Modeling and Simulating Bearings/Bearing Leakages

Bearings are very important machinery components since they dominate machine performance. Almost all machines and mechanisms with a rotating part, from the smallest motor to the largest power plants, from turbomachinery to reciprocating engines, and other industrial equipment our modern society relies upon, could not function without the use of bearings in some form. If one of the bearings fail, not only do the machines stop, but the assembly line also stops, and the resulting costs may be extremely high. For this reason, every bearing manufacturer makes every effort to ensure the highest quality for each bearing and that the end user subjects the bearing to careful use and properly maintains this component.

A bearing can be defined as a machine element which supports another moving machine element (known as a journal). It permits a relative motion between the contact surfaces of the members, while carrying the loads (static and dynamic). Some consideration will show that due to the relative motion between the contact surfaces, a certain amount of power is wasted in overcoming frictional resistance. If the rubbing surfaces are in direct contact, there will be rapid wear. In order to reduce frictional resistance, wear, and in some cases to carry away the heat generated, a layer of fluid (known as lubricant) may be provided. This lubricant is used to separate the journal and bearing, which allows the moving parts to move smoothly and helps to achieve more efficient machine operation. Some of the common bearing types are shown in Figure 1.

Figure 1. Common Types of Bearing Examples. SOURCE: [1]
Figure 1. Common Types of Bearing Examples. SOURCE: [1]
The main purpose of bearings is to prevent direct metal to metal contact between two elements that are in relative motion. This prevents friction, heat generation and ultimately, the wear and tear of parts. It also reduces the energy consumption required for moving parts. Additionally, they also transmit the load of the rotating element to the housing. This load may be axial, radial or a combination of both. Bearings also restrict the freedom of movement of moving parts to a predefined direction. With all these aspects, bearings are clearly important for the operations and the reliability of mechanical products. The right bearing can increase useful life of the machine, and enhance the machine’s overall performance. The wrong bearing can lead to premature failure, increased downtime, and increased wear and fatigue among all components of the machine.

There numerous kinds of bearings available on the market, and each is used for different purposes based on different materials, sizes, shapes, and styles depending on their application. Some of the commonly used bearings in turbomachinery include plain bearings (journal bearings), fixed and tilting bearings, gas foil bearings, thrust bearings, fluid bearings, rolling element bearings, as well as elliptical bearings.
To achieve safer operation of these machines, bearings must be modeled carefully with some of the design considerations including, minimum film thickness, maximum film pressure, maximum bearing temperature, lubricant flow rate, power loss, and others.

All the above-mentioned bearing types and design considerations can be accurately modeled and analyzed in AxSTREAM Bearing™. Additionally, AxSTREAM Bearing™ can be used to determine bearing characteristics that are necessary with regards to rotor dynamic analysis, including bearing stiffness and damping characteristics. AxSTREAM Bearing™ allows designers to determine hydrodynamic and mechanical characteristics for different types of hydrodynamic journals and thrust bearings as well as auxiliary component characteristics such as liquid and gas seals, squeeze film dampers,  and aerodynamic cross-couplings. Figure 2, shows the variety of bearings and auxiliary components available for analyzing in AxSTREAM Bearing™.

Different Types of Bearings
Figure 2. Different Types of Bearings and Auxiliary Components available in AxSTREAM Bearing™

By using AxSTREAM Bearing™, designers can perform numerous analyses using the “Finite Difference Method” which includes, steady-state and transient analysis, whirl stability, and bearing map analysis. In bearing steady-state analysis, designers can determine the pressure distribution, friction coefficient, friction power loss, oil flow rate, oil temperature increment, minimum oil film thickness, temperature distribution, eccentricity and angle, fractional film content, Sommerfeld number, critical mass, and mechanical characteristics (stiffness and damping coefficient). One such examples of journal bearing simulation in AxSTREAM Bearing™ is shown in Figure 3.

