Metal Finishing Guide Book

2013

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cleaning, pretreatment & surface preparation PRIMER ON ULTRASONIC TRANSDUCERS BY JOHN DURKEE, INDEPENDENT CONSULTANT SPECIALIZING IN METAL AND CRITICAL CLEANING, HUNT, TEXAS This article is about technology common to many metal cleaning shops — ultrasonic cleaning systems. Ultrasonic Transducers—How They Work. These equipment components are used in both aqueous and solvent cleaning applications. Chiefly employed for removing solid particulate matter, they are agents of agitation that can dislodge soil components that can't be removed solely by chemical action. In common use for decades, they are becoming (or have become) commodity equipment products despite the best efforts of suppliers to provide differentiation. Ultrasonic transducers produce waves of fluid pressure that bombard part surfaces (and all surfaces under immersion). The waves are produced by diaphragms that vibrate under immersion in fluids. The device producing the vibration is called a "transducer." Frequency of vibration is high—from tens of thousands to hundreds of thousands of oscillations (cycles) per second (cps or Hertz). Consequently, the effect of each cycle of vibration is negligible—but their cumulative and continuous effect can be either positively or negatively dominant. There are two methods by which transducer diaphragms are caused to vibrate. PIEZOELETRIC TRANSDUCERS A piezoelectric material has two unusual and interrelated characteristics. They are basically the reverse of one another: • When a force is applied to a piezoelectric material, a tiny electric current is produced. • When an electric current is passed through piezoelectric materials they deform, i.e., change in size (volume) by a few percent. It is the latter characteristic that produces a vibrating diaphragm. A rigid connector (arm) causes the diaphragm to move slightly when the piezoelectric material changes shape upon application of an electric current (see Figure 1). Repeated application of the electric current, followed by its relaxation, enables a diaphragm to move forward and backward in one direction. Most piezoelectric materials are ceramics, many of which contain silicon, lead, aluminum, or titanium oxides. MAGNETOSTRICTTIVE TRANSDUCERS There is a magnetic analog to the piezoelectric effect. A ferromagnetic material (magnetic Iron) will respond mechanically to magnetic fields. This effect is called "magnetostriction." Magnetostrictive materials transduce or convert magnetic energy to mechanical energy. As with the piezoelectric effect, the reverse is also true. When a magnetostrictive material is magnetized, it elongates—that is, it 70

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