From time to time here on the blog I will try to include some applications notes targeting specific cleaning applications and giving you a taste of the successes and challenges faced in each. Although you may not have a need for the specific information presented, you may, as cleaning technologists, find it interesting what other people are doing. Our first focus on Applications Notes will address bearing cleaning.
Ball and roller bearings and their components are commonly cleaned multiple times throughout the manufacturing process. In precision applications, most notably aerospace, several in-process cleanings are required to facilitate inspection of the component parts of the bearings following various machining and other processes. These inspections may include penetrants and dyes which are used to detect flaws and other surface conditions. Cleaning (frequently ultrasonic) is also employed prior to the nital etch process which is used to measure the surface temper of many precision bearing components. Parts, once inspected, require further cleaning to remove the residuals left by the inspection process.
Ball and roller bearings basically consist of an inner and an outer “race,” the balls or rollers and “separators.” Depending on the bearing, there may also be other components such as seals which also require cleaning prior to assembly. The “races” are the two circular collars that fit one inside the other with grooves in which the balls or rollers roll. “Separators” are used to space the balls or rollers within the races. When the bearing is assembled, the two races are mis-alligned to allow the insertion of the balls or rollers. After the balls or rollers are inserted between the races, the races are brought into alignment and the separator is inserted to space the balls or rollers. Once the rolling components are spaced apart, they prevent axial movement of the races while allowing them to turn one inside the other.
Bearing cleaning is “fussy.” Each manufacturer has its own proprietary process and concepts of how the bearing should be both manufactured and cleaned. The thing that is common to them all is that cleanliness is important! In some cases, cleaning targets the removal of oil, grease or other liquids that may prevent, for example, the penetration of dyes, penetrants and the exposure of surfaces to etches. In other cases, especially as the bearing approaches final assembly, particles that might impede the smooth operation of the bearing or cause damage to the contact surfaces become the major concern.
One intrinsic property of bearings is that they are often made of high carbon steel and “super alloys” which may be subject to being magnetized during the manufacturing process. Metal particles held in place by the forces of magnetism are nearly impossible to remove using any cleaning process. For this reason, de-magnetizing units are used on the input of cleaning systems to prevent particles from being held in place by magnetism.
Bearings and their components made of high carbon steel are subject to oxidation (rusting) during the cleaning, rinsing and drying stages of the cleaning process. Chemistry selections should, therefore, include oxidation inhibitors.
One of the concerns expressed by bearing manufacturers is that cleaning using aggressive agitation or ultrasonics may cause metal to metal impacts between the rolling components and the races that allegedly result in “micro-brinelling,” a localized surface hardening phenomenon. Although it is still a concern, there is no support for the notion that micro-brinelling occurs as a result of mechanical cleaning processes including ultrasonics.
If you have additional specific questions about machines or process for cleaning bearings, I would suggest contacting one of our applications engineers at www.ctgclean.com.
– FJF –