Thrust Block :-

The main thrust block transfers the forward and astern thrust of propeller shaft through the oil film and thrust pads to the engine casing and hull, and limits the axial movement of the shaft. Some axial clearance is essential to allow formation of an oil film in the wedge shape between the shaft collar and the thrust pads. The clearance is also needed to allow for expansion due to temperature rise. The actual clearance required depends on dimensions of pads, speed, thrust load and the type of oil used. If axial clearance is too small, high bearing temperature, power loss and failure can result. A larger than necessary axial clearance will not cause any harm to the thrust bearing pads, but axial movement of the shaft must be limited for the protection of main machinery. Checking of thrust clearance, involves jacking the shaft axially to the end of its travel in one direction and then back to the limit of travel in the other direction. Total movement of thrust (about lmm for 500 mm dia shaft being typical) shaft on a dial gauge gives the clearance. The oil film thickness varies from .013 mm minimum to 0.05mm max. coeff of friction varies from .001 to .005, and load of 35 - 50 MN/m². Use of the feeler between thrust ring and casing also gives the clearance. However use of feeler between thrust pads and collar may cause damage and then can give a false reading.

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