A study of bearing and its types conference Paper

 
3.1 Assumptions in Hydrodynamic Lubricated Bearings
The following are the basic assumptions used in the theory of hydrodynamic lubricated bearings: 
1. The lubricant obeys Newton's law of viscous flow. 
2. The pressure is assumed to be constant throughout the film thickness. 

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International Journal of Advance Research In Science And Engineering http://www.ijarse.com
IJARSE, Vol. No.4, Special Issue (01), March 2015 ISSN-2319-8354(E) 
1676 | 
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3. The lubricant is assumed to be incompressible. 
4. The viscosity is assumed to be constant throughout the film. 
5. The flow is one dimensional, i.e. the side leakage is neglected. 
Unlike rolling bearings which have a limited lifespan, principally due to their fatigue wear through the 
occurrence of high point loads at the contact between running elements and raceways, sliding bearings have, at 
least in theory, an indefinite operational lifespan. As long as an oil film of sufficient thickness is maintained and 
contamination is avoided, the bearing will continue to do its job indefinitely. However, more and more onerous 
demands are being placed on such bearings and their associated methods of lubrication in order to maximise 
performance in terms of efficiency and load carrying capacity. Examples are bearing of electrical generators and 
turbines. Sliding bearings are vulnerable to damage during rapid changes in operating parameters e.g. during the 
start-up process. During running, the faces of the bearing pads are separated from the shaft / collar by a thin oil 
film which provides a buffer in between the two surfaces (hydrodynamic lubrication). While preventing contact 
between the surfaces, this buffer also provides a means of cooling. However, when the machinery is stationary 
this film disappears and contact between the surfaces occurs. Therefore, at start-up, friction between the surfaces 
is inevitable until an adequate film is build up. This makes the bearing susceptible to surface damage. The use of 
a facing (surface coating) can help to limit this susceptibility. 
The operating principle involved can be illustrated by considering a flat surface sliding over a tapered land. 
Motion of flat surface or thrust runner draws fluid such as lubricating oil into wedge shaped zone over the 
tapered land. Pumping into zone of reducing downstream clearance by shearing action of runner then 
pressurizes the oil. The change in separation 
h
adjusts the inflow and outflow to provide the balance of 
integrated pressure over the bearing area with applied load. A group of tapered land thrust pads arranged with 
oil distribution grooves in annular configuration to form a complete thrust bearing.