importance of restraint systems

8/4/2019 Importance of Restraint Systems

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RESTRAINT SYSTEMS AND THEIR IMPORTANCE

IN TODAYS VEHICLES

Ishaya K. Akai

Department of Mechanical Engineering, Waziri Umaru Federal Polytechnic, Birnin Kebbi

ABSTRACT

Every year automakers are engineering more safety devices into their vehicles because of the

rate at which lives are lost in motor accidents today. So this paper is aimed at: explaining how

vehicle body and frame construction works with restraint systems to protect a vehicles

occupants; identifying and locating the most important parts of vehicle restraint systems; and

describing the importance of restraint systems.

INTRODUCTION

Restraint systems, otherwise known as safety systems, are installed in a vehicle to help hold

its occupants in their seats, protecting them from injury during an accident. Restraint systems

include the seat belts and the air bag system, as well as the vehicles body, frame, steering

column, and dash board. Seat belts and air bags are required on all cars and light trucks.

Therefore it is very important that we understand the operation, and if possible, the repair of

these safety systems.

Early vehicles were designed and constructed without taking restraint systems into

consideration. Almost no feature of the interior design of these vehicles provided safeguards

against injury in the event of collision. Doors that flew open on impact, inadequately secured

seats, the sharp-edged rear-view mirror, pointed knobs on instrument panel and doors, flying

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glass, and the overhead structure all illustrated the lethal potential of poor design. A sudden

deceleration turns a collapsed steering wheel or a sharp edged dashboard into a bone-and-

chest-crushing agent. Penetration of the shatter-proof windshield can chisel ones head into

fractions. A flying seat cushion can cause a fatal injury. The apparently harmless glove-

compartment door of the early car had been known to unlatch under impact and guillotine a

child. Roof-supporting structure had deteriorated to a point where it provided scarcely more

protection to the occupants, in common roll-over accidents, than an open convertible. This is

specifically true of the so-called hardtops. Nor were the automobile designed as an

efficient force moderator. For example, the bumper of the early vehicle does not contribute

significantly to reduction of the crash deceleration forces that are transmitted to the motorist;

its function more reflects style than absorb shock.

These weaknesses of early automobile construction were established by the investigation of

several groups, including the Automotive Crash Injury Research of the Cornell University

Medical College, the Institute of Transportation and Traffic Engineering of the University of

California and the Motor Vehicle Research of Lee, New Hampshire.

VEHICLE COLLISIONS

Vehicle collisions, or accidents, normally result from driver error that causes the car or truck

to hit other objects (vehicles, trees, retaining walls, etc.). Tremendous force is generated

when the vehicle, which can weigh almost two tons, crashes to a halt in few meters (or feet)

of travel. See Figure 1.

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Figure 1When a car or truck hits a large object, a tremendous amount of

energy is absorbed to bring the vehicle to a halt. (Daimler Chrysler)

Automobile manufacturers design their vehicles to absorb this force in a controlled manner.

In designing vehicles to absorb the tremendous forces generated when a vehicle suddenly

crashes to a halt, manufacturers incorporate crush zones and side-impact beams in the

construction of their vehicles; and they ensure that before a new car or truck is sent to the

market, they conduct crash tests using dummies to analyze the safety of its design.

Crush zones these are usually located at the front and rear of the body-frame assembly;

they are designed to collapse during a severe impact. The passenger compartment is made

stiffer and stronger than these crush zones, so the occupants are protected from the forces of

the accident. See Figure 2.

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Figure 2 Crush zones are built into the front and rear areas of the vehicles body and frame.

These areas crush more easily than the passenger compartment, increasing occupant safety.Dotted lines show how the body deforms during a major collision. (Saab)

Side-impact beams these are made of high-strength steel, and mounted in the vehicles

door to help prevent intrusion into the passenger compartment. The pillars (body sections that

extend up to the roof panel and are located in front of and behind the doors) are also

strengthened to protect the passenger compartment, especially in the event of a rollover.

Crash tests these are conducted by the auto manufacturer to measure how well the body-

frame structure will protect the vehicles occupants in the event of a major collision. Vehicles

are crashed into standing walls or other vehicles to measure how well the vehicles withstand

and react to the impact forces.

