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This case study discusses oil cleanliness standards for the lubricating oils used at Savannah River Site (SRS), a nuclear materials management facility for the U.S. government Department of Energy. An evaluation of the clearances within machine operating components and the consequences of surface wear and particle contamination interfering with the lubrication film between contact surfaces provide the basis for these cleanliness standards. Under normal operating conditions, maintaining these cleanliness standards is essential to eliminating component wear and ensures continuous functionality of the lubricating oil properties.
Job of Lube Oil
The primary function of lubricating oil is to separate surfaces, reduce friction and absorb heat. Secondary responsibilities include regulating temperature, flushing contaminants, controlling corrosion and providing hydromechanical performance. Improper lubricating oil type, contaminated oil, poor equipment operation, poor maintenance or poor component manufacturing reduce the ability of the lubricating oil to function. These factors can disrupt the hydrodynamic or elastohydrodynamic (EHD) lubrication film between the metal surfaces leading to premature wear of the metal surfaces and high overall operating costs.
When metal-to-metal or particle-to-metal contact exists because of a loss or interference in the lubricating film, adhesive and abrasive wear occurs. This generates more friction, heat and wear particles that further contaminate the oil. Even under ideal conditions of manufacturing, operation and maintenance, other contaminants, such as dirt and moisture, can get into lubricating oil. If dirt or silica particles are large enough they can cause interferences between metal contact surfaces. Moisture, on the other hand, breaks down viscosity and alters the chemical properties of the oil. Analysis of lubricating oil will identify the source of a contaminant, whether the chemical properties of the oil are intact and if machine wear is occurring. It is important to be able to relate results of a lubrication analysis to the oil chemical properties, the various types of metals used in manufacturing the rotating elements, and the operating conditions of a machine. Controlling oil cleanliness minimizes the effects solid particle contamination can have on interfering with the lubricating oil film. It also maintains separation of the metal surfaces.
Origin of Contamination
In addition to wear debris and dirt (silica), solid particulate contamination in oil can come from seals, gasket materials and scale products in reservoirs and oil storage containers. Regardless of the solid contaminant type, when the size of the particulate is greater than the clearance between metal rotating surfaces, abrasion and fretting of the metal surfaces occurs. Once abrasion or fretting starts, the lubricant functions are adversely affected and additional surface damage will result. The continuous contact between particulate and metal generates additional wear debris and larger particulate. To quantify the amount and size of solid particulate contamination in oil, the International Organization for Standardization (ISO) has developed standard ISO 4406:1999. This standard, shown in Table 1, provides the method for coding oil cleanliness based on the solid particulate micron size and the amount of that size particulate present in the oil. Oil cleanliness standards established by ISO in 1999 correlate to the identification of solid particulate measuring 4, 6 and 14 microns in size (represented as R4/R6/R14) according to the quantity of these solid particles found per each milliliter of oil.
Table 1. Allocation of Scale Numbers
The ISO particulate size classification R4/R6/R14 can be compared to tolerances (fits) or clearances between machined components. Methods for calculating the running and sliding fits for machined components are established by the American Standards Association (ASA) B4.1.1 Table 2 provides the typical clearances in microns for various shaft size and shaft/housing combinations and for various machine fit classifications.
Table 2. Class of Fit, Micron Clearances;
Running Class (RC) Fits RC4 to RC8 (ref 2)
Most industrial plant equipment is manufactured to either a Class 2 or Class 3 fit. Precision rotating components such as rolling element bearings are manufactured to closer clearances between contacting metal surfaces.
Table 3. Bearing Component Micron Clearances
Table 3 provides the normal static unmounted manufacturer internal clearances for a typical deep groove ball bearing and a polyoxymethylene (POM) composite journal bearing for the same shaft sizes as shown in Table 2. (It should be noted that internal rolling element clearances will vary between the type of rolling element bearing such as a deep groove ball bearing and a spherical roller bearing. Typically the internal clearances for other types of rolling element bearings will be greater than the values shown in Table 2 for the deep groove ball bearing. It should also be noted that a POM composite bearing consists of materials such as tin, bronze, steel and copper components that will start to show up in lubricating oil should friction occur between rotating surfaces. Bearing manufacturers offer more precision-made bearings with closer internal clearances as well as bearings with greater-than-normal internal clearance. Care should be taken to ensure the correct bearing is selected for a given application and procedures are adhered to for proper installation.)
