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 mirror systems are easily taken for granted, but they have to withstand the same rigorous duty cycle as everything else aboard the vehicle. And it takes a comprehensive battery of grueling tests to insure they will 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, Daimler 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, five 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.
Next is salt corrosion. The expected damage from exposure to salt fog is primarily corrosion of metals, although in some instances salt deposits may result in clogging or binding of moving parts. For these tests, two corrosion and humidity test chambers are employed. With a capacity of 130 cubic feet, each chamber enables testing 44 mirrors (22 pairs) over a typical duration of 10 days. Smaller chambers also perform these same tests. For corrosion testing, the chambers generate a mist that surrounds the samples with a fog that is a solution of 5 percent salt in purified water. One automaker’s specification requires the mirror case to be in salt corrosion for 240 continuous hours!
For most tests, including salt corrosion, the glass must continue to cycle electrically, simulating the driver working the adjustment control. Proper cycling is insured by monitoring the glass and also the current draw (amperes). Some mirrors incorporate turn signals, some have approach lights on the bottom, while others incorporate power folding features. As Bowles points out, “regardless of the features and options, all must operate to OEM specifications.”
The same test chambers described above are also used for non-corrosive humidity testing to determine the resistance of equipment to the effects of exposure to a highly humid atmosphere. For this test, the chambers are usually set for 90 percent humidity at 100 degrees F. This simulates a humidity fog typically encountered on a summer morning near the shore.
Another test uses water spray to simulate a heavy rainstorm or car wash. This test, designed for Ford, uses nozzles to spray the four corners of a mirror while its electrical operation performance is evaluated.
“Although specifications for heat and cold vary from one automaker to another,” explained Bowles, “they commonly require temperatures ranging from -40 degrees C to 100 degrees C.”
To perform these tests, Schefenacker utilizes two microprocessor controlled walk-in chambers measuring 8- x 8- x 10-feet. A typical test holds the temperature at -40 degrees C for six hours, then goes up to +65 degrees C and is held constant for eight hours. These temperature shock tests simulate a sudden change in temperature.
Performance in dust is also tested in this chamber to ensure dust will not penetrate seals, clog openings, degrade electrical circuits or interfere with moving parts. For this test, cement dust of a particular grit size is circulated at a specified velocity.
The Schefenacker lab has also installed a chamber dedicated to vibration analysis in hot, cold and humid environments, allowing the company to see how mirrors operate in different environmental extremes. These combined tests can help determine the nature and source of any issues early on, allowing engineers time to incorporate design changes prior to production and optimize mirror performance.
One vibration test simulates a bumpy road. Samples are mounted in a fixture over a vibration table and tests are made using a laser that reflects light from the glass to a video camera.
An analyzer determines how much the mirror vibrates relative to the amount of vibration in the vehicle itself. These tests are conducted on vibration machines that have a capacity of a maximum weight of 12.5 pounds reaching 100 g’s of acceleration. In addition to testing exterior mirrors, vibration tests are also run on interior mirrors for convertibles that mount to windshields that are not supported by a header.
Vibration tests for durability can lasts up to 20 hours. Following testing, the glass must still operate, the mirror head cannot become unhinged from the pivot point, the attachments must remain intact, and the studs that hold the mirror onto the door must maintain specified torque.
When a mirror system passes the other tests, it is then checked for quiet operation, which Bowles indicates is a major issue these days. A motorized mirror is set up in the anechoic chamber where a microphone records its sound as it is stepped through its motions. Software ascertains whether the mirror’s noise is acceptable – generally around 55 dB on an A weighted scale.
Schefenacker doesn’t expect any bad luck for breaking all these mirrors, rather it expects to insure 10 years of trouble free functionality in all operational and environmental conditions.