Theory and operating considerations
By Joe Escobar
In the arena of parts cleaning, ultrasonic cleaning is making inroads. Environmental impact consideration is leading many companies to look for greener cleaning methods, and ultrasonics can help achieve that goal. Many applications can benefit from this type of cleaning from fuel nozzles, brake parts, and generator components to turbine blades. Ultrasonic cleaners have the ability to clean in narrow crevices as well as inside cavities and small holes that may not be easily accessible through other methods of cleaning. Difficult soils such as carbon deposits are also good candidates for ultrasonics. But there are some considerations to keep in mind when using ultrasonic equipment. We'll take a brief look at how ultrasonic cleaning works as well as some issues to keep in mind to avoid damaging your parts.
It starts with what you can't hear
With any sound, the frequency of its wave determines its tone or pitch. The higher the frequency, the higher the pitch. Ultrasonics uses sound waves that are above 18 Kilohertz (KHz), well above the limits of human hearing. In most industrial ultrasonic cleaning, the frequencies used range between 20 and 50 KHz.
In ultrasonic cleaning, a phenomenon known as cavitation is responsible for the cleaning action. Transducers introduce high frequency, high intensity sound waves into the liquid. Alternating high- and low-pressure waves that are generated by the ultrasonic sound cause millions of tiny bubbles, or cavities, to form and subsequently collapse. During the low-pressure phase of the sound waves, microscopic vapor particles form and expand until the high-pressure phase where they are compressed and implode, or cavitate. At the implosion site when the bubble cavitates, temperatures around 10,000 F and pressures in excess of 10,000 psi are generated. These temperatures and pressures are at the microscopic level where the bubble is imploding, and the process of millions of bubbles expanding and collapsing every second provides the pressures and temperatures that provide the cleaning action in ultrasonic cleaners.
Factors affecting cavitation
Since the cavitation of bubbles is what provides the cleaning in ultrasonics, maximizing cavitation will give you the most cleaning potential. Factors that affect cavitation intensity are the viscosity of the cleaning solution, the frequency applied, the amount of gas present in the solution, and temperature.
The viscosity of the cleaning solution affects cavitation. The lower the viscosity, the more cavitation effect present. Fluids with higher viscosity are sluggish and do not react quickly enough to form cavitation bubbles and the subsequent implosion.
The frequency that the transducers generate affects the cleaning properties of the ultrasonic equipment. The lower the frequency, the larger the vapor bubbles formed will be, and the more intense they will be when they implode. On the other hand, the higher the frequency, the smaller the vapor bubbles will be, and the less intense they will be.
Presence of gas
In order to have the most effective cavitation, the amount of dissolved gas in the liquid must be minimized. Gas that is dissolved in the cleaning solution is released during the bubble growth phase and prevents the violent implosion from occurring.
Temperature is the one factor that has the greatest effect on cleaning. That is because several of the factors above are influenced by temperature. The viscosity of the fluid and the amount of dissolved gas present in the solution are both reduced as the temperature is increased. Performance is usually enhanced by increased temperatures.
When using ultrasonic cleaners, there are several issues to be aware of. One such operating consideration should be resonance.
Avoiding Resonance Damage
Manufacturers have strict time and frequency limits for cleaning their parts with ultrasonics. Sticking to these limits is essential to avoiding resonance damage. Although it may be tempting to leave a part in longer to get it cleaner, the effect that it can cause may be catastrophic. Be sure to follow all the manufacturer's recommended procedures when working with ultrasonic equipment.
With resonance being a critical issue, Pratt & Whitney Tool Support Services and Blackstone~NEY Ultrasonics have addressed the issue in the line of ultrasonic cleaners by randomly varying the frequency and amplitude of the ultrasound. Unlike other cleaners that vary the frequency in a small plus or minus range, this product actually changes the frequency in random steps. This provides the benefit of the part being subjected to randomly varying frequencies. It also prevents the part from getting excited into a resonant vibration that could cause damage.
Just about any object has frequencies at which it naturally vibrates known as resonance frequencies. This phenomenon is easily evident in a crystal glass. If you flick the glass with your finger, it will emit a clear tone as it vibrates. This is its resonant frequency. If an opera singer subjects the crystal glass to a note at its resonant frequency and at a high enough volume or amplitude, the glass will shatter.
The same theory affects parts that are cleaned using ultrasonics. The parts can be excited into resonance during the cleaning process. If they are allowed to remain in this state of resonance, damage can occur, especially in fragile components.
Use the proper equipment
The bottom line if you are considering using ultrasonic equipment for cleaning aircraft parts is to make sure the equipment is approved by the manufacturer. Although a bargain unit may seem harmless, it can cause damage to the part if improper frequencies are used. Even if it is an approved cleaner, be sure to closely monitor the cleaning times in order to prevent damage caused by resonance.