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| Chapter: 1-5
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DESIGN AND CONSTRUCTION OF JOURNAL BEARING DEMONSTRATION RIG
The journal bearing demonstration rig is an apparatus which is used to study how pressure would vary around the section of a journal bearing at various speed of the shaft and loading conditions. The design of the journal bearing was done by the use of standard design procedures carefully stated within this work. The frame, the journal bearing, the journal shaft, the base plate and all relevant components of the apparatus were designed. Other parts not constructed were procured. The fabrication and construction processes were carried out in the workshop. The shaft to be used was machined on the lathe machine to the design specification. So also was the bearing to be used. This was all explicitly discussed in this report. Frame construction was carried out by welding process also stated in the work. The spring damper support was another constructed part. The assembly was done in such a way that the eccentricity between the bearing and the shaft would exist so as to get results. Relevant formulas, derivation and equation models were used in carrying out calculations used to get mathematical results that can be used to compare the experimental results. It is observed in the graph plotted from the experimental results, that the speed is proportionally or linear to the pressure build up in the journal bearing. The pressure tapping (h5), shows the highest reading on the manometer board, ranging from 76cm for 1144rpm to 25cm for 572rpm due to eccentricity of the two centers (shaft and journal bearing).
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.
In 1896, inspired by the work of Osborne Reynolds, Albert Kingsbury conceived and tested a pivoted shoe thrust bearing. According to Dr. Kingsbury, the test bearings ran well. Small loads were applied first, on the order of 50 psi (which was typical of ship propeller shaft unit loads at the time). The loads were gradually increased, finally reaching 4000 psi, the speed being about 285 rpm.
In 1912, Albert Kingsbury was contracted by the Pennsylvania Water and Power Company to apply his design in their hydroelectric plant at Holtwood, PA. The existing roller bearings were causing extensive down times (several
outages a year) for inspections, repair and replacement. The first hydrodynamic pivoted shoe thrust bearing was installed in Unit 5 on June 22, 1912. At start-up of the 12,000 kW units, the bearing wiped. In resolving the reason for failure, much was learned about tolerances and finishes required for the hydrodynamic bearings to operate. After properly finishing the runner and fitting the bearing, the unit ran with continued good operation. This bearing, owing to its merit of running 75 years with negligible wear under a load of 220 tons, was designated by ASME as the 23rd International Historic Mechanical Engineering Landmark on June 27, 1987.
Since then, there has been series of progressive research carried out on bearings bringing to the advent of journal bearings which are not so different from the bearings designed by Osborne Reynolds and Albert Kingsbury which work on the same hydrodynamic lubrication system.
1.3 RESEARCH PROBLEM
The operating conditions of hydrodynamic journal bearings can be described by a set of tribological variables called key operating parameters. For example, the load level of a hydrodynamic journal bearing is described by two parameters: the specific load and the sliding speed. The key operating parameters most directly related to the bearing lubricant-shaft contact are the oil film temperature, oil film thickness and oil film pressure. These three key parameters can be determined by experimental or mathematical means with varying levels of complexity.
Until now, oil film pressure in hydrodynamic journal bearings has been studied mainly by mathematical means, because the experimental determination of oil film pressure has been a demanding or even an unfeasible task. Under real operating conditions, there are typically many practicalities that complicate the experimental determination of true oil film pressure in a certain point or at a certain moment. The oil film may be extremely thin and therefore sensitive to different disturbing factors, for example defects in geometry. In addition, the level of the oil film pressure may be extremely high or have a high level of dynamic variability.
1.4 AIM AND OBJECTIVE OF THE PROJECT
The research into the construction and design of the journal bearing apparatus has several reasons and purposes which need to be achieved and justified.
The main aim of the study was to determine the oil film pressure in hydrodynamic journal bearings carrying realistic loads. In addition, the relationship between the oil film pressure and other key operating parameters of journal bearings such as eccentricity and shaft speed was studied.
The study also included the determination of the relationship between the speed of rotation of the shaft, the pressure around the journal bearing and the oil thickness.
1.5 SCOPE OF THE PROJECT
The design and construction of journal bearing demonstration rig covers a very broad area. This area encompasses the design, design considerations, construction, assembly, working conditions and governing mathematical equations and laws. The design gives in depth details of the construction and fabrication of the apparatus. The working conditions consist of the loading, acting pressures and the variable operating speed which the journal bearing apparatus undergoes. Relevant design calculations and equations to include the Pressure head calculation, Sommerfeld’s equation and number, Petroff’s equation and Reynold’s equation to mention a few, are contained in this work.
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