Crash test dummies auto manufacturers use them to measure the impact forces acting

upon the human body. Sensors in the test dummy record the forces acting upon vital parts of

the body. This allows the manufacturer to estimate how badly people will be hurt during

similar crash conditions. Figure 3

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Figure 3 A crash test dummy is designed to measure forces that would act on the human body during an autoaccident. Damage to the dummy indicates how badly people would be hurt in a similar accident. (Volvo)

Crash test are performed from the front, side, and rear of the vehicle. This allows the

manufacturer to study the effects of major impact forces from each direction. These tests,

though very expensive to perform, but are very important. See Figure 4.

Figure 4 Automakers perform extensive crash tests to enable the safety of new car and truck designs.

AFrontal-impact crash test. BSide-impact crash test. CRear-impact crash test. (Saab)

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ACTIVE AND PASSIVE RESTRAINTS

It has been briefly mentioned that restraints systems hold the driver and passengers in their

seats during an accident. These systems are designed and installed in vehicles to limit injury

during a crash. Most injuries result when people are ejected from their seats or from the

passenger compartment upon impact.

An active restraint system is that type of system that operates without being activated by the

driver or passenger. Air bags and automatic seat belts are examples of passive restraint

systems. Refer to Figure 5.

Figure 5 Modern vehicles are much safer than those produced in the past.Besides strong passenger compartments, seat belts and air bags help

protect the driver and passengers from injury during collision. (Saab)

SEAT BELT SYSTEM

Seat belts are strong nylon straps that hold people in their seats during a collision. Lap belts

extend across a persons lap. Shoulder belts extend over a persons shoulder and chest. Seat

belt buckle allows you to engage and disengage the belt around your body. Seat belt

anchorsprovide a means of bolting the seat belt to the cars body structure. Look at Figure

6. A belt retractor takes the slack out of the seat belt so the belt fits snugly around the body.

Designs vary. See Figure 7.

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A seat belt reminder system lights a dash light and generates an audible tone to warn the

vehicle occupants to buckle their seat belts.

Figure 6Note the basic parts of a seat belt assembly. (Honda)

Figure 7 This vehicle has automatic shoulder belts that are tightened around the

body by small electric motors. Note the location of the retractors. (Honda)

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KNEE DIVERTER

A knee diverter, orknee bolster, is formed into the lower part of the dash to protect the

drivers and front passengers knees from being injured on the metal frame of the dash. The

diverter also prevents the driver and passengers from sliding under the air bag during a

collision. It is usually a thick plastic panel that covers the metal frame of the dash. Refer to

Figure 8.

Figure 8This drawing shows the three primary restraint

devices: seat belt, air bag, and knee bolster (GM)

AIR BAG SYSTEM

An air bag system automatically inflates large nylon bags immediately after the start of a

major collision. The air bag system is designed to supplement the protection provided by seat

belts. Figure 9

The major parts of an air bag system include:

Air bag sensor these are inertia sensors that signal the control module in the event

of a collision.

Air bag module it contains the inflator mechanism and the nylon air bag that

expands to protect the driver and /or passengers during the collision.

Air bag controller this is the computer that operates the system and detects faults.

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Dash warning lamp this is a dash bulb that glows with system problem and goes

out when the problem is over.

Figure 9 An air bag can shoot at speeds up to 320 kilometres per hour (200 miles per hour).This is fast enough to inflate the bag before the human body can fly forward, even in the

worst auto accidents. (Saab)

An electronic air bag system deploys an inflatable nylon bag to help protect the driver

during a collision. It uses impact sensors to detect a severe collision. The sensors feed their

signals to the air bag controller. When at least two impact sensors are energized, the

controller activates the air bag module.

The air bag inflates in about 1/20 th of a second, well before the drivers body flies forward

from the collision. The tough nylon bag can easily absorb the forward inertia of a human

body. This helps protect both the driver and the passenger.

AIR BAG TYPES

Driver-side air bag this type of air bag is used in some older cars and is only

one, which is located in the steering wheel.

