Metal Finishing Guide Book

2013

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Table 1. Visualization of Various Ultrasonic Transducer Applications Magnetostrictive Piezoelectric 20 kHz 40 kHz 68 kHz 104 kHz 170 khz T-Rex eating man Pelican eating fish Hornbill eating beetle Cockatoo eating nuts in shell Finch eating seeds solid media or impact from a pressurized fluid jet apply such different stress to soil elements and the surface on which they lay. To complete this analogy, blast and pressurized jet cleaning technologies might be thought of as being "eaten by a T-Rex dinosaur." (See Table 1). Consider Table 1, in which this analogy is presented in a generalized visual form. Also remember that it's an analogy! The point of this presentation is that the transducer frequency should be chosen to match the nature of the cleaning task. Each choice of frequency will be more useful when applied to a specific type of soil material, and will have different effects on the underlying surface. Said another way, use the right tool (frequency) for each job (cleaning situation). And how is the right tool to be identified? Managers should organize and witness cleaning demonstrations using actual soiled parts with facilities provided by suppliers. These parts should be cleaned using several transducer configurations and the performance evaluated by the normally used cleaning test. Let the details of the application reveal the right choice of frequency. A NEW FREQUENCY SWEEPS CLEAN Selection of a transducer which radiates pressure waves into fluid and onto part surfaces at a selected, constant, and fixed frequency may solve cleaning problems (as above), but also create concern about part integrity. Any single wave frequency can— and is— likely to resonate within the liquid volume as it reflects off the walls which contain the liquid, and the parts. Resonance is the term for coordination of the pressure amplitudes which occur at the constant wave frequency. Pressure values (amplitudes) can combine if the wave frequency doesn't change. This isn't bad, if there isn't some threshold pressure which can harm the parts. But delicate parts will fracture when excited into resonance. This outcome was catastrophic for those removing particles from fragile parts such as those used in disk drives. The solution developed was to force the transducer frequency to vary over a small range by changing the frequency of the alternating current supplied to the piezoelectric crystal. This prevented wave resonance, and application of unwanted high pressure forces to fragile parts. Deliberate variation of frequency around a central value is known as "sweep." The amount is usually 1 or 2 or 3 kHz for a transducer designed to produce pressure fluctuations at 40 kHz. This capability is now a standard feature of nearly all commercial ultrasonic transducer systems — whether to be used with fragile disk drive components or used with sturdy drive gears. 73

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