The Danger of Run Away Engines

High speed operation is dangerous and can lead to a run away engine

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Recently at a local show in New York State, an unattended engine suddenly increased in speed very quickly. Before there was time to react, the machine was joyfully spraying parts over a 25-foot radius. The run away engine went on its merry way until a daring soul removed the spark plug wire and terminated the madness. This little episode raised my interest, and some research on the topic was quite revealing. Engineering books and machining handbooks have a great deal of information concerning cast iron machinery and flywheels. After satisfying my curiosity about the strength of old engines, I became very cautious and aware of the danger relating to a "run away engine."

Many "Ole Tyme" engines were run faster than their rated RPM. This short article gives reasons why this high speed operation should be avoided, offers a way to calculate RPM limits for any particular cast iron flywheeled engine.

Antique gasoline engines had very few problems. One was weight factor. Engines ran slowly because of the stress limitations of large, heavy, bulky parts. Speed of most "Ole Tyme" engines was controlled by means of a governor. The governor maintained a constant speed while at the same time preventing the RPM's from exceeding the maximum rating. Many times operators "tied down" governor weights, thereby allowing operation at the fastest possible speed. Although a highly dangerous practice, it did increase horsepower of the engine. On occasion, increased centrifugal force in the flywheel caused fractured cast iron flywheel parts to sail in all directions inflicting death and injury. (Velocity of the flying parts can easily reach a discharge speed of several hundred miles per hour.) Changes in force within a spoked flywheel rim are proportional by the square of change in the engine RPM. For example: If the RPM of an engine is 200 and the centrifugal force in the flywheel rim is 50 pounds per square inch, and the RPM is raised to 400, the force will then increase to 2500 pounds per square inch. That is a tremendous increase in force and should be kept in mind by all gas engine buffs.

The following is an easy formula to determine the maximum safe RPM for any particular cast iron flywheel:

N=   C x A x M x E x K/D


N=Maximum speed in revolutions per minute

C= 1.0 for electric motors or constant speed machines .90 for steam and gas engines

A=.90 for 4 spokes
   1.00 for 6 spokes
   1.08 for 8 spokes
   1.25 for solid disc type with holes
   1.50 for disc or solid

M=1.0 for cast iron
     1.2 for high grade industrial cast iron.

E=Joint efficiency
1.0 for solid one piece rim

K=Flywheel ratio Take flywheel diameter and divide it by rim thickness (not width) to get percent of flywheel ratio:

1355 for flywheel ratio equal to 1%
1650 for flywheel ratio equal to 2%
1840 for flywheel ratio equal to 3%
1960 for flywheel ratio equal to 4%
2040 for flywheel ratio equal to 5%
2140 for flywheel ratio equal to 7%
2225 for flywheel ratio equal to 10%
2310 for flywheel ratio equal to 15%
2340 for flywheel ratio equal to 20%.

D=Outside diameter of rim in feet

Example: A three horsepower Hercules gasoline engine has a flywheel diameter of 22 inches or 1.83 feet and a rim thickness of three inches.

Answer: Use the formula as shown above.

D=1.83 feet
N=1050 RPM

The maximum theoretical RPM before possible flywheel destruction for this particular engine is 1050.

Some other examples are:

IHC 3 HP "M," Flywheel 1.75 feet, rim 2 1/8-inch. Maximum RPM 1144. New Holland 5 HP Flywheel 2.3 feet, rim 4-inch. Maximum RPM 837. Superior 25 HP 6 spoke oil field engine, flywheel 5.58 feet, rim 5.5-inch. Maximum RPM 362.

The above formula is an approximation of the RPM at which spoked flywheel rim failure can be expected. Maximum RPM can differ with many variables such as: flywheel width, diameter of spokes, types of cast iron used, casting flaws, holes in spokes for pulley mountings, and strain or damage to the casting. However, it does give an accurate estimation of the maximum speed, which should never be approached, or most importantly, exceeded, for safety sake.

Speeds mentioned above are all greater than engine specifications as indicated by the manufacturers. Good mechanics and engineers would never subject their machinery to these RPMs.

Our hobby is concerned with the restoration and preservation of early engines and we all want to enjoy it safely. A few rules that will go a long way in protecting the public and ourselves are: 1) Never leave an engine unattended as we cannot tell when one of our cast iron friends will decide to "run away." 2) Make sure the engine and particularly the governor are in proper working order. 3) Never defeat the purpose of the governor. And 4) keep the entire engine lubricated.

Contact Wayne Graining at 318 Summit St. Boonville, NY 13309