Bearings are essential components in various mechanical systems, providing support and reducing friction between moving parts. Two common types of bearings are journal bearings and thrust bearings. While they share similarities, they possess unique characteristics that make them suitable for specific applications. Let's explore the differences between journal bearings and thrust bearings, including their design features, advantages, and applications.
Journal bearings are primarily used in rotary motion systems to support rotating shafts and minimize friction. They are commonly found in applications such as automotive parts and heavy-duty industrial machines.
Journal bearings are designed with high-grade steel or durable materials like bronze alloys to provide optimal support for turning axles. Depending on the application demands, there are three main types of journal bearings: split bearing, sleeve bearing, and solid bearing.
Each type has its own advantages based on the specific application requirements.
Journal bearings are ideal for slow-speed operations, such as pumps and compressors, where space constraints or environmental concerns make it difficult to install lubrication systems. They are also cost-effective and suitable for use in less critical settings like household appliances. Compared to other bearing technologies, journal bearings require minimal maintenance, reducing downtime and repair costs.
However, regular inspections are still necessary to identify signs of wear, such as loose screws, damaged surfaces, or worn seals. By addressing these issues promptly, users can ensure smooth operation and maintain safety standards.
Thrust bearings, in contrast to journal bearings, are designed to handle axial loads along an axis parallel to its rotation axis. They use two identical rings with grooves that contain ball or roller elements between them. This design allows the inner ring to rotate on its axis relative to the outer ring while carrying heavy loads in one direction.
Thrust bearings are typically made from steel or alloys suitable for high temperatures and wear resistance. Special coatings can be applied to reduce friction and increase their service life. These bearings must have adequate strength and stiffness to handle the load-carrying requirements of the application.
Compared to journal bearings, thrust bearings offer higher rigidity, making them suitable for heavier loading conditions. They also require less frequent maintenance due to their lower operating speeds. Thrust bearings excel in applications where there is limited access for servicing or restricted availability of replacement parts.
Thrust bearings come in various types, including tapered roller and cylindrical type.
Read More: Journal Bearings- An Overview
Journal bearings provide several benefits in many applications. They are known for their high efficiency, superior performance, and low cost.
Journal bearings are highly efficient in reducing friction between moving parts. Their design allows for smooth operation, minimizing energy losses and maximizing the performance of rotating machinery. By reducing friction, journal bearings contribute to increased overall system efficiency.
Journal bearings are known for their durability and long service life. They are designed to withstand heavy loads and operate reliably under demanding conditions. With proper lubrication and maintenance, journal bearings can provide consistent performance over an extended period, reducing the need for frequent replacements.
Compared to other types of bearings, journal bearings are cost-effective. Their simpler design and construction make them more affordable, making them a preferred choice in many applications. Additionally, their long lifespan and low maintenance requirements contribute to overall cost savings in the long run.
Journal bearings offer excellent load capacity, making them suitable for a wide range of applications. They can handle both radial and thrust loads effectively, providing reliable support for rotating shafts. This versatility allows for their use in various industries and machinery types.
One of the significant advantages of journal bearings is their ability to operate in both radial and thrust directions. This flexibility makes them suitable for diverse applications, where the load forces may vary in direction. Journal bearings can efficiently accommodate these dynamic forces, ensuring smooth and reliable operation.
Journal bearings possess good vibration dampening characteristics. They distribute loads evenly over larger contact areas on the shaft's surface, reducing vibrations and minimizing noise generation. This feature is particularly important in applications where noise control is critical, such as medical facilities or music studios.
Thrust bearings have their own advantages when it comes to power transmission and load capacity. They are well-suited for applications requiring high loads and precision, such as industrial machinery with powerful rotating components.
Thrust bearings are specifically designed to handle high axial and radial loads. They excel in applications where heavy loads need to be supported with precision. Whether it's in industrial machinery or automotive transmissions, thrust bearings can withstand substantial forces without compromising performance.
Thrust bearings offer excellent power transmission capabilities. They efficiently transfer power from one component to another, ensuring smooth and reliable operation of the system. This makes them ideal for applications that require precise power transfer, such as in precision manufacturing or high-performance machinery.
