Tractor and Steam Engine Drawbar Horsepower

Box 94, Rt. 1 Nashville, IL 62263

One of the most misunderstood terms in the field of tractors and
steam engines is the Drawbar Horsepower (DBHP) ratings. There often
is a big error made regarding understanding early tractor and steam
engine drawbar ratings. For example, the IH 10-20 or even earlier
8-16 does not show the actual ability of the tractors to produce
drawbar horsepower. At standard engine speeds listed in the
specifications these machines will produce much more than those
drawbar ratings. Of course the ‘hot rodders’ increase
engine RPM and ‘soup them up’ but that is not discussed
here, as we speak only of standard specifications. The steam
engines often have ratings that do not show Drawbar Horsepower.
Their belt HP ratings are realistic and they do have a fairly large
reserve at standard pressure and engine RPM, without any hot
rodding increase in RPM. During the 25 to 30 years of the early
part of this century the reliable manufacturers of internal
combustion and steam engines put conservative ratings in their
specifications and thus there is reserve power available. That is
not now and has not been true for many years, as most makes can
barely reach the power they advertise.

The lower figure of the steam engine ratings, for example the 30
of 30-98 or the 28 from 28-90, and others, are very often accepted
as Drawbar Horsepower. Even some of the printed matter showed the
lower figure as DBHP. However that lower figure, the 28 and the 30
in the above examples, is a rating based on calculations
established much earlier in the steam era and not used for many
years.

Whether Belt HP, Flywheel HP, Power take off HP, or Drawbar HP,
the work being done is measured exactly on the same basis. Whether
33,000 lbs. is moved 1 foot per minute; 3300 lbs. is moved 10 ft.
per minute; 330 lbs. is moved 100 ft. per minute; or 33 lbs. is
moved 1000 feet per minute, it is all the work of 1 HP. Of course
any combination of lbs. moved over a distance equal to any of the
above is one HP. Also whether this is in a straight line or in a
circular motion, the result is the same. Let’s take for example
any internal combustion engine. Its flywheel HP or power take off
HP is usually measured on the basis of one foot radius from the
center of the crankshaft or center of the PTO shaft. In all of
this, the work being done, or in other words the HP developed, is a
factor of speed. Any amount of force large or small that does not
move, is zero Horsepower. Only a moving force represents
Horsepower.

It is extremely difficult to visually judge drawbar HP in the
field! Without measuring, it is almost impossible. Many people have
the idea that measuring it in the field is difficult. Of course
using the ‘33000 lbs. moved 1 foot per minute basis’ and
trying to measure using that, does involve some work and detailed
calculations. However, there is a short cut that is very easy to
use. It is used in the field by Caterpillar Tractor Co. and many
others to accurately measure Drawbar HP. Measurements are easy and
the calculations don’t go beyond arithmetic taught in the 3rd
grade.

Just two simple measurements are made, one is the drawbar pull
in pounds exerted at the drawbar pin, and the second is the time in
seconds it takes to pull the load through a measured distance. A
good measured distance, both in the field or at a show, is 176
feet. However 88 feet is fine if space is limited. Measuring the
pounds pulled at the drawbar is easily done with a hydraulic
cylinder and gauge (or other pull meter).

With those two data readings, a little 3rd grade arithmetic will
give the exact Drawbar Horsepower.

Keep in mind that wheel slippage and rolling resistance are
totally lost and wasted. They are not measured,
and that can be a big fooler! For example the measurement of
Drawbar HP at a tractor pulling con-test often makes the tractor
drivers and owners very angry because it shows so little HP at the
drawbar pin, and they are sure it is very much more HP than it
actually is. They simply do not realize that a great many engine HP
produced by their ‘souped up’ engines is totally lost
THROUGH WHEEL SLIPPAGE. They are actually moving a load very slowly
which represents only little HP at the drawbar pin actually moving
the load while they are wildly spinning their wheels at a fast
pace.

At nearly all the shows the many tractor men love to measure
their tractors on the belt. They often line up to test their
tractors on the belt as they do at Mt. Pleasant. They could have as
much or more fun testing their tractors for Drawbar HP as above
described. Making several tests is often most interesting because
the differences are largely due to changes in wheel slip, to
variations in loads and to using different gear speeds. All this
makes it a very interesting and fun participation for the owners,
drivers, testing crew and for the audience as well.

In closing here is an example of measuring the Drawbar HP of a
gas tractor at a show:

Measured distance.. .88 ft.
Drawbar lbs. pulled. . .3780 lbs.
Time through the 88 ft…. 1754 seconds. That gives you the two
needed measurements and we do this:
60 divided by 1754 seconds=3.428 mph.
3.428 mph X 3780 lbs. pulled= 12,958. Now we divide the answer by a
constant of 375.
12,958 divided by 375=34.55 Drawbar Horsepower at the pin.

The 60 is seconds in 1 minute. Traveling at 60 MPH takes 1
second to travel 88 feet; at 30 MPH it takes 2 seconds; at 20 MPH
it takes 3 seconds; at 10 MPH it takes 6 seconds and so on. So at
17 seconds to travel the 88 feet it is exactly 3,428 MPH. So the
formula is simply: Miles Per Hour time Pounds Pulled divided by the
constant factor 375 and you have Drawbar Horsepower. The 375 is
used with any speed and pull figure.

Finally we say once again that this is an interesting subject
and lots of fun in which to participate, whether it be at a large
show, small show or on your own The Pawnee, Oklahoma, Show plans
include this event in 1987.

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