Shaft Alignment in Relation to Oil Film Stiffness on Sterntube Bearings

Abstract

With an increase in the size and power rate of ships, there is a growing tendency toward the possible absence of oil film due to the misalignment of the propeller shaft at the after end of stern tube bearings.
The authors have developed two different calculating methods on shaft alignment; one is the optimum shaft alignment which makes the uniform distribution of contact stress in the bearing, and the other is to obtain the thickness of oil film on the stern tube bearings under operating condition.
The obtained results are summarized as follows.
1) On the assumption that the supporting point is set up at the after end of the stern tube bearings, the reduction of reaction force at the after end is superior to that of the shaft inclination.
2) If the allowable bending stress of the propeller shaft is large, the reaction force at after end becomes small, and the edge loading is considerably reduced.
3) When the revolution of propeller shaft is high, the oil film is formed enough to protect the bearing metal from damage.When the revolution is low, however, it is difficult to form the oil film, and the metal-to-metal contact will occur.
4) In the case of the ship “A”, the calculated thickness of oil film agrees qualitatively with the measured value, though the quantitative coincidence of the thickness is insufficient.