APPLIED MECHANICS. 



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its teeth t<> travel in throe of the stationary wheel IT, and 

 thus to make the wheel C rotate at half the angular vo- 



locity of A, without putting B in motion. But if C be 

 arrested by the friction of the strap 1 >, the axis of F be- 

 comes fixed, and the rotation of A and O is communi- 

 cated through F to B and H at the same speed, but in 

 the opposite direction. The use of the friction-strap in 

 this apparatus, presents the advantages in gradually 

 generating momentum in the driven machinery, similar 

 to those derived from its. use in destroying the momentum 

 of machinery in motion. Although, for the sake of sim- 

 plicity, we have represented only one bevel-pinion F 

 mounted in the friction- wheel, it is customary to provide 

 at least two on opposite sides of the centre iu order 

 to balance it ; and sometimes four are fitted for the sake 

 of equilibrium and strength. 



DYNAMOMETER. -It often becomes important to 

 inquire what amount of power is communicated through 

 a certain train of machinery. When a steam-engine is 

 employed as the prime mover of any machine, the power 

 communicated can be readily ascertained by the indicator. 

 The engine is first worked alone, or with merely the train 

 of wheel-work, in order that the power necessary to over- 

 come friction may be estimated. It is then worked in 

 connection with the machine, and the driving-power re- 

 quired for the machine is ascertained by subtracting the 

 force neoeMary to overcome friction from the total power, 

 including friction and the resistance of the machine. 

 When machinery is driven by some other power, or when 

 the indicator cannot be conveniently applied, the dyiio- 

 : momeUr (power-measurer) is employed. 



The most simple kind of dynamometer consists of a 

 pulley A (Fig. 267J, fixed on the driving-shaft of the 



machinery whose power is required to be known. This 

 pulley is surrounded by a flexible friction-strap C B D, 



Fig. 107. 



the ends of which may be drawn close together by a 

 screw at I), so as to tighten the strap as much as may be 

 necessary on the circumference of the pulley. From a 

 hook C, attached to the strap, is suspended a scale E, in 

 which sufficient weights may be placed to prevent the 

 strap from being carried round by the pulley in its revo- 

 lution. A loose cord or chain C G, fixed at G, is also 

 provided to keep the strap in its place, in case of the load 

 E being insufficient to counterbalance the friction of the 

 strap. Without this loose cord, a sudden increase of 

 speed or friction might lift the scale and weights, and, 

 whirling them round the pulley, do serious damaje to 

 the machinery. In using this machine, the speed of the 

 shaft is carefully ascertained by counting the number of 

 its revolutions per minute, and the screw D is gradually 

 tightened until the scale and its load are just kept up liv 

 the friction, the imaginary line A C being horizontal. If 

 the tightening of the screw cause the scale to be lifted, 

 without reducing the speed of the pulley, more weight 

 has to be added to the scale ; but if the scale with its 

 load cannot be lifted without retarding the pulley, the 

 weight must be reduced. Having found the weight that 

 is just supported when the velocity is correct, the power 

 may be ascertained as follows : The length of A C being 

 the leverage at which the weight E acts to retard the ro- 

 tation of the pulley, the power passing through the pul- 

 ley must be such as would lift the given weight at the 

 distance A C from its centre, or that would, during each 

 revolution, move the weight E through a space equivalent 

 to the circumference of a circle having A C for its radius. 

 Since twice A C is the diameter of this imaginary circle, 

 and the circumference is 3f times the diameter, 2x3f, or 

 <> times A C, is the space through which the weight is 

 driven during each revolution ; and this quantity multi- 

 plied by the number of revolutions per minute, is the 

 total space through which the resistance is moved during 

 each minute. The power is the weight multiplied by iU 

 velocity, or the distance through which it is moved per 

 minute; and as 33,000 Ibs. moved through 1 foot per 

 minute is the standard horse- power, we have the follow- 

 ing rule for estimating the horse-power as indicated by 

 the dynamometer. 



Rule. Multiply the weight E (in Ibs.) by the length 

 of A C, the lover at which the weight acts (in feet), by 

 i'.. and by the number of revolutions of the pulley pur 

 minute, and divide by 33,000 for the horse-power. 



Example. The length of AC being 2 feet 4 inches, or 

 2J feet, the load E (including the weight of the scale) 

 being 78 Ibs., and the velocity of the pulley 120 revo- 

 lutions per minute, required the power. 



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