Thrust bearings provide dampening capabilities against fluctuations caused by external forces. They absorb shocks and vibrations, protecting delicate components within the system. This feature is particularly important in applications where stability and precision are critical, such as in aerospace or robotics.
Due to their cross-axis contact design, thrust bearings minimize friction and improve energy efficiency. With reduced friction, less energy is wasted as heat, resulting in improved overall system efficiency. This energy-saving characteristic is valuable in applications where energy consumption is a concern, leading to cost savings and environmental benefits.
Thrust bearings have fewer moving parts compared to journal bearings, resulting in lower maintenance costs. With a simplified design, there are fewer components that require regular inspection and replacement. This makes thrust bearings a cost-effective choice, particularly in applications where frequent maintenance or part replacements are challenging or costly.
Recommended Reading: An Ultimate Guide on Thrust Bearing
Design considerations for thrust bearings differ from those of journal bearings. Load capacity is a primary consideration, determining if the bearing can handle the expected loads and the space required for installation. The construction of the thrust bearing, along with material compatibility, is also crucial.
Lubrication selection is vital for reducing friction, protecting against wear and corrosion, and ensuring proper functioning. Regular maintenance checks are essential for maintaining thrust bearing performance and safety. Monitoring operating temperatures helps prevent damage caused by high temperatures, and cooling systems may be necessary to maintain acceptable operating levels.
Proper design considerations, including load capacity, lubrication, and maintenance, are key to ensuring the longevity and optimal performance of thrust bearings.
Maintaining thrust bearings requires regular inspections, lubrication, and cleaning. Following a maintenance schedule is crucial to keep the bearing in peak performance condition. Inspections should occur at regular intervals, with increased frequency for bearings in high-temperature environments. During inspections, check for wear, damage, and tighten all bolts and components according to manufacturer specifications.
Proper lubrication and cleaning are vital for optimal thrust bearing operation. Use recommended lubricants and cleaners approved by the manufacturer to prevent problems. Replace worn-out seals or other components as needed. Before installation, perform preventative measures such as applying grease on the outer surfaces and inspect each part closely for defects.
By following maintenance requirements carefully, thrust bearings can maintain their performance and reliability, minimizing unexpected breakdowns or malfunctions.
The cost comparison between journal bearings and thrust bearings involves both the initial investment and long-term costs. Journal bearings tend to have a higher initial cost due to their complex design. However, they require less maintenance over time and have a longer lifespan compared to most thrust bearings.
Thrust bearings are generally cheaper upfront due to their simpler construction. However, they may need to be replaced sooner due to wear and tear from regular use.
When deciding between the two types, it's crucial to consider the total cost difference. Factors such as installation costs, maintenance requirements, and expected lifespan should be taken into account. Depending on your priorities, such as durability or initial investment, you can make an informed decision that best suits your needs.
When it comes to understanding the differences between journal bearings and thrust bearings, it becomes clear that each type has its own unique characteristics and advantages. Proper consideration of factors such as load capacity, operating speed, size availability, and specific application requirements is crucial for making an informed decision.
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A: Journal bearings are primarily used for rotary motion systems and provide support to rotating shafts while reducing friction. On the other hand, thrust bearings are designed to absorb axial loads applied parallel to their rotation axis, making them ideal for linear applications.
A: Thrust bearings are designed to handle axial loads, while journal bearings are used to support rotating shafts and reduce friction.
A: Journal bearings are used in rotary motion systems to provide support for rotating shafts while minimizing friction between them and the surrounding environment. They help reduce wear and ensure smooth operation.
A: Thrust bearings are used to absorb axial loads applied parallel to their rotation axis. They are commonly used in applications where linear force needs to be supported, such as elevators or wind turbines.
A: A thrust bearing is also known as a thrust washer or axial bearing, reflecting its purpose of handling axial loads.
Journal or plain bearings consist of a shaft or journal which rotates freely in a supporting metal sleeve or shell. There are no rolling elements in these bearings. Their design and construction may be relatively simple, but the theory and operation of these bearings can be complex.
This article concentrates on oil and grease-lubricated full fluid film journal bearings; but first a brief discussion of pins and bushings, dry and semilubricated journal bearings, and tilting-pad bearings.
