The Gearbox: Known Unknowns

Courtesy of Wikimedia.

A product specification needs to encompass everything known about the application, and that includes anything that is “unknown.” No, I am not trying to play word games with you; what you do not know can kill your gearbox very quickly.

The typical specification will include information about the “prime mover,” a fancy way of saying the electric motor or other source of motion that gets the system rotating. It ought to include the “driven equipment” as well, since there are things about the mill, pump, generator, or conveyor that will affect your design decisions.

Information on those items is fairly easy to find on the nameplates or in the supplier catalogs. What is much more difficult to obtain is data on the “system dynamics” that occur when you bolt the drivetrain together.

I learned about re-circulating torque very early in my career because my new employer was embroiled in a nasty warranty dispute over large “bridle roll” gearboxes in a steel processing line. Things were breaking in places the “math” said they should not be breaking, and several “upgrades” did not do much to improve things.

A clever team of engineers from the corporate research center set to work outfitting the line with “telemetry” — a word I had only previously heard about the space program. They got a few patents from their solution to obtaining real time torque data from a moving process line. That data indicated the system had almost FOUR times the torque in it than the electric motor could supply, and that the torque levels did not track with motor amperage.

Where did that power come from? The metal strip itself was acting as a big spring! As the strip was rolled, flattened, and stretched, it released energy into the system. So much energy that shafts, bearings, keyways, gear teeth, and even the housing were unable to withstand it.

Do I have your attention now? The components performed flawlessly once we increased the service factor on the system. Service factor is technical lingo for “we are not really sure of the system dynamics, so we will make the parts bigger.”  This is how we deal with the “unknowns.”

If you look at the standards for enclosed drives, you will find several pages of recommended service factors for common applications. When selecting “catalog gearboxes” these factors often dictate a larger size than the sales department would like to offer. Be very wary of reducing the service factor just to make a sale.

When faced with applications not covered by the service factor charts, make sure all parties involved agree on the value that will be used for the design.

About Charles D. Schultz 511 Articles
Charles D. Schultz is President of Beyta Gear Service and one of Gear Technology's technical editors.

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