of a higher quality.
• Another problem was that the speed of rotation of the rack could not be varied. We concluded that the designers had chosen a speed such that it was slow enough for you to be able to see each tie as it went by. This caused problems when putting ties on the rack while it was rotating, because it rotated too fast. It was also annoying to wait for the rack to complete half a revolution to access ties that you knew were at the back of the closet. This problem could be resolved by introducing a speed controlling mechanism. The rotational speed of a DC motor is proportional to the voltage applied to it. Speed can be controlled by introducing variable battery tappings or resistors.
• It is not necessary to have the bulb on every time you rotate the rack. Having a separate circuit and switch control for the bulb would help the batteries last longer. This can be very easily done without any significant addition to the cost of the tie rack. This would also be helpful in low light situations, where you could turn on the light first and so not have to fumble around in the dark to choose the correct button for direction of rotation.
• We initially considered LEDs to increase the battery life. A LED only turns on when a positive voltage is applied across it. In the tie rack, the two directions of motion are caused by current flowing in different directoins. Therefore we would need 2 LEDs in series, wired opposite to each other. This would ensure that no matter what the direcion of current, one LED would always have a positive voltage across it. The voltage supplied by the battery (3V) is greater than the LED breakdown voltage and so they would emit light. The advantage of a LED is that it produces more light per watt than a bulb and so helps in conserving battery life. In addition, LEDs are solid state components and are difficult to damage with external shocks when compared to light bulbs (for example, if the tie rack was dropped). However LEDs are currently more expensive (price per lumen) than bulbs, and we concluded the added cost was not worth the battery saving benefit of the LED.
• Additional improvements regarding function and design considerations for the product's geartrain can be found at Tie rack geartrain and circuitry analysis.