Low-speed pins and bushings are a form of journal bearing in which the shaft or shell generally does not make a full rotation. The partial rotation at low speed, before typically reversing direction, does not allow for the formation of a full fluid film and thus metal-to-metal contact does occur within the bearing. Pins and bushings continually operate in the boundary lubrication regime.
These types of bearings are typically lubricated with an extreme pressure (EP) grease to aid in supporting the load. Solid molybdenum disulfide (moly) is included in the grease to enhance the load-carrying capability of the lubricant.
Many outdoor construction and mining equipment applications incorporate pins and bushings. Consequently, shock loading and water and dirt contamination are often major factors in their lubrication.
Figure 1. Kingsbury Radial
and Thrust Pad Bearing
Dry journal bearings consist of a shaft rotating in a dry sleeve, usually a polymer, which may be blended with solids such as molybdenum, graphite, PTFE or nylon.
These bearings are limited to low-load and low-surface speed applications. Semilubricated journal bearings consist of a shaft rotating in a porous metal sleeve of sintered bronze or aluminum in which lubricating oil is contained within the pores of the porous metal. These bearings are restricted to low loads, low-to-medium velocity and temperatures up to 100°C (210°F).
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Tilting-pad or pivoting-shoe bearings consist of a shaft rotating within a shell made up of curved pads. Each pad is able to pivot independently and align with the curvature of the shaft. A diagram of a tilt-pad bearing is presented in Figure 1.
The advantage of this design is the more accurate alignment of the supporting shell to the rotating shaft and the increase in shaft stability which is obtained.1
Journal bearings are meant to include sleeve, plain, shell and babbitt bearings. The term babbitt actually refers to the layers of softer metals (lead, tin and copper) which form the metal contact surface of the bearing shell. These softer metals overlay a stronger steel support shell and are needed to cushion the shell from the harder rotating shaft.
Simple shell-type journal bearings accept only radial loading, perpendicular to the shaft, generally due to the downward weight or load of the shaft. Thrust or axial loads, along the axis of the shaft, can also be accommodated by journal bearings designed for this purpose. Figure 1 shows a tilt-pad bearing capable of accepting both radial and thrust loads.
Figure 2. Layers of Journal Bearing Structure
Journal bearings operate in the boundary regime (metal-to-metal contact) only during the startup and shutdown of the equipment when the rotational speed of the shaft (journal) is insufficient to create an oil film. It is during startup and shutdown when almost all of the damage to the bearing occurs.2
Hydrostatic lift, created by an external pressurized oil feed, may be employed to float large, heavy journals prior to startup (shaft rotation) to prevent this type of damage. During normal operation, the shaft rotates at sufficient speed to force oil between the conforming curved surfaces of the shaft and shell, thus creating an oil wedge and a hydrodynamic oil film.
This full hydrodynamic fluid film allows these bearings to support extremely heavy loads and operate at high rotational speeds. Surface speeds of 175 to 250 meters/second (30,000 to 50,000 feet/minute) are common. Temperatures are often limited by the lubricant used, as the lead and tin babbitt is capable of temperatures reaching 150°C (300°F).
It is important to understand that the rotating shaft is not centered in the bearing shell during normal operation. This offset distance is referred to as the eccentricity of the bearing and creates a unique location for the minimum oil film thickness, as illustrated in Figure 3.
Figure 3. Shaft Motion During Startup
Normally, the minimum oil film thickness is also the dynamic operating clearance of the bearing. Knowledge of the oil film thickness or dynamic clearances is also useful in determining filtration and metal surface finish requirements.
Typically, minimum oil film thicknesses in the load zone during operation ranges from 1.0 to 300 microns, but values of 5 to 75 microns are more common in midsized industrial equipment. The film thickness will be greater in equipment which has a larger diameter shaft.
Persons requiring a more exact value should seek information on the Sommerfeld Number and the Reynolds Number. Discussion of these calculations in greater detail is beyond the scope of this article. Note that these values are significantly larger than the one-micron values encountered in rolling element bearings.
