Tractor and Steam Engine Drawbar Horsepower

Content Tools

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.