The Effect of Vertical and Multiplanar Accelerations on Differing Production Seat Belt Sensor Designs

In real world accidents, vehicles are often subjected to multiplanar crash environments such as those seen in multiple impacts or rollover collisions. These various impact environments can directly affect the accelerations experienced by the vehicle and its components. Particularly vulnerable is the vehicle sensitive (inertially activated) crash sensor that is commonly used in seat belt retractor design. Though these designs have been proven effective in the frontal crash environments, they have also been shown to be susceptible to unlocking and/or a delay in lockup by vertical and/or rotational accelerations [1–3]. Such delayed retractor response can result in belt webbing payout and significant occupant motion within the restraint system. This can increase the likelihood and severity of occupant injury within the vehicle and/or injury from partial or full ejection in rollovers. Occurrence of this phenomenon in the real world has been documented and previously published [4, 5]. Evidence of retractor sensor unlocking and resulting spoolout includes identifiable forensic markings on the restraint system components [6] as well as occupant excursion evidence including full ejection and excessive partial ejection of properly belted vehicle occupants. The subject paper will follow previously published test methodologies [1–3] and report on new testing conducted on four differing production seat belt retractor designs. The test methods include linear accelerator tests to document the effects of a vertical pulse on an inertially sensitive retractor and, secondly, rotational accelerator tests wherein the retractors were mounted on a rotating fixture and subjected to various roll rates. Two tested retractors are the ball and cup design with the retractor’s inertial sensor incorporated within the retractor mounted in the vehicle’s roof pillars. The other two tested retractor designs utilize remote mounted (near the vehicle’s center of gravity) inertial sensors. These retractors are biased to the locked position via a spring and are held unlocked only when a solenoid is energized by the vehicle’s remote inertial sensor. For the rotational acceleration tests, the retractors were mounted in similar positions to that seen in their respective production vehicles relative to the vehicle’s center of gravity. Two retractors were then tested concurrently on the same test apparatus such that they were subjected to equivalent multiplanar environments, and thus allowed for direct comparison of their retractor design sensitivities and lockup performance. Additionally, a vehicle pillar mounted retractor was tested with and without a webbing sensitive lockup feature to allow for analysis of this design feature in varying multiplanar environments. The retractors’ performance under both the linear/vertical and rollover test conditions were noticeably different and are analyzed in detail.