SMALL SAWMILLS, THEIR EQUIPMENT, ETC. 61 



the shiipe of two hemispheres joined together at the center by a 

 straight thin rib. By this means the jet is split and returned with- 

 out serious shock. The speed of the rim should be one-half of the 

 jet velocity to give the highest efficiency. The water works by im- 

 pact and pressure and not by weight, as in vertical wheels, and 

 greater speed is directly transmitted, which makes a turbine wheel 

 better adapted for sawmill purposes than a vertical one. 



The advantage of water-power mills is less operating expense. 

 The disadvantages are: Water-power mills are not portable; the 

 source of power not uniform or continuous ; not as capable of control 

 as steam; and in the majority of cases results in low speed and low 

 capacity. 



ENGINE FRICTION BRAKE. 



The engine friction brake is an instrument for measuring the 

 power which an engine can give off for external use. It is usually 

 applied to a flywheel and will give results if the belting is properly 

 adjusted and the shafting in line. The object of a flywheel is to 

 compensate for the irregular turning movements, and to prevent the 

 consequent variation of speed from exceeding certain predetermined 

 limits. The energ-y stored in a flywheel varies as the square of the 

 velocity and directly as the weight. In the case of solid cast iron of 

 good quality the velocity of the rim should not exceed 80 feet per 

 second and in built-up wheels 50 feet. 



STANDARD HORSEPOWER. 



Standard horsepower is 33,000 foot-pounds. A foot-pound is 



1 pound lifted 1 foot per minute or any equivalent amount of force, 



such AS one-half pound lifted 2 feet or 12 pounds lifted 1 inch per 



minute. The horsepower of an engine may be determined by the 



2 X PLAN 

 following formula: H. P.= , When P=mean effective 



pressure, L=length of the sti'okc in feet, A— average net area of 

 piston, and N=number of revolutions per minute. To calculate 

 horsepower, multiply the area of the piston in square inches by the 

 speed of the piston in feet per minute and divide the product by 

 33,000. The result is the horsepower value of 1 pound mean effective 

 pressure, which, if multiplied by the whole mean effective pressure, 

 will give the indicated horsepower. The net effective horsepower is 

 the indicated horsepower less the friction of the engine. 



GEARS. 



The word " diameter " when applied to gears is always understood 

 to mean the pitch diameters. Diameter pitch of the gear is the num- 

 ber of teeth to each inch of its pitch diameter. If a gear has 40 teeth 



