20 POWER AND THE PLOW 



what is known as a brake test is made. To the flywheel is 

 clamped a, friction brake, from which extends a long arm, which 

 is usually supported by a column resting on a platform scale. 

 This arm has the effect of increasing the diameter of the flywheel. 

 The flywheel must revolve against the resistance offered by 

 the brake. The brake arm also tends to revolve and is pre- 

 vented from doing so, the force required to support it being 

 expressed in pounds upon the beam of the scale. The result 

 produced is that of a certain resistance in poiinds, moving at 

 a rate equivalent to the speed of the flywheel, times the cir- 

 cumference of a circle which has as a radius the length of the 

 arm from the centre of the flywheel to the point of support 

 at the outer end. 



By formula, the brake horsepower (b.h.p.) equals: 

 Circumference (2XLX3.1416)XNXF 

 33,000 



in which L = distance in feet from the centre of the crankshaft 

 to the point of support for the outer end of the brake arm; N = 

 number of revolutions of the flywheel per minute; and F = 

 weight shown on the scale beam. 



The b.h.p. must always be less than the i.h.p., due to the 

 friction of the valves, piston, bearings, etc. The best engine 

 is naturally the one which wastes the least power in developing 

 work. For comparison, engineers use a term, mechanical 

 efficiency, which indicates the percentage of the indicated horse- 

 power delivered by the flywheel in a brake test such as just 

 described. In the highest type of steam engines this is some- 

 times 98 per cent. More often it is 85 to 93 per cent., and fre- 

 quently as low as 75 per cent. In the gas engines of smaller 

 type the mechanical efficiency is somewhat lower, ranging from 

 75 to 85 per cent., with occasional records of 90 to 93 per cent, 

 in large stationary plants. 



In plowing with ordinary equipment, the best tractor is 

 one which will deliver the largest percentage of its brake 

 h.p. at the drawbar, all other things being equal. In other 



