Failure analysis and redesign of a helix upper dispenser Seong-woo Woo a, * , Michael Pecht b a SAMSUNG Electronics Co., Ltd., 272, Oseon-Dong, Gwangsan-Gu Gwangju-City 506-723, South Korea b CALCE Electronic Packaging Research Center, University of Maryland, College Park, Maryland, TX 77843-3123, USA Received 28 September 2007; accepted 7 October 2007 Available online 22 October 2007 Abstract Failure analysis was conducted on a fractured helix upper dispenser for a side-by-side refrigerator with ice dispenser system. To reproduce the failure modes and mechanisms causing the fracture, a tailored set of accelerated life testing were applied to sample dispensers. Using bond graphs and state equations, key noise parameters in the assembly, including a variety of mechanical loads, were analyzed. The failure modes and mechanisms found experimentally were identical with those of the failed sample. To correct the problem, the key controllable design parameters of the helix upper dispenser were modified by eliminating gaps and enforcing ribs. The B 1 life of the new design is now guaranteed to be over 14 years with a yearly failure rate of 0.021%. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Reliability design; Load analysis; Accelerated life testing 1. Introduction The basic function of a refrigerator is to store fresh and/or frozen foods. However, refrigerators today also provide other functions, such as dispensing ice and water. As the number of refrigerator parts and their func- tions increase, product reliability can be affected, especially as market pressure for product cost reduction leads to the use of cheaper parts. It is crucial that refrigerator functions are consistently reliable during customer usage. The refrigerator can be designed for reliability by determining proper parameters and their levels [1,2]. However, some seemingly minor parameters may be neglected in the design review, resulting in product failure in use. Products with minor design flaws may result in recalls and loss of brand name value. Furthermore, product liability law requires manufacturers to design products more safely in the European Union [3] and the United States [4]. Preventing such outcomes is a major objective of the product development process – design, pro- duction, shipping and field testing. Conventional methods, such as product inspection, rarely identify the reli- ability problems occurring in market use. Instead, designing for optimal durability and reliability requires 1350-6307/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2007.10.005 * Corresponding author. Tel.: +82 18 505 9084; fax: +82 62 950 6807. E-mail addresses: [email protected](S.-w. Woo), [email protected](M. Pecht). Available online at www.sciencedirect.com Engineering Failure Analysis 15 (2008) 642–653 www.elsevier.com/locate/engfailanal
Transcript
Available online at www.sciencedirect.com
Engineering Failure Analysis 15 (2008) 642–653
www.elsevier.com/locate/engfailanal
Failure analysis and redesign of a helix upper dispenser
Seong-woo Woo a,*, Michael Pecht b
a SAMSUNG Electronics Co., Ltd., 272, Oseon-Dong, Gwangsan-Gu Gwangju-City 506-723, South Koreab CALCE Electronic Packaging Research Center, University of Maryland, College Park, Maryland, TX 77843-3123, USA
Received 28 September 2007; accepted 7 October 2007Available online 22 October 2007
Abstract
Failure analysis was conducted on a fractured helix upper dispenser for a side-by-side refrigerator with ice dispensersystem. To reproduce the failure modes and mechanisms causing the fracture, a tailored set of accelerated life testing wereapplied to sample dispensers. Using bond graphs and state equations, key noise parameters in the assembly, including avariety of mechanical loads, were analyzed. The failure modes and mechanisms found experimentally were identical withthose of the failed sample. To correct the problem, the key controllable design parameters of the helix upper dispenser weremodified by eliminating gaps and enforcing ribs. The B1 life of the new design is now guaranteed to be over 14 years with ayearly failure rate of 0.021%.� 2007 Elsevier Ltd. All rights reserved.
Keywords: Reliability design; Load analysis; Accelerated life testing
1. Introduction
The basic function of a refrigerator is to store fresh and/or frozen foods. However, refrigerators today alsoprovide other functions, such as dispensing ice and water. As the number of refrigerator parts and their func-tions increase, product reliability can be affected, especially as market pressure for product cost reductionleads to the use of cheaper parts.
