BE AKIXOS DR17MR. ] 



APPLIED MECHANICS. 



on the base when tho shaft has been laid in its place. 

 The base has projecting Manges, with holes E, through 



Kg. 220. 



shaft and that of the cast-iron. The shaft, or a piece of 

 iron of equal size, being supported in the hole of the 



Fig. 222. 



which bolts pass for securing the plummer-block to the 

 beam or frame on which it may be fixed. These holes 

 are lengthened to permit a little transverse adjustment 

 if the plummer-block. The cap has generally an oil- 

 cup in its upper part, with a projecting nipple perforated, 

 through which a cotton wick conveys a supply of oil to 

 the surface of the shaft, acting like a siphon by capillary 

 attraction. The brasses C are sometimes made square or 

 octagonal in their outer surfaces, to prevent them from 

 turning round within the plumraer-block ; or, when they 

 are quite cylindrical, small studs or steady-pins H, pro- 

 jecting from them into holes in the cap and base, answer 

 the same purpose. The shaft has collars F, or parts of 

 larger diameter outside the brasses, and the brasses have 

 lips or Hanges, to prevent the shaft from moving longitu- 

 dinally. As the shaft or brasses wear from the constant 

 friction, the cap and upper brass can be more tightly 

 screwed down by the nnts of the bolts D, which are 

 often made double, so that the one nut tightens the 

 other in its place, and prevents it from becoming loosened 

 by the shaking of the machinery. 



Of late years, many bearings, instead of being fitted 

 with gun-metal bushes, have been lined with a soft metal, 

 in which tin is the principal ingredient. 



Fig. 221 represents a transverse section of a bearing 



Fig. 221. 



lined with soft metal ; A the cast-iron cap, and B the 

 base, cast with a hole larger than the shaft. D D soft 

 metal linings, filling the space between the surface of the 



bearing, the soft metal, melted, is poured into the space ; 

 and if the iron have been previously heated to nearly 

 the melting temperature of the soft metal, the latter, on 

 cooling, presents a smooth internal surface, in which 

 the shaft revolves with little friction or wear. When it 

 is inconvenient to form collars on the shaft to prevent 

 longitudinal motion, rings E, bored to fit the shaft, are 

 put on, and kept in their places by tightening-screws 

 pressing against the shaft. 



When it is not convenient to rest a shaft in plummer- 

 blocks supported on walls or beams, they may be made 

 to revolve in hanging bearings screwed up to a beam 

 above. These bearings may be lined as plummer-blocks, 

 with gun-metal or soft metal, and the cap tightened 

 down by a setting-screw, as indicated in Fig. 222. 



PULLEYS OB DRUMS. When it is desired to 

 convey motion from one shaft to another lying parallel 

 to it, each shaft has a pulley or drum attached to it, and 

 round their circumferences a flexible strap or band is 

 tightly strained. Thus e and / (Fig. 223) being parallel 

 Fig. 223. 



shafts at some distance apai-t, a pulley a is keyed on the 

 one, and a pulley 6 on the other, the flexible strap passing 

 round both. On rotary motion being given to the shaft 

 a and its pulley, the friction between the strap and its 

 circumference puts the former in motion, and it conveys 

 the rotation to 6 and its shaft in the same direction. 



When it is desired to reverse the direction of rotation, 

 the strap is crossed (Fig. 224). Also when a different 

 angular velocity of _ 224 



rotation is desired, 

 the pulleys are made 

 of different sizes. 

 If we suppose that 

 the driving pulley 

 has a circumference 

 of 6 feet, and the driven pulley a circumference of 

 3 feet, half the former ; since the strap travels at the 

 same rate with the circumference of the driving pulley, 

 and causes the circumference of the driven pulley to 

 move round with equal velocity, it is clear that the latter 

 must make two revolutions round its axis while tlu 