Although the clearances shown in Tables 2 and 3 appear to provide sufficient separation of metal surfaces for particles in a R4/R6/R14 size classification, when rolling element bearings are installed properly, the clearance is reduced to approximately half the values shown in Table 3. For example, a one-inch rolling element bearing installed on a Class 2 or Class 3 machined one-inch shaft will have an operating or running clearance between 2.5 and 10 microns when installed properly. If the installed one-inch bearing is lubricated with oil containing solid contamination of any source that is equal to or greater than 2.5 microns, abrasive or fretting wear may occur. An operating or running clearance would typically be between these values except for the highly loaded areas of the rotating components. A significant amount of particulate contamination in lubricating oil that is greater than the clearance between contact surfaces will generate noise and vibration, and raise operating temperature due to the heat caused by increased friction. It is therefore important to have good maintenance, good operating practices, trained mechanics and oil free of solid particulate contamination.
Other types of oil-lubricated components such as gears, hydraulic motors, hydraulic pumps and compressors are equally important when considering oil cleanliness and the effect solid particulate contamination can have on the rotating metal surfaces. Enclosed helical, herringbone, bevel, spur and worm gears are manufactured to clearance ranges between 0 to 203 microns. Because of the extreme contact pressures and slower operating speeds of gear rotating drives, higher viscosity lubricating oils with extreme pressure additives are used. Unit contact pressures between particle contaminant and metal surfaces in gears and bearings can exceed 500,000 psi, causing significant surface wear. Hydraulic components including actuators, pumps and motors have tolerances as close as 5 microns for high-pressure components and 20 microns for low-pressure components. Rotary screw compressors operate with rotor end clearances up to 102 microns and rotor-to-rotor clearances up to 76 microns.
The clearances between rotating surfaces for each equipment type and the operating conditions determine the maximum particulate size that can be present in the oil before abrasion or fretting occurs due to particulate interference. Even when clearances are understood for the primary equipment components such as gears or rotors, the same system oil may be lubricating the shaft bearings and seals. One must consider all equipment components in establishing an oil cleanliness code that correlates to smallest clearance maintained between contacting surfaces. Once cleanliness codes are established, oil samples should be taken periodically from operating equipment and storage containers to determine if contamination is present. When contamination is found, the origin or source can be determined by identifying the particle material and type of wear that is occurring.
The majority of rotating equipment at SRS is manufactured to a Class 2 or Class 3 fit typical of most industrial operations. Exceptions to this have been identified in hydraulic components and rotary screw compressors where closer tolerances exist between sliding and rotating elements. Rolling element and journal bearings can be found in all rotating equipment at SRS. Clearances in hydraulic components, rotary screw compressors and bearings are used to establish an acceptable oil cleanliness that minimizes chances of surface wear. Table 4 provides the recommended cleanliness codes for all types and equipment configurations at SRS.
Table 4. SRS Oil Cleanliness Standards
These cleanliness code standards should be used for both new oil shipped to the site and for establishing contamination alarm levels on machines. The allowable level of particulate for each of the cleanliness codes is quantified according to Table 1. Periodic oil samples should be taken to determine if the oil particulate is within or below these cleanliness levels. Anytime the level of particulate concentration exceeds the established cleanliness code, the oil should be filtered to remove contaminants or changed. Removing particulate contamination will prevent surface wear, increase machine reliability and prolong the life of the rotating elements. If contamination is found in new oil, the oil should be returned to the supplier. Table 5 shows typical machines or equipment at SRS where the various ISO viscosity grade oils are used.
The original equipment manufacturer (OEM) for each machine or equipment type will provide recommendations for oils that will conform to one of the ISO oil grade classifications shown in Table 5. Once the ISO oil grade classification is determined, the recommended cleanliness codes can be used for procurement of new oil and for monitoring oil cleanliness in operating equipment. The next higher scale value for each type oil in service and cleanliness code should be used for establishing alarms and for initiation of a process for determining the source of particulate contaminant.