Putting mirror mirror through the wall: breaking mirrors isn't considered bad luck at Schefenacker, it's just part of the daily testing routine

Automotive Industries,  Oct, 2003  

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Automotive mirror systems are easily taken for granted, but they have to withstand the same rigorous duty cycle as everything rise aboard the vehicle. And it takes a comprehensive battery of grueling tests to insure they win have a product life of 10 years or more.

At Schefenacker Vision Systems, USA, Inc., David Bowles is the lab manager and overseas the testing that now supports some 10 major OE programs with GM, Ford, Daintier Chrysler, Mercedes Benz, Hyundai, Honda and Nissan. "These 'shake-and-bake' tests ensure these mirrors can tolerate the worst climate, terrain and dynamic conditions possible," explains Bowles.

Mirrors are heated to 100 degrees C at 90 percent humidity, then frozen to -40 degrees C, subjected to simulated rainstorms, heavy fog, and harsh car washes, then encased in a dust environment and later saturated in a salt mist. They are subjected to severe vibration, and some mirrors are literally pulled apart. "Over a period of several weeks," says Bowles, "these mirrors experience more severe treatment than most cars will in a lifetime."

Typical mirror samples range from 22 to 50 units per test. This past year, the state-of-the-art Schefenacker lab tested nearly 2,000 mirrors in 10 major OE programs. Schefenacker performs these evaluations at its Marysville Mich., facility.

This lab runs 24 hours a day, live days a week and sometimes weekends.

But destructive testing is only a small part of what Schefenacker does to verify OE standards. Most of the tests are to evaluate functionality. Schefenacker has recently made a substantial investment in larger environmental chambers, vibration tables, a new water spray tester and an anechoic sound chamber that performs the sophisticated testing demanded for exterior mirror products.

The lab plays a critical role in new product development with a structured Design Verification Test Plan. Simulated environments give pre-production samples the equivalent of 10 years on the road. More advanced than prototypes, they are subjected to environmental conditions that the production mirror will be expected to endure. Various applications and calculation procedures establish confidence and reliability levels. All this is to determine if adjustments are- needed before the final tooling is complete.

Prior to environmental testing, metrology technicians check dimensional accuracy using coordinate measuring machines (CMMs). Generally a minimum of six pairs of mirrors are measured to ensure that case components are within the tolerances specified from the design group.

"The CMMs measure specific points including the opening, the location of attachment points, and the flush and gap along the edge to ensure proper fit to the car," points out Bowles. "Tolerances on surfaces can range from +/- 0.5mm to 1.5mm."

Pre-test measurements of electrically actuated mirrors include angular travel and speed of the glass movement. Each mirror is secured in a fixture and four probes are attached to variable differential transducers and rest against the glass. As the glass moves, data is fed into a computer workstation to determine whether a mirror meets OEM specifications for minimum and maximum speeds and degrees of movement for glass travel. As the data sheet is printed, any nonconforming numbers appear in red for easy identification and trigger corrective action.

Measurement also determines the 'effort' needed to fold the mirror in toward the door. If the force is too great, a mirror may not fold out of harm's way, such as when a vehicle is in a car wash. Yet if it is too little, a mirror could fold out of adjustment during highway driving. A correction to the pivot design is sometimes necessary.

"The pre-test measurements allow the engineering group and quality group to be proactive by addressing any issues that may arise with meeting OEM specifications," says Bowles. "They also establish a baseline to determine the amount of system or functionality degradation during and upon completion of a product's cycle testing."

Destructive testing is testing in which the part is rendered unusable to prove its strength. The Glass Pull-off Test is an example. This measures the force required to pull the glass out of the mirror, ensuring that the mirror glass does not separate from the housing except when encountering an extremely unusual force--such as a severe accident.

Another destructive test is performed using one of the lab's two mirror impact testers. Each is a metal pendulum with a long arm connected to a massive ball, usually about the size of a softball, swung from above onto the mirror. One of these tests is conducted for testing DaimlerChrysler mirrors chilled to 22 degrees C. They are expected to survive this impact. The other is a European Regulation 46 Impact Tester used for certification of mirrors sold to European automakers or for vehicles exported to Europe. Schefenacker's lab has been certified for this ECE/EC regulation.