It is crucial that refrigerator functions are consistently reliable during customer usage. The refrigerator canbe designed for reliability by determining proper parameters and their levels [1,2]. However, some seeminglyminor parameters may be neglected in the design review, resulting in product failure in use.
Products with minor design flaws may result in recalls and loss of brand name value. Furthermore, productliability law requires manufacturers to design products more safely in the European Union [3] and the UnitedStates [4]. Preventing such outcomes is a major objective of the product development process – design, pro-duction, shipping and field testing. Conventional methods, such as product inspection, rarely identify the reli-ability problems occurring in market use. Instead, designing for optimal durability and reliability requires
1350-6307/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.
A constantAF acceleration factorB the viscous friction coefficientBX durability indexF(t) unreliabilityh testing cyclesh* non-dimensional testing cycles, h* = h/LB P 1ia the applied current, Aif the field current, Aea the applied voltage, Veb the counter-electromotive forceef the field voltage, Vh the test timeJ the momentum of inertia, kg m2
ka constant of the counter-electromotive forceLa the electromagnetic inductanceLB the target BX life and x = 0.01X, on the condition that x 6 0.2m the gear ratioMGY Gyrator in causal forms for basic 2-ports and 3-portsn the number of test samplesn the quotientr the number of failed samplesr the coefficient of gyratorRa the electromagnetic resistanceS0 mechanical stress under normal stress conditionsS1 mechanical stress under accelerated stress conditionsSe effort source in causal forms for basic 1-portsT0 mechanical torque under normal stress conditionsTo motor rotation torque, kN cmti the test time for each sampleT1 mechanical torque under accelerated stress conditionsTf the time to failureTL the ice load torque in bucket, kN cmTPulse the disturbance torque, kN cmx 0.01
Greek symbols
x the angular velocity in ice bucket, rad/sxo the angular velocity in auger motor, rad/sg characteristic life
Superscripts
b shape parameter in a Weibull distributionn the quotient
Subscripts
0 normal stress conditions1 accelerated stress conditions
extensive reliability testing at each development step. However, as a result, the cost of quality assurance andappraisal can increase significantly.
In reliability design, most global companies focus on accelerating life testing (ALT). ALT can help shortenthe product development cycles and identify diverse design flaws. However, failures precipitated by ALT maynot represent those occurring under market conditions because of inconsistencies in the types and magnitudesof the loads applied during testing. Moreover, the numbers of test samples and the test durations are usuallyinsufficient to uncover occasional failure modes. ALT should be performed with sufficient samples and testingtime, with equipment designed to match expected product loads.
Fig. 1 shows the SBS refrigerator with ice dispenser and the mechanical parts of the ice bucket assembly.The assembly consists of the bucket case, helix support, helix dispenser clamp, blade dispenser, helix upperdispenser, and blade, as shown in Fig. 1b.
It was found that the helix upper dispenser in the ice bucket of refrigerators with ice dispenser systems hasbeen fracturing, causing loss of the dispensing function. Thus reproducing the failure mode to assess how toprevent the fracture of the helix upper dispenser was critical. To prevent a recurrence of failure, reliabilitymust be improved based on a knowledge of the exact root causes, thus. The data on failed products in themarketplace are important to understand the use environment of customer of the product and helping to pin-point root causes.
The procedures for reliability design can be summarized as (1) analysis of the marketplace problems; (2)analysis of the loads on the dynamic system; (3) ALTs to predict reliability and durability; and (4) formulationof a corrective action plan. Because the ice bucket assembly is a motor-driven system, it is effective to use thebond graphs method and its state equations in the dynamics analysis of the helix upper dispenser [4,5]. ALTequipment can then be fabricated on the basis of load analysis.
