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Pulling power 18 HP vs 850 HP


Glenn

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My grandfather ran a cotton gin from a big steam tractor in the 1920's (this was near Blanco, Texas). He said it had a gear he called "compound low" that was a slow crawl. He could run one of the rear wheels up against a tree, lock the offside wheel with the mechanical brake, engage low gear and walk the tree over so it would either uproot or snap off. He claimed they could break down blackjack oak up to 6" or so in diameter.

Those tractors could weigh more than 20,000 lbs, which is about the same as some bulldozers.

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Actually HP is a measurement of work performed over time. And is based on colculations made with small pit ponies in Victorian English coal mines.
Modern HP ratings are a mess, as the can be measured at the crank shaft, output shaft or the wheel. Hard to compare apples when some are whole, some are pies and others are sauce.
Most modern manufactures will give you Hp at the crank shaft, this gives them a higher rating, and sounds good to the buyer. Wile old tractors where rated at the wheel, as in the actual work the tractor could perform. Big differance between a 10 Tom tractor rated at 18 HP at the wheel and a 850hp truck rated at the crank shaft.
Oh, and if you ever wandered, a teem of heavy drafts can generate 25hp at the swingle tree. I've seen them pull loads at competitions that a tractor rated at more than twice that couldn't pull back down the track.

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I wonder how much better the  new tractor would fair if it had some lift while it was pulling.  the old steam model had the chain hooked up far higher than what was on the JD.    I remember from horse logging,  you hooked as short as you could to get as much lift on the  front of the log to make it easier for the horses to pull.  bet any money that the JD was really light on the rear axle

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I know that at least with the pommy engines (marshall, Robey, McLaren etc) the stated horsepower of portable and traction engines was not comparable to the rated horsepower of today (ie brake horse power).  The horsepowers of those engines was an indicated or "nominal" horsepower given by a mathematical breakdown of boiler pressure and using calculations of piston dia and stroke etc etc.  It was meant as a means to compare different brands and makes and models. 

This sort of thing also becomes apparent when you compare steam locomotives to diesel/electric or electric loco's, a steam loco can sustain max drawbar pull all day without a problem so long as it has fuel and water, a diesel/electric or electric loco can only sustain max drawbar pull for, in the diesel's case about 5 minutes or less, as the electric traction motors will begin to overheat etc etc at max effort held too long, especially if they are only doing very slow revs. 

On the flip side the modern loco's are easier to control the power available than the steam loco which can at max pull go into a mad wheel slip, but that is not what we were looking at, we are looking at max drawbar pull.

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