TRANSMISSION OF POWER 



299 



tooth space is cut deeper than the working depth. (The 

 working depth of a tooth equals the sum of the addendum 

 and the dedendum, while its total depth equals the sum of 

 the addendum, dedendum, and clearance.) 



342. Ratio of Gear Measurements. Repeated designs and 

 tests of spur gears prove that the dedendum (or addendum) should 

 always have a certain definite ratio or relation to the diametral pitch 

 which is: dedendum times diametral pitch = 1, or, what is the same 



-TOOTH 



FIG. 156. 



thing, dedendum = 1 -r diametral pitch. This relation, of course, 

 holds true for the addendum, since the addendum and the dedendum 

 are equal. It is well to remember this relation, since the diametral 

 pitch is the most important thing to know about a spur gear and all 

 gears are ordered and made according to their diametral pitch. The 

 clearance is also referred to as the diametral pitch. Most gear- 

 makers use the Brown and Sharpe rule which is to make the clear- 

 ance times the diametral pitch equal .157; expressed as a formula 

 this would read : 



F = .157 -4- P 



Where F equals the clearance and P equals the diametral pitch. 



Since we know that the outside diameter is equal to the pitch 

 diameter plus the two addendums, and since the addendum equals 



