796 HYDRAULICS AND ITS APPLICATIONS 



angular velocity, and having corresponding linear dimensions in the 

 ratio s: 1, if both brakes are running full we shall have : 

 A<z = s 2 AI ; v z = s vi ; r 2 = 81^; 



.: AI v<? r 2 = s 5 (Ai vi* 7*1), 



so that the resisting moment, and, therefore the horse-power absorbed, 

 varies as the fifth power of the linear dimensions. 



Experiments show that such a brake as illustrated will absorb a 

 maximum of 29'5 H.P. at 100 revolutions, the disc diameter being 

 18 inches. 



It follows that a brake having a disc 24 inches in diameter will absorb 

 1,100 H,P. at 300 revolutions, while by mounting two or more discs on 

 one shaft the power which may be absorbed is practically unlimited. 



The resistance with a given quantity of water varies approximately as 

 the square of the brake speed, so that the brake is specially well adapted 

 for high-speed work. Also, since the resisting moment at the instant of 

 starting is zero and gradually increases with the speed, the brake is well 

 adapted for the testing of internal-combustion engines or steam turbines 

 of the pressure type, while once having been adjusted it requires no 

 further attention no matter how the speed or power of the prime mover 

 may vary within wide limits. The only drawback is the somewhat large 

 first cost of the apparatus. 



A dynamometer to absorb 6,000 H.P. at 300 r.p.m., built on almost 

 exactly the same lines as the Parsons steam turbine, with circulation of 

 water through the various rings of fixed and moving vanes is illustrated 

 and described in Engineering News, December 30, 1909, p. 726. 



For such high speeds as are common in steam turbines of the impulse 

 type, and for small powers, a simpler type of hydraulic dynamometer 

 gives good results. This consists simply of a series of parallel discs keyed 

 to the power shaft, and rotating with small side clearance between a 

 similar series of stationary discs fixed to the outer casing. As in the 

 previous type of brake, with an uniform speed of rotation the resistance 

 varies with the quantity of water in use, and may therefore be regulated 

 by the opening or closing of the inlet or outlet valve. With a constant 

 quantity of water in the brake the resistance depends on the wetted area, 

 and varies approximately as the 1.8th power of the angular velocity. 

 Assuming it to vary as the square of the velocity, the work done on each 



face of each disc is given by 2 -n-f o> 3 / I A d r, where / is a coefficient of 



-/R 2 

 resistance, probably having a value of about *0040 with roughened metal 



