| Equipment
Reliability Institute your reliability newsletter May, 2001 - vol. 3 |
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Larry George drew many comments, last issue, with his "Measure Field Reliability with Statistics". This time Larry offers "Could Firestone and Ford Have Known?". Larry's piece is followed by some ideas of mine on how to sell training to your boss, his boss, etc. Yes, it takes a little courage to tell them that your people need training. And a "sidebar" mentions some vibration and shock courses I'll be teaching in the next few months. Check them out! Hope you enjoy this issue! Best wishes, |
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Could
Firestone and Ford Have Known? On August 2, 2000, Firestone recalled "…all ATX and ATXII tires of the P235/75R15 size manufactured since 1991 and all Wilderness AT tires of that same size manufactured at Firestone's Decatur, Illinois, plant." On August 30, 2000, the NHTSA recommended that Firestone expand the recall. Firestone declined. Investigation continues. As of February 2, 2001, the NHTSA was aware of 174 fatalities "alleged to be related to a tire failure." Firestone tires were original equipment on some Ford and Mazda SUVs, light trucks, and pickups. The NHTSA Firestone complaints database [1] includes the tire failure date and the vehicle make, model, and year. The difference between failure date and vehicle year is the age of the vehicle at tire failure. The complaints database also indicates whether the failed tire was an original one. Using vehicle production data [2], I made the nonparametric estimates of age-specific tire reliability in figure 1. The last data point for each year is the estimated reliability in the year 2000.
Figure 1. Age-specific tire
reliability estimates of recalled Reliability got worse in tires manufactured in 1996, but you'd have to wait to see the evidence of that. That is because you can't estimate reliability at age five until tires are at least five years old. The NHTSA collected almost all the complaints in the year 2000, so the NHTSA couldn't have made the estimates in figure 1 until late in the year 2000. Table 1. Fatalities by Year of Tire Failure
Presumably, lawyers contacted Firestone and Ford regarding these fatalities, so both companies would have had this data, perhaps in the years in which the tire failures occurred. Shouldn't that have prompted corporate concern? The risk management department of a major medical company told me, "Just don't kill anybody." They explained that their concern was risk, the number of opportunities for failure times the probability of failure per opportunity (unreliability) times expected cost per failure. Firestone and Ford were probably concerned about the ominous trend in fatalities, at least in 1999. Would it have been possible to recognize problems earlier? What If You Had Been Working for Firestone? Imagine that you were working for Firestone in the 1990s. Firestone knows its ships and warranty returns, because generally accepted accounting principles require that data. Ships and returns are statistically sufficient for estimating age-specific field reliability [3], forecasting returns, and providing early warning. In 1991 you would have had the data from 1991; in 1992, you would have had data from 1991 and 1992; and so on. You could have estimated reliability each year. Figure 2 shows the nonparametric least squares estimates, as if they had been computed each year from 1994 through 2000. (I used Ward's vehicle ships and the NHTSA's annual failure counts, for original equipment tires.)
Figure 2. Age-specific tire
reliability estimates by year In the year 2000, problems were obvious, but could they have been recognized earlier? In 1997, the figure would have ended at the sixth year, with a reliability decrease from six nines (0.999999) to five nines (0.99999). That would have gotten my attention. That decrease in reliability could have been because
That reliability decrease should have called for sampling, more analysis, and possible corrective action, if Firestone had been estimating tire reliability from ships and returns data. Firestone would have saved half their costs if the recall had been made in early 1997. What If You Had Been Working for Ford? Imagine that you were working for Ford in the 1990s. Ford knows its own production figures, and it tracks warranty repairs by VIN and symptom, which yields age data sufficient to make the estimates in figure 1. It is not necessary to track warranty repairs by VIN. That requires 1000 times as much data and incurs at least 1000 times as many errors. Suppose some warranty repairs were due to tire related problems. You could have made the estimates in figure 3, which shows estimates made from ships and NHTSA complaints, year by year.
Figure 3. Age specific tire
reliability estimates by vehicle The last data point for each vehicle model is the estimated reliability in the year 2000. The Mountaineer went into production in 1996, and tire problems became evident in its second year, 1997. The Explorer began production in 1990, and tire problems became evident in its sixth year, 1997. These reliability estimates should have encouraged Ford to investigate tire related problems in 1997, if Ford had been estimating age-specific reliability, even from ships and returns data. Conclusions and Recommendations Age-specific field reliability can be estimated without tracking parts and products by serial number. It also helps detect exceptions, process shifts, improvement, or deterioration attributable to calendar intervals. It helps separate the effects of vehicle, make, model, tire type, or plant. If you're trying to control risk like that experienced by Firestone and Ford, estimate age-specific field reliability by calendar intervals. Make actuarial forecasts of returns and put upper confidence limits (UCL) on the forecasts. Use them like control charts. If returns in some calendar interval exceed the UCL, take a sample, estimate reliability from age-at-failure data, search for root causes, and evaluate process improvements. Revise forecasts, estimate risk under alternatives, and act accordingly. References
Mark Felthauser, CCI/Triad, helped with the statistical analyses. Eva Langfeldt, Text Support, edited the article. I am grateful to them for their thoughtful contributions. Contact them through Larry, pstlarry@home.com or 925-447-4969. |
