Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGDOUBLE HELICAL GEARINTERNAL SPUR GEARWORM GEARSINGLE HELICAL GEAREXTERNAL SPUR GEARBEVEL GEAR5.22Figure 5-16.— Gear forms used in shipboardmachinery.Double helical gears have smoother action andless tooth shock than single reduction gears.Since the double helical gears have two sets ofteeth at complementary angles, end thrust (suchas is developed in single helical gears) is prevented.In the double reduction gears used on mostships, a high speed pinion which is connected tothe turbine shaft by a flexible coupling drives anintermediate (first reduction) gear. The firstreduction gear is connected by a shaft to thelow speed pinion which in turn drives the bull(second reduction) gear mounted on the propellershaft. If we suppose a 20 to 1 speed reductionis desired, this could be accomplishedby having a ratio of 2 to 1 between the high speedpinion and the first reduction gear and a ratio of10 to 1 between the low speed pinion on the firstreduction gear shaft and the second reductiongear on the propeller shaft.A typical double reduction gear installationfor a DD 692 class destroyer is shown in figure5-17. In this type of installation, the cruisingturbine is connected to the high pressure turbinethrough a single reduction gear. The cruisingturbine rotor carries with it a pinion whichdrives the cruising gear, coupled to the highpressure turbine shaft. The cruising turbinerotor and pinion are supported by three bearings,one at the forward end of the turbine andone on each side of the pinion in the cruisingreduction gear case.The high pressure turbine and the low pressureturbine are connected to the propeller shaftthrough a locked train double reduction gear ofthe type shown in figure 5-18, First reductionpinions are connected by flexible couplings tothe turbines. Each of the first reduction pinionsdrives two first reduction gears. Attached toeach of the first reduction gears by a quill shaftand flexible couplings (fig. 5-19) is a secondreduction pinion (low speed pinion). These fourpinions drive the second reduction gear (bullgear) which is attached to the propeller shaft.Locked train reduction gears have the advantageof being more compact than other types, forany given power rating. For this reason, all highpowered modern combatant ships have lockedtrain reduction gears. Another type of reductiongearing, known as nested gearing, is illustratedin figure 5-20. Nested gearing is used on mostauxiliary ships but is not used on combatantships. As may be seen, the nested gearing isrelatively simple; it employs no quill shaftsand uses a minimum number of bearings andflexible couplings.FLEXIBLE COUPLINGSPropulsion turbine shafts are connected tothe reduction gears by flexible couplings whichare designed to take care of very slight misalignmentbetween the two units. Most flexiblecouplings are of the gear type shown in figure5-21. The coupling consists of two shaft ringshaving internal gear teeth and an internal100