13
CHAPTER ONE
1.1 INTRODUCTION
Hydrodynamic journal bearings are typical critical power transmission components that carry high loads in different machines. In machine design, therefore, it is essential to know the true or expected operating conditions of the bearings. These operating conditions can be studied both by experimental and mathematical means, for example in test rig experiments, in field or laboratory tests with engines and by calculation or simulation.
Numerous studies of the operating conditions of hydrodynamic journal bearings have been made during the last decades. Still, the case is far from closed. For example, there are a limited number of studies that carry out an in-depth examination of the true operating conditions of bearings in true-scale experiments. There is also a need for experimental studies to verify the theoretical ones.
Fluid friction i.e. viscosity which exists in the lubricant being used is studied alongside the pressure effect which is being generated in the bearing, thus the effect of lubricants with different viscosities are considered.
A simple journal bearing consists of two rigid cylinders. The outer cylinder (bearing) wraps the inner rotating journal (shaft). A lubricant fills the small annular gap or clearance between the journal and the bearing. The amount of eccentricity of the journal is related to the pressure that will be generated in the bearing to balance the radial load. The lubricant is supplied through a hole or a groove and may or may not extend all around the journal. The pressure around the journal is measured on various manometers by means of pressure pipe/tubes. This is done at various speeds to get the relationship between speed and the pressure.
1.2 HISTORICAL BACKGROUND
In the late 1880s, experiments were being conducted on the lubrication of bearing surfaces. The idea of “floating” a load on a film of oil grew from the experiments of Beauchamp Tower and the theoretical work of Osborne Reynolds.
Prior to the development of the pivoted shoe thrust bearing, marine propulsion relied on a “horseshoe” bearing which consisted of several equally spaced collars to share the load, each on a sector of a thrust plate. The parallel surfaces rubbed, wore, and produced considerable friction. Design unit loads were on the order of 40 psi. Comparison tests against a pivoted shoe thrust bearing of equal capacity showed that the pivoted shoe thrust bearing, at only 1/4 the size, had 1/7 the area but operated successfully with only 1/10 the frictional drag of the horseshoe bearing.