MACHINES 545 



around a circle of the same diameter. In this case the 

 mechanical advantage of speed of the machine is 

 5 X2X 3.1416 in. 

 2 X 3.1416 in. 



It is evident that the mechanical advantage of direction 

 or position can not thus be expressed in numbers. 



643. Simple and Compound Machines. Six SIMPLE MA- 

 CHINES are generally recognized: The LEVER, the WHEEL and 

 AXLE, the PULLEY, the INCLINED PLANE, the WEDGE, and the 

 SCREW. Most machines are COMPOUND MACHINES, i.e., they 

 are combinations of two or more of the simple machines. A 

 sewing machine, a typewriter, a clock, a binder, a threshing 

 machine, or an automobile is made up by combining large 

 numbers of simple machines. 



Exercise 103. Study of a Compound Machine 



Examine carefully a sewing machine, a typewriter, a clock, or 

 any other complex machine, noting the simple machines involved 

 and how they are combined. 



644. Friction. FRICTION is the resistance which opposes 

 an effort to slide or roll one surface over another. Every surface 

 is more or less rough. Even the hardest and best polished 

 surfaces are found to be rough, to have uneven surfaces, when 

 examined under a magnifying glass. When we attempt to 

 slide one surface over another, the rough places on one surface 

 catch upon the rough places upon the other. This roughness 

 of the surfaces is the cause of friction. 



The operation of any machine is affected by friction to some 

 extent, (l) Oiling all moving parts which come into contact 

 lessens friction. (2) In general, there is less friction between 

 two surfaces of different material than between surfaces of 

 the same material. For this reason bearings are usually 

 made of different material from that of the axles which rest 

 upon them. (3) Friction is less between two surfaces when one 

 of them rolls upon the other than when one slides upon the 

 other. A ball bearing, therefore, has less friction than a 



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