Introduction
IntroductionIn a typical retractable seat belt assembly, the seat belt retractor plays an important role in controlling webbing extension and retraction, thus meeting safety requirements.
This article introduces several types of retractors. You can learn about them by understanding their components, working principles, benefits, limitations, and applications. After reading this article, we hope you can gain a few tips for selecting a proper retractor for your specific requirements.
What Is a Seat Belt Retractor?
A seat belt retractor allows you to pull the belt out to extend and adjust its length when the car stops or drives under normal conditions. Inside the retractor, a spring provides a rotational force to the spool so that the belt can be retracted and wound up.
There are several types of seat belt retractors with specific structures and characteristics. Each of them is designed for different applications or meets special safety demands.
Types of Seat Belt Retractors
The seat belt retractors can be classified according to different retraction mechanisms and additional safety functions. Regarding their retraction mechanisms, the main types include ELR and ALR. Some retractors are equipped with load limiters or pretensioners to provide additional safety functions.
Emergency Locking Retractor
Structure and Function
The webbing sensor is an important sensitive component in an emergency locking retractor.
Take an emergency locking retractor with an inertial mass, for example. The webbing sensor triggers when the belt is pulled out rapidly. The inertial mass moves with a delay when the spool rotates rapidly, and then the position of the inertial disc changes. The locking pawl then engages with the gear teeth on the metal frame because of the movement of the inertial disc.
Some ELRs are also equipped with vehicle sensors. A vehicle sensor typically consists of a steel ball, a vehicle sensor arm and a vehicle sensor housing. The conditions of the steel ball depend on the vehicle’s movement and orientation. Excessive movement of the steel ball caused by the inertia or the tilt of the vehicle will trigger the locking mechanism.
Advantages
An emergency locking retractor provides comfort and freedom of movement. When a collision occurs and the locking mechanism is activated, the seat belt with ELR restrains the occupant immediately to reduce the risk of injury.
Limitation
The ELR works properly depending on the sensitivity of the sensor. When the sensor detects the speed changes of the belt inaccurately, the safety function of ELR fails. Meanwhile, the design of ELR may increase the cost.
Application
The emergency locking retractors are widely used in the 3-point seat belts for various types of vehicles such as passenger cars and trucks, or other vehicles which have a higher demand for comfort and reliability.
Automatic Locking Retractor
Structure and Function
The main functional components of an ALR include the locking cam and locking plate.
The locking plate in a typical ALR is pushed outward by the webbing wound around the spool, and it moves away from the engagement position of the cam at first. When the webbing is pulled out to a predetermined length, the whole locking system starts to work. The locking plate tilts inward and reaches the cam’s engagement position when the retraction movement occurs. The locking plate is engaged with the cam surface; thus, the belt can only be retracted rather than extended.
Pros and Cons
Once the belt is pulled out to the predetermined length, it won’t be extended again and provides a continuous restraint for occupants.
But the limited adjustability of an ALR makes it difficult to accommodate different body sizes. Sometimes an ALR seat belt may offer less comfort.
Application
According to its properties of providing continuous restraint, an automatic locking retractor is typically used in a 2–point retractable lap belt for some forklifts and golf carts.
Retractor with Load Limiter
Working Principle of Load Limiter
As a safety component to provide additional safety functions, some ELRs are commonly equipped with a load limiter.
The torsion bar load limiter is one of the most common types. There is a torsion bar secured to the spool. Once the vehicle sensor detects the rapid deceleration, the locking mechanism will be activated. But the torsion bar will twist and allow the spool to rotate by a small amount, thus releasing a small amount of webbing farther.
Pros and Cons
The load limiter is designed to reduce the peak force and minimize the injury caused by the belt during a collision.
Sometimes the load limiter results in extra excursion for the occupant, therefore increasing the risk of injury. Meanwhile, some load limiters can only be used once, and the replacement cost is increased.
