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CLUTCHES A clutch is a form of a coupling. It is designed to connect or disconnect a driving and a driven part as a
Figure 11-25.-Types of clutches. means of stopping or starting the driven part. There are two general classes of clutches: positive clutches and friction clutches. Positive clutches have teeth that interlock. The simplest is the jaw or claw type (fig. 11-25, A), usable only at low speeds. The teeth of the spiral claw or ratchet type (fig. 11-25, B) interlock only one waythey cannot be reversed. An example of this type of clutch is that seen in bicycles. It engages the rear sprocket with the rear wheel when the pedals are pushed forward and lets the rear wheel revolve freely when the pedals are stopped. The object of a friction clutch is to connect a rotating member to one that is stationary, to bring it up to speed, and to transmit power with a minimum of slippage. Figure 11-25, C, shows a cone clutch commonly used in motor trucks. Friction clutches may be single-cone or double-cone. Figure 11-25, D, shows a disc clutch, also used in autos. A disc clutch also may have several plates (multiple-disc clutch). In a series of discs, each driven disc is located between two driving discs. You may have had experience with a multiple-disc clutch on your car. The Hele-Shaw clutch is a combined conical-disc clutch (fig. 11-25, E). Its groove permits cooling and circulation of oil. Single-disc clutches are frequently dry clutches (no lubrication); multiple-disc clutches may be dry or wet (either lubricated or operated with oil). Magnetic clutches are a recent development in which the friction surfaces are brought together by magnetic force when the electricity is turned on (fig. 11-25, F). The induction clutch transmits power without contact between the driving and driven parts. The way pressure is applied to the rim block, split ring, band, or roller determines the names of expanding clutches or rim clutches. In one type of expanding clutch, right- and left-hand screws expand as a sliding sleeve moves along a shaft and expands the band against the rim. The centrifugal clutch is a special application of a block clutch. Machines containing heavy parts to be moved, such as a rolling mill, use oil clutches. The grip of the coil causes great friction when it is thrust onto a cone on the driving shaft. Yet the clutch is very sensitive to control. Diesel engines and transportation equipment use pneumatic and hydraulic clutches. Hydraulic couplings (fig, 11-25, G), which also serve as clutches, are used in the hydraulic A-end of electric-hydraulic gun drives. SUMMARY In this chapter we discussed the following elements and mechanisms used in naval machinery: Two types of bearings are used in naval machinery: sliding and antifrictional. Springs are another element used in machinery. Springs can be twisted, pulled, or stretched by force and can return to their original shape when the force is released. One basic mechanism of machines is the gear differential. A gear differential is a mechanism that is capable of adding and subtracting mechanically. Other basic mechanisms include linkages, couplings, cams and cam followers, and clutches. |
 
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