In some difficult or extreme bearing operating environments, such as vacuum, high temperature and dry-running, conventional oil and grease fluid lubricants have limited effectiveness. Under such conditions, conventional fluid lubricants either fail early or never are considered as an option.
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Solid lubricant coatings, however, offer a solution for diverse applications exhibiting extreme and difficult running conditions. The coatings effectively reduce friction, have high hardness and wear resistance, and strongly bond to the bearing steel, providing long operating endurance. Because the coatings are only 1 to 2 microns thick, they are extremely smooth (steel mirror surface). The coatings are deposited on finished bearing components, including those made of hardened bearing steel, at a temperatures below 200ºC — a temperature that does not affect the supporting steel's mechanical properties. There is no need for post machining.
The solid lubricant coatings entered field testing in 1998 and are now in a full application and development stage, providing engineering, maintenance and supervisory personnel responsible for bearing reliability with solutions that were not possible with traditional lubrication technology.
There are three prominent solid lubricant coatings; metal-alloyed diamond-like-carbon, hard graphite, and metal-doped molybdenum disulfide.
This metal-alloyed coating is deposited as a multi-layer coating in a low vacuum by means of physical vapor deposition. It consists of approximately half diamond structure and half graphite. The diamond structure provides hardness and wear resistance; graphite gives lubricity. A metallic interlayer between the coating and the steel substrate ensures strong bonding.
Pure graphite offers optimum lubricity in wet or humid environments. A high energy beam under vacuum deposits graphite as a high density coating, which results in a hardness exceeding that of hardened bearing steel. A metallic interlayer between the coating and the steel substrate works in combination with the high hardness to provide long service life in water lubricated, highly loaded rolling contact conditions.
Metal doped molybdenum disulfide
Molybdenum disulfide is a conventional, well-known solid lubricant having a layered structure. A traditional molybdenum disulfide layer is rather soft and degrades in humid conditions.
Consequently, traditional molybdenum disulfide coatings do not function well as a solid lubricant for rolling bearings.
The new breed of molybdenum disulfide coatings — which benefit from metal doping — is three to five times harder than the pure molybdenum disulfide coating and provides high humidity resistance. Metal doped molybdenum disulfide coatings have a metallic interlayer and offer an extremely low coefficient of friction in dry running conditions and extended service life in highly loaded rolling bearings.
Several characteristics make the solid lubricant coatings effective in rolling bearings:
- The coatings can be deposited either on rolling elements such as rollers and balls or on a bearing's rings. For most applications, coating the rolling elements provides adequate lubrication.
- The coatings have high thermal resistance and can be used in bearing environments up to 400º C.
- The coating's dynamic coefficient of friction is less than one-quarter that of steel. Low friction reduces heat generation and wear, and provides long service life for rolling and sliding contacts.
- The coatings are hard — 30 to 50 percent harder than hardened bearing steel. This higher hardness increases wear resistance and results in long service life.
- The coatings can be used in combination with conventional fluid lubricants and environmentally friendly "green" lubricants for reliable and improved performance in applications calling for that type of multi-lube solution.
- Rolling bearings coated with the solid lubricants demonstrate a significantly longer life and four times the load carrying capacity of rolling bearings with traditional solid lubricant coatings.
Bearing coatings at work
When conventional fluid lubricants are not an adequate solution, solid lubricant coatings enable bearing users to overcome technical challenges to reduce downtime and extend lube intervals (or, in some cases, eliminate them). The range of performance issues that lubricant coatings address extends to water lubrication, fuel lubrication, high contamination and clean conditions, among others.
The following examples from four industries address three equally demanding application challenges: marginal lubrication, dry running and vacuum conditions. Because the solid lubricant coatings are so new, some examples are still in the development stage.
Marginal lubrication in a paper mill
Paper mills often experience problems in the calendar section, where paper is glazed between rolls. The bearings in these rolls operate in an environment of varying loads, ranging from virtually zero to full load. During loading, the bearing rollers must accelerate, an action that often causes smearing problems. A breakdown from catastrophic bearing failure costs approximately $80,000. Standard oil-lubricated bearings in this application need replacement at least once a year. Bearings coated with solid lubricant, made of metal alloyed diamond-like carbon, together with oil lubrication, have run without any bearing problems for three years--a span exceeding the machine's normal three-year maintenance interval.
Dry-running in a chemical plant
A chemical processing facility's polyethylene reactor uses six rolling bearings, one of which repeatedly showed lube-related failure within 72 to 200 days. No grease or oil lubricants are allowed inside this reactor. The only possible lubricant is ethylene gas, which is not sufficiently effective. Each bearing failure causes a production stoppage and results in production loss valued at approximately $100,000. A solid lubricant coated bearing now in development is made of metal alloyed diamond-like carbon and is deposited on the bearing's rolling elements. The solid lubricant coating is expected to improve bearing performance and meet the facility's requirement for one-year minimum bearing operating life.
Dry-running in a textile plant
Solid lubricant coated bearings equipped with ceramic rolling elements solved a particularly demanding challenge at a textile drying processing facility. Each dying machine uses two deep groove ball bearings. The processing equipment uses carbon dioxide, a gas that would dissolve conventional oil and grease bearing lubricants. One solution to protect fluid bearing lubricants would be to design a completely sealed system, with an external supply of fresh oil provided to the bearings. The sealing system, however, is expensive and not reliable. The target bearing life is one year — the drying machine's maintenance interval. The combination of a solid lubricant and ceramic rolling elements provides the lowest wear rate and longest endurance life for this demanding application.
Some applications require coatings to be used in combination not only with special bearing materials, but also with conventional fluid lubricants. An example focuses on bearings with ceramic rolling elements used in a robot for semiconductor processing in vacuum conditions. During operation, the bearings experience an oscillating movement. The combination of vacuum and oscillation results in fretting damage to the bearings. Fretting is a wearing away or corroding of the bearing metal under small oscillating motion. In a vacuum, conventional grease has a high evaporation rate and would contaminate the vacuum system. Grease alone could not survive long under fretting wear. A solid lubricant coated bearing together with special vacuum grease is now under development to address the complex application challenge. The grease has a low evaporation rate to ensure that the system stays clean and the solid lubricant coating provides service life sufficient to prevent the fretting damage. The bearings are currently undergoing a bearing life test of 100 million cycles.
Know what's available
When applications present special bearing lubrication challenges, knowing what solutions may be available can be a critical knowledge resource. Research in bearing lubrication and coatings will continue, no doubt leading to advancements that help keep bearings running longer, more smoothly and reliably. Ask your bearing supplier to keep you updated on the latest advances. You may discover solutions that yield business efficiencies associated with reduced downtime and greater overall equipment dependability.