TRIBOLOGY: THE SCIENCE OF COMBATTING WEAR By William A Glaeser (Member, STLE), Richard C Erickson (Member, STLE), Keith F Dufrane (Member, STLE) and Jerrold W Kannel Battelle Columbus, Ohio Reprinted with permission of Dr Sheldon R Simon, Manager, Battelle Technical Inputs to Planning Program, Battelle, Columbus, Ohio PART 6 HYDRODYNAMIC LUBRICATION (Continued) Automobile tires A smooth automobile tire in contact with a wet road surface is somewhat analogous to a journal bearing system with an extremely large clearance. The region of the tire just ahead of the contact forms a hydrodynamic wedge. If a sufficient quantity of water accumulates in this region and the speed of the tire is high enough, a hydrodynamic water film may develop with lifts the tire off the road and causes a dramatic loss of traction called hydroplaning. Fortunately, modern treat designs and road grooving methods interrupt the hydrodynamic wedge, and impede the formation of hydrodynamic film pressure. Thrust bearings These bearings are used to support an axial load on a rotating shaft or plate. Aircraft engines, turbine generators, and most machines generally contain at least one thrust bearing. The illustration in Fig. 1 shows the contacting face of a typical tapered pad thrust bearing system. In this drawing, 8 tapered pads and 2 circumferential oil grooves are shown; but other bearings may be flat, have 3 or more pads, and contain fewer or more oil grooves. The hydrodynamic film pressure distribution for one pad from this bearing is shown in Fig. 1. Fig. 1 Oil film pressure distribution in a tapered pad thrust bearing. The range of operating conditions of a thrust bearing can sometimes be extended by supporting the pads on pivots. These are called tilting pad thrust bearings. For low-speed operating conditions i.e. 50 to 700 rpm thrust pads can be mounted on a series of preloaded springs. These spring-mounted thrust pad bearings are used in applications where they can be immersed in lubricant to simplify lubricant delivery and cooling, e.g. in very large machinery such as hydroelectric power generation turbines.
With proper lubrication, both the centrally pivot pad and spring-mounted thrust pad can be designed to carry high loads and perform over a wide range of operating conditions, including both forward and reverse rotation. Oil impregnated porous bearings Produced by powder metallurgy, pores of the bearing are impregnated with lubricating oil prior to installation. Their porosity serves as a built-in wick. For example, when the journal begins to rotate, oil is drawn out of the pores, passed through the hydrodynamic wedge, and returned to the bearing walls by hydrodynamic pressure and capillary forces, where it is filtered and heat is rejected. This cyclic action offers a low cost, low maintenance bearing system for such equipment as sub fractional horsepower motors used, for example, in light duty exhaust fans. Gas lubricated bearings The use of a gas as a lubricant is sometimes referred to as aerodynamic lubrication. Gas bearings represent a special kind of hydrodynamic lubrication that offers three major advantages: The viscosity of compressed gas is less sensitive than liquids to temperature extremes. The viscosity of many gases actually increases with temperature, enabling gas bearings to perform well in extremely hot o cold environments. The lower viscosity of compressed gases yields very low frictional resistance, and hence, low power loss at very high speeds and high temperatures. The inertness of several common gases reduces the danger of chemical attack by the lubricant. However, a major cost disadvantage is incurred when gas bearings are used in applications which can also use oil lubricated components. The tight clearance required to form a hydrodynamic wedge for such a low viscosity substance requires extremely close tolerances, and therefore, high manufacturing costs. Consequently, the use of gas bearings is restricted to applications where less expensive alternatives won t function adequately e.g. long life gyroscopes. Foil bearings Foil bearings are relatively inexpensive gas bearings which use a thin metal foil as the load bearing element. The foil conforms to the shaft and allows very small bearing clearances. Piston rings Piston rings are flexible components which slide along the bore of an engine cylinder to seal combustion gases inside the chamber and guide the piston through its stroke. As the piston travels through the cylinder the ring tilts as shown in Fig. 2. This tilting action creates a wedge for accumulating a thin layer of oil between the ring surface and the cylinder wall, thus generating hydrodynamic life. Lifting protects the cylinder and the piston ring from wear, and significantly reduces friction through most of the stroke. Modern design techniques and materials selection have further reduced piston ring and cylinder liner wear, and therefore, have increased engine life to within acceptable limits for all but the most severe service conditions.
