Volume 2

Encyclopaedia of agriculture by the most eminent authorities / edited by C.E. Green and D. Young.

  • Green, C. E. (Charles Edward), 1866-1920.
Date:
1907-1909
Catalogue details

Licence: In copyright

Credit: Encyclopaedia of agriculture by the most eminent authorities / edited by C.E. Green and D. Young. Source: Wellcome Collection.

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    directly a horse puts its weight on to the traces it is exerting a force, but it is not giving out power until it starts to move forward. To find the power being exerted we must multiply the pull in lbs. shown on the dynamometer or spring balance by the speed in feet per minute. This gives us the power in foot-pounds per minute, and if we divide the figure so obtained by 33,000 we get the horse-power exerted. This figure of 33,000 foot-pounds per minute is supposed to represent the rate at which a strong horse can work steadily for eight hours a day, and was determined by Watt, the inventor of the steam-engine, from a study of the brewers’ dray horses in London, which were capable of maintaining a steady pull of 150 lb. at a rate of 220 feet per minute—2J miles per hour. To take a practical example: an agricultural motor is drawing a six- furrow plough at a speed of 3 miles per hour {i.e. 264 feet per minute); if we put a spring balance between the motor and the plough, we find the pull to be, say, 15 cwt. {i.e. 1680 lb.), then the horse-power that is being exerted on the plough is— 1680x264 , 4- 1Q1 TTT) 33,000 It is important to remember that the dyna- mometer by itself does not give the power but only the tractive force; thus the same motor might give a dead pull of 25 cwt. or more in starting the plough out of the ground; in the same way the horse is capable of exerting a much greater pull than 150 lb., and frequently does so when starting a heavy load; though the power in each case might be less. Dynamometers are frequently employed for measuring the brake horse-power of engines, and the type generally employed is known as “ absorption dynamometers,” or more simply “ brake dynamometers.” A common form con- sists of a belt or rope wrapped round the fly- wheel or pulley of the engine to be tested, with a spring balance at each end, or sometimes with a dead weight at one end and the spring balance at the other; the difference between the two spring balances, or between the dead weight and the spring balance, gives the force applied at the rim of the fly-wheel or pulley. If the speed of engine is known, the horse-power is easily calculated thus: H.P. = force applied at rim in lbs. multiplied by the circumference of the wheel in feet, multiplied by the revolutions per minute, and divided by 33,000. The illustration shows one arrangement commonly employed. When the engine is at rest the whole of the weight is taken by the spring balance; when the engine is running, the friction of the rope on the pulley tends to lift the weight W and relieve the spring balance. The load applied is easily altered by adding weights on to W. There are many other forms of dynamometers, such as the Torsion dynamometers ” for measuring twisting forces; “ Transmission dynamo- meters ” for measuring the power being transmitted by belting, shafting, etc., but these will seldom come under the notice of the agriculturist.