# A Review of Seatbelt Technologies and Their Role in Vehicle Safety
## 3 Results
### The Load Limiter  
Load limiters are designed to control the force exerted by the belt on the passenger's body, thus preventing chest injuries caused by excessive pressure. These devices allow a certain elongation, by unwinding, of the belt strap, reducing the maximum tension applied. The first simple force limiters make pleats sewn on the belt strap. These pleats are released when a threshold force acts on the strap. By gradually releasing the pleats, the strap will gradually lengthen and the force in the strap will dissipate during the release, limiting the force with which the belt acts on the passenger's chest. Another solution consisted of making pre-cut metal clips, used for one of the belt anchor points. Their mode of operation is similar to that of sewn pleats; under the action of a threshold force, the clip gives way and allows the belt to lengthen. Recent generation force limiters are torsion bar-type mechanisms, which deform plastically and allow the controlled unwinding of the strap, or through deformable elements, under a certain load, absorbing part of the impact energy. A significant advancement in load-limiting technology involves the integration of predictive load detection mechanisms. This approach combines real-time acceleration data with occupant size parameters to estimate the forces acting on the restraint system. By comparing these predicted loads against a predefined threshold, the system dynamically adjusts the activation and deployment characteristics of load-limiting devices, ensuring an optimal balance between restraint effectiveness and occupant safety [28]. There are several types of force limiter systems:  
* With a constant force limiter, when we have a single torsion bar in the retractor mechanism;
* With progressive force limiter on the rib cage. To obtain the progressive limiter function, it was necessary to implement the second torsion bar and the elements that make the transition between them;
* With a degressive force limiter on the rib cage, it has additional elements in its construction compared to the progressive version;
* With adaptive force limiter on the rib cage. This has in its composition, in addition to the two torsion bars with different diameters, a system for changing the degressive or direct load on the small torsion bar. In Figure 10, the main components of a load limiter are presented as follows: (a) drive wheel; (b) spindle; (c) torsion bar; (d) locking mechanism; (e) metallic case. Load limiters are particularly important in frontal impacts, where the forces applied to the chest can be high. This system reduces the risk of seatbelt-related injuries, such as chest or internal organ injuries, especially in high-impact impacts. Figure 10: Load limiter components: (**a**) drive wheel; (**b**) spindle; (**c**) torsion bar; (**d**) locking mechanism; (**e**) metallic case. In [29], an analysis was performed measuring the pretensioner and load limiter forces at two levels, the first with a threshold of 4.5 kN, the other up to 7 kN. The forces were measured using transducers placed at the belt level, but the actual force during an accident may be different. The presence of load limiters significantly reduces the risk of serious and fatal injuries in the whole body and load limiters with a higher load-limiting force contributed significantly to reducing fatal injuries although these effects need to be studied in combination with different delta-v values. These findings are also confirmed in previous studies by [30; 31]. Other authors mention the need to adapt the deployment of load limiters for different types of impacts, depending on the age of the occupants [32]. The authors in [33] suggest that, while seatbelts are highly protective against fatal and most severe injuries in multi-occupant crashes, older occupants do not receive the same seatbelt benefits as their younger counterparts for severe injuries, Abbreviated Injury Scale (AIS)3+ scale, and torso injuries.