| Motor Oil Viscosity Explained |
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| Motor Oil Viscosity Explained Out of all the characteristics a lubricant possesses Perhaps the most important is its viscosity. The viscosity of a fluid and how that viscosity reacts to heat will determine how well a fluid can perform the basic functions as a lubricant. What is viscosity? When a fluid is subjected to external forces, it resists flow due to its internal molecular friction. Viscosity is a measure of that internal friction. Viscosity can be referred to as the measurement of a fluids resistance to flow. Viscosity can be viewed in two different ways. The first is a fluids tendency to flow as it is visually seen. One can think of this as the time it takes to watch a fluid pour out of a container. The term for this kinematic viscosity is centistokes (cst). Another is a Sabolt Universal Second (SUS or SSU). When comparing the viscosity of two different fluids, keep in mind that centistokes and Sabolt Universal Second are different units, and therefore they can't be directly compared. Te temperature of the fluid being tested will also affect the outcome. Kinematic viscosity of a lubricant is determined at 100C (212F) and/or 40C (104F) depending on the grading system being used. For general comparison purposes, the temperature at which the viscosity was determined for the two products must be the same. The second is a fluids tendency to flow as is indicated by measured resistance. You can think of this as the energy required to move an object though a fluid. It takes little energy to stir water with a spoon. However, significantly more energy is required to stir honey with that same spoon. The term for this is dynamic viscosity or absolute viscosity and is expressed in units known as centipoises (cp) Whether it is reported in centistokes, SUS or Centipoises units, the higher the number assigned, the more viscous or thicker the fluid is, and a fluids viscosity or thickness is directly proportional to its internal friction and resistance to flow. There are other ways to refer to a fluids viscosity. Some of the more common terms are thin, light or low. These terms suggest how a relatively free-flowing fluid such as water flows. Terms such as thick, heavy of high suggest that the fluid demonstrates a strong resistance to flow. Honey is a good example of a fluid with a high viscosity. A fluids viscosity is important because it is directly related to its load carrying capabilities. The greater a fluids viscosity, the greater the loads it can withstand. The viscosity of a fluid must be adequate to separate moving parts under normal operating conditions (temperature and speed). Knowing that a fluids viscosity is directly related to its ability to carry a load, one would think that the more viscous a fluid, the better it is. The fact is, the use of a high-viscosity fluid can be just as detrimental as using too light an oil. Too low (thin or light) = Metel-to-metal Contact (friction and wear), poor sealing and increased oil consumption. Too high 9thick or heavy) = Increased fluid friction, reduced energy efficiency, higher operating temperature and equipment starting difficulties particularly at cold temperatures. The key is to select a fluid that is not too light and not too heavy. Fluids (lubricant stocks) thicken as they are cooled. As their temperature continues to decrease, they will eventually reach a point at which they become no longer fluid. As they thicken, their load-carrying ability increases, but their ability to be circulated becomes significantly impaired. As fluids are heated, they thin, which reduces their ability to prevent metal-to-metal contact. Therefore,it is important that equal temperatures be used when discussing or comparing viscosity of fluids. Single Viscosity Grade Fluids SAE Single-grade (sometimes referred to as straight grade or mono-grade) oils indicate their viscosity performance at either high or low temperature. They are designed for use when operating temperatures are relatively constant. Examples of high temperature, single grade motor oils would be SAE 30, 40, or 50 and SAE 80, 90, or 140 for gear lubricants. Examples of single grade oils with low temperature performance are SAE 5W, 10W, or 15W for motor oils and 75W, 80W, and 85W for gear lubricants. The presence of a W in oils grade (such as 10W or 80W) indicates low temperature performance. Anytime you see the W, think of it as standing for winter or cold. Multi-Viscosity Grade Fluids Multi-grade oils are concerned with performance at both high and low temperatures. They are designed for use when operating temperatures vary significantly. For example, if a trucker leaves Florida with a load of oranges bound for Minnesota, the trucks engine may have SAE 30 single-grade oil in it, which is appropriate for the ambient temperatures in Florida. Upon reaching the subzero temperatures of Minnesota, the driver finds that the SAE 30 is too viscous to allow for starting of the vehicle. By draining the SAE 30 and installing an SAE 15W, he finds his starting problems are resolved and he continues his work. However, returning to Florida causes the SAE 15W to thin to the point that engine damage occurs. This could be avoided by changing back to higher viscosity oil prior to a return to the warmer climate. The need for such frequent oil changes is not only inconvenient, it raises the cost of operation. Multi-grade oils allow for operation in changing climates without the need for changing the engine oil, Typically multi-grade engine oils are 5W-30, 10W-30, 15W-40 and 20W-50. Typical multi-grade gear lubricants are 75W-90, 80W-90, and 85W- 140. How are multi-grade (multi-viscosity oils made? Some exist naturally. For example, many synthetic base stocks have sufficient viscosity stability (index) to qualify as multi-grade oils. For mineral base oils, it is another story. They are created by first selecting a base stock that possesses the desired low-temperature properties. To that stock, chemical compounds known as viscosity index improver's are added. These viscosity index improver's minimise the thinning of the oil as temperatures increase. This allows the lighter base stock to now also meet the higher temperature requirements. Multi-Grade oils offer the best of both worlds. They provide low temperature fluidity as well as the high temperature stability required for operation in a changing environment. When selecting a lubricant, there are three basic rules of thumb to consider:
There is no advantage in using heavier oil than is needed. In fact, it can be a disadvantage. Remember, Too low (thin or light) = Metal-to-metal contact (friction and wear), poor sealing and increased oil consumption. Too high (thick or heavy) = Increased fluid friction, reduced energy efficiency, higher operating temperatures and equipment starting difficulties particularly at cold temperatures. |