Journal Bearing Simulation
Figure 3. Journal Bearing Simulation in AxSTREAM Bearing™

Another important factor that may limit bearing efficiency and performance is lubrication. Inappropriate lubrication of a bearing’s rolling elements leads to bearing failure and may affect the overall machine’s performance due to bearing leakages and other phenomena. Bearings fail for many reasons, but improper lubrication is at the top of the list, according to a variety of studies. Lubrication is a key factor that can make or break a bearing’s service life. Some research in the bearing industry has stated that improper lubrication accounts for around 80% of all bearing failures [2].

As mentioned earlier, bearings must be lubricated to prevent metal to metal contact between the rolling elements. In addition, lubrication protects the bearing against corrosion and wear, dissipates generated heat, seals out solid and liquid contamination, and reduces bearing noise. A properly lubricated bearing will have the best chance of reaching its maximum service life. Therefore, an efficient lubrication system is essential to minimize the risk of bearing damage. To achieve this, the necessary amount of lubricant with the appropriate pressure should circulate throughout the entire system, which includes all of the bearings and other components in the system. If the required flow does not circulate properly to each corner of the system or rotating components in the machine, then cavitation will occur due to adverse pressure and, excessive heat will be generated due to poor mass flow rate. This will lead to major damage of the machine’s components and reduce the life of the entire machine.

Lubrication failure can be the result of unsuitable lubrication, insufficient lubricant quantity or viscosity, lubrication contamination, excessive temperatures, over-lubricating, and lubricating oil leakage from the bearing case seals. To avoid these failure issues, designers should carefully consider lubrication system models to ensure a long, reliable life for the bearings and other components in the machine.

These systems for different applications (such as steam and gas turbine engine lubrication systems, automotive engine lubrication system, and others) can be modeled very accurately and analyzed using a 1D thermal-fluid network analysis tool, such as AxSTREAM NET™, developed by SoftInWay.

Using AxSTREAM NET™, designers can evaluate the necessary oil flow rate that is required for the entire lubrication system, which includes bearings and other components. Additionally, designers can estimate the system pressure levels, temperatures, and heat flows by modeling the network of a particular lubrication system using AxSTREAM NET™.  In AxSTREAM NET™, various elements can be used to model bearings including where the oil flow is required to provide the required cooling. Additionally, designers can account for the temperature and obtain the necessary mass flow rate required to cool down the bearings.

An example of a system being modeled in AxSTREAM NET™ is shown in Figure 4. Here a 16 cylinder marine diesel engine’s inner lubrication system is shown, which contains bearings, gear components, pistons, cylinders and the camshafts and crankshaft. In the inner lubrication system of the engine, oil is supplied to the bearings, cylinder walls, and the gears. To learn more about this system being modeled in AxSTREAM NET™, check out our previous blog here: Modelling and Analysis of a Submarines Diesel Engine Lubrication System.

Diesel Engine (Inner Engine) Lubrication System Modeled
Figure 4. Diesel Engine (Inner Engine) Lubrication System Modelled in AxSTREAM NET™

As presented throughout this article, bearings play an important role in ensuring the reliability and increasing the overall lifetime of a machine by reducing the friction between the moving parts and providing smooth machine operation. This helps in preventing excess heat generation, wear and tear of the parts, and reduces power consumption. Bearings also protect the parts that support the  machine’s rotation and maintain the correct position for the rotating components. It’s important to remember that minor issues and defects as well as improper lubrication can lead to significantly shortened system lifespans. While modeling bearings, designers should carefully account for all the design considerations for the bearing used, which includes minimum film thickness, maximum film pressure, maximum bearing temperature, lubricant flow rate, power loss and other factors. Thus, it is essential that designers model an efficient lubrication system that operates properly, even under various operating conditions to the minimize risk of bearing damage. To address all these bearing design and lubrication challenges efficiently and maximize productivity, designers can use tools such as AxSTREAM Bearing™ and AxSTREAM NET™.

Are you looking to expand your bearing simulation capabilities or reduce project times? Or are you looking to model and simulate bearing leakages and lubrication systems accurately? Reach out to us at Info@Softinway.com to schedule a demo of AxSTREAM Bearing™ and AxSTREAM NET™ today!

References:

[1] https://www.thermalspray.com/common-types-bearings-industrial-applications/

[2]Koyo Seiko Co. Ltd., Rolling Bearings: Failures, Causes