Chapter 7 — BASIC FIRE CONTROL 



fed into the computer. If the information is 

 sufficient and correct, the computer tells how 

 to get to firing position and when to fire. The 

 electromechanical computer solves the problem 

 by means of a maze of interconnecting machinery. 

 Taken as a whole, the computer is complex and 

 difficult to comprehend. Basically, however, the 

 components are simple machines consisting of 

 gears, levers, cams, springs, etc., that perform 

 mathematical functions more rapidly than man 

 could possibly do. 



Mechanical Computing Devices 



Of the many devices for computing fire 

 control data, the easiest ones to understand 

 are mechanical devices that perform basic mathe- 

 matical operations. 



Solving the trigonometric functions of a right 

 triangle is an important mathematical operation 

 because fire control computations are based on 

 the right triangle. Three of the six natural 

 functions of an angle in trigonomet"y enter into 

 the fire control problem. They are the sine, 

 cosine, and tangent. The remaining functions — 

 secant, cosecant, and cotangent — seldom are used. 

 Other mathematical computations performed by 

 mechanical computing instruments include alge- 

 braic functions, simple addition, subtraction, and 

 multiplication. 



Mechanical computing devices depend on 

 rotary and linear motion in solving problems. 

 When an electrical signal is received by a 

 synchromotor, for instance, the motor turns 

 a shaft, providing an output of rotary motion. 

 This motion is then transmitted along various 

 shafts and through gear trains to some other 

 point in the computer. As long as the path of 

 motion is along shafts and gears, the motion 

 is rotary. If the rotary motion is used as an 

 input to a component such as a lever multiplier 

 or lever differential, the rotary motion is con- 

 verted to linear motion. Linear motion is trans- 

 mitted by pushrods and links, and may be 

 reconverted to rotary motion. 



Figure 7-4 shows the parts of a bevel gear 

 differential which combines two inputs into a 

 single output that is either the sum or the differ- 

 ence of the inputs. Aroxind the center of the 

 mechanism are four bevel gears meshed together. 

 Bevel gears on either side are called end gears; 

 those above and below are called spider gears. 

 The spider gears mesh with the end gears to 

 perform the actual addition or subtraction. They 



follow the rotation of the two end gears, turning 

 the spider shaft a number of revolutions propor- 

 tional to the sum or difference of the revolutions 

 of the end gears. 



Assume that the left side of the differential 

 is rotated while the right side is held fast. The 

 moving end gear drives the spider gears, making 

 them rotate on the stationary end gear. This 

 motion rotates the spider in the same direction 

 as the input and turns the output shaft a number 

 of revolutions proportional to the input. If the 

 right side is rotated while the left side is held 

 stationary, the same result occurs. When the 

 two inputs rotate in the same direction, the 

 differential adds; when they rotate in opposite 

 directions, the differential subtracts. The output 

 of a gear differential equals half the sum or 

 difference of the inputs. 



Another type of differential is the lever 

 differential, shown in figure 7-5. This device 

 performs the same functions as the gear dif- 

 ferential—it adds and subtracts, and the output 

 is proportional to the inputs — but the motion 

 used in accomplishing the job is linear. The 

 upper left drawing in figure 7-5 shows the 

 lever differential receiving equal inputs. If both 

 inputs push upward one-fourth inch, the output 

 shaft also moves upward the sams distance. 

 The drawing in the lower left shows unequal 

 inputs, and the triangle on the right represents 

 the values entered on the lower left drawing. 

 (When comparing the drawings, remember that 

 none of the joints are anchored. The entire 

 assembly is capable of moving.) In the lower 

 left drawing, consider input A to be zero and 



OUTPUT 



INPUT 



INPUT 



(OUTPUT) 



A=ZERO INPUT 

 B=+4 INPUT 

 C= + 2 OUTPUT 



INPUT 



NPUT 



OUTPUT MOVEMENT EQUALS 1/2 

 THE SUM OF THE INPUTS OR 1/2 

 THE DIFFERENCE BETWEEN THE 

 INPUTS 



Figure 7-5.- 



71.103 

 Lever differential. 



119 



