This column is about technology common to many metal cleaning shops—ultrasonic cleaning systems. These equipment components are used in both aqueous and solvent cleaning applications. Chiefly used for removing solid particulate matter, they are agents of agitation which 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.

Good Vibrations
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—change in size (volume) by a few percent.

Figure 1: Diagram of piezoelectric effect (click to enlarge).

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. This is shown in 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.

Magnetostrictive 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—changes dimension in one direction. As in Figure 1, that dimensional change can be used to cause a diaphragm to move—though driven by a different factor.
Most magnetostrictive materials are metal alloys of nickel or contain significant quantities of nickel compounds.

Magnetostrictive transducers are not used at frequencies above 30 kHz. The main reason is that the difficulty and cost of controlling the motion of the relatively large mass of material associated with magnetostrictive transducer elements becomes too severe at frequencies above that level.

Table I: Comparison of Piezoelectric and Magnetostrictive Transducers (Click to enlarge).

The two methods of generating pressure waves for cavitation are compared in Table I.

Making a Choice
Some may inform managers that the choice is between the higher purchase price and longer maintenance life of magnetostrictive transducers versus the opposite for piezoelectric transducers, or to achieve a lower level of operating noise.

That’s a false choice. The choice should be totally based on the character of the parts.

• No one would consider use of magnetostrictive transducers for cleaning of disk drive components where piezoelectric transducers are commonly used. The components would “dance” in the water bath and be destroyed.
• No one would consider use of piezoelectric transducers for removal of scale prior to painting of small engine blocks for lawn mowers. Nothing would be removed.

What About Frequency?
In next month’s column we’ll review the science a manager should choose to use one frequency of ultrasonic waves versus others. And in the February column, we’ll review how to size the power input.

Editor’s Note: In last month’s column, Figures 7, 8, and 9 were incorrect. Please visit here for the correct images. We are sorrry for any inconvenience this may have caused.

John B. Durkee, II, PhD, PE, is a consultant in metal and critical cleaning. You can contact him at 830-238-7610;
(e-mail) jdurkee@precisioncleaning.com; or (fax) 612-677-3170.