I was trying to determine what size cultivator to buy for my tractor and was given this response when I was questioning why they were recommending a 15 to 25 hp cultivator for my 32hp rated engine. I found the response interesting so I thought I would share it.
In the beginning, tractors were rated by drawbar HP and belt HP, so tractors 100 years ago were labelled a 12-20 for example. The belt HP was the higher number.
As the use of a belt to power stationary equipment died out and PTO driven implements came into vogue (roughly by the 1930's), tractors had engine, PTO and drawbar HP specifications. The drawbar HP was the true pulling power, but it was also the lowest number and sales departments largely opted to use PTO HP numbers to promote tractors.
You could say the PTO number is a "middle of the road" number but it was also not honorable 90, 80 or 70 years ago to sell tractors by engine HP since* there were a multitude of tractor makers and many of theme were fleecing the public with low prices and inflated or unproven claims on performance. Then Nebraska University started the Nebraska Tractor Tests to test every tractor so by mid-century, a test report was largely expected by the buying public for every new tractor model made.
At the Nebraska Tractor Test Lab, engine HP is tested but then weight is added to the tractor to maximize traction for pulling power. The ability of the tractor pull under maximum load is tested at this point.
Without sufficient tractor weight, tractors pull less (more wheel slippage). Engine HP alone does not take tractor weight into account. There is natural loss of power through the drivetrain, which the difference between the engine HP and drawbar HP measures the loss. The PTO HP number is typically bigger than drawbar HP since there is less drivetrain power loss through the PTO.
Through the second half of last century, sales literature mostly listed the PTO HP first to differentiate the power between different models.
Then -- by the end of the century, tractor makers were largely again selling tractor by engine HP. The model number systems also often reflected the engine HP. Unlike their grandfathers, new, younger tractor buyers are increasing becoming unfamiliar with this science of tractor power and traction.
Regarding tillage implements, since tractors were so long measured by PTO HP, power requirements of tillage implements tended to also be measured the same way. Perhaps now the implement specs need to be converted to engine HP -- BUT this is hardly universal knowledge at this point, so I'll rebut the point that "pulled implements were rated (or have been rated) by engine HP".
In the end, pulled implements can't be rated by engine HP. If the tractor doesn't weight enough, it doesn't matter how big the engine is - the wheels will just spin. The industry will have to work through this self-imposed "knowledge gap" on tractor power.
Shame on the tractor makers for selling tractors by engine HP.
Mike
Buckeye Tractor Co.
P. O. Box 97
11313 Slabtown Road
Columbus Grove, OH 45830
