Lubrication Theory and Practice
Suspension of Contaminants
The basic functions of a lubricant are friction reduction, heat removal and suspension of contaminants. Designing a lubricant to perform these functions is a complex task, involving a careful balance of properties both in the base oil and the performance enhancing additives. (Lubrizol)
Simply stated, friction is reduced by maintaining a film of lubricant between surfaces that are moving with respect to each other, thereby preventing the surfaces from coming into contact and subsequently causing surface damage.
Friction is a common element in daily life. One can walk up a steep ramp without slipping back because of high friction between shoe soles and the ramp surface. One can slide down a ski run because friction between packed snow and skis is low. Both cases illustrate friction between ordinary surfaces.
The coefficient is roughly constant for any pair of surfaces. For nonlubricated metal of ordinary surface finish and cleanliness, exposed to the atmosphere, the value may be about 1. For the same metal contaminated by handling, the value will drop to about 0.3 to 0.1. For well-designed and well-lubricated systems, the coefficient may be as low as 0.005. Under very special conditions, values as low as 0.000005 have been attained. By contrast, the coefficient for clean metal surfaces in a vacuum may be as
high as 100 to 200 or more, and cold welding due to adhesion can occur.
Lubrication is of two general types based on the operating environment; that is, load and speed of the equipment and viscosity of the lubricant. Smooth surfaces separated by a layer of lubricant do not come into contact and, hence, do not contribute to frictional forces. This condition is called hydrodynamic lubrication. Boundary lubrication, on the other hand, arises when there is intermittent contact between surfaces, resulting in significant frictional forces. (Lubrizol)
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Keeping a liquid film intact between surfaces moving with respect to each other is generally done mechanically, as by pumping. In a cylindrical journal and bearing, the rotary shaft acts as a pump to maintain the lubricant film. The journal floats on a film of oil with an equilibrium thickness established between oil input and oil leakage (mostly at the bearing ends). The equilibrium thickness of the oil film can be altered by:
- Increasing load, which squeezes out oil;
- Increasing temperature, causing more oil leakage;
- Changing to a lower viscosity oil, which also causes more oil leakage;
- Reducing journal speed, which generates a thinner oil film.
Lubrication of a journal rotating in a cylindrical bearing offers the classic example of the hydrodynamic theory of bearing friction, as described by Osborne Reynolds in 1886. The theory assumes that under these conditions, friction occurs only within the fluid film, and is a function of fluid viscosity. (Lubrizol)
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As pressure or load increases, viscosity of the oil also increases. As the lubricant is carried into the convergent zone approaching the contact area, the two surfaces deform elastically due to lubricant pressure. In the contact zone, the hydrodynamic pressure developed in the lubricant causes a further increase in viscosity that is sufficient to separate the surfaces at the leading edge of the contact area. Because of this high viscosity and the short time required to carry the lubricant through the contact area, the lubricant cannot escape, and the surfaces will remain separated. Load has little effect on film thickness because at the pressures involved, the oil film is actually more rigid than the metal surfaces. Therefore, the main effect of a load increase is to deform the metal surfaces and increase the contact area, rather than decrease the film thickness. (Lubrizol)
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The simple assumptions made in discussing fluid film lubrication are hardly ever valid in practice. Under certain conditions such as shock loading, steady heavy load, high temperature, slow speed, and critically low viscosity, the lubricant system no longer remains in the hydrodynamic regime. A situation arises wherein there is intermittent contact between the surfaces, resulting in a significant rise in temperature and subsequent destruction of the contacting surfaces. Under these circumstances, the fluid film is no longer capable of adequately protecting the surfaces, and other approaches must be employed such as adding film-forming additives. (Lubrizol)
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Viscosity is one of the most important properties of a lubricating oil. It is one factor responsible for the formation of lubricating films under both thick and thin film conditions. Viscosity affects heat generation in bearings, cylinders and gears due to internal fluid friction. It affects the sealing properties of oils and the rate of oil consumption. It determines the ease with which machines can be started at various temperatures, particularly cold temperatures. The satisfactory operation of any given piece of equipment depends on using an oil with the proper viscosity at the expected operating conditions.
The basic concept of viscosity is where a plate is being drawn at uniform speed over a film of oil. The oil adheres to both the moving and stationary surfaces. Oil in contact with the moving surface travels at the same velocity, V, as that surface, while oil in contact with the stationary surface is at zero velocity. In between, the oil film can be visualized as many layers, each being drawn by the layer above it at a fraction of velocity V proportional to its distance above the stationary plate. A force F must be applied to the moving plate to overcome the friction between the fluid layers. Since this friction is related to viscosity, the force necessary to move the plate is proportional to viscosity.
Viscosity can be determined by measuring the force required to overcome fluid friction in a film of known dimensions. Viscosity determined in this manner is called dynamic or absolute viscosity. Dynamic viscosity is usually reported in poise (P) or centipoise (cP, where 1 cP = 0.01 P), or in SI units as pascal-seconds (Pa-s, where 1 Pa-s = 10 P). Dynamic viscosity, which is a function of only the internal friction of a fluid, is the quantity used most frequently in bearing design and oil flow calculations.
Because it is more convenient to measure viscosity in a manner such that the measurement is affected by oil density, kinematic viscosity is normally used to characterize lubricants. Kinematic viscosity of a fluid equals its dynamic viscosity divided by its density, both measured at the same temperature and in consistent units. The most common units for reporting kinematic viscosity are stokes (St) or centistokes (cSt, where 1 cSt = 0.01 St), or in SI units as square millimeters per second (mm2/s, where 1 mm2/s = 1 cSt).
