FARM POWER AND MACHINERY. 



245 



FARM POWER AND MACHINERY 



Steam Engine. 



When steam engines were in- 

 vented, the rate at which they could 

 work was compared to that at which 

 a horse could work. A "foot- 

 pound" of work is defined as the 

 work done when "a force of 1 lb. is 

 exerted through a distance of 1 foot." 

 An engine or horse is said to be 

 working at the rate of 1 horse power 

 when it does 33,000 foot-pounds of 

 work in one minute. 



To calculate the rate at which a 

 1,600-lb. horse works when it exerts 

 a pull of 150 lbs. at the rate of 2 1-2 

 miles an hour: 



There are 5,280 feet in a mile, and 

 60 minutes in an hour. Therefore, 

 2 1-2 miles an hour is 2 1-2 times 

 5,280, divided by 60, which equals 

 220 feet a minute. The horse exerts 



a force of 150 lbs. over a distance of 

 220 feet in one minute. Therefore, 

 it does 150 times 220, or 33,000 foot- 

 pounds of work in one minute, which 

 equals 1 h.p. 



If the horse were exerting a force 

 equal to half its weight, 750 lbs., in 

 going up hill, it would exert a force 

 of 750 lbs. through 220 feet pej 

 minute. It would therefore do 750 

 times 220, or 165,000 foot-pounds, of 

 work in one minute, which equals 

 5 h.p. This is one advantage a horse 

 has over mechanical engines: A 

 horse can work at five to ten times 

 its ordinary rate for a short time, 

 but the engine gains because it can 

 keep it up an unlimited length of 

 time at the same speed and force 

 of exertion. 



Mean Effective Pressure. 



The mean effective pressure of a 

 steam engine is easily measured from 

 indicator ca^d, but an exact calcula- 

 tion otherwise is more complicated. 

 It is the result of subtracting the 

 mean or average back pressure on 

 piston from mean forward pressure. 

 The greater distance the piston tra- 

 vels before the steam is cut off, the 

 greater will be the mean effective 



pressure. For instance, with an ini- 

 tial steam pressure of 80 lbs. per 

 square inch and a cut off of 1-5, the 

 mean effective pressure is about 31 

 lbs. With 1-3 cut off, it is about 47 

 lbs., and with 1-2 cut off, about 60 

 lbs., and with 3-4 cut off about 71 

 lbs. The average standard simple 

 steam engine has a fixed cut off at a 

 little over 1-2 the stroke. 



Rated Horse Power. 



This can be calculated by the rule: 

 Horse-Power equals P x L x A x N, 

 divided by 33,000. 



Where P equals the mean effective 

 pressure in lbs. per square inch: L 

 equals length in feet of piston stroke: 

 A equals the area of piston in square 

 inches: N equals the number of 

 strokes per minute. 



Example: An engine has a mean 

 effective pressure of 30 lbs. per 

 square inch; the length of the stroke 

 is 1 foot; the area of the piston is 

 55 square inches, and the piston 

 makes 240 power strokes per minute. 

 The Horse-Power is: 



30 X 1 X 55 X 240 =12 H.P. 



33.000 



Indicated Horse Power. 



Indicated horse power is measurea 

 by what is known as an indicator 

 card. It is an actual test under work- 

 ing conditions. By the use of the In- 

 dicator the steam pressure on the pis- 

 ton is measured through the entire 

 length of the stroke. If there is any 

 fault in the engine due to poor con- 

 struction or wear such as a leak past 

 the piston rings or in the valves it will 

 be shown by this method. In this 



case the horse power actually deliver- 

 ed by the engine will necessarily be 

 less than the rated horse power. Re- 

 member also that the rated horse 

 power depends on the number of re- 

 volutions or speed of the engine. 

 Manufacturers sometimes use a rated 

 speed higher than is advisable in 

 actual use, thus the actual or indicat- 

 ed horse power is lower than the rated 

 horse power. 



