How many- 

 kinds of mo- 

 tion are con 



THE, ELEMENTS OF MACHINERY. Ill 



power is capable of exerting directly can be converted into those which aro 

 necessary for the purposes to whicli the machine is applied. 



248. The varieties of motion wliich occur in 

 machinery are divided into two classes, viz. : 

 chinef y""""" KoTARY and Kectilinear Motion. 

 wuntisRota. 249. In Rotary Motion, the several parts 

 ry Motion? rgvolve round an axis, each performing a com- 

 plete circle, or similar parts of a circle, in the same time. 

 250. In Rectilinear Motion, the several parts 



What is Rec- , . ^^ ^ ,. • \ j. 



tiiinear Mo- of a mQvmg Dody proceed in parallel straignt 

 lines Avith the same speed. 



Examples of rotary motion are seen in all kinds of wheel work, and exam- 

 ples of rectihnear motion in the rod of a common pump, the piston of a steam- 

 engine, the motion of a straight saw. 



„„ .. „ . In rotary and rectilinear motion, if the parts move con- 



WhatisRecip- •' . • n j • i 



rocating Mo- staHtly in the same direction, the motion is called contumed 



*■""*' rotary, or continued rectUinear motion. If the parts move 



alternately backward and forward in opposite directions, passing over the 



same spaces from end to end continually, the motion is called reciprocating 



motion. 



How are rota- 251. The method by which a power having one of these 



ry and recipro- motions may be made to communicate the same or a different 



conv7rted°ia'to kind of motion, involves a lengthy description of a great 



each other? variety of machinery; butlhe most simple and common plan 



of converting rotary motion into rectilinear, and rectihnear motion back agaui 



into rotary, is by means of what is called a Crank. 



y^^f. is a 252. The Crank is a double winch, or han- 

 crank? ^^e, and is formed by bending an axle so as 

 to form four right angles, facing in opposite directions. 



It is represented complete in Fig. 96. Attached „ 



to the middle of C D, by a joint, G, is a rod, H, 



which is the means of imparting power to the crank. 



This rod is driven by an alternate motion, like the 



brake of a pump. The bar C D is turned with a — ^ 



circular motion round the axle A F.* ~~~ 



.^^ ^ ,, . The disadvantage attending the 



What disad- " " 



vantages at- use of the crank is, that it is incapa- 



ofthe crlnkT ^^® ^^ transmitting a constant force 



to the resistance. This is illustrated in Fig. 97. In No. 1, 



* The terms axis, axle, arbnr, and shaft, in mpohanics, are generally understood to 

 mean the bar, or rod, which passes throu2:h the center of a wheel. A gudgeon is the pin, 

 or support, on which a horizontal shaft turns ; the plus upon which an upright shaft turns 

 are called pivots. 



