MECHANICAL PRINCIPLES OF MACHINES 23 



and machines acting with great power take the name of 

 engines. 



Workshop tools are divided into two classes, hand-^ools and 

 machine tools. The former class includes hammers, chisels, 

 files, ratchet braces, span- 

 ners, etc. The latter class 

 includes lathes, planing, 

 shaping,drilling, and slotting 

 machines, used in the fit- 

 ting shop ; and punching and 

 shearing machines, bending 

 rolls, and steam hammers, 

 used in the smith's shop. 

 The compressed air at- 

 tachment (Fig. 8) is a good 

 example of a power tool. 



FIG. 8. A power tool (compressed 

 air attachment) tightening nuts 

 on a freight car. Compressed air 

 may be utilized in this way to 

 screw on nuts and thus save the 

 mechanic's strength. 



24. Force and Work. 



To understand the princi- 

 ples underlying the use of tools and machines, it is necessary 

 chiefly to understand the differences between force, work, 

 and energy. Force is that which tends to produce, to change, or 

 to destroy the motion of a body. The' force may be the strength 

 of man or animal, or of steam, or electricity, etc. Tools and 

 machines when stationary are in what is called a state of in- 

 ertia. The overcoming of resistance through any distance, such 

 as putting tools or parts of machines in motion, is called work. 

 Work is done when a force produces or destroys motion. 



25. Estimating the Work Done. In estimating the work 

 done two factors are employed distance and force (weight) 

 the units of which are the foot and the pound respectively. 



