Working principle of car seat belts

Working principle of car seat belts

Currently, the seat belt system is an indispensable part of every car. Although it is a small part, it is of great importance, helping you to improve your survival in a traffic accident.

1. The idea of ​​a seat belt system

  • The seat belt’s basic working principle is simple: It holds you in place so you don’t fly forward and hit the windshield or hit the dashboard when the car suddenly stops. But why is this happening? It can be summarized that: due to inertial force. So what is inertia?
  • Inertia is the tendency of an object to maintain its motion when anything counteracts this movement. In other words, inertia is the object’s resistance to changes in speed and direction of motion. Everything wants to keep their motion naturally.

  • If a car had a speed of 50 km / h, the inertia would always want to keep it moving at 50 km / h in that direction. Air resistance and friction with the road slows it down, but the engine’s power compensates for the lost energy to overcome road surface friction and air resistance.
  • Anything in the car, including the driver and the passenger, has its own inertia, according to the car’s inertia. The car increases the rider’s speed according to its speed. Imagine that you are traveling at a steady speed of 50 km / h. Your speed and the car speed are almost the same, so you feel yourself and the car moving like a single block.

  • If suddenly the car hits a telephone pole, it will immediately prove to you that your inertia and the car are completely independent of each other. The force of the telephone pole applied to the car causing it to suddenly stop, but your speed will remain the same. Without a seat belt, you will be “thrown” into the rim of the steering wheel or fly up and hit the windshield at a speed of 50 km / h. Just like a telephone pole that stops your car, the dashboard, windshield or road surface will stop you by holding you back with tremendous force.

2. The duty of the safety belt system

  • In the previous section, you learned that when a car suddenly stops, the passenger is suddenly stopped along. The job of the seat belt is to distribute that stopping force into a healthy part of the body to minimize danger.
  • Traditional seat belts are lapbelt (usually pulled over your hips) or shoulder belt (pulled over the shoulder). These two types of seat belts are fastened to the bodywork to keep the torso fastened to their seat.
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  • When the seat belts are correctly fastened, they provide full stopping force to the chest or pelvis, which are the strongest areas of the body. Because seat belts act on a wide horizontal band along the human body, the stopping force is not concentrated in a small area but is dispersed, so it does not pose a great danger.
  • What’s more, the seat belt is constructed of a more flexible material than the dashboard and windshield. They are slightly stretchable, meaning the stop won’t be too sudden. So if there is a collision you can only move a little, and of course, still not leave your chair.

  • In addition to the seat belt system, people also let the car have collision energy absorption zones. These areas are very “soft”, located in the front and rear of the vehicle. When a collision comes from the front or rear, the front or rear end of the vehicle will cower. Instead of having the entire car come to a sudden stop when hitting an obstacle, it absorbs part of the impact force on the very crushed part of the impact area, like you squeeze an empty soda can. The cabin of the car is tougher, does not flatten and so we avoid the “squeeze” situation.
  • If then, the car has not stopped yet, then the crumpled front end would be an obstacle to the obstacle. However, crumbling areas will only protect you while you are in the cabin, and of course if you wear your seat belt.

  • The simplest type of seat belt, seen at thrill rides at amusement parks. It consists of a thin wire attached to the body of the car. These ropes always hold you in the chair, very safe but slightly uncomfortable. The car’s seat belts have very good elasticity. You can lean forward while the string is in tension. But if there is a collision, the seat belt will suddenly hold you tighter to the seat.

3. Dilation and contraction: Two prerequisites

  • In the normal seat belt system, a cloth strip is connected to a tension mechanism. The central element of the tensioning device is a rotating tube attached to the end of the string. The inside of the tensioner is a spring that provides a force to rotate the spool. As a result, the spinner instantly coils, allowing the string to re-tension whenever the string falls.
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  • When you pull the string out to tie it, the spinner will rotate counter-clockwise and it will rotate the return spring to rotate in the same direction. In fact, the rotating tube works to release the tension of the spring. A spring always wants to retain its original shape, so it resists twisting motion. If you release the cord, the spring will pull tight, turning the spool clockwise until the seat belt reaches a certain tension. The tensioner has a locking mechanism that prevents the spool from rotating in the event of a car crash. Currently, there are two common locking systems:
  • The system is activated by the movement of the vehicle.
  • The system is activated by the movement of the seat belt.
  • The first system locks the spool when the vehicle suddenly decelerates (when hitting an obstacle, for example). The diagram below shows the simplest type of this design. The central working factor of this structure is a weight. When the car comes to a sudden stop, the weight’s inertia causes it to shake forward. A tab located at the other end of the weight immediately inserts the teeth of the gears connected to the spool. Because the cam claw is retained, the gear cannot rotate counter-clockwise, so the spool cannot be rotated. When the seat belt loosens after impact, the gear rotates in the same clockwise direction and the cam lugs are released from the gear.

  • The second system locks the spool when something tugs on the string. The main working element of this design is a centrifugal clutch – lever (1) with a rotating pin installed with a rotating spool. When the spool rotates slowly, the lever does not rotate around its axis. A spring holds it in place. However, if the seat belt is suddenly jerked, the spindle is strongly rotated, the centrifugal force causes the heavy object at the end of the lever to shoot out. The eject lever pushes a cam yoke (5) into the space of the tension mechanism. This cam is connected to a locking pin by a slide pin in the small groove (4). When the cam moves to the left, the latch moves along the groove of the latch. This pulls the lock pin into a ratchet (2) that snaps into the spindle. The brake immediately locks the ratchet’s teeth to prevent it from turning counter-clockwise, keeping the spool from falling off.

  • On some of the newer seat belt mechanisms, the pretensioner pre-collision tensioner also has a construction that works to tighten the belt, but is slightly different from the two designs above. Here is the working principle of this structure.
  • The idea of ​​this mechanism is to tighten the seat belt any time it loosens up in the event of a collision. In contrast to the conventional locking mechanism in a tensioner, which holds the string in tension under all circumstances, the pretensioner only actually pulls the string when necessary. This force makes us in the most appropriate position for a collision. Pretensioner always works with conventional key mechanisms, not replacing them.
  • There are a number of different pretensioner systems on the market. Some pull the whole tension mechanism back, some just spin the spool. Usually, the pretensioner is connected to a centralized processor along with airbag control. The processor will monitor all the signals from the mechanical or electronic sensor when it detects a sudden deceleration of the collision.
  • When a collision is detected, the processor activates the tensioners which then activate to the air bag. Some pretensioners are designed using electric motors or coils, but most popular designs today use fire particles to stretch seat belts. The diagram below shows a typical pattern of this design.
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  • The central element of this pretensioner is a closed chamber containing combustion gas. Inside this closed chamber there is a small space containing explosive particles. This detonator is controlled by two wires connecting the combustion chamber to the central processing unit. When the processor detects a collision, it immediately delivers an electric current through the two poles of the igniter to spark the ignition of the explosive particles that burn the available gas. The high-pressure combustion gas pushes the piston up at a very high rate. A gear rod structure connected to the piston causes the gears to rotate and the reel to rotate. The speed of the tooth bar is large, making the spool roll very strongly, stretching the entire belt.
  • For many years, seat belts have undoubtedly proven to be the most important safety device in cars. However, that doesn’t mean they are 100% safe for us. With scientific advancement, advanced technologies will allow safer and more accurate safety devices. In the future, the cars will be provided with better seat belts and airbags with completely new safety technologies. However, the government will have to focus on the bigger problem, which is forcing people to use safe devices when using cars.

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