CLUTCHES AND REVERSE GEARS
Clutches are normally used on direct-drive propulsion engines to provide a means of disconnecting the engine from the propeller shaft. In small engines, clutches are usually combined with reverse gears and are used for maneuvering the ship. In large engines, special types of clutches are used to obtain special coupling or control characteristics and to prevent torsional (twisting) vibration.
Diesel-propelling equipment on a boat or a ship must be capable of providing backing-down power as well as forward power. There are a few ships and boats in which backing down is accomplished by reversing the pitch of the propeller. Most ships, however, back down by reversing the direction of rotation of the propeller shaft. In mechanical drives, reversing the direction of rotation of the propeller shaft may be accomplished in one of two ways. You can reverse the direction of engine rotation or use the reverse gears.
Reverse gears are used on marine engines to reverse the rotation of the propeller shaft during maneuvering without reversing the rotation of the engine. They are normally used on smaller engines. If a high-output engine has a reverse gear, the gear is used for low-speed operation only and does not have full-load and full-speed capacity. For maneuvering ships with large directpropulsion engines, the engines are reversed.
The drive mechanism of a ship or a boat is required to do more than reduce speed and reverse
Figure 8-6.-Typical steam turbine and reduction gear.
the direction of shaft rotation. It is frequently necessary to operate an engine without having power transmitted to the propeller. For this reason, the drive mechanism of a ship or boat must include a means of disconnecting the engine from the propeller shaft. The devices used for this purpose are called clutches.
The arrangement of the components depends on the type and size of the installation. In some small installations, the clutch, the reverse gear and the reduction gear may be combined in a single unit. In other installations, the clutch and the reverse gear may be in one housing and the reduction gear in a separate housing attached to the reverse-gear housing.
In large engine installations, the clutch and the reverse gear are sometimes combined and are sometimes separate units. They are located between the engine and a separate reduction gear,
or the clutch may be separate and the reverse gear may be combined.
In most geared-drive, multiple-propeller ships, the propulsion units are independent of each other. An example of this type of arrangement is shown in figure 8-7
In some installations, the drive mechanism is arranged so that two or more engines drive a single propeller. This is accomplished by having the driving gear, which is on or connected to the crankshaft of each engine, transmit power to the driven gear on the propeller shaft.
Friction clutches are commonly used with smaller, high-speed engines, up to 500 horsepower (hp). Certain friction clutches, however, in combination with a jaw-type clutch, are used with engines up to 1400 hp; and pneumatic clutches with a cylindrical friction surface are used with engines up to 2000 hp.
Friction clutches are of two general stylesdisk and band. In addition, friction clutches can be classified as dry or wet types, depending on whether the friction surfaces operate with or without a lubricant. The designs of both types are similar, except that the wet clutches require a large friction area. The advantages of wet clutches are smoother operation and less wear of the friction surfaces. Wear results from slippage between the surfaces during engagement and disengagement and, to a certain extent, during the operation of the mechanism. Some wet-type clutches are periodically filled with oil. In other clutches, the oil is a part of the engine-lubricating system and is circulated continuously.
Twin-Disk Clutch and Gear Mechanism
One of the several types of transmissions used by the Navy is the Gray Marine transmission mechanism. Gray Marine high-speed diesel engines are generally equipped with a combination clutch and a reverse and reduction gear unit, all contained in a single housing at the after end of the engine.
The clutch assembly of the Gray Marine transmission mechanism is contained in the part of the housing nearest the engine. It is a dry-type, twin-disk clutch with two driving disks. Each disk is connected through shafting to a separate reduction gear train in the after part of the housing. One disk and reduction train is for reverse rotation of the shaft and propeller, and the other disk and reduction train is for forward rotation.
Figure 8-8.-Diagram of airflex clutch and reverse-reduction gear.
Airflex Clutch and Gear Assembly
On the larger diesel-propelled ships, the clutch, reverse, and reduction gear unit has to transmit an enormous amount of power. To maintain the weight and size of the mechanism as low as possible, special clutches have been designed for large diesel installations. One of these is the airflex clutch and gear assembly used with engines on LSTs.
A typical airflex clutch and gear assembly for ahead and astern rotation is shown in figure 8-8 There are two clutches, one for forward rotation and one for reverse rotation. The clutches, bolted to the engine flywheel, rotate at all times with the engine at engine speed. Each clutch has a flexible tire (or gland) on the inner side of a steel shell. Before the tires are inflated, they will rotate out of contact with the drums, which are keyed to the forward and reverse drive shafts. When air under pressure (100 psi) is sent into one of the tires, the inside diameter of the clutch decreases. This causes the friction blocks on the inner tire surface to come in contact with the clutch drum, locking the drive shaft with the engine.
Hydraulic Clutches or Couplings
The fluid clutch (coupling) is widely used on Navy ships. The use of a hydraulic coupling eliminates the need for a mechanical connection between the engine and the reduction gears. Couplings of this type operate with a small amount of slippage.
Some slippage is necessary for operation of the hydraulic coupling, since torque is transmitted because of the principle of relative motion between the two rotors. The power loss resulting from the small amount of slippage is transformed into heat that is absorbed by the oil in the system.
Compared with mechanical clutches, hydraulic clutches have a number of advantages. There is no mechanical connection between the driving and driven elements of the hydraulic coupling. Power is transmitted through the coupling very efficiently (97 percent) without transmitting torsional vibrations or load shocks from the engine to the reduction gears. This arrangement protects the engine, the gears, and the shaft from sudden shock loads that may occur as a result of piston seizure or fouling of the propeller. The power is transmitted entirely by the circulation of a driving fluid (oil) between radial passages in a pair of rotors. In addition, the assembly of the hydraulic coupling will allow for slight misalignment.