APPLIED MECHANICS. 



[FEED-PUMP CRAVK. 



which are fitted with a governor, to which it in attached 

 by meant of proper connecting rods ; and by it* meani 

 the speed of the engine U regulated with great nicety, 

 a* more or loss steam U permitted to pau the valve. 



Fl. 171 



FEED-PUMP. The feed-pump of an engine is for 

 the purpose of supplying the boiler with water to take 

 the place of that which is boiled off, and passes away in 

 the form of steam, after working the engine. It consists 

 of a barrel C (Fig. 173), in the upper part of which a 

 plunger G is worked alternately upwards aud downwards 

 through a stuffing-box. At the lower end of the barrel 



Tig. 171 



ia a valve-box, containing a suction-valve B, covering a 

 pipe A by which the water is drawn from a convenient 

 reservoir, and a discharge-valve D, past which the water 

 has to_ flow in its progress to the boiler by the feed- 

 pipe E. When the plunger is raised, the valve D being 

 kt-pt down in its seat by the pressure of water in the 

 boiler (equivalent to that of the steam} communicated 

 through the feed-pipe E, the vacuum left by the rise of 

 the plunger is filled by water entering from A and raising 

 the valve B for its passage. On the descent of the 

 ]il linger, the water being forced out of the barrel, presses 

 down the valve B, but raises the valve D, and flows on- 

 wards to the boiler. At some convenient part of the 

 feed-pipe E there is always fitted a stop-cock, or shut-oil' 

 valve, for completely cutting off communication between 

 the pump and the boiler in case of the valves being de- 

 ranged. But water being almost totally incompressible, 

 it would be extremely hazardous to close this communi- 

 cation while the pump is in action ; for in that case the 

 barrel must be burst open, or some part of the machinery 

 that works the pump must be broken. It is, therefore, 

 nmal to provide also a relief-valve, constructed exactly 

 like a safety-valve on the feed-pipe, to permit the efflux 

 of the water when it* ordinary passage is closed. The 

 cover V of tin- feed-valve box should be capable of being 

 readily moved to give access to the valves ; and it is 

 often made of considerable size, hollowed out to contain 



Fig. 174. 



air, which is compressed by the influx of water during 

 the descent of the plunger, and react* to force the water 

 onwards to the boiler while the plunger ascends. In 

 all cases, indeed, where water under considerable pres- 

 sure is exposed to the recurring action of a propelling 

 force, as in the feed-pump, an air-vessel should be pro- 

 vided to act as a spring, relieving the blow on the 

 and regulating its motion to a gradual flow instead of a 

 sudden movement. 



CRANK AND CONNECTING-ROD. Having now 

 described the cylinder in which, and thepwton on which, 

 the steam acts to put the machinery in motion the slide 

 by which the alternation of the course of the steam U 

 effected the throttle-valve by which its quantity is re- 

 gulated and the feed-pump by which the necessary 

 supply of water is maintained in the boiler ; we have to 

 inquire how the reciprocating motion of the piston is 

 converted into the rotary movement required for driving 

 machinery, and how this rotary movement produces the 

 reciprocating motion of the slide and feed-pump, and 

 governs the action of the throttle- valve. 



To the end A (Fig. 174) of the piston-rod there is 

 jointed the connecting-rod A B, having an eye at B 

 working on the crank-pin, or pin fixed to the crank an 

 arm B C projecting from the main shaft or spindle C. 

 The crank-pin can move round in a circle, the diaincU r 

 of which is exactly equal t<> the 

 length of stroke, or distance 

 through which the piston travels 

 in the cylinder. As the piston 

 descends from F to G, making 

 the down-stroke, the crank is 

 B caused to revolve from E round 

 to D, one half-revolution. Again, 

 while the piston ascends fn.in < ; 

 to F, making the upstroke, the 

 crank revolves from D round to 

 E, another half-revolution. Kach 

 revolution, then, of the crank 

 requires a double stroke of the 

 piston ; and to effect it, the 

 upper and lower portions of the 

 cylinder have each to be filled 

 and emptied of steam ; or, as 

 their capacities are equal, the 

 cylinder has for each revolution 

 to be twice filled and emptied. 

 It is obvious that, at every dif- 

 ferent point of its revolution, the 

 crank is acted on by a different 

 force, owing to the varying ob- 

 liquity of the connecting-rod. 

 At the two extreme points, D 

 and E, where the crank is in a 

 line with the connecting-rod, 

 the effect of the piston to cause 

 it to revolve is reduced to no- 

 thing ; for it merely pushes or 

 pulls it against the central shaft. 

 These points are technically 

 called the dead centres, because 

 there the force of the piston is dead or ineffective. But 

 to make up for the total want of action at those points, 

 we find that at some other points the effect of the force 

 passing through the connecting-rod to turn the crank, is 

 greater than the pressure on the piston, in consequence 

 of the obliquity of its action. 



Again, the piston, during a revolution or double 

 stroke, passes through a distance equivalent to twice the 

 diameter of the crank-circle ; while the crank-pin passes 

 over the circumference of that circle, more than 3 times 

 its diameter. The influence of the pressure on the piston 

 to turn the crank may be best conceived by a graphical 

 delineation of the force in the following manner : If 

 we divide the circle described by the crank ]>in round 

 the centre C into any number of equal parts (Fig. 176), 

 and draw a straight lino A B equal to the half-circum- 

 ference divided into corresponding parts, A B represents 

 the distance through which the crank-pin moves during 



