APPLIED MECHANICS. 



[BEAM ENGINES. 



parallel motion is also fitted there to give rectilineal 

 motion to the pump-rod. 



riff. 107. 



BEAM ENGINES. In marine engines, where it is 

 desirable to keep the weight as low as possible, the ar- 

 rangement of beam ia different. Instead of one beam 



Fig. 203. 



above the cylinder and crank, two beams are fitted be- I 

 low, one on each sido of the cylinder, and the parallel | 



motion employed in that case is somewhat different from 

 tint which wo have described, although the principle* 

 cmlxxlied are very similar. 



The alternate movements of the steam to and from 

 the opposite ends of the cylinder, are effected in con- 

 densing engines in the same manner as non-condensing 

 engines, by a slide worked by an eccentric, or by valves 

 worked by cams. In single-acting engine* valves are 

 generally employed, and they are worked by levers struck 

 by pins or tappets fixed to the air-pump rod, as it as- 

 cemls and descends along with the piston. 



The single-acting beam-engine is almost exclusively 

 employed for pumping. The form of engine most com- 

 monly used for this purpose is what is called the Cornish 

 engine, because at the mines in Cornwall these engines 

 have been employed with groat economy and efficiency 

 for draining to great depths. 



The steam is admitted above the piston at considerable 

 pressure, and the supply is cut off when the piston has 

 performed a small part of its down stroke, so that the 

 rest of its stroke is effected by the expansion of the 

 steam. When the piston has reached the bottom of the 

 cylinder, the steam is permitted to pas* from its upper 

 to its lower side, so that it does not press it in either 

 direction ; but the weight of the pump-rods at the other 

 end of the beam, loaded, if necessary, with additional 

 weight, causes the piston again to ascend to the top of 

 the cylinder. The steam is again admitted above the 

 piston, while that which is below it flows into the con- 

 denser ; and thus the alternation is continued, the 

 valves which control the movements of the steam being 

 worked by levers and tappets from the air-pump rod. 



Fig. 208 represents a double-acting condensing beam- 

 engine working a crank. Here the weight of the piston 

 and air-pump bucket, and their rods at one end of the 

 beam, is balanced by that of the connecting rod at the 

 other end, and the steam is alternately admitted by the 

 slide to the upper and lower sides of the piston, and 

 thence to the condenser, so that each end of the beam 

 is alternately pushed upwards and pulled downwards by 

 the pressure on the piston. A the steam-pipe from the 

 boiler, supplying steam to the slide-jacket or space round 

 the slide B. C the cylinder fitted with piston, and 

 piston-rod connected to one end of 

 the beam M by the p;ir;illol motion L. 

 E the condenser, communicating with 

 the slide-jacket by the eduction pipe 

 D. F the injection-cock, admitting 

 water into the condenser by a jet 

 from the cold cistern in which it is 

 placed. G the air-pump, worked 

 by a rod from the parallel motion, 

 and discharging into the hot well 1), 

 in which is placed the feed-pump, 

 worked by a rod from the beam, so as 

 to draw water from the hot well, and 

 propel it by a feed-pipe to the boiler. 

 K the cold-water pump, worked by 

 a rod from the beam, drawing water 

 from a well or stream, and supplying 

 the cold cistern for condensation. N 

 the connecting-rod, worked from one 

 end of the beam, and giving rotary 

 motion to the crank O. P the fly- 

 wheel, fixed on the crank-shaft, and 

 revolving with it, with the necessary 

 momentum for bringing the crank 

 over the dead centres, or highest and 

 lowest points of its revolution, where 

 the connecting-rod is ineffective to 

 turn it. Q the governor, caused to 

 revolve by a strap or band, and suitable 

 gearing connecting it with the crank- 

 shaft. The steam-pipe A is fitted with 

 a throttle-valve connected with the 

 governor by levers and rods, so that 

 the velocity of the engine is controlled 

 by the throttling of the steam, as already described.* 



Bet antr, p. 862. 



