الفهرس Only 14 pages are availabe for public view
 |  | from | 124 |  |  |
| | | from | 124 | | |
Abstract
In high speed rotating machinery using hydrodynamic circular bearings, instabilities due to the oil whirl create serious problems. In such applications non-circular bearings are usually preferred because of their superior dynamic performance characteristics compared with those of circular bearings. The performance characteristics of the non-circular bearings have been investigated by many researchers, but the analysis were predominately isothermal in approach.
In this study, a thermohydrodynamic solution for five types of journal bearings (circular, two-lobe, offset, three-lobe, four-lobe types) is performed by solving the generalized form of Reynolds equation simultaneously with the energy equation, taking into consideration the viscosity variation across the film thickness. Two parameters are used for evaluating the thermal effects on the performance characteristics of the five bearing types; Peclet number, Pe, and the viscosity-temperature coefficient a. The finite difference technique is used to solve both the Reynolds and energy equations to obtain the static performance parameters such as; the load carrying capacity, the attitude angle, the friction coefficient, the side leakage flow and the temperature variation. The eight oil-film stiffness and damping coefficients are determined applying the finite perturbation technique. Using these coefficients, the critical mass, the whirl ratio and the charts of stability are determined.
It is found that the pressure and the load carrying capacity decrease with the increase of both Peclct number and viscosity-temperature coefficient. The results also show that, the increase of Pe and a causes an increase of the attitude angle, the side leakage and the friction coefficient. Comparing with the classical isothermal solution, the thermohydrodynamic solution decreases the stability region and increases the whirl ratio of the five bearing types. It may be concluded that considering the thermal effects in the solution of these types of bearings has a marked influence on their performance characteristics.
On the other hand, in classical hydrodynamic lubrication analysis, the lubricants are assumed to behave as Newtonian fluids. Advances in technology and in many practical lubrication applications necessitate the development of improved lubricants where the Newtonian fluids assumption is not satisfactory engineering approach to lubrication problems. Couple stresses may appear in lubricants containing additives or in lubricants with long-chain molecules and they have a significant effect on the bearing behaviour. The dynamic characteristics of the non-circular bearings lubricated with Newtonian fluids have been studied by many researchers. In this study, the static and dynamic characteristics of the five bearing types lubricated with couple stress fluids are obtained. On the basic of the Stokes couple stress fluid model, the modified Reynolds equation in dynamic state is derived and then numerically solved. The effects;of couple stresses on the perfonnance of
the journal bearing are analysed by defining a new parameter J , which this parameter plays an important role in the study of couple stress in lubricants. The film pressure is solved using the finite difference technique. The static and dynamic characteristics are evaluated and the stability conditions are studied for the five types of journal bearings for different values of the couple stress parameter 7 .
According to the results obtained, it is concluded that the presence of couple stress increases the load carrying capacity and decreases the friction coefficient. Also it is found that the couple stress increases the stiffness and damping coefficients and the stability for the five types of journal bearings studied in this thesis. The results obtained are compared with the characteristics of the five bearing types lubricated with Newtonian fluids. It is observed that the static and dynamic characteristics of the five types of bearings are improved by using a lubricant with a couple stress compared to a Newtonian lubricant.