TRANSMISSION OF POWER 295 



or rawhide, are let into and fixed in the iron rim of the gear. 

 The gear so formed is termed a mortise gear, and is always the 

 quicker running gear of the pair. It is in such a case as this 

 that the teeth are termed cogs and they are usually made of 

 hornblende or beech, both of which are compact in grain and 

 take a smooth surface. Machine gears which are subject to 

 much vibration and shock are frequently made of phosphor- 

 bronze (an alloy of copper, tin, and phosphorus), gun-metal, 

 steel, or malleable cast iron, because these materials have a high 

 tensile strength and greater elasticity than ordinary cast iron. 



338. Relation between Speeds and Diameter. In the 



mechanical world or in speaking of machines, the expression 

 " geared to 75 " is often heard. This means that one turn of 

 the driving wheel will cause the circumference of the drive to 

 pass over 75 in. 



To illustrate : A bicycle sprocket with a circumference of 30 in. and 



orv o 



a rear wheel of 80 in. would give this ratio of speed: = - = 2%; 



30 3 



i.e., one turn of the pedal would turn the rear wheel 2% times. 

 The gear of the wheel is found by multiplying this number by the 

 diameter of the wheel, say 27 in.; 27 X f = 72 in. 



The proportion between the speeds and the diameters of gears is 

 just the same as the proportion between the speeds and the number 

 of teeth. This means that we can find the ratio of the speeds of two 

 gears just as well if we know their diameters as if we know the num- 

 ber of their teeth. Suppose the diameters of two gears are 12 in. 

 and 24 in. respectively. Then the ratio of their speeds would be as 

 2 is to 1, if the 12 in. gear is the driver. If the 24 in. gear is the 

 driver the ratio would be as 1 is to 2; i.e., if the 24 in. gear is driving 

 and turns once, the 12 in. gear would turn twice. 



Sometimes it is easier to figure the ratio of the speeds of 

 gears from their diameters, but as the diameter used is the 



