NAVAL ORDNANCE AND GUNNERYVOLUME 2, FIRE CONTROLCHAPTER 19SURFACE FIRE CONTROL PROBLEM
 HOME          INDEX Chapter 19 SURFACE FIRE CONTROL PROBLEMA. GeneralB. Analytical solutionC. Graphic rangekeepingD. Mechanical solution-generalE. Basic mechanismsF. Mechanical solution-basic rangekeepersG. Mechanical solution-establishing the horizontal plane
 E. Basic Mechanisms19E1. Introduction The study of any computer should be based on an understanding of the various basic mechanisms of which it is composed. These devices perform the mathematical computations essential to solution of the fire control problem. The present discussion will be limited to the most important basic mechanisms.Mechanisms are represented by conventional symbols which facilitate their presentation on functional diagrams. Since the understanding of such diagrams is essential to the officer performing gunnery duties, these symbols are included in the figures depicting each mechanism.19E2. ShaftsThe computer receives inputs, performs computations, and transmits outputs. These quantities are in various units such as yards of range, degrees of elevation, knots of ship speed. Since, however, each of these quantities can be expressed as a number, it is possible to translate all quantities to shaft rotation. This procedure not only provides mechanical representation of the quantity; it also makes it easy to change or modify the quantity by suitable rotation. As the value of an input (such as own-ship speed) changes, the values of the computed quantities will change accordingly. These changes must be carried instantaneously and continuously to all the mechanisms affected by the change of input. These changing values are carried from one mechanism to another by shaft rotation.Turning a shaft changes the value of the angle, speed, or distance represented by the position of that shaft. Rotation in one direction increases the value represented, and is considered positive; rotation in the other direction decreases the value, and is negative. From this, it follows that every shaft has a zero position, even though, as is sometimes true, the shaft may be restrained from reaching the zero position.One revolution of a shaft can represent any convenient amount of change in the given quantity. For example, one revolution of the elevation shaft may represent a 3-degree change in elevation. On a range shaft, one turn may represent 100 yards of range. The value that a shaft carries in one revolution is called shaft value. Total value carried by a shaft is the shaft value multiplied by the number of revolutions made by the shaft from its zero position.
 It is sometimes necessary to add or subtract a constant from a value carried by a shaft. For example, in the computer the value of range minus a constant is used. In this instance the crank input is range, but the mechanism is positioned for range minus K. The constant K can be set into the transmission line by a sleeve coupling or a clamp, as is shown in figure 19E1. Here a sleeve coupling joins two shafts which must introduce into the mechanism range in yards minus a constant of 50 yards. Each shaft has a counter showing its position. Both shafts are initially positioned at 50 yards. If one clamp is loosened, the input shaft can be turned until its counter reads zero without moving the crankshaft. If the clamp is now tightened, with the value of 50 yards on the crankshaft and zero on the input shaft, the two shafts will turn together when the crank is turned, but the input to the mechanism will always be 50 yards less than the crank input. This use of a clamp is called “putting a constant offset on the line.” The offset can be plus or minus.19E3. GearsGears are wheels with mating teeth cut so that one can turn the other without slipping. If two mating gears are the same size, they will have the same number of teeth. One revolution of the driving gear will turn the driven gear one revolution, because each tooth of the driving gear will push one tooth of the driven gear across the line between their centers.If two gears are of different sizes, the smaller is called a pinion, the larger a spur. When a spur gear and a pinion mesh together, their shafts turn at different speeds. If the spur gear has twice as many teeth as the pinion, one revolution of the driving gear will turn the pinion two revolutions.The ratio between the number of teeth on the driving gear and the number of teeth on the driven gear is called the gear ratio. Gear ratios are often used for no other reason than to change the shaft values, by multiplication or division. If a shaft has a value of 500 yards per revolution, this can be reduced to 50 yards per revolution by a 1:10 gear ratio.Although there are many different types of gears, most of those used in fire control installations are of three types: spur gears, bevel gears, and sector gears.Spur gears are used to connect parallel shafts and transmit motion in opposite directions, as in figure 19E2.Bevel gears can be designed to transmit motion between shafts at almost any angle to one another, as indicated in figure 19E3. By using bevel gears, several shafts at different angles can be driven by one driving shaft.Sometimes only a part of a gear is needed, where the motion of the pinion is limited. In this case, space is saved by the use of a sector gear, illustrated in figure 19E4. A special type of sector gear is the rack, frequently used to convert rotary motion to linear motion. A rack is a straight bar with gear teeth cut in it, as shown in figure 19E5. If the rack drives a gear, it converts linear motion into rotary motion. The rack is restrained by guide rails which permit motion in one line only. The position of a rack indicates a value by the amount it has moved from zero.It must be understood that motion of a shaft, rack, gear, or dial has no meaning except that assigned by the designer. It is not difficult to design an assembly so that proper values are entered and transmitted. For example, the conversion constant 0.563, which converts knots to yards per second, is applied by a gear ratio. (A close approximation can be obtained by a gear ratio of 22:39, which equals 0.564.) The mechanism shown in figure l9E6 will serve to illustrate some of the principles presented in this section. The sequence starts with an input shaft, one turn of which represents 400 knots. The ratios of the gears have been designed to use the 4-inch motion of the rack, which is driven by a screw with 20 threads to the inch. Since each revolution of the screw shaft represents 5 knots, each inch the screw (and therefore the rack) moves equals 100 knots, and 4-inch travel of the rack represents 400 knots. It is possible to transform the output of the rack (linear motion) to shaft rotation as shown. If an output shaft carrying a gear with pitch diameter of 1 inch is used, one revolution of this output shaft will represent                      or 3.14 inches of linear motion of the rack. The value 4/3.14 turns of the output shaft will then represent 400 knots.