A Study on the Relation between Shape of Slope and Bearing Characteristics of Oil Lubricated Aft Stern Tube Bearing
In this paper, result of alignment calculation study on the relation between shape of slope and bearing characteristics and the mutual relations among the bearing characteristics by different calculation methods are related.
As the shape of slope, three kinds (Single, Partial and Double Slope) and as the calculation method, three kinds (Simple, Spring and Oil film Support method) are taken up for study.
The shafting for study is quite ordinary type which is direct driven by diesel engine and provided with aft and fore stern tube bearings and one intermediate shaft bearing.
Key Words: Stern Tube Bearing, Shaft Alignment, Slope Boring, Relative Inclination, Thickness of oil film
In the shafting with oil lubricated stern tube bearing, slope is usually provided on the aft stern tube bearing. Its purpose is to decrease the locally distributed load on the aft part of the bearing due to the cantilever weight of propeller.
As the shape of slope, generally three types, Single, Partial and Double Slope are considered. The dimension of slope is usually decided by investigating the bearing characteristics based on the alignment calculation of shafting. As the method of alignment calculation, three kinds, Simple Support, Spring Support and Oil Film Support calculations are considered.
In the actual design, I think shape of slope is usually selected without special comparative investigation for each ship and each shafting. Each shipyard seems to have individual favorite type based on the experiences. On the other hand, dimension of slope seems to be decided based on the calculation of Simple Support.
I myself also have applied one of the types (Single Slope) without altering for each ship and each shafting and decided the dimension of slope based on the calculation of Simple Support for almost all cases.
However, difference of bearing characteristics between the three types of slope and mutual relation in bearing characteristics between the three types of calculation method have remained as an interesting theme for me.
At this opportunity I tried to study on this theme which had been a matter of concern for a long time. The result of study is introduced in the following.
Note: In this paper, some abbreviations are used. Regarding the abbreviation, please refer Fig.3.
2. OUTLINE OF SHAFTING FOR STUDY AND CALCULATION MODEL
Outline of shafting for study and calculation model are as shown in Fig.1. The shafting is quite ordinary type which is direct driven by long stroke low speed diesel engine and provided with aft stern tube bearing (AB), fore stem tube bearing (FB), one intermediate shaft bearing (IB). The propeller shaft diameter is 710 mm.
In the calculation model, IB and M/E Bear are treated as Simple Support and FB is treated as Spring Support. The reason why FB is treated as Spring Support is so that the support point load of FB do not become minus value in any case changing the slope of AB. The offsets of FB, IB and M/E Bear are fixed as 0(zero) in order to make the study simple. (This shafting is of actual ship. In the actual ship, offset of FB is raised a little and offsets of IB and M/E Bear lowered a little)
3. SHAPE AND DIMENSION OF SLOPE
The shapes and dimensions of slope for calculation are as shown in Fig.2. These slopes are prepared based on the following consideration.
1) Change of the offset is to be in unit of 0.05 mm.
2) Knuckle point of Partial Slope and Double Slope is to be set on the half position of bearing length.
As it can understand from the figures in Fig.2, applicable range of Partial and Double Slope is limited. In this study, Partial and Double Slope are applied for the range of AB/A Offset (AOS) -0.05 〜 -0.2 mm.
4. CALCULATION METHOD
Calculation program is that calculation of oil film characteristics by solving Reynolds equation and calculation of continuous beam with multi support point are combined with. In the program, FEM is used to calculate oil film characteristics and continuous beam. The calculation model is as explained in the above (section 2) and the following conditions are used for the calculation of Spring Support and Oil Film Support.
*NKK Design & Engineering Corporation
3, Benten-cho, Tsurumi-ku, Yokohama, 230-0044 JAPAN
FAX: 045-502-6892, E-mail: Yoshiih@nss.tsurumi.nkk.cojp
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