An air bag is a safety feature designed to protect passengers in a head-on collision. Most cars today have driver's side airbags and many have one on the passenger side as well. Located in the steering wheel assembly on the driver's side and in the dashboard on the passenger side, the air bag device responds within milliseconds of a crash. After the initial impact, a folded nylon bag becomes rapidly inflated with nitrogen gas. This acts as a cushion for passengers, preventing them from hitting the steering column and dashboard, something that often causes traumatic injuries. While air bags have been around for awhile, their widespread use has only occured within the past decade. They have significantly reduced the number of deaths and serious injuries from automobile crashes. According to the Insurance Institute for Highway Safety, the number of deaths from collisions when both seatbelts and airbags were used was 24% less than when only seatblets were used. The National Highway Traffic and Safety Administration (NHTSA) now requires cars made after 1998 to contain both driver side and passenger air bags. This regulation and the popularity that air bags now enjoy from the safety conscious public mean that the air bag is a safety device that is here to stay.
The air bag system can be broken down into two main components. These
are the impact sensor and the air bag module unit. The impact sensor
does what its name implies, senses impacts. The sensors are set to a
sensitivity level where they will only deploy in an accident that is equal
to or greater than a 12 mph crash into a concrete wall. The sensors
function by detecting automobile deceleration. When the automobile
decelerates at a rapid rate, the sensors are tripped. This deceleration
detecting is the job of two or more deceleration sensors, placed at the
front of the car. When the sensors go off, they send an electrical
current to the air bag module unit, causing it to deploy.
The air bag module unit consists of an inflator assembly, a nylon bag,
and a breakaway cover. This unit is typically located in the steering
wheel column on the driver's side or in the dashboard on the passenger's
side. The electrical current from the sensors travels to the inflator
assembly, where it causes a tiny initiator to be fired. This inititator
creates a spark which ignites a propellant, which in most air bags is
sodium azide. The reaction creates nitrogen gas. Cinders are removed
and the gas is cooled through a filtration screen also inside the
assembly. The nitrogen gas is what causes the air bag to inflate. This
inflation occurs in an average of only 30 milliseconds. When an occupant
plunges into the air bag, the gas if forced backwards through vents, a
process which takes another forty-five milliseconds. The whole sequence
from initial detection of a crash, until the air bag is fuly deployed,
happends very quickly. This quickness is required in order to protect
those inside of the car, and was one of the major obstacles that needed
to be overcome in the development of the air bag.
The road to the development of an effective, reliable, and unobtrusive
air bag has been a bumpy one. First deceived in the 1950's, it took over
30 years for the air bag to gain the support of the public.
The air bag was first conceived by John W. Hetrick in 1952. Like many
inventions, he created the idea as a result of an event that had occured
in the spring of 1952. He states:
"In the spring of '52, my wife, my seven-year-old daughter, Joan, and I
were out for a Sunday drive in our 1948 Chrysler Windsor. About three
miles outside Newport we were watching for deer bounding across the
road. Suddenly, there was a large rock in our path, just past the crest
of a hill. I remember hitting the brakes and veering the car to the
right. We went into the ditch but avoided hitting both a tree and a
wooden fence. In that respect we were very lucky.
As I applied the brakes, both my wife and I threw our hands up to keep
our daughter from hitting the dashboard. There was soft mud in the
ditch, so the car wasn't damaged, and no one was hurt.
During the ride home I couldn't stop thinking about the accident. I
asked myself, 'Why couldn't some object come out to stop you from
striking the inside of the car?' As soon as I got home that night I sat
down at the kitchen table and drew some sketches. Each evening for the
following two weeks, I'd add or subtract something from the sketches
(Sherman 49-50)."
Hetrick applied an event he had observed while in the Navy to the design
of the air bag. He was repairing a torpedo which had a canvas covering.
When the compressed air that was in the torpedo was released, it quickly
inflated the canvas shooting it to the ceiling.
With this knowledge, he developed his design until he was able to obtain
a patent on the device on August 5, 1952. The idea was similiar to the
air bag designs of today. Compressed air is stored in a container and
when a spring-loaded weight senses the car decelerating at a rapid enoguh
rate, it opens a valve that allows the pressure in this container to fill
a bag. The idea was ingenious, but Hetrick's air bag never would have
functioned properly. This achievement would require years and years of
designing and testing by some of the countries top automotive designers,
backed by the countries top automotive companies.