Dynamic viscosity in centipoise can be converted to kinematic viscosity in centistokes by dividing by the fluid density in grams per cubic centimeter (g/cm3) at the same temperature. Kinematic viscosity in square millimeters per second can be converted to dynamic viscosity in pascal-seconds by multiplying by the density in grams per cubic centimeter and dividing the result by 1000.
Other viscosity systems, including Saybolt, Redwood, and Engler, have also been used because of their familiarity to many people. The instruments developed to measure viscosity in these systems are rarely used. Most viscosity determinations are made in centistokes and converted to values in other systems.
The viscosity of any fluid changes with temperature, increasing as temperature decreases, and decreasing as temperature rises. Viscosity may also change with a change in shear stress or shear rate. To compare petroleum base oils with respect to viscosity variations with temperature, ASTM Method D 2270 provides a means to calculate a viscosity index (VI). This is an arbitrary number used to characterize the variation of kinematic viscosity of a petroleum product with temperature. The calculation is based on kinematic viscosity measurements at 40 and 100°C. For oils of similar kinematic viscosity, the higher the viscosity index, the smaller the effect of temperature. The benefits of higher VI are:
1. Higher viscosity at high temperature, which results in lower engine oil consumption and less wear.
2. Lower viscosity at low temperature, which for an engine oil may result in better starting capability and lower fuel consumption during warm-up. The measurement of absolute viscosity under realistic conditions has replaced the conventional viscosity index concept in evaluating lubricants under operating conditions.
Another factor in viscosity measurements is the effect of shear stress or shear rate. For certain fluids, referred to as Newtonian fluids, viscosity is independent of shear stress or shear rate. When viscosity is affected by changes in shear stress/shear rate, the fluid is considered non-Newtonian. Kinematic viscosity measurements are made at a low shear rate (100 s-1).
Other methods are available to measure viscosity at shear rates that simulate the lubricant environment under actual operating conditions. Different instruments used to measure kinematic viscosity are:
- Capillary Viscometers: measure the flow rate of a fixed volume of fluid through a small orifice at a controlled temperature. The rate of shear can be varied from near zero to 106 s-1 by changing capillary diameter and applied pressure. Types of capillary viscometers and their mode of operation are:
Glass capillary viscometer: Fluid passes through a fixed-diameter orifice under the influence of gravity. The rate of shear is less than 10 s-1. All kinematic viscosities of automotive fluids are measured by capillary viscometers.
High-Pressure Capillary Viscometer: Applied gas pressure forces a fixed volume of fluid through a small-diameter glass. This technique is commonly used to simulate the viscosity of motor oils in operating crankshaft bearings. The viscosity is called high-temperature high-shear (HTHS) viscosity and is measured at 150°C and 106 s-1. HTHS viscosity is also measured by the tapered bearing simulator. Rotary viscometers use the torque on a rotating shaft to measure a fluid's resistance to flow. The Cold Cranking Simulator (CCS), Mini-Rotary Viscometer (MRV), Brookfield Viscometer and Tapered Bearing Simulator (TBS) are all rotary viscometers. Rate of shear can be changed by changing rotor dimensions, the gap between rotor and stator wall, and the speed of rotation.
- Cold Cranking Simulator: The CCS measures an apparent viscosity in the range of 500 to 200,000 cP. Shear rate ranges with engine cranking data at low temperatures. The SAE J300 viscosity classification specifies the low-temperature viscometric performance of motor oils by CCS limits and MRV requirements.
- Mini-Rotary Viscometer (ASTM D 4684): The MRV test, which is related to the mechanism of pumpability, is a low shear rate measurement. Slow sample cooling rate is the method's key feature. A sample is pretreated to have a specified thermal history which includes warming, slow cooling, and soaking cycles. The MRV measures an apparent yield stress, which, if greater than a threshold value, indicates a potential air-binding pumping failure problem. Above a certain viscosity (currently defined as 60,000 cP by SAE J 300), the oil may be subject to pumpability failure by a mechanism called "flow limited" behavior. An SAE 10W oil, for example, is required to have a maximum viscosity of 60,000 cP at -30°C with no yield stress. This method also measures an apparent viscosity under shear rates of 1 to 50 s-1.
- Brookfield Viscometer: Determines a wide range of viscosities (1 to 105 P) under a low rate of shear (up to 102 s-1). It is used primarily to determine the low-temperature viscosity of automotive gear oils, automatic transmission fluids, torque converter and tractor fluids, and industrial and automotive hydraulic fluids. Test temperature is held constant in the range -5 to -40°C. The Scanning Brookfield technique measures the Brookfield viscosity of a sample as it is cooled at a constant rate of 1°C/hour. Like the MRV, this method is intended to relate to an oil's pumpability at low temperatures. The test reports the gelation point, defined as the temperature at which the sample reaches 30,000 cP. The gelation index is also reported, and is defined as the largest rate of change of viscosity increase from -5°C to the lowest test temperature.
- Tapered Bearing Simulator: This technique also measures high-temperature high-shear rate viscosity of motor oils (see High Pressure Capillary Viscometer). Very high shear rates are obtained by using an extremely small gap between the rotor and stator wall. Physical requirements for both crankcase oils and gear lubricants are defined by SAE J300. (Lubrizol)
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Another important function of a lubricant is to act as a coolant, removing heat generated by either friction or other sources such as combustion or contact with high-temperature substances. In performing this function, the lubricant must remain relatively unchanged. Changes in thermal and oxidative stability will materially decrease a lubricant's efficiency in this regard. Additives are generally employed to solve such problems. (Lubrizol)
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Suspension of Contaminants:
The ability of a lubricant to remain effective in the presence of outside contaminants is quite important. Among these contaminants are water, acidic combustion products, and particulate matter. Additives are generally the answer in minimizing the adverse effects of contaminants. (Lubrizol)
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