Tractor and Steam Engine Drawbar Horsepower


| May/June 1987



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).