We investigated the reliability of a helix upper dispenser. First we analyzed ‘‘uncontrollable’’ mechanicalload conditions of an ice bucket assembly using robust design schematic, bond graph, and state equations.
Fig. 1. SBS refrigerator and ice bucket assembly. (a) SBS refrigerator and (b) Mechanical parts of the ice bucket assembly.
We then proposed new ALT methodologies for robust designs. After a sequence of reliability testing, includ-ing ALT, we evaluated the reliability of the improved helix upper dispenser with BX life. Finally, we proved theeffectiveness of these methodologies to design for reliability.
2. Field application problems
In the marketplace, products in the dispenser system of the refrigerator were cracking and fracturing (seeFig. 2) under unknown customer usage conditions. Field data indicated that the damaged products may havetwo structural design flaws: (1) a 2 mm gap between the blade dispenser and the helix upper dispenser, and (2)a weld line around the impact area of the helix upper dispenser. Due to the gap, the rotating blade dispenserimpacts the fixed helix upper dispenser. Because of the weld line, a crack may occur. The temperature of theproduct was below �20 �C.
3. Load analysis
The mechanical icemaker system in a side-by-side (SBS) refrigerator with a dispenser system consists ofmany structural parts. Depending on the customer usage conditions, these parts receive a variety of mechan-ical loads in the icemaking process. Icemaking involves several mechanical processes: (1) the filtered water ispumped through a tap line supplying the tray; (2) the cold air in the heat exchanger chills the water tray; and(3) after ice is made, the cubes are harvested, stocking the bucket until it is full. When the customer pushes thelever by force, cubed or crushed ice is dispensed.
In the United States, the customer typically requires an SBS refrigerator to produce 10 cubes per use and upto 200 cubes a day. Ice production may be influenced by uncontrollable customer usage conditions such aswater pressure, ice consumption, refrigerator notch settings, and the number of times the door is opened.When the refrigerator is plugged in, the cubed ice mode is automatically selected. A crusher breaks the cubedice in the crushed mode.
Normally, the mechanical load of the icemaker is low because it is operated without fused or webbed ice.However, for Asian customers, fused or webbed ice will frequently form in the tray because they dispense icein cubed mode infrequently. When ice is dispensed under these conditions, a serious mechanical overloadoccurs in the ice crusher. However, in the United States or Europe, the icemaker system operates continuouslyas it is repetitively used in both cubed and crushed ice modes. This can produce a mechanical overload.
Fig. 3 overviews the schematic of the ice maker. Figs. 4 and 5 show a schematic diagram of the mechanicalload transfer in the ice bucket assembly and its bond graph. An AC auger motor generates enough torque tocrush the ice. Motor power is transferred through the gear system to the ice bucket assembly – that is, to thehelix upper dispenser, the blade dispenser and the ice crusher.
Key Control ParametersC1: AC auger motor specificationsC2: Ice bucket assembly materials & size C3: Ice tray materials & size C4: Lever material & size
Fig. 3. Robust design schematic of ice maker.
ArmatureSupply
ea
ia
Field supply
Motor GearClamp & Crusher
TL
ω
To
ωo
ef if
ia
ea
To
ωo
if
ef
TL Jω
Tpulse
B
Fig. 4. Schematic diagram for mechanical ice bucket assembly.
When Eq. (1) is integrated, the angular velocity of the ice bucket mechanical assembly is obtained as
yp ¼ 0 1½ �ia
x
� �ð2Þ
The mechanical stress (or life) of the ice bucket assembly depends on the disturbance load TPulse in Eq. (1). Theaccelerated life testing applies the stress between low and high to the breakdown stress. The life–stress model(LS model) [7] can be modified as
4. Theoretical background of new definition of BX life for the accelerated life test
The characteristic life g by maximum likelihood estimation can be defined as
gb �P
tbirffi n � hb
rð5Þ
As product (or part) reliability improves, there are usually no failures in the test. Thus, it is not appropriateto evaluate the characteristic life in Eq. (5). When the number of failed samples is below four, it follows thePoisson distribution [8]. At a 60% confidence level, the characteristic life can be redefined as
gb ffi 1
r þ 1� n � hb ð6Þ
In order to introduce the BX life, the characteristic life in the Weibull distribution can be modified as
LbB ffi x � gb ¼ x
r þ 1� n � hb ð7Þ
In order to assess BX life with about a 60% confidence level, the number of test samples is derived from Eq.(7). That is
n ffi 1
x� ðr þ 1Þ � 1
h�
� �b
ð8Þ
on the condition that the durability target, h* = h/LB P 1.
