Journal bearings contain a shaft or journal that rotates freely in a supporting metal shell. There are no rolling elements present in these bearings. The building and designing may be relatively simple, but the theory of these bearings can be complex.
Journal bearings are meant to include sleeve, shell, plain, and babbitt bearings. The babbitt term refers to the layers of softer metal that form the bearing shell’s metal contact surface. These metals overlay strong steel support from the more complex rotating shaft.
In this article, we tend to focus on oil and grease lubricated fluid film journal bearings.
Journal Bearing lubrication
Bearing lubrication plays an essential role in the performance and life of bearings. The essential task of lubricant is to separate parts moving relative to one another to reduce friction. A lubricant that a bearing manufacturing company uses will operate in a specific condition and provide a load-bearing protective film.
The true condition is when this film separates the friction surfaces. Also, providing this load-bearing film, the lubricant allows for the dissolution of heat, and it prevents overheating of the bearing. Also, it protects from corrosion, moisture, and contaminants.
Lubricants used in journal bearing should have the following characteristics:
- Good film strength to support loads
- Non-corrosive and compatible with adjacent components
- Maintains a stable viscosity over a range of temperature
- Stable structure to provide a long service life
- Provides a barrier against moisture and does not leak out
When cooling is required, waste or contaminants require to be flushed away from the bearing; oils are used in a journal bearing. The high-speed bearing is constantly lubricated with oil rather than grease. Oil is supplied to the bearing either by a pressurized oil pump system or an oil ring by bearing and bushes suppliers. Grooves in the bearing shield are used to distribute the oil through the surfaces of bearings.
The viscosity grade need is dependent upon bearing RPM, load, and oil temperature. The bearing speed is frequently measured strictly by the RPM of the shaft. They have no consideration of the surface speed of the shaft.
Typically rust and oxidation prohibited extra systems are used in oils employed in these applications. Antifoam and pour point depressant additives are required to be present. Antiwear hydraulic oils are used as long as the zinc AW component’s temperature limit is not exceeded. Also, when excessive water is not present.
R&O oils tend to have good water separation characteristics. It is beneficial, and hydraulic oil properties would be helpful only during startup and shutdown, assuming a proper operation bearing.
The grease method is used to lubricate journal bearings when the bearing’s cooling is not a factor. Usually, when the bearing operates relatively low speed. Grease is beneficial if shock loading occurs or if the bearing repeatedly starts and stops or reverses direction.
It is almost always used to lubricate pins and bearing bushings. It is because it provides you a thicker lubricant than oil to support static loads. Also to protect against vibration and shock loading are common in many of these applications.
Lithium soap or lithium complex viscosity is the most common thickener used in greases. It is excellent for most journal-bearing applications—the viscosity of grease changes with pressure, load, and speed. Within a rotating journal bearing, as it rotates faster, the apparent viscosity of grease will decrease, and it approaches the thickness of the base oil that is used in grease.
From both ends of the bearing shell, the pressure is lower, and apparently, the viscosity remains higher. It results in broad grease at the bearing ends, acting as a built-in seal to reduce the ingression contaminants.
Shell life of a lubricant
Synthetic oils are steady materials, and they are not anticipated to oxidize or polymerize at room temperature for 10 years. The ester linkage may be a concern to a minute degree of hydrolysis in the presence of moisture that becomes more acidic if it’s present. Greases can age in more complicated ways, and they could be affected by a change in structure. The significant oil would be evident, and it would remain the grease would stiffen. Also, it gets softer over some time.
Journal bearings have a simpler design, and it is not as difficult to lubricate as journal element bearings. The viscosity that is matched to the operating conditions and a clean and dry lubricant will serve to form a full fluid lubricating film that provides you excellent bearing life. There are different things to consider when selecting a lubricant, such as speed, environment, temperature and desired life, and more. Also, aside from those already mentioned, there are various characteristics of greases and oils that are considered a must. Bearing manufacturing companies believe that lubrication is one of the most crucial specifications for manufacturers to consider.