The pressures encountered in the contact area of journal bearings are significantly less than those generated in rolling bearings. This is due to the larger contact area created by the conforming (similar curvature) surfaces of the journal and the shell.
The mean pressure in the load zone of a journal bearing is determined by the force per unit area or in this case, the weight or load supported by the bearing divided by the approximate load area of the bearing (the bearing diameter times the length of the bearing). In most industrial applications, these values range from 690 to 2,070 kPa (100 to 300 psi).
At these low pressures, there is virtually no increase in the oil viscosity in the bearing contact area due to pressure. Automotive reciprocating engine bearings and some severely loaded industrial applications may have mean pressures of 20.7 to 35 MPa (3,000 to 5,000 psi). At these pressure levels, the viscosity may slightly increase. The maximum pressure encountered by the bearing is typically about twice the mean value, to a maximum of about 70 MPa (10,000 psi).
Oil whirl is a phenomenon that can occur in high-speed journal bearings when the shaft position within the shell becomes unstable and the shaft continues to change its position during normal operation, due to the fluid forces created within the bearing. Oil whirl may be reduced by increasing the load or changing the viscosity, temperature or oil pressure in the bearing.
A permanent solution may involve a new bearing with different clearances or design. Oil whip occurs when the oil whirl frequency coincides with the system's natural frequency. The result can be a catastrophic failure.3
Oils are used in journal bearings when cooling is required or contaminants or debris need to be flushed away from the bearing. High-speed journal bearings are always lubricated with oil rather than a grease. Oil is supplied to the bearing by either a pressurized oil pump system, an oil ring or collar or a wick. Grooves in the bearing shell are used to distribute the oil throughout the bearings' surfaces.
The viscosity grade required is dependent upon bearing RPM, oil temperature and load. The bearing speed is often measured strictly by the revolutions per minute of the shaft, with no consideration of the surface speed of the shaft, as per the 'ndm' values calculated for rolling bearings. Table 1 provides a general guideline to selecting the correct ISO viscosity grade.
The ISO grade number indicated is the preferred grade for speed and temperature range. ISO 68- and 100-grade oils are commonly used in indoor, heated applications, with 32-grade oils being used for high-speed (10,000 RPM) units and some outdoor low-temperature applications.
Note in the table that the higher the bearing speed, the lower the oil viscosity required; and that the higher the operating temperature of the unit, the higher the oil viscosity that is required. If vibration or minor shock loading is possible, a higher grade of oil than the one indicated in Table 1 should be considered.
Bearing Speed Bearing / Oil Temperature (°C) (rpm) 0 to 50 60 75 90 300 to 1,500 - 68 100 to 150 - ~1,800 32 32 to 46 68 to 100 100 ~3,600 32 32 46 to 68 68 to 100 ~10,000 32 32 32 32 to 46Table 1. Journal Bearing ISO Viscosity Grade Selection
Another method of determining the proper viscosity grade is by applying minimum and optimum viscosity criteria to a viscosity-temperature plot. A generally accepted minimum viscosity of the oil at the operating temperature for journal bearings is 13 cSt, although some designs allow for an oil as thin as 7 or 8 cSt at the operating temperature.
The optimum viscosity at operating temperature is 22 to 35 cSt, for moderate-speed bearings if no shock loading occurs. The optimum viscosity may be as high as 95 cSt for low-speed, heavily loaded or shock-loaded journal bearings.
Using this method requires some knowledge of the oil temperature within the bearing under operating conditions, which can be difficult to determine. Fortunately, an accurate oil temperature is not needed for most viscosity determinations. It is common to determine the temperature of the outer surface of the pipes carrying oil to and away from the bearing.
The temperature of the oil inside of the pipes will generally be higher (5 to 10°C, 10 to 18°F) than the outer metal surface of the pipe. The oil temperature within the bearing can be taken as the average of the oil entering versus the temperature exiting the bearing.4
A third and more complex method is to calculate the oil viscosity needed to obtain a satisfactory oil film thickness. Persons wishing to learn more about this method should seek information regarding the Sommerfeld equation and either eccentricity ratios or Reynolds Numbers.4
If the oil selected is too low in viscosity, heat will generate due to an insufficient film thickness and some metal-to-metal contact will occur. If the oil is too high in viscosity, heat will again be generated, but due to the internal fluid friction created within the oil. Selecting an oil which is too high in viscosity can also increase the likelihood of cavitation.
