Effectiveness of a Mechanical Pretensioner on the Performance of Seat Belts

A pretensioner is a device that actively withdraws a portion ofbelt (about 10-15 cm) during the impact, in order to keep the car occupant adherent to his seat and prevent contacts with the steering wheel, or with dashboard components. In its general form, a pretensioner is composed by a front-installed sensor (with an intervention threshold among 8-15 g) and by an effector connected to the belt system itself. A research has been performed on a mechanical pretensioner, on a pyrotechnical pretensioner and on standard automatic seat belts, with and without slack. Each device has been tested by simulation in a frontal impact at 35 mph with a GM Hybrid II dummy. The laboratory results show a significant reduction in HIC vs Standard belts, especially when poor wearing conditions are present (slack). Thc rcduction in forward displacement of head is particularly important for the interaction with the steering wheel (ordashboard components for the passenger) in small car interiors, in order to offer a better protection to occupants of economical, !arge diffusion cars. One can individuate some differences in the functioning of the pyrotechnical and the mechanical system, mainly in the direction of the retraction force, that is door-oriented in the first on, inside-oriented in the other one. Other characteristics of the pretensioner, not evaluable in this study, can present relevant effects in preventing dangerous side effects of seat bell action. It will be very interesting to investigate the performance of pretensioners -both pyrotechnical and mechanicalin real car accidents. PRETENSIONING AND DECELERATION PATTERN In a frontal impact, deceleration is already demonstrable during braking phase, if present; however, i t can become dangerous for a restrained occupant only after the contact with foreign struc­ tures. An ideal safe vehicle is composed by a rigid core, designed as a shell to protect its occupants, and by progressively deformable periferic struc­ tures, projected to absorb the whole amount of the inertial force, or a fraction as higher as possible. Its interiors are out of contact (or fully padded), and the occupant is perfectly adherent to his seat. In a such conceived motorvehicle, the only limiting factor to the effectiveness of restraint system is the conservation of the structural ge­ ometry of the car interior ("survival space"). In the actual car, this dampening effect is always partial and disomogeneous, due to lack of structure and/or to the presence of not deformable mechanical devices (e.g., engine or steering com93 ponents); each model of car has its own decelera­ tion pattern, characterized by a sequence of spikes that can reach dangerous levels. Car interiors are traumatic if impacted, and moreover the occu­ pants are allowed a certain degree of movements by the restraint system. In a lateral crash, there is no way to reduce the deceleration force before the involvement of the passengercompartement; on the opposite side, the dissipation of deceleration forces is easier in rollover and multiple crashes 6•17• Both laboratory and epidemiological re­ search have conclusively proved the effectiveness of seat belts in preventing or ameliorating the lesions caused by car accidents 9•10; nevertheless, it is weil known how in some frontal impacts the face of the driver can hit some parts of the steering wheel. The so produced face lesions play an im­ portant role in the described "redistribution of lesions" 10•11 following seat belts use. Furthermore, if the belt system is malposi­ tioned, the driver can move forward against the belt itself, or slip under the belt (submarining) 2• This action can produce lesions of various kind ("seat belt syndrome"), generally located at the abdomen, thorax or side of neck 4•7•8•18• Abdominal lesions regard usually the perito­ neum and the small bowel, with pathogenetic mechanisms involving pressurc f rom the belt, compression of gas-filled ansm, or sudden decel­ eration alone3•7•16. Thoracic lesions usually consist in fractures of sternum and/or ribs or clavicula, while neck lesions involve vascular structures like thc jugular vein or the carotid artery 18. The elimination, as far as possible, of these side effects is a must for oncoming generations of seat belts, whose performance can bc improved by the following means: • Reducing the cinetic cnergy to dissipate (inter­ vention outside car structure: c.g., speed limit or deformable barriers); • Optimizing the dissipation pattern of decelera­ tion (intervention on car structure); • Raising the compliance of the rcstraint system. Tue last objectivc can be pursucd with belt pretensioning. During the impact, a portion of the belt is extracted from the system, due to delay from the first contact and the locking of the retractor, to the narrowing of spires around the rctractor ("film spool effcct"), and then to the stretching of the belt undcr the pressure of the body. Thc sum of these effects is referred to as "web pay-out". The "ignition" of a restraint systcm starts with a defined rctard from the initial impact; this histeresis, that is characteristic of each single car model, is around 25-35 msec for three-point auto­ matic belts. The web-clamp systems 1 arc simple devices that can passively prevent most ofthe spool effect, but not other components of web pay-out. In its general form, a pretensioner is com­ posed by a crash sensor, a power source and an effector connected to the belt rctractor. The pyrotechnical pretensioner has a small explosive charge, that is ignited by an electronic control system when an impact is detected. The electromechanic sensor is based on an inertial mass (usually a sphere), thatfordecelerations over a given threshold switches an electric contact and modifies the logical status of a microprocessor. A central check control, always active to relieve 94 eventual failure of the circuitation, sees this changed signal and drives the explosion of the charge, connected with the retractor by means of a steel wire. Pyrotechnical pretensioners are in­ stalled on some Mercedes cars. Tue mechanical pretensioner is powered by a prcloaded torsional spring bar, which can be mounted transversally under the seat 5• Since the torsional bar is hinged as a pendulwn, the bar itself is the crash sensor; during the impact, one end of the bar will move forward and an overbent knee­ joint mechanism will collapse (Picture 1 ) . This device too has already been adopted and installed on some upper-class European cars. Both kinds of pretensioners are activated in frontal impacts, with collision angles no wider than ±30°. Another type of mechanical pretensioner, named proconlten (programmed construction ten­ sion), is installed as optional on the Audi 80 sedan. This interesting device is powered by the displace­ ment o f the engine during the impact; a steel wire provides both pretensioning of seat belts and re­ traction of the steering wheel 13• Picture 1: Design and operating system of the mechanical belt pretensioner. A pretensioner device must have an inter­ vention time no longer than 20-25 msec; other­ wise, the occupant body inertia would rise to values so high to vanish the pretensioning effect. The threshold for the activation is triggered usu­ ally at 8 g, depending on the specific standards of the car manifacturer. Adding active components to traditional seat belt systems can be ad justed on the peculiar decel­ eration pattern of the assigned car model, so to smooth its spikes and furtherly prevent head trauma. MATERIALS AND METllOD Tue experimental study is aimed to evaluate the performance of a mechanical pretensioner in a simulated car impact, in relation to the perform­ ance of a pyrotechnical pretensioner and of Stan­ dard three-point automatic seat belts. All the tests were performed at the Autoliv Development laboratories in Värgärda (Sweden), during 1 988. lt was not used a Standard car crash pulse as foreseen by the ECE 16 regulation, but the charac­ teristic car crash pulse (Picture 2) of a currently marketed and recently projected European upperclass car. Tue instrumentation was set as following: • Acceleration sled, in conformity to vigent rules; • Soft seat, currently producted, belonging to the same car model which the car crash pulse is referred to; • Dummy General Motors Hybrid II, equipped with decelerometers; • High speed film recorder (1000 photograms/ sec).