The full-film condition denotes the presence of enough lubricant to ensure complete separation of the moving surfaces. It also is known as hydrodynamic full film. The EHD condition refers to a hydrodynamic film formed by applied pressure or load. It is predominantly found in rolling element bearings. Sometimes referred to as thin-film lubrication, the boundary layer condition is usually the result of an insufficient lubricant supply. Although lubrication is present, it does not exist in sufficient quantities to prevent metal-to-metal contact.
In rolling element bearings, for instance, the load on a roller causes the roller to move toward a stationary element, or a raceway. This load creates a pressure area that deforms elastically, resulting in a "Hertzian" contact area.
This pressure can go as high as 200,000 pounds per square inch (psi) ," compressing the lubricant into a thin film. The viscosity of the oil increases where this fluid film acts as a solid and allows the ball to roll without metal-to-metal contact. When the viscosity is "wrong," the load-carrying ability of the lubricant is affected negatively. Additionally, if the oil degrades to a point where it is too thick to penetrate between these surfaces, the oil supply might not be adequate to prevent sacrificial contact.
Oil starvation. The use of too little lubricant can be catastrophic. This is commonly the result of incorrect filling, incorrect oiler settings and unrecognized leakage. Without enough oil to prevent friction, "thermal runaway" can happen very quickly to a steel bearing.
As the temperature of the bearing increases, the ball and race both expand, creating an even tighter fit. This increases the temperature even more, and the cycle continues to a rapid, catastrophic failure.
A less obvious cause of oil starvation is high viscosity, resulting from oxidation or degradation, or improper oil selection. If the oil is too thick, it cannot penetrate the small clearances of a rolling element bearing, particularly at higher speeds.
Excessive lubrication. It is a common mistake to believe that more is better, especially when it comes to oil sump lubrication. Too much oil can adversely affect the operation of flinger rings, slingers and direct bearing contact. Churning can lead to higher operating temperatures, increased oxidation and reduced equipment efficiency.
Chemical Processing Plant Case History
The Albemarle chemical processing facility in Pasadena, Texas, increased pump reliability by improving oil sump lubrication. Beginning in 1998, a failure history was kept on an ITT Goulds 3196 MT process pump used in the production of bromide. Operating outdoors, the pump runs 24 hours a day/seven days a week and was suffering from a very high seal failure rate ," 11 occurrences per year, at an average rebuild cost of $3,345.
After evaluating the failure mode, most often the mechanical seal, the plant determined the seal failure to be a symptom of other problems. Numerous bearing failures helped direct the attention to the moisture contamination of the oil.
In 1999, the first change was implemented. The plant switched from mineral oil to synthetic oil for the superior demulsification characteristics. Some improvement was seen, but problems continued.
In late 1999, bearing isolators and a sleeveless shaft for more rigidity were installed. Water contamination problems continued until desiccant cartridges were installed. Although the cartridges worked, they lasted only two to three weeks before becoming saturated. After the plant installed an automatic moisture removal unit in August of 2000, the oil samples showed no contamination. Seal failures were reduced from 11 per year in 1998 to five in 2000 and two in 2001. These seal failures are thought to be related to cavitation. The plant achieved a savings ," in rebuild costs alone to just one piece of equipment ," of $23,415 in just one year (2001).
Three steps to improvement
By taking a three-step approach incorporating prevention, detection and correction, chemical facilities can vastly improve the reliability of lubrication ," and that of related equipment.
Step 1: prevention
Prevention of contamination. When specified properly, housing components, including oilers, seals and vents, can be very effective in preventing contamination. For many years, constant-level oilers have been essential in maintaining oil levels. Most of these are vented to the surrounding atmosphere, which can lead to contamination ingression to the housing sump.
By switching to a nonvented oiler, plants can reduce ingression significantly. Bearing housing seals, more often recognized as isolators, have been producing positive results in reducing oil leakage and contamination ingression.
Prevention of degradation. Lubricant life is reduced significantly when exposed to high operating temperatures. The oxidation rate of oil doubles every 18 Degrees F. This can be significant when considering pump operating temperatures are frequently near, or above, 140 Degrees F. By simply lowering the operating temperature of the oil to 122 Degrees F, plants could realize a 50 percent reduction in the rate of oxidation ," doubling the oil's effective life. The most basic methods to reduce (or maintain) oil operating temperatures include:
Using oil with the correct viscosity. Too high or too low of a viscosity will raise the oil's temperature.
Using quality oil. Do not buy cheap oil to save money ," it will end up costing you more.
Using the right amount of oil. Maintain proper oil levels ," too much or not enough will increase the oil's temperature.
Keeping the oil clean. Contaminated oil operates at a higher temperature than clean oil.