Gearbox: Defining Loads

There was a time when no thinking person drove a car with more than 100,000 miles on a long-distance trip. A standard warranty was 12 months or 12,000 miles. A three- or four-year-old car was in the back row of the used car lot. Today, we assume a new car will easily provide reliable service into the 100,000 mile plus range. The average age of a car in the United States is something like 11 years. Similarly, industrial gearboxes are now expected to last multiples of the once-accepted “5 to 7 years.” Better materials and higher quality components are part of the prescription for longer service life, but equally as important is a thorough understanding of the “duty cycle” the equipment will experience.

Duty Cycle

The service factor charts (see previous posting) in the standards have columns for the expected “hours-per-day” of operation, but there is a lot more to duty cycle than how long the drives are under power. I summarize it as “how big of a hammer do you have and how often do you hit it?” Pretend you are an automobile company; getting past that ten-year warranty is much easier when the driver is a law abiding adult, rather than some fool kid who has to replace his tires every three months. Our industry’s appreciation of duty cycle can be traced to warranty issues. Several influential technical papers explain just how much service life can depend on operating conditions. One that comes to mind concerned two identical power plants. The one that was started and kept running a nearly full load seldom experienced a failure; its neighbor, which was used only during times of peak demand, needed frequent repairs. The damage was traced to high starting loads; in other words, frequent application of that proverbial hammer. Wind turbine gearbox designers are credited with bringing Miner’s Rule analysis of variable load applications into common use. Your product specification is incomplete without an understanding of the expected “duty cycle.” If you or your customer do not have this information, it would be wise to use the keywords in the Gear Technology (geartechnology.com) search engine and read the technical papers available.