Simply stated, power is a rate of doing work. And work is defined as exerting a force through a distance. So, if you exert a force of one pound through one foot of distance you've performed one foot-pound of work. (This is NOT the same thing as a foot-pound of torque! The units happen to be the same, but they're quite different things.) If you pushed with one pound of force through one foot of distance in one second, the power you've applied is one foot-pound per second. If you did it in half a second, then the power is doubled to two ft-lb/sec. If it took you two seconds, that would be half a foot-pound per second.
The units may vary, but power is always force x distance / time. One horsepower is 550 ft-lb/sec or 33,000 ft-lb/min.
So far, we've only discussed power in linear terms. But engines and motors are rotary devices. Without getting into details, for rotary devices power equals torque x angular velocity. When dealing with English units, one horsepower equals (torque (in foot pounds) x rpm)/(33,000 ÷ 2π) or (torque x rpm) / 5252.
All a dynamometer does is apply a load (linear force or torque) to an engine or vehicle. If you know both the force/torque and speed/rpm, it's simple to calculate power.
When you look at the Nebraska tests, you'll see that horsepower is measured in two different ways: PTO and drawbar. PTO horsepower is simple to measure, while drawbar horsepower is not. But of course drawbar horsepower is for most purposes the more important measurement, since it measures the actual rate at which work can be done pulling an implement. Which is of course what really matters.
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Today's Featured Article - Old Time Threshing - by Anthony West. A lovely harvest evening late September 1947, I was a school boy, like all school boys I loved harvest time. The golden corn ripens well and early, the stoking, stacking,.... the drawing in with the tractors and trailers and a few buck rakes thrown in, and possibly a heavy horse. It would be a great day for the collies and the terrier dogs, rats and mice would be at the bottom of the stacks so the dogs, would have a busy time hunting and killing, all the corn was gathered and ricked in what we c
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