EVERYTHING THAT YOU WANTED TO KNOW ABOUT CONTAMINATION BUT WERE AFRAID TO ASK…
ELEMENT TYPES
Surface Elements: Surface elements work by direct interception of particles larger then the pore size of the media. Dirt is trapped on the upstream side of the media with the holding capacity limited by the number of pores in the media. When new, media resistance to flow is small, but as the element builds up dirt, the resistance to flow rapidly increases. The capacity of most surface element media is usually measured in grams.
Depth-Type Elements: These elements work by both direct interception of particles and absorption (molecular attraction) of particles. These elements use several types of media to achieve the goal of holding particles. The fluid must take a longer path through the element before exiting. Normally, these elements have large holding capacities and initially have a higher resistance to flow. However, a drop in pressure occurs as contamination builds up or is trapped.
ELEMENT SYSTEMS
Full-Flow Elements: Designed to filter fast flowing oil, full-flow elements protect downstream components from larger sized particulate matter. These elements typically hold particles from 20 to 40 microns in size. Their ability to pass large volumes of fluid makes full- flow elements unsuitable for smaller particles without becoming large and expensive.
Partial-Flow Elements: Sometimes called kidney loops, these elements are designed to filter small streams of fluid, returning the filtered fluid to the sump of the system. These elements typically hold particles from 1-10 microns. Smaller quantities of flow allow the elements to remove slighter particles without affecting the normal functions of the system.
THE FUNCTIONS OF OIL
Clean: This function is to transport undesirable materials in oil to the element or hold the particles in suspension until the oil is changed. This is the characteristic that keeps internal parts clear of varnish and sludge buildup.
Cool: An important function of oil is to cool the machine parts with which it comes into contact. Some systems require all cooling to be performed by only the oil.
Seal: The film of oil that bridges the gap between two surfaces to keep fluids in while keeping air and dirt out is called the seal. For example, in a hydraulic ram or spinning shaft, the thin film of oil creates a seal that bridges the gap and isolates the two sides.
Lubricate: Lubrication is the function of oil that allows two moving parts to slide past each other with little friction. This film of oil is important in reducing wear and providing ease of movement within a machine.
CLEANLINESS LEVELS
Since fluids can have such small particles in suspension and because maintaining good cleanliness is economically important, the oil industry has developed standards for quantifying particulate buildup. The current standards fall under ISO 4406c. These standards are used by manufacturers and engineers to determine how clean fluid should be to provide acceptable performance and service life. The chart shown details the level of particles of a given size in a milliliter of fluid and the grade or number assigned to indicate the ISO class of the fluid. Hydraulic oils that are used with servo valves should have a 16/14/12 level or better while in service. The first number is class code for 14 microns and greater particles, the middle number indicates the class for 6 microns and greater, while the last number is the class for 10 microns and greater. At this cleanliness level, sticky or binding valves should not pose a problem and oil should be in serviceable condition for an extended time. This method can also be used to evaluate contamination control systems or elements in a system.
New oils: As surprising as it may seem, most oil purchased today has not been filtered before sold. Yes, it is newly refined and looks translucent, but it has many more particles than would be expected in new oil. Typically, new oils being delivered to customers are class 22/18/16 and sometimes worse. This puts the end user in a position of having to filter the oil before it is put into service to assure it will not cause premature wear on the machinery.
THE PARADIGM SHIFT
Particles in the oil film moving within the clearances between the moving parts are called Clearance-Sized Particles or CSPs (refer to Fig. 1). For an engine like a Caterpillar 3516 or 3520, the average film thickness is around 3-5 microns between the piston rings and cylinder walls depending on compression and temperatures. A particle (or dust) small enough to work within the film thickness is a non-wear issue. In contrast, particle sizes large enough to wedge themselves within the film thickness but slightly more prominent (i.e., 4-10 microns) create the most abrasive wear. According to a Society of Automotive Engineers (SAE) International article, " Correlating Lube Oil Filtration Efficiencies With Engine Wear, “The researchers found clearances in the Diesel and Gasoline Engines varied between 2 and 22 Microns during engine operations. That means particles in the 2 to 22 Micron size range will most likely damage engine parts.”
Fig. 1 Moving Parts Clearance Sized Particles