38 FARM MACHINERY 



cream separator has machine-cut gears. Very large gear 

 wheels have each tooth inserted in a groove in the gear 

 wheel rim. Such a tooth is called a cog; hence the term 

 cog is often applied to all forms of the gear tooth. Cogs 

 may be made of metal or wood. 



Like pulleys, gear wheels are spoken of as the driver 

 and the driven. To find the speed ratio of gear wheels, 

 the following rule may be used: 



Rule : Revolution of driver per minute, multiplied by 

 the number of teeth in driver, equals the revolution of the 

 driven per minute, multiplied by the number of teeth in 

 driven. 



56. Shafting. Where several machines are to be 

 operated from one power unit, it is necessary to provide 

 shafting on which pulleys are placed. Shafting should 

 be supported by a hanger at least every 8 feet, and the 

 pulleys placed as near as possible to the hangers. Thurs- 

 ton gives the following formula for cold-rolled iron shaft- 

 ing: 



55 



when H.P. is the horse power transmitted, d is the 

 diameter of shaft in inches, R the revolutions per min- 

 ute. Steel shafting will transmit somewhat more power 

 than iron, and some difference may be made for the way 

 the power is taken from the shaft; but the above rule is 

 considered a safe average. 



57. Friction. It has been stated that a machine will 

 not deliver as much energy as it receives because a cer- 

 tain amount must be used to overcome friction. Friction 

 is the resistance met with when one surface slides over 

 another. Since machines are made of moving parts, 

 friction must be encountered continually. In the majority 

 of cases it is desired to keep friction to a minimum, but in 