5. Laboratory experiments
Generally, the operating conditions for the mechanical ice bucket assembly in an icemaker are �15 to�30 �C temperature, 0–20% relative humidity, and 0.2–0.24 G vibration. The dispenser is used an averageof approximately 3–18 times per day. Under maximum use for 10 years, the dispenser incurs about 65,700usage cycles (Table 1). Data from the motor company specifies that normal torque is 0.69 kN cm and maxi-mum torque is 1.47 kN cm. Assuming the quotient n = 2, the acceleration factor is approximately 5 in Eq. (4).
The test cycles and the numbers of samples [9] used in the ALT were calculated as follows:
n ffi ðr þ 1Þ � 1x� LB
AF � h
� �b
ð9Þ
The test cycles and test sample numbers calculated in Eq. (9) were 59,200 cycles and 10EA, respectively. TheALT was designed to ensure a B1 of 10 years life with about a 60% level of confidence that it would fail lessthan once during 59,200 cycles [8].
1ing cycles of the ice dispenser
Operating cycles (times)
1 day 10 years
Normal Worst Normal Worst
ker 1–3 18 10,950 65,700
Fig. 6. Duty cycles of disturbance load TPulse on the band clamper.
Fig. 7. Equipment used in accelerated life testing.
Fig. 6 shows the duty cycles for the disturbance load TPulse. Fig. 7 shows the ALT equipment for the repro-duction of the failed structural parts in the field.
The equipment in the chamber was designed to operate to about �30� of temperature. The controller out-side can start or stop the equipment and indicate the completed test cycles, and the test periods, such as sampleon/off time. To apply the maximum disturbance torque TPulse, two parts – the helix upper dispenser and theband clamper – were bolted together. When the controller outside the chamber gives the start signal, the augermotor rotates the clamp helix dispenser, the helix upper dispenser and the blade dispenser. At this point, therotating blade dispenser will impact the fixed helix upper dispenser to the maximum mechanical disturbancetorque (1.47 kN cm). Depending on the operating condition of the equipment, the blade dispenser will providemaximum torque to the helix upper dispenser 4–6 times in 5 s.
6. Results and discussion
6.1. Validity of the accelerated life test and failure analysis
Fig. 8 shows the failed product in the field and a sample after accelerated life testing. In the photo, the shapeand location of the broken pieces in the failed market product are identical to those in the ALT results. Fig. 9represents the graphical analysis of the ALT results and field data on a Weibull plot. For the shape parameter,the estimated value in the previous ALT is 2.0. However, the final value obtained on the Weibull plot was 4.8.As the ratio of characteristics life, g1/g2, gives the acceleration factor, AF is approximately 2.2 on the Weibullplot.
We conclude that these methodologies are valid to reproduce the fielded failures because (1) the locationand shape of the fractures in both market and ALT results are extremely similar; and (2) on the Weibull,the shape parameters of the ALT results, b1 and market data, b2, are very similar.
The fracturing and cracking of both the fielded products and the ALT results occur in the contact area ofthe blade dispenser (Fig. 8). These structural flaws generate the concentrated mechanical stress when the bladedispenser, made of stainless steel, meets the polycarbonate helix upper dispenser at a right angle (Fig. 10). Dueto the 2 mm gap between the blade dispenser and helix upper dispenser and the impact (1.47 kN cm) ofthe blade dispenser, the concentrated stress of the blade dispenser is approximately 36.9 kPa, based on finite
Fig. 8. Failed product in field and ALT. (a) Failed product in field. (b) Failed sample in accelerated life testing.
