Converting Tractor Power Ratings

Manufactures can use anyone of the four power ratings to advertise their tractors. When manufactures use engine or brake power to rate a tractor they are not providing any useful information because the useable power will be less than that. When the tractor is rated by engine or brake power, the PTO power will be less because of the losses in the power train. The drawbar power will be less because there are additional losses in the transmission and drive train and a tractor is not able to apply all of the torque of the drive wheels to the soil.

Actual PTO and drawbar power ratings can be determined if the tractor has been tested by the Nebraska Tractor Test (NTT) Station or by the Organisation for Economic Cooperation and Development (OECD). When actual PTO and drawbar ratings are not available, the 86% rule can be used to estimate them from the engine rating.

Studies have shown that if a large number of tractors are compared over many different traction conditions, the PTO power will be approximately 86% of the engine power, and the drawbar power when tested on a concrete track will be approximately 86% of the PTO. The 86% rule permits an estimate of PTO and drawbar power. It is not intended to be a substitute for actual data.

Problem: Estimate the amount of power available at the PTO and the drawbar for a tractor rated at 125.0 engine power (EHp).

Solution: Using the 86% rule:

PTOHp = EHp x 0.86 = 125.0 hp x 0.86 = 107.5 or 110 PTOHp DbHp = PTOHp x 0.86 or EHp x 0.86 x 0.86 or EHp x 0.7396 DbHp = EHp x 0.7396 = 125 hp x 0.7386 = 92.45 or 92.4 DbHp

The 86% rule can be used in other ways also. For example, if the drawbar power is known, the PTO and engine power can be estimated.

Problem: What is the estimated PTO power if the tractor produces 50.0 power at the drawbar?

Solution: From the discussion of the 86% rule, we know that the PTO power will be larger than the drawbar. Therefore:

DbHp 50.0 DbHp

Lugging Ability 85

1300 1400 1500 1600 1700 1800 1900 2000 2100 RPM

1300 1400 1500 1600 1700 1800 1900 2000 2100 RPM

FIGURE 7.2. Engine torque curve with lugging ability.

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  • FIORENZA PISANI
    What is tractort rating in agricultural engineering?
    5 months ago

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