How to measure horsepower
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Posted by Nate on June 19, 1998 at 10:49:38:
A couple days ago there was a discussion about how to measure horsepower. This is how it's done. PTO horsepower and automobile horsepower are measured the same way. A variable braking device (often a water pump or a generator) is attached to the powered output shaft. On a car
this may be the crankshaft or the rear wheel. This braking device is allowed to be free in the direction of rotation (it would spin around if you didn't hold it back). An arm is attached to the
braking device with a place to put weights 1 foot from the centerline of the output shaft. Bit by bit the engine is throttled up and weight added to keep the brake from spinning. When the
throttle is maxxed the weight hanging on the shaft is noted and this is the ft-lbs of torque at whatever the engine speed happens to be. The load on the brake can then be changed (ie. more
or less water supplied to the pump) to vary the engine speed. The weight on the arm is then adjusted to re-balance it and a new torque reading taken. In the case of a tractor there is usually
just one speed, (the governed speed). This torque number is then run through the formula HP = Torque x RPM / 5252. This gives the horsepower at whatever speed was used for the
calculation. Drawbar horsepower is measured by taking the max pull at different speeds (this isn't the weight being pulled it's the tension in the cable) and multipying it by the speed of the
tractor in feet per second and diviving by 550. So HP = Pull(lbs) x Speed(fps) / 550. Horse horsepower is measured like drawbar horsepower. BTW all these horsepowers are the same, each just as real as the next. The only difference is how many more horsepower are we going to waste before we use it? In the case of drawbar HP; none, in the case of automotive crankshaft HP; quite a bit, not to mention you don't get to sit at the rated HP speed all the time like you do in a tractor!
Nate
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