ESS aids automotive test
Greg Reed, Contributing Technical Editor -- Test & Measurement World, 3/1/2005
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| Wayne Tustin of the Equipment Reliability Institute. |
Tustin received a BSEE in 1944 from the University of Washington, and he worked at Boeing Airplane Co. where he was part of a team that modal tested the XB52 model of the B52 bomber. From 1948 to 1961, he worked for MB Electronics in New Haven, CT, a manufacturer of shakers for vibration testing. Since 1962, he has supplied technical training to both government and industry. We recently spoke about environmental testing.
Q Describe the activities of the Equipment Reliability Institute.
A We're a specialized engineering school. We help clients avoid in-the-field failures and achieve all possible reliability. Much of this assistance addresses gathering more representative field data and helping the client to develop more effective environmental testing, especially vibration and shock testing. We also offer formal vibration training at client facilities, in short open courses to which clients can send individuals, and through distance learning over the Internet.
Q What are the most effective environmental stress screening (ESS) tests for hi-rel applications?
A Rapid heating/rapid cooling over a temperature range of about 100°C combined with multi-axis random vibration. The latter is generally accomplished with a number of pneumatic impactors or repetitive shock actuators.
Q What are ESS, HASS, and HALT?
A ESS was "invented" in 1979 by the Navy's Willis J. Willoughby. Combining these tools (stresses) saved time over alternating between a chamber and shakers. Additional time was saved by increasing severity with highly accelerated stress screening (HASS). Note, however, that both ESS and HASS are post-production procedures for finding weak designs, weak parts, and faulty workmanship in products that might later fail in service. Highly accelerated life testing (HALT) uses the same tools but uses them earlier, as part of development.
Q How do shock and vibration testing techniques impact automotive engineering?
A Automobile manufacturers want vehicles and onboard equipment to outlast the warranty period, which in some cases is 10 years and 150,000 miles. Multi-axis vibration tests of entire vehicles and large components use multiple electrohydraulic (servohydraulic), long-stroke, low-frequency shakers. Vibration tests on smaller units such as engine controls, entertainment systems, and so forth use electrodynamic, one- or two-stroke shakers at frequencies up to perhaps 500 Hz.
Q What criteria do you suggest for purchasing shock and vibration test equipment destined for hi-rel applications?
A If you need to perform automotive or aerospace vibration and shock testing, then I suggest you not invest in what has come to be called "shock test machines." You may wonder how you will do half-sine and trapezoidal and other classical shock tests. The answer is: You won't. They're gradually disappearing. They never represented the real world. Instead, put your money into an electrodynamic shaker. Then, you can not only do sine and random vibration tests, but also some classical shock pulses, and you can better replicate real-world, highly oscillatory shock pulses.
