60 



MECHANICS. 



I 



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 when not made of 

 uniform material, in consequence of the relative number 



Fig. 62. of teeth working together not being such as 

 to equalize the wear of all alike. If the 

 number of teeth on a wheel is divided with- 

 out a remainder by the number of the pinion, 

 then the same teeth will repeatedly engage 

 each other, and they will often wear uneven- 

 ly. 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 eifected by first taking a 

 number for the wheel that will be evenly yv.. 63. 



divided by the number on the pinion, and 

 then adding one more tooth to the wheel. 

 This will effect a continual cliange, 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. 



Cog-wheels are most usually made 

 v>^ith the teeth on the outside or cir- 

 cumference of the wheel ; these are termed spur-wheek. 

 If the teeth are set on one side of the wheels, they are 

 termed crown-wheels. When they are made so as to work 

 together obliquely, they are called bevel-wheels, Vi'i m fig. 62. 

 Where the obliquity U email, ihc motion may be com- 



Bevel-w?ieels. 



Vniversal joint. 