Passenger- side air bag many new vehicles are equipped with dual air bags: a

driver-side air bag and a passenger-side air bag. The passenger-side air bag

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deploys from the right side of the dash. It is much larger than a drivers-side air

bag, since it is relatively far from the passengers and may have to protect two

persons simultaneously. See Figure 10.

Figure 10The passenger-side air bag is much larger than other air bags.It must be able to protect two people from striking the dash and

windshield during a frontal impact. (Volvo)

The driver- and passenger-side air bags will only deploy during frontal impacts. Steering

wheel and dash mounted air bags may not deploy in side impacts, rear impacts, or rollover

situations. A collision must occur within about 30o of the vehicles centerline for these air

bags to inflate. This is illustrated in Figure 11.

Side-impact air bags they can be located in the door panels or on the outside edge of each

front seat. They are smaller in sizes, compared to the drivers-side and passengers-side air

bags. These small air bags deploy when the vehicle is hit from the side. They generally do not

deploy during a frontal impact. Side-impact air bags are becoming more common and are

used by several auto manufacturers. Figure 12

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Figure 11 Drivers and passengers air bags will normally deploy if impact is within 30o of a vehiclescenterline. (GM Trucks)

Figure 12 Side-impact air bags are becoming more common. They help hold your body in the seat when thevehicle is hit from the side. This unit deploys from the side of the seat cushion. (Volvo)

When a car is hit from the side, injury usually results when the occupants shoulder and head

fly through the side window glass. A side-impact air bag system senses the side thrust of the

impact and deploys a small air bag to cushion the person as he or she is propelled sideways.

See Figure 13

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Figure 13 Side-impact air bags normally use their own sensors to detect

side-impact forces. The only deploy when the impact forces would tend tothrow the body sideways and into the door. (Volvo)

Window air bags they are designed to drop down like curtains over the side window

glass. This helps protect the occupants from head and facial injuries caused by impact with

the door glass.

Rear seat air bags they fit into the rear cushion of the front seats. They inflate to protect

the passengers in the rear seat from injury during a frontal collision. Although not very

common, the can be found in a few expense luxury cars.

AIR BAG MODULE

The air bag module consists of a nylon bag and an igniter-inflator unit enclosed in a metal

and plastic housings. The drivers-side air bag module is packaged in the centre of the

steering wheel pad. The passengers-side air bag module is mounted under a small door

formed in the right side of the dash pad. Look at Figure 14.

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Figure 14 Cutaway (cross-section) shows inside of an air bag module. TheAir bag is folded neatly under the steering wheel cover. The air bag igniter

Generates a small electric arc across two metal pins when an electrical signal

is sent to igniter from the controller. The arc fires an igniter charge, causing

the gas-generating pellets to burn. The burning pellets generate a rapidlyExpanding gas that inflates the air bag. (Breed Automotive Corporation)

The air bag itself is a strong, reinforced nylon sack attached to the metal frame of the

module. It is tightly folded for storage in the steering wheel pad, the dash, the door panel, or

the side of the seat. See Figure 15.

Air bag vent holes allow for rapid deflation of the air bag after deployment. These small

holes are formed around the outer edge or back of the bag.

The air bag igniter generates a small electric arc when an electrical signal is sent to it from

the air bag controller. The arc forms across two small pins in the igniter charge. This flash of

the igniter charge causes the gas-generating pellets to burn, generating a huge volume of

expanding gas, Figure 15.

The air bags propellant charge, or expanding gas, is usually produced by the burning of

sodium azide pellets in the air bag module. The burning pellets form nitrogen gas, which

inflates the air bag.

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Figure 15 Line drawing shows the details of a common air bag module. (Toyota)

The large volume of nitrogen gas can inflate the air bag in a fraction of a second. This action

forces the steering wheel cover to split open and the air bag shoots out at about 320kilometers

per hour (200 miles per hour). This is fast enough to cushion the forward thrust of the human

body as it flies forward after the collision. The air bag also protects the drivers or

passengers heads from flying objects resulting from the accident. Immediately after the air

bag is inflated, the gas is vented out the small holes on the sides or rear of the bag. This

prevents the occupants from being pinned in their seats. It also allows the driver to see out of

the wind shield right after deployment.

