tailieunhanh - Friction and Lubrication in Mechanical Design P02

Tower reported the results of a series of experiments intended to determine the best methods to lubricate a railroad journal bearing. Working with a partial journal bearing in an oil bath, he noticed and later measured the pressure generated in the oil film. Tower pointed out that without sufficient lubrication, the bearing operates in the boundary lubrication regime, whereas with adequate lubrication the two surfaces are completely separated by an oil film. Petrov [5] also conducted experiments to measure the frictional losses in bearings. He concluded that friction in adequately lubricated bearings is due to the viscous shearing of. | 10 Chapter 1 lubrication theory 6 7 . Tower reported the results of a series of experiments intended to determine the best methods to lubricate a railroad journal bearing. Working with a partial journal bearing in an oil bath he noticed and later measured the pressure generated in the oil film. Tower pointed out that without sufficient lubrication the bearing operates in the boundary lubrication regime whereas with adequate lubrication the two surfaces are completely separated by an oil film. Petrov 5 also conducted experiments to measure the frictional losses in bearings. He concluded that friction in adequately lubricated bearings is due to the viscous shearing of the fluid between the two surfaces and that viscosity is the most important property of the fluid and not density as previously assumed. He also formulated the relationship for calculating the frictional resistance in the fluid film as the product of viscosity speed and area divided by the thickness of the film. The observations of Tower and Petrov proved to be the turning point in the history of lubrication. Prior to their work researchers had concentrated their efforts on conducting friction drag tests on bearings. From Tower s experiments it was realized that an understanding of the pressure generated during the bearing operation is the key to perceive the mechanism of lubrication. The analysis of his work carried out by Stokes and Reynolds led to a theoretical explanation of Tower s experimental results and to the theory of hydrodynamic lubrication. In 1886 Osborne Reynolds published a paper on lubrication theory 4 which is derived from the equations of motion continuity equation and Newton s shear stress-velocity gradient relation. Realizing that the ratio of the film thickness to the bearing geometry is in the order of 10 3 Reynolds established the well-known theory using an order-of-magnitude analysis. The assumptions on which the theory is based can be listed as follows. The pressure is constant