Oil Bath / Spalsh Oil Lubrication
Decisions in connection with the selection of lubricants must parallel decisions in connection with the supply of the lubricant to the bearing for maintaining conditions that will prevent rapid deterioration of the lubricant and bearing. An oil sump applicable to horizontal, inclined, and vertical axis arrangements provides a small pool of oil contained in contact with the bearing, as shown in the below picture.
The liquid level in the stationary condition is arranged to just reach the lower portion of the rolling elements. Experience has shown that higher levels lead to excessive lubricant churning and resultant excessive temperature. This churning in turn can cause premature lubricant oxidation and subsequent bearing failure. Lower liquid levels threaten oil starvation at operating speeds where windage can redistribute the oil and cut off communication with the working surfaces. Maintenance of proper oil level is thus very important and the provision of a sight glass is recommended.
Oil bath systems are used at low-to-moderate speeds where grease is ruled out by short relubrication intervals, hot environments, or where purging of grease could cause problems. Heat dissipation is somewhat better than for a greased bearing due to fluid circulation, offering improved performance under conditions of heavy load where contact friction losses are greater than the lubricant churning losses. This method is often used when conditions warrant a specially formulated oil, not available in a grease. A cooling coil is sometimes used to extend the applicable temperature range of the oil bath. This usually takes the form of a water-circulating loop or, in some applications, the fitting of one or more heat pipes.
Wick-feed and oil-ring methods of raising oil from a sump to feed the bearing are not generally used with rolling bearings, but occasionally, shaft motion is used to drive a viscous pump for oil elevation, thus reducing the sensitivity of the system to the oil level. A disk dipping into the sump drags oil up a narrow groove in the housing to a scraper blade or stop that deflects the oil to a drilled passage leading to the bearing. A major limitation of all sump systems is the lack of filtration or debris entrapment. Fitting a magnetic drain plug is advantageous for controlling ferrous particles, but otherwise, sump systems are only suitable for clean conditions.
Circulating Oil Lubrication
As the speeds and loads on a bearing are increased, the need for deliberate means of cooling also increases. The simple use of a reservoir and a pump to supply a lubricant flow increases the heat dissipation capabilities significantly. Pressure feed permits the introduction of appropriate heat exchange arrangements. Not only can excess heat be removed, but also heat can be added to ensure flow under extremely cold start-ups. Some systems are equipped
with thermo-statically controlled led valves to keep the oil in an optimum viscosity range.
Another equally important point is that a circulating system can be fitted with a filtration system to remove the inevitable wear particles and extraneous debris. Finer filtration is introduced to existing circulating systems with beneficial effects; however, increased pressure drops, space, weight, cost, and reliability have to be considered.
Circulating systems are used exclusively in critical high-performance applications, of which the main shaft support bearings of an aircraft gas turbine engine constitute prime examples. Subjected to heavy thrusts at near limiting speeds, the angular-contact ball bearings generate considerable frictional heat. This heat must be removed effectively together with leakage heat conducted to the bearing cavity from the surrounding engine components.
Heavily loaded bearings running at moderate speeds can be supplied with oil jets aimed at the rolling elements. At higher speeds, bearing windage deflects the jets, and lubrication and cooling become ineffective. This problem can be avoided by routing the oil to pickup scoops on the shaft with centrifugal force taking the oil via drilled passages to the inner ring, as shown in the below figure. Much of the flow passes through axial slots in the bore of the inner ring, removing heat as it does so. Only a small portion of the lubricant is metered to the rolling contacts through grooves between the inner ring halves. Separate drilled holes may be used to supply the cage lands.
Adequate space should be provided on both sides of the bearing to facilitate lubricant drainage. Often, space is at a premium, so a system of baffles can be substituted to shield the lubricant from the windage, permitting it to be scavenged without severe churning. When the lubricant pump is activated at the same time as the main machinery, these baffles act as a dam and retain a small pool of lubricant in the bottom of the bearing to provide lubrication at start-up until the circulating flow becomes established.
Hydrocarbon-based fluids are satisfactory for circulating lubricant systems operating at temperatures of about 274° C (525° F). Hydrocarbon oxidation starts at room temperature; oxidation becomes significant at 175° C (347° F). Incipient thermal decomposition starts at about 300° C (572° F) and becomes a significant problem at about 449° C (840° F). The use of an inert cover gas to exclude oxygen can extend the working range to the latter limit. Beyond 449° C (840° F), fluorocarbon-based fluids are serviceable, but conspicuously lack the lubrication properties of hydrocarbon-based fluids, and have superior oxidation stability. Up to this time, they have not been able to reach the temperature limits inherent in the tool steels used in aircraft gas turbine-bearing applications.
Air-oil / Oil-mist /Oil-spot Lubrication
A separate class of lubrication arrangements can be used when minimal bearing friction is essential at moderate-to-high speeds and where loads are sufficiently low so that heat removal is not a major concern. The lubricant is delivered to the bearing as a fine spray or an air-entrained mist in just sufficient quantities to maintain the necessary lubricant films in the contacts. Lubricant churning is virtually eliminated, and the volume of lubricant is so small that it can be discarded after a single passage through the bearing. Scavenging, cooling, and storage facilities are unnecessary. The one-time exposure to high shear stresses and temperatures relaxes the oxidation and stability requirements of the fluid to some extent. The necessity for satisfactory air quality in the workplace requires that the exhaust droplets be reclassified and lubricant collected before discharge. It has also been shown that the spray does not need to be continuous. Trace injection of minute quantities of lubricant at intervals of up to 1 h is sufficient to keep precision spindle assemblies running at friction torque levels unobtainable by any other method.
