82 MECHANICS. 



that the preceding tooth ceases to touch its correspond- 

 ing tooth on the other wheel. 



3. There must he sufficient space hetween the teeth 

 not only to admit those of the other wheel, but to al- 

 low a certain degree of play, which should he equal to 

 at least one tenth of the thickness of the teeth. 



4. The pinions should not he very small, unless the 

 wheels they act on are quite large. In a pinion that 

 has only eight teeth, each tooth begins to act before it 

 reaches the line of the centres, and it is not disengaged 

 as soon as the next one begins to act. A pinion of ten 

 teeth will not operate perfectly if working in a wheel 

 of less than 72 teeth. Pinions of less than six teeth 

 should never be used. 



5. To give strength to the teeth of wheels, make the 

 wheels themselves thicker, which increases the breadth 

 of the teeth. 



6. Wheel- work is often defective in consequence of 

 the relative number of teeth working together not be- 

 ing such as to equalize the wear of all alike. If the num- 

 ber of teeth on a wheel is divided without a remainder 

 by the number of the pinion, then the same teeth will 

 repeatedly engage each other, and they wdl often wear 

 unevenly. The number should be so arranged that 

 every tooth of the pinion may work in succession into 

 the teeth of the wheel. This is best effected by first 

 taking a number for the wheel that will be evenly di- 

 vided by the number on the pinion, and then adding 

 one more tooth to the wheel. This will effect a contin- 

 ual change, so that no two shall be engaged with each 

 other twice until all the rest have been gone through 

 with. This odd tooth is called the hunting-cog. 



