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The Case of the Strewn Skiers
Kenneth Russell, Professor Emeritus, MIT -- Design News, September 3, 2007
The motorized ski tow eliminated laborious uphill hikes and thereby revolutionized downhill skiing. Early tows amounted to a rope threaded around a pair of pulleys and driven by a dismounted automobile engine and transmission. Modern tows span miles and operate at great heights. Failure of a rope tow might land the skier on his rear end. Failures of modern tows are a lot more serious.
The Scene of the Crime
The case at hand involved a chairlift in New Hampshire. A fractured housing caused the chair to separate from the cable and fall to the terrain far below. The two teenage male passengers were seriously injured and their parents sued the ski area. I was retained by the ski area in my usual role of metallurgist.
The Investigation
The figure, above right, is a drawing of the housing assembly. The housing is connected to the cable by a pair of arms that extend through the larger holes. The chair swings from a 3-inch long bronze bushing that is tightly clamped between the ears of the housing by a steel bolt and nut. The chair is suspended from a bronze bushing clamped between the ears of the housing. The housing fractured through both ears, which released the bolt and bushing and put the chair and passengers into free-fall.
The initial study was by a plaintiff expert who blamed poor design and assembly of the housing assembly. He claimed over tightening of the bolt caused the ears to fracture. He was apparently ignorant of the presence of the bronze bushing which separated the ears and precluded any such failure.
I loaded an exemplar (duplicate) housing to failure, which occurred at 17,250 lb. This load is nearly 100 times the weight of the two passengers, which is one huge factor of safety. But, was the subject housing of the same strength?
My studies said it was. Radiography and optical and scanning electron microscopy showed a sound microstructure remarkably free of cracks and porosity. A small specimen machined from the subject housing and loaded to failure had a strength consistent with that of the exemplar. So, the housing was plenty strong enough. The test showed only 6 percent ductility, which is to be expected of the aluminum-silicon alloy used.
The Smoking Gun
The plaintiff expert saw my report and changed his tune. He dropped the over-tightening claim and concluded the failure was due to sympathetic oscillations set up in the cable instigated solely by the powering of the lift. These oscillations caused the chair to strike a tower and fracture the housing. He stated without proof the boys could not possibly have caused the swinging.
Anyone who has ever used a playground swing knows better. During the case the Boston Globe reported a Lake Tahoe accident in which teenagers “swinging the chairs” dumped people from 40 ft and injured 17 of them. But, this was a plaintiff expert so his clients had to be guiltless.
The expert also claimed, wrongly, the low ductility of the alloy made it inappropriate for use. Cast alloys seldom have high ductility. The design engineer understands this fact and deals with it. A more ductile casting material could have been used, but at a cost in other qualities. The Al-Si casting alloy has a remarkable ability to fill complex molds and produce porosity-free castings. Another alloy might have performed better in simple test specimens, but have produced incomplete bracket castings, rather like waffles made with too little or too viscous a batter.
The plaintiffs lost their case against the ski area. My client said my testimony was a big part of the decision. The plaintiffs then hired a new expert and sued a bunch of other parties including the National Ski Assn. of which the original defendant was a member. I never heard the outcome of these suits.
Talkback
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One might wish to consider the possibility of an assembly technician assembling the bracket...
- 2007-11-12 12:49:04 -
"I loaded an exemplar (duplicate) housing to failure, which occurred at 17,250 lb. This...
- 2007-09-22 19:16:38 -
My own experience with chairlifts is that they will rock particularly in windy conditions but...
- 2007-09-12 14:23:13
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