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It takes courage to say to one's superiors, "My people need training in _______" (naming the subject). Among the unpleasant rejoinders you might get: We can't afford it. There's no budget for training. You're already expended this year's training budget. Teach them yourself. You took _______ in college, didn't you? How have you "gotten by" up till now? Why not continue? Perhaps readers can add to that list, citing similar rejoinders they have received. Let's hope you get a more helpful rejoinder, something like "Put your requirements in writing, in a form I can pass up the line. There must be several ways we can obtain that training. List the strongest needs (from management's point of view) first, down to the weakest needs. Estimate costs of obtaining that training "outside". If you think that any of our present staff can teach what you need, work with that person to estimate hours needed ´ hourly rate including overhead + all materials needed. Also estimate immediate and long-range costs of continuing as we are, without that training." Obtaining training "inside" Does anyone in your organization want to teach about ________? If you find such a person, question his/her practical experience in the field. Look at what he/she has published. Will that kind of material help your people? Does he/she have time to prepare lectures? Handout materials? Estimate what that will cost (several hours away from other duties ´ that person's hourly rate, for each projected class hour). Also estimate the chances that this person will be an effective teacher. Obtaining training "outside" A. Here you might consider sending individuals to a local university short course, if the instructor has actually worked in the field named. Check up on that instructor. Look at what he/she has published. Will that kind of material help your people? Ask the university "Who took this course last year?" Ask fellow members of your local or national engineering society for their opinions. B. Alternately, you might consider sending individuals to a distant short course. In addition to fees, estimate travel and living costs, as well as time off work. If your management funds either A or B, management probably hopes that the now-educated attendee(s) will be able to teach others. Estimate what that will cost (several hours away from other duties, preparing classroom materials ´ hourly rate, for each projected class hour). Also estimate the chances that this now-educated attendee will be an effective teacher. Bringing in specialized training This may prove most effective and least costly, particularly if "tailored" to meet your peoples' needs. Hopefully, other departments will share in the costs as well as the benefits. For example, if your people perform environmental tests, speak with designers who have had difficulty meeting (or even understanding) product test requirements. Speak with production and quality engineering people concerned with stress screening. Distance Learning You are no doubt aware that colleges and universities as well as specialized schools offer instruction via "the Net" or via CD-ROM, with individuals studying at their own pace, on their own time or on "company time". This approach can be cost-effective, but you need to monitor participant progress over many weeks. Getting by without specialized training Certainly it's possible to continue as you are doing, but at what cost? Your best people want to feel that they are learning new material, and may leave if they perceive of themselves as stagnating. Do your designers and test people communicate well? Participating together in training enhances on-the-job communications. Perhaps you are not fully using your existing lab, and must go "outside" to a commercial test lab. How many hours of "outside" testing did you use last year? At what hourly rate? Do you perform needed developmental tests outside? Or do you defer tests until products are mature, resulting in very expensive changes? What did those changes cost last year? Right now Wayne Tustin is "studying up" on the vibration and shock measurement and testing issues pertaining to computer hard drives, preparing for a private course. Have you heard about intense noise developmental testing and production screening of printed wiring boards? It's an alternate method (to electrodynamic shakers and to pneumatic repetitive-shock machines) for flexing PWBs during HALT, HASS, ESS, etc. You can send him an e-mail to tustin@equipment-reliability.com if you want to learn more. Or you can phone him at 805/564-1260. He would like to hear from you. |
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Questions our readers have asked... Bill Shust's client asked "How can we avoid fatigue failures on our loaders?" Onboard monitoring of a construction loader during feedlot operations identified suspect weld locations and lengthened service life dramatically. I installed strain gages and pressure transducers
on a loader, and recorded up Static stalls, truck loading and yard cleanup operations by several operators were recorded. Relatively lightweight flaked corn and alfalfa are normally loaded with this unit. Therefore, occasional and unintended side-contact of the bucket with the concrete storage bin was originally suspect as the most severe field event. However, lateral bending strains caused by side-contact proved significantly lower than vertical bending strains measured during and after typical truck loading operations.
Fortunately, free-body diagrams of the equipment were used to also show that a relatively simple change in the retraction hydraulic circuit could greatly reduce peak-to-peak strains (by close to one-half) of the more damaging cycles. The fatigue model predicted that the resulting strains would improve service life by almost a factor of ten. A full report including photos, video logs, test data files, and SoMat DataXplorer software was transmitted to the customer on CD. Prior to forming Objective Engineers, Mr. Shust managed test programs and conducted mechanical testing for over 15 years in the railroad, highway truck, and power generation industries. He has BS and MS degrees in Mechanical Engineering from Michigan Technological University. If you want to contact him, please send an e-mail to shust@equipment-reliability.com |
to 55 minutes of continuous data as well as ASTM rainflow histograms.
Using a SoMat eDAQ equipped with a 450 Mbyte PCMCIA memory card,
strains and pressure were recorded at 1000 samples/second. The 16-bit
analog to digital converters of the eDAQ yielded data resolution
of approximately 0.1 microstrain or 0.1 psig. The recorder was located
in the operator's cab near the driver. Data was downloaded to a
notebook PC over a LAN cable, typically during operator breaks.
Strain
gages had been placed 1/2-inch from various weld toes, for later
application of British Standard 5400 fatigue analysis method, commonly
used with structural steel. Measured vertical strains were more
than twice the infinite-life strain ranges for BS5400 weld Classes
F2 and G. Fatigue analysis showed that relatively few cycles of
these larger strain ranges contributed the majority of fatigue damage.
Further examination of time history relationships showed that excessive
residual pressure in a hydraulic cylinder resulted in these few
but large-amplitude cycles. Hence the typical service life of this
new equipment would fall short of customer expectations in several
welds. |
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ERI is seeking an authority on European EMC directives to teach USA EMC practitioners. Please e-mail Wayne Tustin |
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Check our Vibration and Shock Glossary. You will find important words and their definitions. This list evolved from Wayne's 50 years of work experience and it is constantly updated. |
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