26. State about thrust shaft & bearing:

a. Thrust shaft:

-The thrust shaft is installed between of crankshaft and intermediate shaft and together with this shaft;  it transmits power of the main engine to the propeller.

-Thrust shaft made of forged steel, has one or two collars. In medium or high-speed engines, it is installed in the reduction gear, which is located in the after part of the main engine.

a.     Thrust bearing:

-Thrust bearing is designed to absorb thrust force from the propeller and transmits it to the hull of the ship.

-There are three kinds of thrust shaft bearing corresponding to the thrust shafts, these are:

+Thrust shaft with tapered roller bearing

+Thrust shaft with one collar

+Thrust shaft with two or more collars

*Tapered roller bearing

-A tapered roller bearing is normally installed on small ships in a reduction gear. This kind of bearing has light weigh, small size and small friction coefficient, but for the big ships it is difficult to inspect, repair and change the roller bearing.

*Thrust bearing (with multi- collars thrust shaft):

-This is a traditional type used in the past such as old steam ship. Because one collar could not withstand heavy thrush force from the propeller so two, three or more collars are provided to transmit thrush force to bearings, which are fixed tightly to hull of the ship. Because big size in structure and  difficulty in repairing so it is rarely used nowadays.

*Thrust bearing (with one collar thrust shaft)

-Due to dimension of the thrust with many collars is so large, so the change in design of thrust bearing was developed with one collar thrust bearing.

-When shaft rotating, between surface of the collar and surface of the pad creates an oil wedge. Because of that, the oil firm is always maintained between two surfaces and this kind of bearing can withstand heavy thrush force.

-When shaft working, L.O temperature is increased. To maintain the temperature in the bearing case lower 700 C, a L.O cooler must be fitted. Besides that, a heater also is installed to reduce viscosity of L.O in cold weather.

b.     Effect of a worn-out thrust bearing

-Misalignment of the thrust shaft and shafting system.

-Too small space between the collar and its bearing.

-Short of oil in the quantity.

- These causes may result in heating up and wearing out.

-With a worn-out thrust bearing, the crankshaft will be approached forward in ahead turning and afterward in astern turning, thus cause the following problems:

+Heat will be produced by friction between webs and the crank pin metals or the side face of the main bearings.

+Slant movements of the piston will increase wears of the piston and the cylinder liner in fore and aft direction. The misalignment of the connecting rod will increase the temperature of the piston pin and crank pin metal.