Abstract: Radial sliding bearings have been used in power transmission systems such as steam turbines, gas turbines, and gearboxes, and have the advantages of large load-bearing capacity, low power consumption, impact resistance, and high operating accuracy. In order to ensure the improvement of the bearing capacity and positioning accuracy of the sliding bearing, it is necessary to control the temperature rise of the sliding bearing. With the increasing demand for high speed and high load sliding bearings in engineering applications, the design of sliding bearings has become the focus of attention.
Keywords: fluid dynamic pressure; Sliding bearings; optimal design
1. Preface
In ship power plants, the shafting power transmission system is a very important component, and the safety and reliability of the shafting affect the power performance of the ship. Due to the fact that sliding bearings are an important component of ship shafting, they play a role in positioning and load-bearing. Cooling the sliding bearings can reduce bearing wear and also increase the service life of the bearings. By using a reasonable lubrication method, not only can the friction performance of sliding bearings be improved, but also the temperature rise of bearing shells can be reduced. Therefore, it is necessary to optimize the design of the bearing structure in order to improve the working performance of the bearing.
2. Mathematical model of flow field in sliding bearings
When solving the flow field in sliding bearings, it is necessary to fully consider the cavitation effect and viscosity temperature effect. However, the temperature rise of the oil film is affected by the flow characteristics of the lubricating oil, and the flow characteristics of the lubricating oil can change the viscosity of the lubricating oil. Therefore, when calculating the flow field of sliding bearings, factors such as the flow characteristics, viscosity temperature effect, and viscosity dissipation of the lubricating oil should be considered. If the speed of the sliding bearing is high, there will be turbulence in the flow field at this time.
2.1 Hole model
Due to the important role of cavitation effect in sliding bearings, this article mainly adopts a "full cavitation" model for cavitation simulation of sliding bearings, and the fluid is mainly composed of three parts: lubricating oil, steam, and insoluble gas. The essence of Cavitation is the mass transfer phase transition between gas and liquid phases.
If the speed of the sliding bearing is high and the clearance is large, the flow field of the sliding bearing will fluctuate and also exhibit turbulent properties. In laminar flow, the fluid mainly undergoes a smooth stratified flow where adjacent fluid clusters are not mixed with each other. However, when molecules collide and exchange, the laminar flow can manifest as a regular flow. Turbulence is a complex, unsteady, and random vortex motion, and in addition to collisions between molecules, fluid micro clusters can exchange mass and momentum through intense pulsation mixing. The basic characteristics of turbulence can be manifested as randomness or pulsation.
2.2 Discretization of equations
This article mainly uses the control volume method to solve the transport equation of the flow field in sliding bearings. The basic idea of the control volume method is to divide the calculation area into a series of grids, and each grid node has at least one mutually exclusive Control volume, then control the differential equation with solution on each control volume for integration, and then a set of discrete equations will be obtained. Therefore, the control volume method is also called the Finite volume method, and its physical significance mainly lies in the Conservation law of physical field variables within a finite Control volume.
3. Studying the static characteristics of sliding bearings
The heat transfer inside sliding bearings is relatively complex, and sliding bearings mainly involve convective heat transfer between the lubricating oil and the inner wall of the bearing pad, as well as convective and radiative heat transfer between the outer wall of the bearing pad and the outside world. At this time, the viscous dissipation of the lubricating oil generates frictional heat and other phenomena. Convective heat dissipation is the main form of heat dissipation in high-speed bearing lubrication systems, accounting for about 90% of the total heat exchange. In addition, due to the large temperature gradient inside the sliding bearing, temperature can affect the viscosity of the lubricating oil. The oil supply temperature and pressure have a direct impact on the static characteristics of sliding bearings, and by selecting appropriate oil supply pressure and temperature, the working performance of sliding bearings can be improved. There are two main reasons for the formation of cavities in sliding bearings: one is that when the lubricating oil flows into the non load bearing area, the oil film pressure continuously decreases. When compared with the saturation pressure of the same temperature sliding oil, the pressure at this time decreases, and when the lubricating oil undergoes phase transformation and produces cavities, it is called strong cavitation; The second type mainly includes insoluble bubbles in the lubricating oil. When the lubricating oil enters the non bearing zone, the external pressure of the bubbles continuously decreases, and Z will eventually cause the diameter of the bubbles to increase, leading to the rupture of the oil film. Under different working conditions, the cavity pressure of East China bearings will also vary, so it is necessary to conduct a profound study on the impact of cavity pressure on sliding bearings.
The static characteristics of sliding bearings mainly refer to the distribution of parameters such as bearing capacity, friction coefficient, misalignment angle, oil film pressure, and temperature of the bearings under different loads during stable operation. After passing through the oil supply hole, lubricating oil flows into the sliding bearing cavity through a certain pressure and temperature, and the pressure and temperature at this time are called the oil supply pressure and oil supply temperature. The rotation of the journal can introduce lubricating oil into the convergence gap, resulting in fluid dynamic pressure. At this time, the combined force of the oil film pressure is balanced with the load on the journal, and the balance position is mainly biased towards one side. During the actual operation of bearings, as the oil film pressure in the divergent section continuously decreases, the air dissolved in the lubricating oil will form bubbles, and Z will eventually cause the oil film to rupture. In addition, if the pressure is lower than the saturation pressure of the lubricating oil at temperature, the lubricating oil will change into steam, so Cavitation will occur in the loose section of the sliding bearing.
The rotation of the journal causes the oil film to be subjected to wall shear force, and generates frictional resistance and frictional heat on the journal. One function of lubricating oil is to separate the bearing pads from the journal, thereby avoiding solid dry friction between the two. Another function is to take away the heat generated by fluid friction and work. Due to the influence of temperature on the viscosity of lubricating oil, when the temperature of lubricating oil increases, the viscosity will decrease. If the temperature of the lubricating oil inside the bearing is too high, the viscosity of the lubricating oil will decrease, and it will also reduce the bearing capacity of the oil film, ultimately causing lubrication failure. Therefore, it is necessary to control the high temperature of the oil film Z of the sliding bearing. The viscosity temperature relationship of lubricating oil can be calculated by viscosity, and some studies show that within a certain temperature range, the viscosity and temperature of lubricating oil show a Exponential distribution law. During the high-speed rotation of the journal, the lubricating oil will generate viscous dissipation and frictional heat under the action of shear force, and this will cause the oil film temperature to continuously increase. After the oil film temperature increases, the viscosity of the lubricating oil will continuously decrease, and the decrease in viscosity will cause its load-bearing capacity to continuously decrease.
4. Conclusion
Through a reasonable analysis of the heat transfer process inside sliding bearings and the cooling mechanism of lubricating oil heat transfer, it is concluded that the heat conduction process inside bearings is essentially a process of combining parameters such as lubricating oil flow rate, viscosity temperature characteristics, eccentricity, etc. After calculation and comparison, if the viscosity temperature effect and energy equation of the lubricating oil are not fully considered, it will affect the bearing characteristics. This article analyzes the influence of oil supply pressure, oil supply temperature, and cavity pressure on the static characteristics and cavity distribution of sliding bearings, in order to improve the friction coefficient of sliding bearings.
More about Marginal MG-800 Bimetallic Self-lubricating Bearing:
High load, low speed, low friction, wear resistance, a long working life span on preventing the mating parts from holding-on. The product is widely used in mining machinery, automobile, building machinery, agriculture equipment, rolling steel industry etc.
