The primary requirement for hydrodynamic lubrication (oil wedge) is that oil of correct viscosity and sufficient quantity be present at all times to flood the clearance spaces.
The oil wedge formed in a hydrodynamic bearing is a function of speed (RPM), load (cylinder pressure), and oil viscosity (at operating temperature). Under fluid film conditions, an increase in viscosity or speed increases the oil film thickness and the coefficient of friction, while an increase in load decreases them. The separate consideration of these effects presents a complex picture that is simplified by combining viscosity Z, speed N, and unit load P, into a single dimensionless factor called the ZN/P factor. Although no simple equation can be offered that expresses the coefficient of friction in terms of ZN/P, the relationship can be shown by a curve such as that in figure 8-15. A similar type curve could be developed experimentally for any fluid film bearing.
In figure 8.15, in the zone to the left of c, fluid film lubrication exists. To the left of a, boundary lubrication exists. In this latter zone, conditions are such that a full fluid film cannot be formed, some metallic friction and wear commonly occur, and very high coefficients of friction may be reached.
The portion of the curve between points a and c is a mixed film zone including the minimum value of f corresponding to the ZN/P value indicated by b. From the point of view of low friction, it would be desirable to operate with ZN/P between b and c, but in this zone any slight disturbance such as momentary shock load or reduction in speed might result in film rupture. Consequently, good practice is to design with a reasonable factor of safety so that the operating value of ZN/P is in the zone to the right of c. The ratio of the operating ZN/P to the value of ZN/P for the minimum coefficient of friction (point b) is called the bearing safety factor. Common practice is to use a bearing safety factor on the order of 5.
In an operating bearing, if it becomes necessary to increase speed, ZN/P will increase and it may be necessary to decrease oil viscosity to keep ZN/P and the coefficient of friction in the design range. An increase in load will result in a decrease in ZN/P, and it may be necessary to increase the oil viscosity to keep ZN/P and the coefficient of friction in the design range.
Film thickness can be related to ZN/P in the manner shown in Figure 8.16. In general, film thickness increases if ZN/P is increased — for example, if the load is reduced while the oil viscosity and journal speed remain constant. With a proper bearing safety factor, the film thickness will be such that normal variation in speed, load and oil viscosity will not result in the reduction of film thickness to the point at which metal-to-metal contact will occur.
It is important to note that oil viscosity changes with temperature. Automotive oil viscosity is commonly measured in terms of Centistokes. The “dynamic viscosity” of an oil is a measure of the internal friction of a fluid, and it is typically recorded at two temperatures (100 degrees F and 212 degrees F) in accordance with the ASTM. The viscosity of an oil is recorded at a low and at a high temperature in Centistokes. Because oil loses viscosity as temperature increases, the “trend” of an oil can be plotted from the low and high temperature Centistoke measurements. These measurements are referred to as the Kinematic viscosities. The operating oil temperature of an engine or gear set will determine which viscosity oil to use.
For example an NHRA Pro Stock engine makes over 1,400 HP and uses a 0W-5 weight engine oil. A NASCAR Nextel Cup Engine only makes 850 hp but uses a 5W-20 weight oil. How can a more powerful drag motor use a lighter weight oil? The NHRA Pro Stock motor runs at a cool 100 degrees F. A NASCAR Nextel Cup engine runs around 220 degrees F. Our Driven XP0 racing oil is 11.5 Centistokes at 100 degrees F, and the XP0 oil is popular with the NHRA drag racers. Joe Gibbs Racing uses the XP1 in our un-restricted NASCAR Nextel Cup engines, and the XP1 is 9.5 Centistokes at 212 degrees F. As you can see, the operating viscosity of the 5W-20 XP1 oil is lighter than the operating viscosity of the 0W-5 XP0 oil due to the difference in operating temperature.
All material referenced from Lubrication Fundamentals, Second Edition, By D.M. Pirro and A.A. Wessol, Published By Marcel Dekker, Inc., Copyright 2001 Exxon Mobil Corporation