Tractor Draw Bar HP!!

Looking at it from an engineering perspective consider this: 4WD indeed incurs higher driveline HP losses due to the extra gears, bearings, U-joints, etc. Also 4WD makes for a heavier tractor. These losses however can be offset by the reduced tire slippage and drive tire flex losses 4WD provides (you’re dividing the power among 4 driving wheels instead of 2).

Clearly 4WD increases a tractor’s overall pulling ability. However for heavy pulling in actual field conditions, I suspect 4WD does a slightly better job of delivering engine HP to the drawbar than 2WD. Otherwise why do virtually all tractors greater than about 150 HP have 4WD?

Incidentally engine torque in and of itself means nothing - you can get ANY torque you wish thru gearing. It’s engine HORSEPOWER (torque x RPM) that does the work!!

• Nebraska Tractor Test Laboratory

Mathematically:

Drawbar HP = Drawbar pull x speed / 33,000 [pull measured in pounds, speed in feet/minute]

Under ideal conditions maximum drawbar HP will be around 85% of engine max HP. The rest of the engine HP is “lost” in moving the tractor itself, in frictional losses in the driveline, in running accessories like the hydraulic pump, etc.

However, I have seen some posted hp ratings where the engine hp on some tractors was close to the PTO(as you all said) but others the PTO was significantly less. I mentally made a note that whomever designed that rig didn't do it right and I surely wouldn't want to buy that tractor. So as procurement criteria, I would want to know all three: engine, drawbar, and PTO.

Mark

PTO HP divided by .86 approximates BHP

PTO HP multiplied by .86 approximates DBHP

Because PTO HP is easily calculated from a dyno pull and somewhat consistent from test to test, deriving the other figures from PTO HP is most often done.

Transmission type: Simple gear drive transmissions are the most efficient – they lose only a few % of the engine flywheel HP passing thru them. Powershifts don’t do quite as well. Hydrostatics are the worst – may lose as much as 15% - 20% of the engine’s flywheel power.

PTO design: Depending on the mechanical design PTO output may be limited to some fraction of the available engine HP. Example: I’m uncertain of the exact numbers but recall working several years ago with an older Steiger equipped with a hydraulic PTO (PTO motor-driven from the tractor hydraulics). The engine was rated at 325 HP, but the PTO was limited to 120(?) HP. Doesn’t necessarily mean it’s an inefficient design – only that the PTO mechanism is incapable of safely transmitting full engine HP.

Tires: Same as with cars, older (bias ply?) tires lose more to HP to slippage and internal flexing than do modern tractor radials. This can amount to a difference in several % in drawbar HP all other things being equal.

Weight: As with any vehicle, the heavier it is the more power is required to move it forward. So if a tractor is overballasted during the drawbar test HP will unnecessarily be be lost simply moving the tractor along (though the effect of this one will be relatively minor overall).

So simply comparing published HP ratings without consideration to stuff like the above CAN be misleading!

I never thought about the part where the designer said that we'll just rate this function for x hp rather than it just happened to come out as x hp after all was said and done with the design.

Mark

The actual calculation encompasses many variables which are hard to quantify so estimates are usually made. Rather than "guess" the best way is to use the old rule of thumb; 86% of PTO hp. In your case, about 49 DBHP.