Mobile Hydraulics - Oils
Oil must serve several functions within the hydraulic system: deliver power, lubricate components, dissipate heat, and carry away contaminates. To perform these functions hydraulic oils contain specific additives to enhance their ability to stand up under the pressure, temperature extremes, and other operating conditions to which they are subjected. To a large extent the life of the hydraulic system is directly tied to the life of the oil. If oil is kept clean and below 140° F the entire system benefits.
Viscosity is the measure of how a fluid resists flowing. Oil viscosity is measured in SSU or Saybolt Seconds Universal. This is sometimes also referred to as SUS. This number represents the amount of time it takes 60 milliliters (about two ounces) of fluid at a given temperature (usually 100° F) to flow through an orifice of a given size (.060"). Oil has a higher viscosity at low temperatures, a lower viscosity at high temperatures. For hydraulic oils, viscosity at startup (cold) should not exceed 7500 SSU. Viscosity at 100° F should be in the 75–200 SSU range. When you look at the specifications for oil, unless otherwise stated, the SSU specified is at 100° F.
Do not attempt to thin oil with kerosene or diesel fuel for winter operation. Instead switch to a lower viscosity oil or add an approved thinning agent designed for that purpose.
Lubricity refers to the ability of oil to maintain a protective film on metal surfaces. Without this oil film, metal to metal contact would create friction, resulting in excessive wear and heat generation. Film thickness is related to viscosity. High viscosity fluids are thicker, forming a thicker film on internal components. Although automatic transmission fluid (ATF) is frequently used as hydraulic fluid, it is actually a poor choice because it loses film strength at high pressures and temperatures. On the positive side ATF has excellent thermal stability.
In truth, there is much more to oil than petroleum. Oils contain additive packages specific to their function. Motor oil, for example, contains high temperature and detergent additives. Quality hydraulic oils must contain high pressure, anti-rust, anti-wear, and anti-foaming agents. All are necessary for the oil to do its job. Base hydraulic oil selection on frequency of use, maximum PSI, climate, and how essential the piece of equipment is.
It is important to remember that these additives are heat sensitive. The ideal operating temperature for hydraulic systems is 100°–140°F. Temperatures over 180° can contribute to oxidation, robbing the oil of its ability to perform. As additives “cook out” they leave behind varnishes, which can cause valves to stick and degrade performance. These oils feel sticky to the touch rather than slick. Heat also affects performance efficiency. As a rule, system efficiency suffers approximately 1% for each 10°F over 130°F. At 180°F that represents a 5% efficiency loss and a two-thirds reduction in the projected useful life of the oil.
One important function of the hydraulic oil is to deliver contaminates to the filter where they can be removed from the system or to the reservoir where they can settle out rather than be held in suspension.
The number one enemy of hydraulic systems is contamination. We use the word “contamination” rather than “dirt” because contamination can take many forms. There is particulate contamination; chemical contamination; and biological, or microbial, contamination. The latter occurs when there is water present in the system in which biological agents can grow.
Chemical contamination includes diesel fuel and kerosene used to thin the oil, water, cleaning chemicals, and liquid calcium chloride. Water is the most common, entering the system through the tank breather as the oil level rises and falls during normal system operation. Pressure washing, if directed toward the reservoir, also introduces water. Oil containing high levels of water has a milky appearance. This is referred to as “emulsified” water. It has been estimated that as little as one percent of water in hydraulic oil can reduce pump bearing life by as much as 90 percent! The presence of water also accelerates the breakdown of the additive package and promotes the formation of acidic byproducts which lead to corrosive wear. Water is also a major contributor to the process of oxidation, the reaction of oxygen to the carbon and hydrogen elements of hydraulic fluids, which results in the formation of sludge and contributes to corrosion.
Particulate contamination includes objects large and small in various concentrations. Examples include silt, sand, welding splatter, rust particles, metal shavings, teflon tape, fibers from rags, bolts, and hand tools. Some particles are large enough to bring a pump to an immediate and violent stop, breaking gear teeth and shearing input shafts. Others, especially in high concentration and under pressure, have the effect of sandblasting the internal parts of the hydraulic components. In either case the end result is increased wear and heat, decreasing system efficiency and component life.
How does contamination enter the system? It may be built-in, induced, ingressed, or internally generated.
Built-in contamination occurs during the manufacturing and assembly procedure and includes welding slag and splatter, dust from storage, paint chips, teflon tape particles, and contaminates from “new” oil. Hint: Store oil drums on their sides and filter new oil as it is put into the reservoir to reduce built-in contamination. Store designated oil handling pans, buckets, and funnels upside down in a dust free cabinet.
Induced contamination occurs when a system is opened for service and dirt is allowed in. Also included is water from pressure washing.
Ingressed contamination is that which is drawn into the system during normal operation, usually via the reservoir breather or through cylinder rod seals.
Internally (wear) generated contaminates consist of wear particles from pumps, cylinders, and hydraulic motors; rubber compounds from hoses and seals; and varnishes from the breakdown of oil additives.
In analyzing contamination two factors are considered: size and concentration. The unit of measure for particle size is the micron (μ). Concentration is measured in the number of particles per milliliter, 1/1000 of a liter. (A milliliter of oil is about the size of a sugar cube.)
So, how large is a micron? The visibility threshold, the smallest object that can be seen with the naked eye, is approximately 40μ. Table salt is 100μ, a human hair has a diameter of 70μ, and a red blood cell is 7μ. Generally, in mobile hydraulics, we employ a 10 micron filter.
Oil sampling analysis is a useful tool for determining both the concentration and composition of contaminates. The report details the type of particles found and the concentration, in parts per milliliter, of 5 and 15 micron size particles. A typical oil analysis report shows the concentration of wear metals (iron, chromium, and aluminum, for example); contaminate metals (sodium and potassium); additive metals (magnesium and calcium); and non-metallic contaminates like water and fuel. These are shown in parts per milliliter and a final score is determined for the overall system cleanliness. This score is based on a cleanliness code developed by the ISO, the International Organization for Standardization. This system assigns a code based on parts per milliliter and establishes minimum cleanliness levels. An improvement in particle contamination of one ISO cleanliness code can result in a 10% to 30% increase in component life. The ISO recommendation for a typical open center gear pump system is 19/17/14. The first number (19) in the code reflects the allowable number of particles of a size equal to or greater than (≥) 4μ. The second number (17) reflects the allowable particle count ≥ 6μ, and the third ≥ 14μ. This translates to a total particle count of no more than 5000/ml of 4μ or larger, 1300/ml of 6μ or larger and 160/ml of 14μ or larger particle size. The 5000/ml would include ALL particles larger than 4μ. In addition, the report, by identifying the specific contaminates helps to isolate the source of the contamination. Oil sample analysis can be an extremely useful tool for prolonging system life.
Another term used in discussing oil and filtration is “beta ratio.” The beta rating of filter is a numerical representation of the filter’s effectiveness in removing particles of a specific size on the first pass. The beta rating number looks something like this: β10 = 20. In this example the filter is allowing one in 20 particles of a 10 micron size to pass. The filter is, therefore, 95% efficient. If the efficiency rating were 4 instead of 20 the filter would be only 75% efficient; one particle in four would pass. If the efficiency rating were 50 the filter would be 98% efficient; one particle in 50 would pass. There are two numbers in the beta rating, the particle size and the particle count.
According to ISO standards, a beta ratio of 75 is considered the “absolute rating.” Any ratio higher than βx=75 cannot be statistically verified.