THE. ELEMENTS OF MACHINERY. 



101 



What is the 

 most frequent 

 method of 

 transmitting 

 motion through 

 ft conibinatioa 

 o; wheels? 



When great power is required, wheels and axles may be combined to- 

 gether in a manner similar to that of the compound lever already explained 

 (§ 207). By such a combination we gain the advantage of using a very large 

 wheel with a small axle, without their inconveniences. 



218. The most frequent metliod of transmitting motion 

 through a combination of wheels, is by the construction of 

 teeth upon their circumference, so that the teeth of each 

 wheel falling between those of the other, the one necessarily 

 pushes forward the other. When teeth are thus afiBxcd to 

 the circumference of a wheel, they are termed cogs; upon an 



axle, they are termed leaves, while the axle itself is called a pinion. 



jj, ^- Fig. 75 represents a combination 



of wheels and axles for the trans- 

 mission of power. If the teeth on 

 the axle of the wheel c act on six 

 times the number of teeth on the 

 circumference of the second wheel, 

 the second will turn only once for 

 every six turns of the first. In the 

 same manner the second wheel, by 

 turning six times, turns the third 

 wheel once ; consequently, if the proportion between the wheels and their 

 axles be preserved in all three, the third turns once, the second six times 

 and the first thirty-six times. Now, as the wheel and axle act in all respects 

 like a simple lever, and a combination of wheels and axles as a combina- 

 tion of levers, there is no difficulty in understanding how a mechanical ad- 

 vantage is gained by this contrivance. The power is to the weight as the 

 product of the diameter of all the axles is to the product of the diameter of 

 all the wheels. Thus, if the diameter of all the axles be expressed by the 

 numbers 2, 3, and 4, and the diameters of the wheels, c, /, and g, be expressed 

 by the numbers 20, 25, and 30, then power will be to the weiyht as 2X3X 

 4=24, is to 20X25X30=^15,000 ;— or a power of 24 at the first wheel will 

 balance 15,000 at the axle of the last wheel. 



219. One of the most familiar instances of combined wheel- 

 work is exhibited in clocks and watches. One turn of the axle 

 on which the watch-key is fixed, is rendered equivalent, by a 

 train of wheel-work, to about 400 turns, or beats, of the bal 

 ance-whcel ; and thus the exertion, during a few seconds, of 



the hand which winds up, gives motion for twenty-four, or thirty hours. By 



What arc fa- 

 miliar illustra- 

 tions of com- 

 pound wheel 

 work? 



FlQ. 76. 



Fig. 77. 



as by teeth, 

 turoing-lathe. 



increasing the number of wheels, 



time-pieces are made which go for 



a year, or a greater length of time. 

 "Wheels may be connected and 



motion communicated from one to 



the other, by bands, or belts, as well 

 This principle is seen in the spinning-wheel and common 

 A spiuamg- wheel, as a c, Fig. 70, of thirty inches in circum- 



