i6 



FARM MACHINERY 



and the lever, where the inclined plane is wrapped around 

 a cylinder and engages a nut. The pitch of a screw is the 



distance between a point on one thread 



to a like point on the-next, or, in other 

 words, it is I inch divided by the num- 

 ber of threads to the inch. Thus, 8 

 threads to I inch is 1/8 pitch, 24 threads 

 1/24 pitch. There is a great gain of 

 power in the screw because the load is 

 moved a short distance compared with 

 the power. A single-pitch thread ad- 

 vances along the length of the screw 

 once the pitch at each turn ; a double 

 pitch advances twice the pitch. The 

 part of a bolt containing a screw thread 

 on the inside is spoken of as a nut. 

 The name burr is often given to the 

 nut, but burr applies more particu- 

 larly to washers for rivets. The tool 

 used in making the thread in a nut is 

 called a tap, and the one for making 

 outside threads a die. 



30. A pulley consists primarily of a 

 grooved wheel and axle over which 

 runs a cord. 



A simple pulley changes only the di- 

 rection of the force. By a combination 

 of pulleys the power may be increased indefinitely. The 

 .wheel which carries the rope is called a sheave, the cover- 

 ing and axle for the sheave the block, and the whole a 

 pulley. A combination of blocks and ropes is called a 

 tackle. With the common tackle block, the power is 

 multiplied by the number of strands of rope less one. 

 The mechanical advantage may be obtained in another 



FIG. 4 SIMPLE PUL- 

 LEY, WHICH ONLY 

 CHANGES THE DI- 

 RECTION OF A FORCE 



