Page 79 - Introduction to Marine Engineering

66 Steam turbines and gearing

3. HP and LP thrust bearing weardown.
4. Main thrust bearing weardown.
5. Turning gear engaged (this would prevent starting of the turbine).
Such 'turbovisory' systems, as they may be called, operate in two ways.
If a tendency towards a dangerous condition is detected a first stage
alarm is given. This will enable corrective action to be taken and the
turbine is not shut down. If corrective action is not rapid, is unsuccessful,
or a main fault condition quickly arises, the second stage alarm is given
and the main trip relay is operated to stop the turbine.

Gearing

Steam turbines operate at speeds up to 6000rev/min. Medium-speed
diesel engines operate up to about 750rev/min. The best propeller
speed for efficient operation is in the region of 80 to lOOrev/min. The
turbine or engine shaft speed is reduced to that of the propeller by the
use of a system of gearing. Helical gears have been used for many years
and remain a part of most systems of gearing. Epicyclic gears with their
compact, lightweight, construction are being increasingly used in marine
transmissions.

Epicyclic gearing
This is a system of gears where one or more wheels travel around the
outside or inside of another wheel whose axis is fixed. The different
arrangements are known as planetary gear, solar gear and star gear
(Figure 3.12).
The wheel on the principal axis is called the sun wheel. The wheel
whose centre revolves around the principal axis is the planet wheel. An
internal-teeth gear which meshes with the planet wheel is called the
annulus. The different arrangements of fixed arms and sizing of the sun
and planet wheels provide a variety of different reduction ratios.
Steam turbine gearing may be double or triple reduction and will be a
combination from input to output of star and planetary modes in
conjunction with helical gearing (Figure 3.13).

Helical gearing
Single or double reduction systems may be used, although double
reduction is more usual. With single reduction the turbine drives a
pinion with a small number of teeth and this pinion drives the main
wheel which is directly coupled to the propeller shaft. With double