Dynamically loaded lubricated journal bearings in multibody mechanical systems

• Main Author: Alshaer, Bassam Jameel
• Format: Dissertation
• Language: English
• Subjects: engineering, mechanical
• ISBN: 9780599813977; 0599813970

This research addresses the analytical evaluation of dynamic pressures and forces produced by the lubricant in a lubricated long journal bearing under dynamic loading. An evaluation of accuracy of the widely adopted Ott's solution is conducted. The results show that when the journal center is moving toward the center of the bearing and the eccentricity ratio is high, the force formulation according to Ott's solution give inaccurate results. Differences as high as 93% is found between Ott's analytical solution and the numerical solution for certain conditions. Starting from Reynolds partial differential equation of lubrication theory, a new formulation for the dynamic characteristics of lubricated long journal bearing under dynamic loading is presented. The lubricant forces calculated at certain instants of time showed exact match with the numerical solution, which is not the case with Ott's solution. The new formulation is utilized in evaluating the transient and steady state characteristics of lubricated long journal bearing under unidirectional constant load and the results are compared with the Ott's solution.
Clearances exist in different kinds of joints in multibody mechanical systems, which could drastically affect the dynamic behavior of the system. If the joint is dry with no lubricant, impact occurs, this results in wear and tear of the joint. In practical engineering design of machines, joints are usually designed to operate with some lubricant. Lubricated journal bearings are designed so that even when the maximum load is applied, the joint surfaces do not come into contact with each other. In this research, a general methodology for modeling lubricated long journal bearings in multibody mechanical systems is presented. This modeling utilizes a new method of solving for the forces produced by the lubricant in a dynamically loaded long journal bearing. A perfect revolute joint in a multibody mechanical system imposes kinematic constraints, while a lubricated journal bearing joints imposes force constraints. As an application, the dynamic response of a crank-slider mechanism including a lubricated journal bearing joint between the connecting rod and the slider is considered and analyzed. The dynamic response is obtained by solving the constraint equations and the forces produced by the lubricant simultaneously with the differential equations of motion, a proper set of initial conditions numerically. The results are compared with the previous studies performed on the same mechanism as well a dry clearance joint. It is shown that in a multibody mechanical system, the journal bearing lubricant introduces damping and stiffness to the system. This damped system lowers the applied reaction moment required to run the crank with constant speed compared to a dry joint simulation. The earlier studies previous predict that the order of magnitude of the reaction moment is twice that of a perfect revolute joint. The proposed model predicts that the reaction moment is within the same order of magnitude of the perfect joint simulation case.