CONVERTING POWER TO DRIVE

Figure 8-4.—Typical gas turbine reduction restarted. This allows the engine to operate in the opposite direction. This operation takes time and presents a difficult situation if sudden changes in direction are required. To eliminate this stopping-starting situation and to make a smoother transition from forward to reverse in less time, reverse-reduction gears, clutches, and controllable-pitch propellers are used. Figure 8-3 shows a typical diesel engine. GAS TURBINE DRIVE Gas turbines are used on patrol craft, destroyers, cruisers, frigates, amphibious craft, and auxiliary oilers. Compared to other propul- sion units, they offer a high power-to-weight ratio. Gas turbine gear drive units consist of gas turbines, reduction gears, and controllable-pitch propeller units. Figure 8-4 shows a typical gas turbine reduction gear arrangement. gear module arrangement. CONVERTING POWER TO DRIVE 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) 8-3