Precision aircraft bearings cleaned using co-solvent process...
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The Aurora Bearing Company produces precision rod-end and spherical bearings. Recently, the company adopted a new non-CFC precision cleaning process using materials that are effective, readily available and meet environmental regulations. The company's products are used in industrial applications, military equipment and commercial and military aircraft.
"This new approach to precision cleaning allows us to remove oils and contaminants from finished bearings and satisfy production cleaning standards," said Robert Peshia, plant services superintendent. He explained that absolute cleanliness is necessary for precision bearings, which are individually inspected before they are re-lubricated and packed for shipment. This quality control process has been in place since the company's inception in 1972.
Mr. Peshia recalled that until about ten years ago his company used a vapor degreaser with 1,1,2-trichloroethylene solvent to dissolve and flush away oils, greases, particulate material and metal fines generated during the bearing manufacturing process.
In 1987, when the company added precision stainless-steel bearings for aircraft to its product line and found that the existing vapor degreasing process was not suited for cleaning this product. "At that time, we purchased a small two-sump vapor degreaser that used a chlorofluorocarbon (CFC) as the solvent," Mr. Peshia noted. "This unit worked well and allowed us to reliably clean stainless steel aircraft bearings."
As CFCs rose and ozone-depleting solvents were limited in supply, Aurora Bearing installed an aqueous cleaning system for use with both its general industrial and aircraft product lines. Mr. Peshia reported that a complication arose because of differing needs. A rust-preventing additive in the aqueous cleaning process that was required for standard steel bearings formed an unsatisfactory residue on stainless steel aircraft components. This residue had to be removed manually, adding cost and production delays.
According to Mr. Peshia, the company learned about an alternative, non-ozone depleting process that was successfully used by another bearing manufacturer. This cleaning method consisted of a co-solvent vapor degreasing system using a solvating agent from Petroferm Inc. and a perfluorocarbon rinse agent from 3M.
Mr. Peshia contacted Petroferm. One of the representatives suggested an even more effective co-solvent process based on the company's solvating agent and a new rinse agent, 3M™ HFE (hydrofluoroether) fluid.
The solvating agent, a low volatility, high solvency organic fluid, lifts soils from part surfaces while the rinsing agent removes the solvating agent and suspended soils. This co-solvent process is analogous to conventional CFC two-sump vapor degreasing, since it uses a mixture of the solvating and rinsing agent in the boil sump and pure 3M HFE -7100 in the rinse sump.
Solvating agent SA-70 was specified for the Aurora Bearing co-solvent cleaning application because of its effectiveness in dissolving water and removing it from parts along with cutting oils and other contaminants.
The rinse agent used in the co-solvent process is a member of a new family of hydrofluoroether compounds commercialized in 1996. The HFE-7100 fluid was given an acceptable without restrictions listing under the EPA's Significant New Alternatives Policy (SNAP) program for replacement of ozone-depleting substances in specific solvent applications.
The 3M product offers a balance of performance, environmental and workplace safety properties. In addition to an ozone-depletion potential of zero, the product has a shorter atmospheric lifetime and lower global warming potential than CFCs and many fluorinated compounds. In addition, it is non-flammable and low in toxicity.
The high density of the HFE formulation and its low viscosity and surface tension values are important properties in cleaning applications, especially for penetrating, cleaning and rinsing components having complex geometries. Because of these properties, the fluid drains effectively from cleaned components such as mechanically complex bearings. A low heat of vaporization aids in rapid drying of the rinse agent and minimizes liquid dragout from the cleaning process.
Sample parts from Aurora Bearing Company were cleaned initially at Petroferm to determine parameters for an optimized co-solvent process. Petroferm assisted in preparing the Aurora Bearing vapor degreaser for co-solvent cleaning by adding ultrasonic agitation to the boil sump, complementing an ultrasonic transducer already in place in the rinse sump.
In the Aurora Bearing co-solvent system, contaminated parts are first immersed in the boil sump, which contains an approximate 50/50 mixture of co-solvent and hydrofluoroether rinse agent. Soils are dissolved primarily by the co-solvent, although the rinse agent also contributes to cleaning.
After boil sump cleaning, parts are immersed in the rinse sump. This sump contains nearly 100-pct-rinse agent, which serves to remove the solvating agent and contaminant residues from parts.
Following rinsing, parts are moved into the vapor zone above the rinse sump for a final rinse with hydro-fluoroether condensate. The parts basket is then lifted into the freeboard zone where remaining HFE-7100 fluid evaporates and is returned to the sump, leaving parts clean and dry.
A typical co-solvent cleaning cycle at Aurora Bearing requires between six and nine minutes, depending on the duration of each phase. Mr. Peshia has settled on a two- to three-min boil sump dwell, an equivalent period in the rinse sump and one-min dwell periods in the vapor zone and in the freeboard area to minimize rinse agent dragout. Occasionally, operators will also hold cleaned parts above the boil sump for a brief interval to minimize transfer of contaminated solvating agent to the rinse sump.
Fluid consumption in the co-solvent process depends somewhat on operator technique. Mr. Peshia found that loss rates were reduced through control of air flow and drafts, and deliberate, unhurried movement of part baskets to minimize vapor dispersal and turbulence in the fluid sumps. He estimates that rinse agent consumption is approximately one liter every three days.
Mr. Peshia has equipped the vapor degreaser boil sump with filtration and finds that the system performs with minimal downtime. Aurora Bearing has made minor adjustments to chiller coils, raising their temperature slightly to minimize frost buildup.