Quality Cleaning for Gearing Applications

Just as there are many different methods for cleaning gears, there are many considerations to be aware of as well. Read on for an in-depth description.

Gears are precision components that place great demands on cleaning. The cleaning industry offers various solutions in order to fulfil these demands both economically and ecologically. Contamination from the manufacturing process—chips and residues from machining media, for example—may result in excessive wear and functional impairment for gear wheels. The quality of subsequent manufacturing processes such as hardening may also suffer from contamination on the surface of the parts. Despite its great significance, cleaning is usually allotted only a subordinate role. However, the cleaning step in the manufacturing process often offers considerable potential for reducing costs and optimizing quality. Figure 1

The Right Cleaning Agent 
Gears are usually cleaned in a wet chemical batch process with water-based cleaning agents or solvents. The effectiveness of aqueous media—which are available as alkaline, neutral, and acidic cleaning agents—is based upon an organic or inorganic builder and tensides. The latter are capable of “pushing” themselves between the contamination and the material to be cleaned as surface-active substances and dislodge non-polar contamination such as oil and grease, as well as polar contamination (e.g. emulsions, salts and particles). Cleaning is followed by a single or multistage rinsing process, for which deionised water is frequently used during the last rinsing cycle(s). This prevents the adhesion of residues left by the cleaning agent or surface spots on the gear wheels, which might impair processes such as hardening. Figure 2

In the case of solvents, basic differentiation is made among non-halogenated hydrocarbons (HC) and modified alcohols, chlorinated hydrocarbons (CHC), and polar solvents. Non-halogenated hydrocarbons (HC) provide good dissolving performance for animal, vegetable and mineral oils, and grease, and demonstrate outstanding materials compatibility. Consequently, most of the processing oils and greases used in modern manufacturing processes can be reliably washed away with these cleaning agents. Particles which cannot be dissolved by the solvent such as chips are removed along with the oil because they are no longer able to adhere to the surface. Polar contamination like salts from emulsions cannot be removed with hydrocarbons. Chlorinated hydrocarbons (CHC)—i.e. the traditional degreasers—assure especially effective degreasing of metals, as well as drying. They’re the preferred solvent for processing gear wheels contaminated with chlorine-containing oils. Polar solvents unite the advantages of aqueous cleaning and the use of solvents. By taking advantage of well-balanced grease and water-dissolving characteristics, these cleaners are capable of simultaneously removing non-polar contamination like grease and oil, as well as polar contamination (aqueous coolants and lubricants, polishing pastes, salts, chips, and other solids).

State-of-the-art for cleaning with solvents involves closed systems that fulfill requirements for reducing the emission of volatile compounds: e.g. the VOC directive. They’re usually offered as single-chamber systems and are laid out for multistage cleaning processes such as cleaning, steam degreasing, rinsing, and drying. Figure 3

Faster Cleaning with Ultrasound
In order to be able to achieve the desired cleaning results within short periods of time, the effectiveness of the cleaning medium is enhanced by means of various physical processes that demonstrate effects of varying magnitude. Regardless of the medium, ultrasound is frequently used for cleaning gears. This process can be taken advantage of to wash away particulates as well as film-like contamination, and high degrees of cleanliness are achieved. Ultrasonic waves develop their full cleaning effectiveness in a liquid bath. The cleaning effect is based on cavitation: When a liquid is subjected to ultrasonic sound, the great intensity of the sound waves results in successive phases of “underpressure” and “overpressure.” The liquid is broken up by the underpressure, which causes the formation of microscopically small cavities. During the subsequent overpressure phase these cavitation bubbles are rendered unstable and collapse (implode), and they generate hydraulic impacts with considerably high energy densities. As a result, micro-currents are caused in the liquid.
When these currents strike a surface, they “blast” contamination off and rinse it away. Generally speaking, the lower the frequency the larger the cavitation bubbles, and thus the more energy is released. Figure 4

The Cleaning Basket—A Decisive Factor
The basket within which the gear wheels are cleaned influences the effectiveness of the utilized system technology, treatment time, the cleaning temperature, and the medium. As a prerequisite for quick, reliable removal of contamination, the gear wheels must be readily and uniformly accessible to the cleaning agent and the mechanical washing process must develop its full effectiveness, so that film-like contamination and particulates can be washed away as efficiently as possible. This is made possible through the consistent use of round wire. As opposed to closed containers or baskets made of perforated sheet metal, cleaning baskets made of round wire are also distinguished by significantly better draining characteristics. And this means that considerably less contamination and cleaning agent is carried over. This results in a longer service life for the cleaning bath, and thus improves cleaning system availability and efficiency. Figure 5

Integrated Aqueous Gear Wheel Cleaning
Specially developed aqueous systems are available for fully automated gear wheel cleaning. Thanks to their modular design, isolation of the supply module from the cleaning system and a small footprint, they can be integrated into existing manufacturing environments in a trouble-free fashion and adapted to meet individual requirements. In this way, for example, the systems can be equipped with individually configurable cleaning, rinsing, passivation, and drying chambers. Cleaning by means of spraying and immersion is possible in the cleaning chamber, as is additional support by means of ultrasound and flushing under pressure. Gear wheels are fed to the system via fully automated conveyor belts and gripper systems, as well as robots or gantries.

Dry Cleaning with Vibration
Vibration cleaning involves an innovative decentralized process for cleaning individual parts, with the help of which oil, cooling lubricants, chips, and other contamination can be removed without the use of any media. The parts is clamped to a vibration generator either manually or with an automated handling system, and is caused to vibrate during the cleaning process. The adhesive forces between the surface of the part and the contamination are overcome by this vibration, and the contamination enters a state of suspension in which it can be exhausted. Oscillation frequency, oscillation amplitude, cleaning time, and exhaust power are freely programmable, making it readily possible to fulfill various requirements. Compared with conventional wet chemical cleaning, good cleaning results are obtained with this dry process. Furthermore, it’s energy saving and ecological as well. The quality of removed processing media is not influenced, which can thus be returned to the production process. Beyond this, the compact systems of modular design fulfil the demands placed upon lean manufacturing and are distinguished by comparatively low investment and operating costs.

The Plasma Process for Micro-Cleaning
Gear wheels made of various materials—steel, non-ferrous metals, plastics, and ceramics, for example—can be treated either in batch processes or as individual parts with plasma technology. In this case contamination can even be reliably removed from complicated shapes with tight radii, undercuts, drill holes, and slots. Plasma is a gaseous mixture of atoms, molecules, ions, and free electrons. Depending upon the application, various plasma gases can be used, by means of which the surface is simultaneously cleaned and activated.
This dual function is based on a physical and a chemical reaction involved in the process: The atoms released in the plasma “bombard” the surface of the gear wheel to be cleaned. They function like a miniature sand-jet in the nano range, thus removing thin organic contamination such as oil and grease. At the same time free ions and electrons are deposited on, and enter into, a chemical bond with the surface. Consequently, surface tension is adjusted to an ideal value for subsequent processing, for example coating, bonding or surface finishing.