Elevator (History)

Statistically speaking, cable-borne elevators are extremely safe. Their safety record is unsurpassed by any other vehicle system. In 1998, it was estimated that approximately eight millionths of one percent (1 in 12 million) of elevator rides result in an anomaly, and the vast majority of these were minor things such as the doors failing to open. Of the 20 to 30 elevator-related deaths each year, most of them are maintenance-related — for example, technicians leaning too far into the shaft or getting caught between moving parts, and most of the rest are attributed to other kinds of accidents, such as people stepping blindly through doors that open into empty shafts or being strangled by scarves caught in the doors. In fact, prior to the September 11th terrorist attacks, the only known free-fall incident in a modern cable-borne elevator happened in 1945 when a B-25 bomber struck the Empire State Building in fog, severing the cables of an elevator cab, which fell from the 75th floor all the way to the bottom of the building, seriously injuring (though not killing) the sole occupant — the elevator operator. However, there was an incident in 2007 at a Seattle children's hospital, where a ThyssenKrupp ISIS machine-room-less elevator free-fell until the safety brakes were engaged. This was due to a flaw in the design where the cables were connected at one common point, and the kevlar ropes had a tendency to overheat and cause slipping (or, in this case, a free-fall). While it is possible (though extraordinarily unlikely) for an elevator's cable to snap, all elevators in the modern era have been fitted with several safety devices which prevent the elevator from simply free-falling and crashing. An elevator cab is typically borne by 2 to 6 (up to 12 or more in high rise installations) hoist cables or belts, each of which is capable on its own of supporting the full load of the elevator plus twenty-five percent more weight. In addition, there is a device which detects whether the elevator is descending faster than its maximum designed speed; if this happens, the device causes copper (or silicon nitride in high rise installations) brake shoes to clamp down along the vertical rails in the shaft, stopping the elevator quickly, but not so abruptly as to cause injury. This device is called the governor, and was invented by Elisha Graves Otis. In addition, a oil/hydraulic or spring or polyurethane or telescopic oil/hydraulic buffer or a combination (depending on the travel height and travel speed) is installed at the bottom of the shaft (or in the bottom of the cab and sometimes also in the top of the cab or shaft) to somewhat cushion any impact. However, In Thailand, in November 2012, a woman was killed in free falling elevator, in what was reported as the "first legally recognised death caused by a falling lift".

Q1: What types of elevator hold the highest safety record?
A1: cable-borne elevators
Q2: In the late nineties, what was considered the rate of cable-borne elevator problems? 
A2: eight millionths of one percent (1 in 12 million) of elevator rides
Q3: What was the severity of the majority of elevator failures recorded?
A3: minor things such as the doors failing to open
Q4: What is an example of a maintenance-related elevator death?
A4: technicians leaning too far into the shaft or getting caught between moving parts
Q5: Before the Twin Towers attack in 2001 , what was the only reported freefall accident attributed to a modern cable-borne elevator?
A5: 1945 when a B-25 bomber struck the Empire State Building