The lube oil in a hydrodynamic journal bearing serves two functions. The primary function is to develop an oil film under the shaft that has a pressure distribution sufficient to lift the weight of the journal (shaft) off the surface of the bearing. The second major function is to take heat away from the oil film, hence, away from the bearing and the journal or shaft. Without the proper flow of oil through the bearing, the oil film itself, the bearing surface and the rotor can have significant temperature increases. There are several consequences of rising temperature. The first consequence is that warmer oil is less viscous and can reduce the thickness of the oil film leading to contact between the rotating journal and the bearing. A second consequence is that with increasing temperatures, the journal may grow in diameter more than the bearing, possibly causing metal to metal contact between the journal and the bearing. A third consequence is that heat, particularly in the presence of water, can chemically alter the oil.
The Effect of Viscosity on Bearing Performance
The stability of a bearing can be described by the Sommerfeld Number and the S-Omega Stability Curve. Dr. Mel Giberson released a Tech Note in June of 2016 which went into the subject in detail. The bearing comes more stable as the oil temperature increases and viscosity decreases. Warm oil in a proper temperature range is desirable. Oil temperatures that are inconsistent or hot are not desirable. Therefore, in order to control the lube oil supply temperatures, some oil systems are equipped with oil coolers to cool the oil when in operation as well as with oil heaters to heat the oil in preparation for operation.
Oil and Water
Water is almost always present in lube oil. The water can be dissolved, emulsified, or free. The amount of water that can be held in the oil is dependent on temperature and pressure. As the oil heats, the water is released as free water.
Water can come from a variety of sources. The oil that comes directly from the supplier usually contains water and particulate matter. The air inside any oil reservoir has moisture which can dissolve into the oil. Other sources include worn seals that permit steam from adjacent steam turbine seals to enter adjacent bearing oil seals.
All forms of water in the oil are problematic, but free water is the most destructive. The viscosity of water is approximately 2.3% of the viscosity of hydraulic oil (0.75 cs vs. 32 cs). Free water can also create sludge that bonds to the Babbitt surface. This sludge can build up enough to change the shape of the bearing surface, and therefore, can reduce the load carrying capacity of the oil film in a journal bearing or a thrust bearing
Controlling the Oil Flow
A properly designed oil system for hydrodynamic bearings preferably uses a positive displacement pump rather than a centrifugal pump. Positive displacement pumps work by moving oil from the inlet of the pump to the outlet via screw pump, a vaned pump, or a piston pump, for example. The pump controls the flow. The oil piping and other system characteristics provide resistance to that flow, and it is this resistance that creates the pressure in the system.
Centrifugal pumps work by having an impeller with vanes that rotates. As the impeller rotates, any liquid inside the impeller will be centrifugally forced to the outside. The flow through the pump is controlled primarily by the physical size of the impeller (OD and ID), the density of the liquid, and the rotational speed, as well as the suction and discharge pressures. Since the mass flow of the lubricant (not the supply pressure) is critical to the proper functionality of the bearing, the preferred pump is a positive displacement pump for a simple reason: Assuming the piping is filled, as soon as a positive displacement pump starts to turn, oil is flowing because the discharge pressure of the pump is immediately developed. In the case of a centrifugal pump, the pump speed must reach nearly full speed before sufficient pressure is developed to move the oil, and this lag time often is sufficient to starve a bearing and cause failure. Again, the pressure in the oil film of a hydrodynamic journal bearing is not created by oil pump. Instead, the pressure distribution in the oil film is created by the presence of a viscous oil, the geometry of the journal bearing, and rotation of the shaft (journal).
Achieving the proper lubricant flow rate through a bearing is critical. If oil stays in the bearing too long, it will overheat. If most of the oil flows in and then right out again, the oil does not have enough time to remove heat from the journal and bearing surfaces. There must be a good balance, and this is achieved by TRI design practices and methods.
For certain bearing designs, particularly tilting pad bearings, the seals at the ends of a bearing provide an important contribution to the oil flow balance through a bearing. Bearings can be prematurely worn when the shaft is not parallel with the bearing bore. This misalignment can either come from a poor job at assembly time or due to not understanding how the machine changes during operation. Some machines have uneven thermal growth that take them out of alignment when they heat up. Others machines have vacuum conditions that pull on the turbine shells and the bearing standards taking them out of alignment.