Airbags are one of the most important advancements in automobile safety. In an accident, the airbag rapidly inflates on impact to help arrest the movement of the driver and passenger.
Stored in compartments in the steering wheel, dashboard, and doors, airbags are standard safety devices in automobiles and trucks globally. In a steering wheel, the airbag is mounted inside a housing, concealed behind a set of molded plastic covers that must function properly and look good.
These covers have tear seams designed to break open instantly when the airbag inflates after an impact. The thickness of these seams is critical to proper airbag performance; too thick and the airbag may not fully deploy in an emergency; too thin, they may separate if the cover is bumped or pressed during normal driving.
Airbag tear seams are typically 0.05" to 0.08" (1mm to 2mm) wide with a center thickness of 0.02" to 0.06" (0.5mm to 1.5mm). The cross-sectional profile of the seam can be flat on both sides, V-shaped on both sides, or flat on one side and V-shaped on the other. Its size and shape make it challenging to measure the seam’s thickness.
The tear seam must be designed to resist accidental damage, tampering, and wear throughout many years of use. Tear seams can be molded directly into the part or scored into the seam using a laser or other tool. Once created, manufacturers carefully measure the seam’s thickness as part of their quality control process to ensure it falls within specifications – if the seam doesn’t function correctly, the airbag may not inflate properly, or at all.
Hall effect thickness gages offer a quick and nondestructive alternative to cutting and mechanically measuring airbag tear seams. The gages are small, lightweight instruments that make fast, accurate, and repeatable measurements of nonferrous material. A Hall effect sensor has an output voltage that varies when exposed to a magnetic field. In thickness gaging applications, a Hall effect sensor is incorporated into a small probe along with a strong magnet that creates a magnetic field around the sensor. Wall thickness is measured by placing a target, such as a small steel ball, disk, or wire, on one side of the test piece and the magnetic probe on the opposite side. As the distance between the target and probe changes, the voltage across the Hall effect sensor varies in a predictable way. This change in voltage is used to calculate the thickness reading.
Hall effect gages can potentially measure any non-ferrous material with geometry that permits placing a probe tip on one side of a wall and a small target such as a steel ball on the other, up to a maximum thickness of approximately 1" (25mm). Materials that can be measured include all types of plastics and composites, aluminum, titanium, and other non-ferrous metals, glass, wood, paper products, and certain non-ferrous stainless-steel alloys. Measurement accuracy can approach ±1% of wall thickness and is typically ±3% or better.
When measuring the thickness of an airbag tear seam, hold the probe tip on one side of the seam and place the target on the other side – the distance between the probe and the target represents the seam’s thickness.
Because of the seam’s shape, a special chisel-shaped probe tip and target disk set are typically used to help ensure accurate thickness measurements. The recommended probe for this application has a narrow, pointed tip designed to fit into the airbag tear seams. A sharp-edged target disk is recommended for V-shaped seams, and a square-edged target disk is recommended for flat seams.
The tear seam must be designed to resist accidental damage, tampering, and wear throughout many years of use.”
Before taking a thickness measurement, the Hall effect gage needs to be calibrated. When using target disks for calibration, make sure that the target is aligned perpendicular to the probe tip. If the target is misaligned, measurements could be inaccurate.
When making measurements, the probe is usually placed on the outside of the airbag cover, and the target disk is placed on the inside, making it possible to freely move the probe. It’s important that the disk remains aligned with the probe tip and that both the probe and the disk are in firm contact with the seam’s surface to ensure an accurate thickness measurement.
As with any Hall effect thickness application, the instrument measures the distance between the probe tip and target, so if either the probe tip or the target is not securely in contact with the surface of the tear seam, or if the target disk is misaligned with respect to the probe, the displayed thickness will be incorrect. With proper calibration and target disk orientation, you can expect a measurement accuracy of ±3% (or better).