M.Sc Thesis

M.Sc StudentDov Avishai
SubjectExperimental Investigation on the Different Parameters
Affecting the Performance of a Water-Lubricated
DepartmentDepartment of Mechanical Engineering
Supervisors PROF. Yuri Kligerman
PROF. Morel Groper


Sliding bearings operating in full-hydrodynamic lubrication regime exhibits low friction coefficient and extended life. Most often, these bearings termed "hydrodynamic bearings" use oil as the lubricating fluid, choice made thanks to the oil's typical high viscosity which provides the bearing a high load carrying capacity.

As a result of environmental and economic reasons, shafts of modern ships and submarines are supported by sea water lubricated bearings. This technology requires the use of bearings carefully designed for several reasons: (1) the lower viscosity of the water in comparison with oils limits the load carrying capacity in the mostly desired full hydrodynamic regime, (2) the high corrosiveness of the sea water requires selecting suitable materials, most often some type of polymer or composite material, and (3), the necessity for effectively flushing the bearing for cooling and removal of impurities present in sea water possess an additional engineering challenge. These factors influence the water lubricated bearing performance in particular and the operation of the vessel's propulsion system in general.

The solution of the Hydrodynamic Lubrication (HL) problem defined by the Reynolds equation subjected to the typical assumptions allows the   calculation of the hydrodynamic pressure field in the thin water film. Consequently, knowledge of the pressure field permits the calculation of the relevant bearing's performance parameters including the load carrying capacity, the friction coefficient, and the tangential and axial flowrates.   However, in a polymer made journal bearing with a relatively low elastic modulus as typical used for marine water lubricated bearings, the elastic deformation affects the hydrodynamic pressure and vice versa thus an Elasto-Hydrodynamic Lubrication (EHL) model should be applied. Consequently, an EHL model solved using the commercial package COMSOL Multiphasic? software was developed and further validated.

Using the dimensionless form of the Reynolds equation while maintaining the Sommerfeld number and the L/D ratio permits to test a scaled down bearing while obtaining identical conclusions as for the full scale bearing.  In this study the geometric scaling ratio between the vessel's bearing dimensions and the experimental bearing was selected as 1:5.3. A test rig carefully designed to allow testing of water lubricated elastomer or composite made bearings was developed. This test rig allows the comparison and validation of the theoretical model with experimental data and postulates additional possibilities for better understanding the operation of those bearings.  Experimental obtained data allowed the estimation of the bearing's friction regime (i.e. boundary, mixed or full hydrodynamic lubrication). In addition a first attempt was made to quantify the effect of the bearing's operation regime on the acoustic emission into the water.