58 THE DIRECT-CURRENT MOTOR CH. Ill 



of hoisting machinery we are at liberty to select values for 

 v and d that will give us the best value of .!/. Taking d 

 as small as is practically convenient, we can get any re- 

 quired value of v by the use of worm gearing if necessary. 

 We are not here limited for space, and can make the 

 gear wheel on the main shaft as large as we please. 



For instance, in Example 18 the ratio =-' has to be 



(JL 



10*6. By the use of worm gearing we can make v = 60, and 

 we get 3f=6'37. with a rope drum 30 inches in diameter. 

 If we reduce the diameter of the rope drum to 21 inches the 

 induction factor may be reduced to 4-25 without altering 

 any of the conditions. There is, however, a practical 

 limit to the reduction of 3/; namely, the speed of the 

 motor. In the example before us this would be 1,150 

 revolutions per minute, for v = 60 and cZ = 36. If the 

 diameter is reduced to 24 inches the speed of the motor is 

 increased to 1,725; this will involve an increase in the 

 velocity of the line of contact of the toothed wheels, and 

 experience shows that the tendency of the gears to cut 

 increases with the speed, giving us a practical limit to the 

 speed of the motor. 



In railway work single reduction gearing is now 

 universally used, and since there is a practical limit to the 

 reduction of the number of teeth in the pinion, it follows 



ftj 



that to increase the ratio - we must either increase the 



d 



diameter of the gear wheel or decrease the diameter of the 

 driving wheel. We are here limited by the necessary 

 clearance between the gear wheel and the ground, for the 

 diameter of the gear wheel is limited by that of the driv- 

 ing wheel. 