The high- and low-pressure zones, which are created within the oil on each side of the area of minimum film thickness, can cause oil cavitation in these bearings. Cavitation is a result of expansion of dissolved air or a vapor (water or fuel) in the low-pressure zone of the bearing.
The resulting bubble implodes, causing damage, as it passes through the high-pressure portion of the bearing. If the implosion or collapse of the vapor bubble occurs next to the metal surface, this can cause cavitation pitting damage to the metal. If the implosion of the bubble occurs within the oil, a micro hot spot or micro-dieseling can occur, which may lead to varnishing within the system.
Typically, a rust and oxidation (R&O) inhibited additive system is used in the oils employed in these applications. Antifoam and pour point depressant additives may also be present. Antiwear (AW) hydraulic oils may also be used as long as the high-temperature limit of the zinc AW component is not exceeded and excessive water is not present.
R&O oils tend to have better water separation characteristics, which is beneficial, and the AW properties of a hydraulic oil would be beneficial only during startup and shutdown, assuming a properly operating bearing.
Grease is used to lubricate journal bearings when cooling of the bearing is not a factor, typically if the bearing operates at relatively low speeds. Grease is also beneficial if shock loading occurs or if the bearing frequently starts and stops or reverses direction.
Grease is almost always used to lubricate pins and bushings because it provides a thicker lubricant than oil to support static loads and to protect against vibration and shock-loading that are common in many of these applications.
Lithium soap or lithium complex thickeners are the most common thickeners used in greases and are excellent for most journal bearing applications. The grade of grease used is typically an NLGI grade #2 with a base oil viscosity of approximately 150 to 220 cSt at 40°C.
Greases for low-speed, high-load, high temperatures and for pins and bushings may use a higher viscosity base oil and be formulated with EP and solid additives. Greases for improved water resistance may be formulated with heavier base oils, different thickeners and special additive formulations.
Greases for better low-temperature dispensing may incorporate a lower viscosity base oil manufactured to an NLGI #1 specification. Bearings lubricated by a centralized grease dispensing systems typically use a #1, 0 or 00 grade of grease.
The apparent viscosity of grease changes with shear (pressure, load and speed) that is, greases are non-Newtonian or thixotropic. Within a rotating journal bearing, as the bearing rotates faster (shear rate increases), the apparent viscosity of the grease decreases and approaches the viscosity of the base oil used in grease.
At both ends of the bearing shell, the pressure is lower and therefore the apparent viscosity remains higher. The resulting thicker grease at the bearing ends acts as a built-in seal to reduce the ingression of contaminants.
The greasing procedures for journal bearings and pins and bushings are not as well-defined or as critical as for rolling bearings because the grease is not subjected to the churning action created by the rolling elements.
The volume of grease to inject and the frequency of application are dictated more by trial and error. Generally, most journal bearings cannot be overgreased. Caution must be taken when pumping grease into a bearing that is fitted with seals, so they are not damaged or displaced by the force and volume of the incoming grease.
The harshness of the environment, shock loading and especially the operating temperature will be major factors in determining the frequency of relubrication.
Journal bearings are generally a simpler design and not as difficult to lubricate as rolling element bearings. The proper viscosity matched to the operating conditions and a clean and dry lubricant will usually suffice to form a full fluid lubricating film and provide excellent bearing life.
References
Strecker, William. 'Troubleshooting Tilting Pad Thrust Bearings.' Machinery Lubrication magazine, March-April .
Strecker, William. 'Failure Analysis for Plain Bearings.' Machinery Lubrication magazine, July-August .
Berry, James. 'Oil Whirl and Whip Instabilities within Journal Bearings.' Machinery Lubrication magazine, May-June .
Tribology Data Handbook. Chapter 61, Journal Bearing Design and Analysis. Khonsari, M. CRC Press, .
Editor's Note:
Portions of this article have been previously published in the Society of Tribologists and Lubrication Engineers (STLE) Alberta Section, Basic Handbook of Lubrication, Second Edition, .
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