MECHANICS. 



CHAPTER Xtl. Methods of regulating 

 Machinery Nature of a Fly -Wheel 

 its power of regulating force its 

 power of accumulating force In- 

 stances Nature and Properties of 

 the Governor. 



(103.) IN applying force to impel ma- 

 chinery, for the purposes of manufac- 

 ture, the mere transmission of the ef- 

 fect of the power to the working point 

 is not the only end to be attained. It 

 is most frequently necessary that the 

 action of the working point should be 

 steady and uniform, and not subject to 

 irregular or desultory changes, occa- 

 sioning jolts in the" machinery, and 

 sudden inequalities in 1he work. The 

 want of uniformity, in the performance 

 of a machine, may arise from either of 

 three causes. First, a want of uni- 

 formity in the action of the power, or 

 Jlrst mover, which impels the machine. 

 Thus, for instance, in the single-acting 

 steam-engine, where the elastic force 

 of steam acts upon the piston during 

 its descent, but which action is sus- 

 pended during the ascent. Secondly, 

 a want of uniformity in the resistance, 

 or load upon the machine ; and, thirdly, 

 because the machine, in the different 

 positions which its parts assume during 

 the motion, transmits the impelling 

 power to the working point with greater 

 or less effect. 



One of the most simple and effectual 

 methods of equalizing these irregula- 

 rities, is by the use of a FLY-WHEEL. 

 A FLY-WHEEL is a heavy disc, or hoop 

 balanced on its axis, and so connected 

 with the machinery, as to turn rapidly 

 round with it, and so as to receive its 

 motion from the impelling power. Let 

 us suppose a case in which the impel- 

 ling power is perfectly uniform, but the 

 resistance or load is irregular and inter- 

 mitting. Thus, suppose an overshot 

 water-wheel, (Treatise I. Chap.V.) urged 

 by a regular and uniform fall of water, 

 applied to work a common suction- 

 pump. (PNEUMATICS, 40.) Here the im- 

 pelling power is constant and uniform, 

 but the resistance only acts during the 

 ascent of the piston, and the machine 

 is unloaded during its descent. As the 

 impelling power during the descent of 

 the piston has nothing to overcome ex- 

 cept the inertia of the machine, and the 

 friction of the parts, it will urge the 

 piston down with a rapidly accelerated 

 force, so that at the end of the stroke 



the piston will have acquired a consi 

 darable velocity. But, in the ascent of 

 the p ston, the impelling power is op- 

 posed by the column of water which 

 the piston has to raise. (PNEUMATICS, 

 40.) This continually retards the 

 wheel, and when the piston has reached 

 the summit of the stroke, all its former 

 acceleration is destroyed, and the 

 same hobbling, irregular motion is con- 

 tinued. If a FLY-\VHEEL be attached to 

 such a machine, almost all this irre- 

 gularity will be removed. When the 

 heavy mass of the fly-wheel has been 

 put in rapid motion by the impelling 

 power, it will produce two very obvious 

 effects : by virtue of its inertia, it will 

 oppose a considerable resistance to any 

 sudden acceleration, and also to any 

 sudden retardation of its motion ; that 

 is, it has a disposition to continue the 

 motion which has been imparted to it, 

 and to resist the reception of more. 

 By this, on the ascent of the piston, the 

 weight of the column of water is dragged 

 up, not alone by the energy of the prime 

 mover as before, but by the moving 

 force which has been imparted to the 

 fly-wheel, and which that wheel endea- 

 vours to keep. On the other hand, 

 when the piston descends unloaded, the 

 action of the prime mover upon it, 

 which before caused its sudden and 

 rapid acceleration, is now intercepted 

 by the fly-wheel, which, by its great 

 inertia, refuses to receive that rapid 

 degree of acceleration which had been 

 before produced. 



The power of a fly-wheel to resist 

 acceleration is proportional to the 

 square of its diameter, and, therefore, 

 by sufficiently increasing its size and 

 weight, we may be enabled to equalize 

 the most desultory and irregular mo- 

 tions in the machinery. 



In the example which we have just 

 given, there was a variable resistance 

 opposed by an uniform power. The 

 reverse of this often happens, and a va- 

 riable power is opposed to a constant 

 resistance. Thus, in the single-acting 

 steam-engine already alluded to, when 

 the piston has been forced down by the 

 pressure of steam, it is usually drawn 

 up again by a weight suspended from 

 the opposite end of the beam. In this 

 case the mover is very unequal and de- 

 sultory, and would never serve any 

 purpose in which uniformity of action 

 is necessaiy. But, if a fly-wheel be 

 attached to the machine, the momen- 

 tum which it acquires during the de- 



