FAST AM) LOOSE PULLETS. ] 



APPLIED MECHANICS. 



887 



192,48x^=72, 



48 X = 



revolutions per minute ; the diameters being 16, 12, 8, 

 and 4 inches in each. 



Occasionally, a strap may be employed when it is 

 desired that the motion shall be changed transversely, as 

 indicated in Fig. 228. In such a case it is necessary to 

 have very wide pulleys, a and b, because the two parts of 

 the strap c and d must pass obliquely over their surfaces. 

 This mode of employing a strap is, however, very disad- 

 vantageous, for the strap must be continually slipping 

 longitudinally along the face of each pulley, and thereby 

 be subjected to friction and wear. 



In a manufactory where numerous machines are em- 

 ployed, it is essential to have a ready means of throwing 

 any of them into action or out of action at pleasure. 

 When the machine is driven by a strap, this operation ia 

 conveniently effected by what are called ihefast and loose, 

 or the live and dead pulleys ; that is to say, two pulleys of 

 equal size placed side by side on the shaft, one being 



Fig. 229. 



firmly fixed to the shaft by a key so as to revolve with it, 

 while the other revolves freely on the shaft. "When it is 

 desired to put the machine in action, the band is put on 

 the fast pulley, and thus gives motion to the shaft or re- 

 ceives motion from it. For throwing the machine out of 

 action, the band is shifted to the circumference of the 

 loose pulley, which revolves idly on the shaft. The 

 shifting of the strap from the one pulley to the other is 

 effected by means of the striking gear, a forked lever/e d 

 (Fig. 230) moving on a pivot e, and holding the strap c 

 within its forked part d. By pulling round the handle / 

 of the lever, the band, being pressed edgeways by one 

 side of the fork, is pushed gradually on to the fast pulley 

 a or the loose pulley 6 at pleasure. It would be difficult 

 thus to shift a tight strap when at rest ; but while it is 

 in motion, with the pressure of the fork directed on it, 

 it is gradually edged off from its former position. For 

 this reason, the fast and loose pulleys should always be 

 placed on the driven shaft, because, were they placed ou 

 the driving shaft, the band when on the loose pulley, 

 being at rest, could not easily be shifted. 

 Kg. 230. 



In order that straps, which are generally made of 

 leather, india-rubber, or gutta-percha, may act well with- 

 out slipping ineffectively along the surfaces of the pulleys, 

 they should be tightly strained into their place. The 

 pulleys should be turned true and smooth on their cir- 

 cumferences ; for the smoother the pulley, the more 



driving friction appears to take place between its sur- 

 face and that of the strap. When the strap is not 

 sufficiently tight, or does not take sufficient hold of the 

 pulley to drive the machinery with which it is connected, 

 it must be made of greater breadth, and strained more 

 tightly. Sometimes, when a strap slips, powdered resin 

 is strewn upon its inner surface with good effect. The 

 greater the arc of the circumference which a strap em- 

 braces, the firmer is its hold, or the less tendency has 

 it to slip ; and the greater the width of the strap, tho 

 firmer also is its hold. The diameter of the pulley does 

 not affect the slip of the strap, except when the diameter 

 is very small ; while the strap is rigid, and not easily bent 

 closely round the circumference. 



Were straps perfectly flexible, it would be found that, 

 with a given breadth and tightness, the driving power 

 would be the same for all diameters of pulleys. 



It is often convenient to use cylindrical bands or cords, 

 made of catgut, or gutta-percha, instead of flat straps 

 (Fig. 231). In such cases, tho pulley is grooved in the cir- 



Fig. 231. 



cumference, and the band wedges itself into the groove, 

 and thus exerts sufficient friction on the circumference for 

 the communication of power from one pulley to another. 

 The principal advantage of bands of this kind is, that 

 by means of guide pulleys, A and B, grooved in the 

 circumference, the band may be led in almost any 

 direction on the plane of the pulley, or oblique to it. 



Straps are best suited for machinery driven at high 

 speeds, with small pressures ; they are cheap, con- 

 venient, and smooth and noiseless in their action ; and 

 when any undue strain comes upon the machinery, they 

 slip, and thus act as a safeguard against damage. But 

 for all machinery intended to communicate heavy strains 

 through their parts, as in cranes, and whenever certainty 

 of connection between one moving part and another is 

 required, as in clockwork, recourse must be had to 

 gearing, or toothed wheels. 



TOOTHED WHEELS. Were we to mount two wheels 

 on parallel axes, so that their circumferences touch each 

 other, by causing the one to revolve we should also cause 

 the other to revolve, pro- 

 vided the friction between 

 their surfaces at the point 

 of contact were sufficient to 

 overcome such resistance as 

 might be presented to the 

 rotation of the driven wheel. 

 Practically, this friction is 

 not sufficient in ordinary 

 cases, and it becomes neces- 

 sary to fit the circumferences with teeth or cogs, or pro- 

 jections and recesses at corresponding intervals, so that 

 each tooth of the one fits successively in each recess of 

 the other. By this arrangement, the direction of rotation 

 is reversed, as indicated by the arrows in Fig. 232. 

 When it is desired that the direction of rotation be re- 

 tained the same, it is twice reversed by tho intro- 



Fig. 232. 