1000 10000010000
0.05
0.10
0.50
1.00
5.0
10.0
50.0
90.0
99.0
0.01
Cycles, Times
Unr
elia
bilit
y, F
(t)
β1=4.7785, η1=1.0262Ε+4β2=4.0710, η2=2.2215Ε+4
: Reproduce of Failure
: Field Failure
Fig. 9. Field data and results of ALT on Weibull chart.
element analysis. Under �20 �C, it is particularly fragile due to the weld line near the impact area of the helixupper dispenser.
6.2. A corrective action plan and life prediction
Table 2 and Fig. 11 show the improved design of the helix upper dispenser based on the ALT results. Theconfirmed values of AF and b in Fig. 7 were 2.2 and 4.8. The calculated test sample number in Eq. (10) was10EA. Failure analysis identified the root cause of the failed product as the 2 mm gap between the blade dis-penser and the helix upper dispenser, and the weld line.
When the gap between blade dispenser and helix upper dispenser was eliminated in this modified design, themechanical concentrated stress of the sample, based on finite element analysis, was reduced from 36.9 kPa to21.3 kPa.
This approach was very effective in reproducing the fracture of the product from the marketplace. All sam-ples in the first ALT (n = 10) failed within 11,600 cycles, as shown in Table 2. However, the life of the newlydesigned samples do not fail within the target life of B1 10 years.
In order to improve the reliability of the newly designed helix upper dispenser, a second ALT was imple-mented with a key controllable design improvement – no gap in the samples. Based on the first ALT, the AFand b values in the second ALT were 2.2 and 4.8. The test cycles and test sample number calculated in Eq. (9)were 62,000 cycles and 6EA, respectively. For the second ALT, all samples were failed within 38,000 cycles. Asshown in Fig. 9, the second ALT results show the sample life lengthened. However, the failed test samples inmission test cycles were still found, as shown in Table 2.
The BX life of the sample was calculated as:
TableResult
Factor
Values
BX ffih �AF
LB� x � n
r þ 1
� �1b
ð10Þ
The B1 life of the samples in the first and second ALTs was 1.4 and 4.7 years, respectively. Thus, the B1 life ofthe newly designed samples was 3.4 times that of the current design.
For the failed samples, the key controllable design improvement in the third ALT was to add ribs on theside and front of the impact area, as shown in Table 2. These redesigned samples were implemented for thethird ALT. The test cycles and test sample number calculated in Eq. (9) were 62,000 cycles and 6EA, respec-tively. In the third ALT results, the samples did not crack and fracture until 75,000 cycles of testing.
Table 3 shows the results obtained from the third ALT. The B1 life of the redesigned samples using Eq. (10)was 14 years. When the design of the current product was compared with that of the newly designed one, theB1 life expanded about fourteen times, from 1.4 years to 14 years. The design improvements of eliminating thegap and reinforcing the ribs were very effective in enhancing the reliability of the sample.
Table 3 shows the acceleration factor is 2.2. Assuming the normal torque and maximum torque of the sam-ple are 0.69 and 1.47 kN cm, the quotient, n, is 1.01. The cycles to failure of the sample in Eq. (3) can be mod-eled as follows:
T f ffi A � ðT PulseÞ�1:01 ð11Þ
7. Conclusions
To improve the reliability of the helix upper dispenser in SBS refrigerators, we have examined the failuremodes and mechanisms for fractured dispensers and predicted the life of the helix upper dispenser with variousimprovements using accelerated life testing. The following general conclusions were obtained:
(1) Based on the claimed marketplace product returns and ALT reproduction, the root causes of the failedhelix upper dispenser in the SBS refrigerator include the combination of customer usage conditions andthe fragile structure of the helix upper dispenser. Specific flaws were found to be gap, the weld line, andthe impact of the stainless steel blade dispenser on the polycarbonate helix upper dispenser.
