MACHINERY. 



product of the numbers of teeth in all the drivers, 

 and divide the result by the product of the numbers 

 of teeth in all the followers. In this rule, the 

 diameters, or the radii, or the circumferences, ol 

 the wheels may be substituted for the numbers ol 

 the teeth. 



Sometimes it is desirable to vary the speed of 

 a part of a machine at differ- 

 f] fl ent parts of a process, while 

 I \ \ 7 the driving -power continues 



1 1 J ' the same. One way of effect- 

 ing this is by conical pulleys 

 or drums, as in the fig. 44. 

 According as the band is 

 shifted along from one end to 

 the other and this the ma- 

 chine is made to do itself 



Fig. 44. 



the relative speed of the two pulleys will vary 

 to the desired degree. 



CHANGE OF ONE KIND OF MOTION INTO 

 ANOTHER. 



By means of a rack, b, and pinion, a (fig. 45), a 

 rotary motion is converted 

 into motion in a straight 

 line. Both motions in this 

 arrangement are necessarily 

 alternating. 



Fig. 46 represents a con- 

 tinuous rotary motion con- 

 verted into an alternate 



Fig. 45- 



rising and falling. The projections, called cams 

 or wipers, of the wheel W, depress the end of the 

 lever bearing the hammer, H, so that it is made 

 to rise and descend on 

 the anvil A thrice during 

 each revolution of the 

 wheel. The stampers 

 of fulling - mills, and 

 those used for crushing 

 oil - seeds and other 

 substances, are in a 

 similar way lifted and 

 dropped by the wipers of 

 a wheel coming in con- 

 Fig. 46. tact with a pin project- 

 ing from the stamper. 



For converting an alternating straight motion 

 into a continuous revolution, the most common 

 means is the crank. A crank is an arm or bend 



on an axle or shaft. 



Fig. 47. 



In the figure, the shaft 

 extends on both sides of 

 the bend, so that two 

 wheels could be turned, 

 and the motion is com- 

 municated by a con- 

 necting rod, S. If the 

 portion to the left of the 

 rod were removed, the 

 arrangement would be 

 what is called a winch, 

 and might be driven by 

 the hand. 



In turning a machine with a winch, the hand 

 may exert more or less pressure at every point of 

 the circuit ; but it is only about the middle of 

 each of the two semicircles, while the handle is 

 being pulled towards the body, and again while it 

 is being pushed away, that the full strength of the 



arms is effective. In a crank worked by a rod, 

 there are two positions in which the effect of the 

 pressure of the rod is reduced to nothing namely, 

 when the crank-arm C is in the same straight 

 line, upwards or downwards, with the rod S. A 

 push or pull of the rod in these positions can 

 only press the shaft against its bearings. The 

 effect is greatest when the rod and crank-arm are 

 at right angles, and it decreases gradually on 

 both sides of that position, until at the top and 

 bottom it is reduced to nothing. In order to carry 

 the crank over these dead points, as they are 

 called, a heavy wheel, called a fly-wheel, is fixed 

 on the shaft ; this receives part of the force of the 

 rod while at its best, acts as a reservoir, and by 

 its stored-up energy carries the shaft round when 

 the rod is powerless. 



In marine-engines and in locomotives, a fly- 

 wheel is unnecessary, and would be objectionable, 

 owing to its weight and the room it requires. In 

 these cases, the power is generally applied to the 

 shaft at two points by two connecting rods, each 

 proceeding from a separate cylinder; and the two 

 cranks or bends of the shaft are made at right 

 angles to one another, so that while the one is at 

 one of its dead points, the other is at one of its 

 best positions. 



The crank can effect the converse of the change 

 now described that is, the conversion of a rotary 

 motion into an alternating rectilinear one. But 

 it is more usual, when an alternating motion of 

 only small power is required, to take it from a 

 revolving shaft by means of an eccentric. One of 

 the most simple and elegant contrivances for con- 

 verting motion in a curve into motion in a straight 

 line is the Parallel Motion, invented by Watt. 



FRICTION. 



When one body rubs against another as it 

 moves, a certain force is felt to resist the motion. 

 This resistance is called friction. As a consider- 

 able proportion of the motive-power in all opera- 

 tions is spent in overcoming the friction of the 

 parts of the machine upon one another, and is 

 thus lost for the useful work, it is of great import- 

 ance to understand the nature of this obstructive 

 force, with a view to reduce it to the least possible 

 amount. Accordingly, a great many careful ex- 

 periments have been made on this subject, and 

 the result is a number of precise and valuable 

 facts or laws regarding friction, which are now 

 considered certain and reliable. The more im- 

 portant may be thus stated and illustrated. 



When a block of oak say, a cubic foot, which 

 weighs about 60 Ibs. is placed on a horizontal 

 table of cast-iron, the two surfaces being flat and 

 smooth, it requires a force of nearly the weight 

 of the block, or 24 Ibs. pulling horizontally, to 

 make it slide along the table. This measures the 

 friction between the two surfaces. Another block 

 of the same size and shape laid on the same table, 

 would require the same force to draw it ; and if 

 the two were laid side by side, and fastened to- 

 gether so as to become one block, it would evi- 

 dently require double the force, or 48 Ibs. to draw 

 the double block ; the amount of the friction being 

 thus still | of the weight, or of the pressure between 

 the two surfaces. But suppose that, instead of 

 being laid side by side, the second block were laid 

 on the top of the first, what is to be expected? 



m 



