CHAPTER ONE
INTRODUCTION
The Mitchell tilting pad apparatus is a hydrodynamic measuring instrument developed in the early 1880’s in the laboratory of Beauchamp Tower in England. Tower was employed to study the friction in railway journal bearings and come up with the best method of lubricating them. The Mitchell tilting pad apparatus is used broadly in two different experiments namely;
1) Determination of the load carrying capacity of the slider bearing.
2) Confirming the theory of the hydrodynamic lubrication.
Tilting pad journal bearings are a source of both static support and dynamic stiffness and damping. Tilting pad journal bearings have a number of pads, typically four or five. Each pad in the bearing is free to rotate about a pivot and cannot support a moment. As a result, the destabilizing forces are greatly reduced or eliminated, and the bearings are no longer a potential source of rotordynamic instability. This feature has made tilting pad journal bearings the standard fluid-film bearing for most high-speed applications. High-speed rotordynamic applications often have rotors that pass through one or two bending critical speeds as the machines are accelerated to the operating speed. The damping from the fluid film bearings is required to safely pass through these bending critical speeds as the rotating element is accelerated. The damping also helps suppress potentially destabilizing forces from sources such as radial seals, balance pistons, impeller eye seals, internal friction fits, and unbalanced electromagnetic forces.
2
1.1 AIMS AND OBJECTIVES OF THE PROJECT
The project designing, constructing and testing on the Mitchell tilting pad apparatus is aimed at achieving the following objectives:
1) To verify the hydrodynamic theory of lubrication as it was propounded by Beauchamp Tower in 1880 AD.
2) To determine the load carrying capacity of the tilting pad slider bearing
3) To provide the fluid mechanics laboratory of the Mechanical Engineering Department with a hydrodynamic fluid analyzing apparatus.
4) To activate and motivate the students potentials into practically solving problems facing mankind.
5) To run tests with the apparatus and compare the results obtained with the established or ideal standards.
1.2 PROJECT JUSTIFICATION
Engineering is known to be to be practice-oriented discipline. Inotherwords, no useful Engineering endeavor can exist in theory only, it must be applied to touch and transform life through meaningful practice. Therefore, this project task given to us is to ensure the knowledge we students gained throughout our five year degree programme and channeled towards the construction of Mechanical Engineering equipments.
1.3 APPLICATIONS AND USES
The Mitchell tilting pad apparatus is applicable in experiments which does not require;
3
1) Hydrostatic lubrication
2) Boundary lubrication
3) Solid lubrication
4) Elastohydrodynamic form of lubrication.
It only finds its application useful in full film or fluid lubrications in motion, in which there is a situation that the load carrying surfaces of the bearing are separated by an adequate supply at all times of a relatively thick film of lubricant, so as to prevent metal to metal contact and that the stability thus obtained can be explained by the laws of fluid mechanics. The Mitchell tilting pad is a Mechanical Engineering apparatus with the above aims and objectives.
1.4 LIMITATIONS
The Mitchell tilting pad apparatus is limited to use with the specified oil viscosity (SAE20W/50) or Whitz oil which is typical automobile engine oil. The Mitchell tilting pad apparatus is also limited to operation when there is power failure or insufficient supply of power that drives the electric motor, other limitations are stipulated below;
1) The use of lubricants that do not obey Newton’s law of viscous flow.
2) The use of compressible lubricants
3) Fluid pressure varying in the axial direction.
From the limitations stated above, it is necessary to identify the Newton’s viscous effect, which states that the shear stress in the fluid is proportional to the rate of change of velocity with respect to ‘y’.