Passenger-side and side-impact air bags are very similar in design. The passenger-side air bag

is much larger than a side-impact bag and, therefore, requires more gas for proper inflation.

An exploded view of a passenger-side air bag is shown in Figure 16.

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HYBRID AIR BAGS

A hybrid air bag uses a small explosive charge and a pressurised gas cartridge to inflate the

air bag. The small, metal gas cartridge contains inert argon gas pressurised to 20,700 kPa

(300 psi). When the air bag controller sends current to this type air bag module, it fires a tiny

amount of pyrotechnic material (rapidly burning substance) that forces a plastic bullet

through the gas cartridge. The cool pressurised argon gas then blows out to inflate the air bag,

Figure 17.

A hybrid air bag is designed to help prevent minor skin burns that can result from the hot

nitrogen gas generated by burning sodium azide pellets.

MECHANICAL AIR BAGS

All parts of a mechanical air bag system are contained in the steering wheel module. During a

front-end collision, a metal ball in the module slams forward, striking a lever arm. The other

end of the lever arm the pushes a firing pin into the igniter material. The igniter material

burns, igniting the sodium azide pellets. The gas generated by the burning pellets quickly

inflates the air bag.

Mechanical air bags are designed for after-market installations. Older vehicles, which were

not originally equipped with air bags, can be retrofitted with the mechanical air bag system to

increase driver protection during an accident.

AIR BAG SENSORS

Air bag sensors detect a collision by measuring vehicle deceleration during a collision. They

are inertia sensors that detect a rapid change in speed or velocity. One or more sensors are

commonly incorporated in air bag systems. The trend is to use fewer sensors than in the past.

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Impact sensors they are mounted in front and, in some cases, on the side of a

vehicle to detect a collision. Front impact sensors are often located in the engine

compartment, on or near the radiator support. Figure 18.

Figure 18 Typical locations of the air bag impact sensors.

They are often mounted near the radiator support. (Ford)

Side impact sensors are mounted in the doors or in the B pillars (pillars

behind the front doors).

A safing sensor orArming sensor it is a back-up sensor designed to ensure that the

vehicle is actually in a collision. It provides a fail-safe system to prevent accidental

bag deployment. For the inflation of the air bags, the safing sensor and at least one

impact sensor must be closed. Figure 19.

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Figure 19 note the basic circuit for air bag sensors. A safing sensor

And two primary impact sensors are wired in series. This requires that the

Safing sensor and at least one of the impact sensors be closed to fire air bag. (Ford)

Magnet-and-ball sensor this is used in some air bag systems as the impact sensor.

A small permanent magnet is used to hold a steel ball away from the electrical

contacts in the sensor. During a severe collision, the rapid deceleration throws the

steel ball forward, overcoming the force of the magnet. The ball rolls forward and

strikes electric contacts. This closes the sensor circuit to signal the controller of a

possible collision requiring air bag deployment. See Figure 20.

Figure 20 An air bag system sensor closes when exposed to rapid deceleration forces.

This sensor uses a magnet to hold a steel ball. If impact is great enough, the steel ball is thrown

away from the magnet and into the two metal contacts. This closes the circuit and signals the

controller that the vehicle is in a collision. (Ford)

Coil spring sensors a coil spring sensor uses a small metal weight attached to a

metal coil spring. During a severe frontal impact, the weight is thrown forward with

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enough force to overcome spring tension. This weight touches the sensor contacts

and closes the electronic control module (ECM).

Seat cushion sensors they detect the weight of a person sitting in the passenger

seat. If no one is sitting in the passenger seat, the air bag system may not deploy the

passenger air bag. This saves the considerable cost of replacing an air bag without it

protecting someone.

Accelerometer sensor many late model vehicles are equipped with one central

accelerometer (inertia sensor) instead of separate impact and safing sensors. The

accelerometer measures changes in motion or deceleration and is sometimes

mounted in the air bag controller. Some accelerometers contain thin wafer of

semiconductor material that is deflected and warped by rapid deceleration. The

bending of the semiconductor produces a piezo-electric, or pressure generated

electrical signal, that fires the air bag.