Automakers were interested in the air bag after Hetrick patented the idea
(It is interesting to note that Hetrick never got any compensation for
his invention). Both Ford and General Motors began to experiment with
air bag designs in the late 1950's. They quickly realized the
complexities that would be involved. There were two initial obstacles
that would have to be overcome if the air bag were to become a success.
The first of these was accurately and reliably detecting a collision and
the second was inflating the air bag within forty milliseconds. Both
automakers felt that these two tasks were insurmountable with the current
level of technology and lost interest in the idea.
The air bag gained new enthusiam in 1966. One reason for this was the
successful testing of a new detonating valve, created by the U.S. Army.
This could be applied to air bag design. In addition, researchers at
Eaton (an automotive research center) had been developing the air bag and
had had success with testing pre-inflated air bags. The design that they
came up with was similiar to that of Hetricks. It consisted of a nine to
ten foot nylon bag that attached to a container holding nitrogen gas.
When a sensor was triggered, a detonator would release the gas into the
bag. The detonator exploded with the power of a .22-caliber rifle
shell. The tests that Ford/Eaton conducted showed significant progress
in solving the two problems mentioned above. The sensors were both fast
and reliable and it was shown that the bags could be inflated quickly.
With the success of these tests, Ford decided that it would use the
safety device in its 1971 line of fullsize Fords and Mercurys. However,
this plan was quickly derailed by Ford's chief body engineer, Stuart
Frey. Frey claimed that there were two main problems with the air bag.
One was that there was performance problems in the components intended
for mass production. The second was that childsize dummies received what
could be fatal blows from air bags during crash tests. Other problems
that he cited were that the bags were not effective during angular crashes
and the windshield often broke when the bags were deployed. The design
team was sent back to the drawing board.
At the same time, General Motors was at work on its own design for the
air bag. As at Ford, General Motors faced numerous problems including
unacceptable sound levels, dummies that poorly mimicked humans, and the
need to completely redesign the car's interior in order to be able to
accomodate the bags. Yet, GM had several advantages over Ford. These
involved the experience of many of its subsidiaries. GM owned the AC
Electronics division which had been at work developing accelerometers in
the Boeing 747. This expertise was useful in the development of crash
sensors. The company's work in hypervelocity re-entry physics gave its
auto division knowledge of the gas dynamics of bag filling. The
experience that was gained in systems-analysis helped the automaker to
create models to predict the motions of passengers during a collision.
All of this knowledge allowed GM to develop an effective air bag system.
GM installed this system in 1000 Chevrolet Impalas in 1973. The move
gave the company the lead in air bag technology and it was hoped that
this field trial would pave the way for the eventual installation of air
bags in 10,000 full-size Oldsmobile, Buick, and Cadillac cars a year
during the period 1974-1976. This was the first time that air bag
systems were offered to the public. GM set up the tooling to produce
100,000 cars with air bags a year. However, the air bag did not sell.
During this period, only 10,321 air bag equipped cars were sold. As a
result, the air bag program at GM ended and it would be eight years
before the air bag was seen again.
The air bag was reborn after Mercedes-Benz offered it as an option in its
1984 models. Within two years, air bags had become standard equipment on
all of Mercedes-Benz's automobiles. Due to the success that the company
had, Ford announced the following year that it would offer an optional
driver's side air bag on 1986 Ford Tempo and Mercury Topaz models. GM
also began to relook at the idea of air bags in its cars.
The air bag crusade gained further momentum when the government passed a
law that required that in 1990, all cars sold in the US had to be
equipped with either an air bag or automatic seat belts. Automatic seat
belts were often viewed as a nuissance by the public and therefore,
automakers often choose the air bag. In 1991, Congress enacted the
Intermodal Surface Transportation Efficiency Act. This ordered the
National Highway Traffic Safety Administration to require both driver's
side and passenger air bags in all new vehicles by 1998.
The air bag has had a long and difficult journey. After repeated
failures, it has finally come to be one of the most important safety
features in automobiles today. It has been credited with saving
thousands of lives and it is likely to be present in automobiles for
decades to come.
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