(2) Key controllable design improvements involved gap elimination and rib reinforcement. These wereshown to be effective in enhancing the reliability of the helix upper dispenser.
(3) After a sequence of reliability testing, the yearly failure rate and B1 life of the redesigned helix upperdispenser, based on the results of ALT, were 0.021% and 14 years, respectively.
(4) The inspection of the failed product, load analysis, and three rounds of ALT, were very effective inreproducing the fracture in the helix upper dispenser claimed in the marketplace and in improving itsreliability.
References
[1] Taguchi G. TAGUCHI on robust technology development. New York: ASME Press; 1992. p. 16–62.[2] Council Directive of 25 July 1985 on the approximation of the laws, regulations and administrative provision of Member States
concerning liability for defective products.[3] Restatement of the law, third, torts: product liability. The American Law Institute; 1923.[4] Karnopp DC, Margolis DL, Rosenberg RC. System dynamics: modeling and simulation of mechatronic systems. 3rd ed. New York:
John Wiley & Sons, Inc.; 2000. p. 113–49.[5] Rosenberg RC. State space formulation for bond graph models of multiport systems. J Dyn Syst Meas Control, Serial G – Trans
ASME 1971(1):35–40.[6] Shin WJ, Ha HG. Improvement on the speed-response of dc motor using bond graph modeling method. J Korean Inst Commun Sci
1991;16(4):309–18.[7] ASM International. Packaging, Electronic Materials Handbook, vol. 1. Ohio: ASM International; 1989. p. 887–94.[8] Lee SY. Reliability engineering. Seoul: Hyung Seol; 2003. p. 9–15.[9] Ryu DS, Chang SW. Novel concept for reliability technology. Microelectron Reliab 2005;45:611–22.
Dr Woo has a BS and MS in Mechanical Engineering, and he has obtained PhD in Mechanical Engineering fromTexas A&M. He major in energy system such as HVAC and its heat transfer, optimal design and control ofrefrigerator, reliability design of thermal components, and failure Analysis of thermal components in marketplaceusing the Non-destructive such as SEM & XRAY.
In 1992.03–1997 he worked in Agency for Defense Development, Chinhae, South Korea, where he hasresearcher in charge of Development of Naval weapon System. Now he is working as a Senior ReliabilityEngineer in Side-by-Side Refrigerator Division, Digital Appliance, SAMSUNG Electronics, and focus onenhancing the life of refrigerator as using the accelerating life testing. He also has experience about Side-by-SideRefrigerator Design for Best Buy, Lowe’s, Cabinet-depth Refrigerator Design for General Electrics.
Dr Michael Pecht has a BS in Acoustics, an MS in Electrical Engineering and an MS and PhD in EngineeringMechanics from the University of Wisconsin at Madison. He is a Professional Engineer, an IEEE Fellow and anASME Fellow. He has received the 3M Research Award for electronics packaging, the IEEE Undergraduate.
Teaching Award, and the IMAPS William D. Ashman Memorial Achievement Award for his contributions inelectronics reliability analysis. He has written eighteen books on electronic products development, use and supply
chain management. He served as chief editor of the IEEE Transactions on Reliability for 8 years and on theadvisory board of IEEE Spectrum. He is chief editor for Microelectronics Reliability and an associate editor forthe IEEE Transactions on Components and Packaging Technology. He is the founder of CALCE (Center forAdvanced Life Cycle Engineering) and the Electronic Products and Systems Consortium at the University ofMaryland. He is also a Chair Professor. He has been leading a research team in the area of prognostics for the past10 years, and has now formed a new Electronics Prognostics and Health Management Consortium at the Uni-versity of Maryland. He has consulted for over 50 major international electronics companies, providing expertise
in strategic planning, design, test, prognostics, IP and risk assessment of electronic products and systems.