Fig; 2 Piston and ring in the bore of an engine cylinder. Benefits and limitations of hydrodynamic lubrication systems Hydrodynamic bearing systems have specific advantages and disadvantages, especially when compared with elastohydrodynamic devices, e.g. rolling element bearings. What hydrodynamic systems offer compared to rolling element bearings Low noise. Since an oil film separates the moving components in a hydrodynamic bearing, very little noise is produced. This factor is important to the saleability of appliances and other devices intended for commercial, residential, and military markets. Rolling element bearings, on the other hand, because they consist of several balls or rollers that can vibrate, often create unacceptable levels of noise. Size. The advantage of the compact geometry of hydrodynamic bearings is illustrated in Fig. 3. Rolling element bearings require greater space to house the rolling elements and races. Conformability. The ability of hydrodynamic bearing materials to conform to minor misalignments resulting either from assembly or changes during service provides a favourable degree of forgivingness to the components e.g., main bearings in diesel engines. Rolling element bearings are available which provide self-aligning characteristics, though other performance capabilities may be limited and costs increased. Embeddablity. Embeddability which often accompanies conformability describes the ease by which foreign particles may embed below the bearing surface in the soft overlay, thus reducing abrasive damage to the mating surface. Since rolling element bearing steels are too hard to provide embeddability, they must be protected from contaminants through the use of shields or filtered lubricants. Fig: 3 Six comparison between a hydrodynamic bearing and a ball bearing. Long Life. During extended operation under monotonic loading, rolling element bearings will often fail due to contact fatigue. However, hydrodynamic bearings are not subject to contact fatigue under monotonic loading conditions. Consequently, extremely long service life can be expected from well-designed and maintained hydrodynamic bearings.
Journal bearings subjected to cyclic loading as in compressors, out of balance shafting, or internal combustion engines are subject to fatigue. Shock load resistance. Hydrodynamic oil films can adjust their thickness and pressure distribution in response to shock loading. Therefore, hydrodynamic bearings can generally withstand shock loading for extended periods without failure. The bearings in a reciprocating engine are constantly subjected to repeated cyclic loads which are transmitted through an oil film. Since the components of a rolling element bearing are in closer contact with each other, shock loads must be absorbed by the elasticity of the bearing material. Such loadings can, therefore, significantly accelerate bearing deterioration. Limitations of hydrodynamic bearings Limited low speed capability. HDL films break down at low operating speeds. This leads to the onset of mixed hydrodynamic and boundary film lubrication in which somewhat higher friction and wear rates prevail. Consequently, rolling element bearings or bearings designed for boundary lubrication perform best in applications involving slow speed operation. Lubricant maintenance. The performance characteristics of HDL components are dependent on the condition of the lubricant employed. Lubricant maintenance recommendations must be carefully observed to avoid loss of adequate oil supply or viscosity, chemical changes or excessive contamination which affect performance. In certain applications, rolling element bearings often require less frequent or less extensive lubricant maintenance for comparable performance. Costs Initial costs. The cost of a complete hydrodynamic bearing system depends, of course, on the requirements of each application. Among the factors to be considered are: Lubricant delivery and filtration system Bearing material, geometry, and surface finish Precision requirements Cooling Lubricant delivery and filtration system. As it has been shown in Part 5, hydrodynamic systems can be lubricated by several techniques some rather simple and others more complex. Choosing the most appropriate approach depends mainly on the operating requirements and environment of each application. Bearing material, geometry, and surface finish. The type of material selected will have a direct impact on costs. While some materials are expensive e.g. tin, Teflon and graphite fibers others are comparatively less expensive e.g., lead and low tin bronze. The need for high tolerances will also boost costs, as will intricate designs and very smooth surface finishes. Precision requirements. In certain applications e.g., in high-speed bearings a high level of precision is often required. Fabrication procedures to achieve a more precise bearing could involve grinding, lapping or burnishing operations and/or individual matching of components, thus creating higher manufacturing costs. From all these basic considerations, initial cost projections can be compiled. Maintenance and replacement costs. The performance of hydrodynamically lubricated components depends on the lubricant used. Thus, to achieve full benefit from the lubricant the system must be constantly maintained to avoid loss of adequate supply oil supply, reduction in viscosity, and to prevent chemical changes that affect performance. Inadequate system monitoring and improper lubricant maintenance are probably the main causes of repair and replacement expenditures. Fluid levels, oil filters, and various other critical parts need to be inspected regularly. If neglected or inadequately checked, interrupted lubricant supply or contamination can lead to wear and eventually to component failure. Thus a situation can be created where failure of one small bearing can shut down an entire operation, resulting in untold financial losses not only in repair or replacement but in production and labor as well.
Editor s note: Further reading on HDL, see: (1) Fuller, D.D., Theory and Practice of Lubrication for Engineers, 2 nd Ed., John Wiley and Sons (1986) (2) (2) Booser, E.R. and Wilcock, D.F., Hydrodynamic Lubrication, Lubr. Eng., 47, 8, pp645-647 (1991)