The basic characteristics of a propulsion unit usually make it necessary for the drive mechanism to change both the speed and the direction of shaft rotation. The engine in many installations includes a device that permits a speed reduction from the engine to the propeller shaft so that both the engine and the propeller may operate efficiently. This device is a combination of gears and is called a reduction gear.

REDUCTION GEARS

Engines must operate at relatively high speeds for maximum efficiency. Propellers must operate at lower speeds for maximum efficiency. Therefore, reduction gears are used to allow both the engine and the propeller to operate within their most efficient revolutions per minute (rpm)

Figure 8-5.-Locked-train-type gearing.

ranges. A typical steam turbine reduction gear is shown in figure 8-5

The use of reduction gears is by no means limited to ship propulsion. Other machinery, such as ship's service generators and various pumps, also have reduction gears. In these units, as well as in shipboard propulsion units, engine operating efficiency requires a higher rpm range than that suitable for the driven unit.

Reduction gears are classified by the number of steps used to bring about the speed reduction and the arrangement of the gearing. A gear mechanism consisting of a pair of gears or a small gear (pinion) driven by the engine shaft, which directly drives a large (bull) gear on the propeller shaft, is called a single-reduction gear. In this type of arrangement, the ratio of speed reduction is proportional to the diameter of the pinion and the gear. For example, in a 2-to-1 single-reduction gear, the diameter of the driven gear is twice that of the driving pinion. In a 10-to-1 single-reduction gear, the diameter of the driven gear is 10 times that of the driving pinion.

Steam propulsion-type ships built since 1935 have double-reduction propulsion gears. In this type of gear, a high-speed pinion, connected to the turbine shaft by a flexible coupling, drives an intermediate (first reduction) gear. This gear is connected by a shaft to the low-speed pinion that, in turn, drives the bull gear (second reduction) mounted on the propeller shaft. A 20-to-1 speed reduction might 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 and the secondreduction gear on the propeller shaft.

For a typical example of a double-reduction application, let us consider the main-reduction gear shown in figure 8-6. The high-pressure and low-pressure turbines are connected to the propeller shaft through a locked-train type of double-reduction gear.

NOTE: This type of reduction gear is used aboard many naval combatant ships.

First-reduction pinions are connected by flexible couplings to the turbines. Each of the first-reduction pinions drives two first-reduction gears. A second-reduction (slow speed) pinion is attached to each of the first-reduction gears by a quill shaft and flexible couplings. These four pinions drive the second-reduction (bull) gear that is attached to the propeller shaft.