PRINCIPLES OF NAVAL ENGINEERING 



DOUBLE HELICAL GEAR 



SINGLE HELICAL GEAR 



INTERNAL SPUR GEAR 



EXTERNAL SPUR GEAR 



WORM GEAR 



BEVEL GEAR 



5.22 



Figure 5-16.— Gear forms used in shipboard 

 machinery. 



Double helical gears have smoother action and 

 less tooth shock than single reduction gears. 

 Since the double helical gears have two sets of 

 teeth at complementary angles, end thrust (such 

 as is developed in single helical gears) is pre- 

 vented. 



In the double reduction gears used on most 

 ships, a high speed pinion which is connected to 

 the turbine shaft by a flexible coupling drives an 

 intermediate (first reduction) gear. The first 

 reduction gear is connected by a shaft to the 



low speed pinion which in turn drives the bull 

 (second reduction) gear mounted on the pro- 

 peller shaft. If we suppose a 20 to 1 speed re- 

 duction is desired, this could be accomplished 

 by having a ratio of 2 to 1 between the high speed 

 pinion and the first reduction gear and a ratio of 

 10 to 1 between the low speed pinion on the first 

 reduction gear shaft and the second reduction 

 gear on the propeller shaft. 



A typical double reduction gear installation 

 for a DD 692 class destroyer is shown in figure 

 5-17. In this type of installation, the cruising 

 turbine is connected to the high pressure turbine 

 through a single reduction gear. The cruising 

 turbine rotor carries with it a pinion which 

 drives the cruising gear, coupled to the high 

 pressure turbine shaft. The cruising turbine 

 rotor and pinion are supported by three bear- 

 ings, one at the forward end of the turbine and 

 one on each side of the pinion in the cruising 

 reduction gear case. 



The high pressure turbine and the low pres- 

 sure turbine are connected to the propeller shaft 

 through a locked train double reduction gear of 

 the type shown in figure 5-18, First reduction 

 pinions are connected by flexible couplings to 

 the turbines. Each of the first reduction pinions 

 drives two first reduction gears. Attached to 

 each of the first reduction gears by a quill shaft 

 and flexible couplings (fig. 5-19) is a second 

 reduction pinion (low speed pinion). These four 

 pinions drive the second reduction gear (bull 

 gear) which is attached to the propeller shaft. 



Locked train reduction gears have the advan- 

 tage of being more compact than other types, for 

 any given power rating. For this reason, all high 

 powered modern combatant ships have locked 

 train reduction gears. Another type of reduction 

 gearing, known as nested gearing, is illustrated 

 in figure 5-20. Nested gearing is used on most 

 auxiliary ships but is not used on combatant 

 ships. As may be seen, the nested gearing is 

 relatively simple; it employs no quill shafts 

 and uses a minimum number of bearings and 

 flexible couplings. 



FLEXIBLE COUPLINGS 



Propulsion turbine shafts are connected to 

 the reduction gears by flexible couplings which 

 are designed to take care of very slight mis- 

 alignment between the two units. Most flexible 

 couplings are of the gear type shown in figure 

 5-21. The coupling consists of two shaft rings 

 having internal gear teeth and an internal 



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