AIR BAG CONTROLLER

The air bag controller or air bag control module uses inputs from the impact and safing

sensors to determine if air bag deployment is needed. If at least one impact sensor and the

safing sensor are closed, the controller sends a high current pulse to the air bag module. The

pulse produces a small electric arc in the air module, igniting the pyrotechnic material to

produce gas expansion and bag inflation. Figure 21

The air bag controller also generates trouble codes and energizes a warning lamp if it detects

something wrong with the system. Refer to Figure 22.

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Figure 21 General arrangements of the parts in an air bag system.

Note the location of the controller. (Honda)

Figure 22 Diagram shows how the controller operates the air bags. Note

that the controller has self-diagnostic capabilities and will generate a

trouble code when an air bag system problem is detected. (Honda)

A smart restraint system uses additional inputs to affect the operation of the air bags. It uses

conventional impact sensor inputs, as well as data from the seat sensors, side door impact

sensors, yaw sensors, wheel speed sensors, seat belt sensors, and even collision-predicting

sensors. This allows the smart system to adjust the speed and pressure applied to the air bags

to better protect the vehicles occupants from injury. For example, a small child would

require less air inflation pressure than a very large adult.

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CONCLUSION

The technology found in typical late-model vehicles helps to prevent thousands of highway

deaths each year. Many high-speed accidents, which were often fatal in yesterdays lower-

tech cars, now result in only minor injuries. This improved safety record is primarily due to

the superior structural body/frame designs and advanced restraint systems found in todays

vehicles. Air bags, front and rear crush zones, stronger pillars, and reinforced passenger

compartments have all contributed to improved vehicle safety. One most unfortunate thing is

that drivers, who are ignorant of the importance of restraint systems, tend to destroy them or

refuse to use them.

Some developing countries, including Nigeria, are now enforcing the use of seat belt systems.

This has tremendously helped to prevent thousands of deaths each year during accidents.

At this juncture, I want to suggest to all vehicle manufacturers to ensure that restraint systems

are installed in all vehicles produced today, and also plan total phase-out of vehicles without

restraint systems by the year 2020. Enlightenment campaigns should be embarked upon by

governments and vehicle manufacturers to educate people on the importance of restraint

systems in vehicles. This would make them ensure that restraint systems are installed in any

vehicle they want to purchase.

SUMMARY

Crush zones located at the front and rear of the body-frame assembly are

designed to collapse during a severe impact.

Crash tests are used by the auto manufacturer to measure how well the body-

frame structure will protect the driver and passengers in a major collision.

Crash test dummies are used to measure the forces acting upon the human

body during a collision.

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Seat belts are strong nylon straps that hold people in their seat during a

collision.

A knee diverter is formed into the lower part of the dash to prevent the

drivers and front passengers knees from being injured on the metal frame of

the dash.

An air bag system automatically inflates a nylon bag immediately after the

start of a major collision.

Most vehicles are equipped with a passenger-side air bag, which deploys

from the right side of the dash.

Side-impact air bags can be located in the door panels or on the outside

edges of the front seats.

The air bag module comprises a nylon bag and an igniter-inflator unit

enclosed in metal-plastic housing.

A hybrid air bag uses a small explosive charge and a pressurised container of

gas to inflate the air bag.

All parts of a mechanical air bag system are contained in the steering wheel

module.

Air bag sensors detect a collision by measuring vehicle deceleration.

The trend is to replace several safing and impact sensors with one central

accelerometer that measures changes in motion or deceleration.

A smart restraint system uses additional inputs to affect the operation of the

air bag system.

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REFERENCES

1. A. Graham bell (2006) Four-Stroke Performance Tuning Great Britain.

2. Robert Bentley (2005) BMW 3 Series (E 46) Service Manual Great Britain.

3. Chris, Johnson (2006) Modern Automotive Technology Shop Manual USA.

4. James, E. Duffy (2004) Modern Automotive Technology Tinley Park, Illinois.

5. Microsoft Encarta (2008) Air Bags

6. Tom, Denton (2004) Automobile Electrical and Electronic Systems Great

Britain.

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importance of restraint systems

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