Retractor with Pretensioner
Structure and Working Principle
Another component used with the ELR, called the pretensioner, works differently. The pyrotechnic pretensioner is one of the most typical types. When the collision is detected by the sensor, the gas generator is activated and produces a force to push a piston rapidly. Then, the piston drives the spool in the retractor to rotate quickly and remove the slack of the seat belt.
Advantage
With the help of a pretensioner, the belt is tightened up, and the occupants are secured in a proper position instantly. It maximizes the protection of the airbag and reduces the risk of occupants colliding with the vehicle interior.
Limitation
Most pyrotechnic pretensioners have complex structures and are designed to be used once, thus increasing the repair and replacement cost. On the other hand, the pretensioner won’t be activated until the collision intensity reaches a certain condition.
ELR vs ALR
| ELR | ALR | |
| Activated Conditions | Rapid Webbing Movement or Sudden Deceleration | Webbing Is Pulled Out to the Predetermined Length |
| Additional Safety Function | Load Limiter, Pretensioner | Usually None |
| Flexibility of Webbing | High flexibility to retract and extend | Limited movement |
| Comfort | Higher Comfort | Lower Comfort |
| Application | Typically Used in Passenger Car Seat Belts, Truck Seat Belts… | Typically Used in Forklifts Seat Belts, Golf Carts Seat Belts… |
| Restraint Situation | During Emergencies | Continuous Restraint |
How to Select the Right Seat Belt Retractor?
When you are looking for a seat belt retractor manufacturer, the following tips may help you to choose the right retractors to meet your requirements:
Application
According to different installation spaces and safety requirements of various types of vehicles, it is essential to select a suitable retractor for them.
For instance, passenger cars and trucks require higher comfort and more flexibility of movement for occupants. Therefore, emergency locking retractors are commonly adopted in these vehicles.
In addition, automatic locking retractors are typically used in some industrial vehicles, such as forklifts. The reason is that the drivers need to be continuously restrained during vibration or other situations, so the ALRs provide continuous restraint to meet security requirements.
Function
Each type of seat belt retractor features different retraction mechanisms and safety functions. You can make better decisions after understanding their differences.
Dimension
Before choosing a suitable retractor, you should check the mounting space in the specific vehicle. The dimensions of the retractor housing must be properly matched with the mounting space.
Additionally, it is vital to confirm the dimensions of its components. Take the spiral spring as an example. The number of pre-wound turns influences the available length of seat belt webbing.
FAQ
1. Are seat belt retractors universal?
No. According to the specific applications, structures of the internal car frame, and safety requirements, you must select suitable seat belt retractors to meet your functional or business requirements.
2. What quality tests should a seat belt retractor undergo?
Here are several typical tests for seat belt retractors:
- Test for retraction force
- Fatigue test for durability
- Automatic locking function test
- Emergency locking response time test
- Vehicle sensor sensitivity test
All of these quality tests are conducted using professional equipment. Sometimes, there are some slight differences according to different enterprises.
3. Do all retractors require the load limiter?
No. Load limiters are sometimes required in passenger cars. Moreover, the different retraction mechanisms of the retractors influence whether load limiters are needed. For example, the load limiters are commonly adopted in ELRs, but they are seldom used in ALRs. Finally, the load limiters may be necessary according to your client’s customized requirements.
4. Why is a seat belt retractor regarded as part of the complete restraint system?
A retractor isn’t just a simple mechanism for retracting and extending seat belt webbing. It provides some efficient and safe functions during your driving. For some manufacturers, a seat belt assembly with a retractor is not only competitive but also offers customers more security.
Final Thought
As a functional and safety mechanism, a seat belt retractor is an essential part of a complete seat belt assembly. It serves an important function in controlling webbing movement and providing extra protection for occupants.
You can choose an ELR or an ALR based on your needs for retraction mechanisms. Moreover, you can install a load limiter or a pretensioner depending on the performance and customization requirements.
DEMS is a professional seat belt supplier with extensive experience in retractor manufacturing. We are capable of optimizing the products and providing OEM services for you. Contact our team for more detailed information or to discuss your customized solutions.
