Reliability Engineering Snapshot TM

The Illustrated Newsletter For Failure Analysis, Non Destructive Evaluation, and Predictive Maintenance

Machine Design

November 1999

"Rules of Thumb" are just that, general guidelines that need to be reviewed on an individual basis. Not to do so, can spell trouble. A case in point for "making the pinion gear harder than the ring gear." How much harder?

The 10 ft. diameter spur ring gear in the picture to the left has a mating pinion. The rule of thumb says that the smaller pinion gear should be harder than the much larger ring gear. This is because the smaller pinion turns many more times than the ring gear, and logic dictates that the pinion will probably wear out faster than the ring gear. To counter this affect, the surface hardness of the pinion is usually made harder than the ring gear.

But how much harder is the question. In the picture to the right it is painfully obvious that the pinion is too hard. One can see that the face of the bottom tooth is no longer straight; it has taken on the shape of the pinion tooth profile. This takes the phrase "wearing-in" to the extreme. This is not the type of wearing-in one wants. The original tooth profile can be seen on the backside of the top tooth. The pinion is so hard that the ring gear metal has actually yielded and rolled out the sides.

The picture at the left shows the deformed tooth profile of the ring gear. The white vertical bars on either end of the picture denote the location and width of the pitch line. The relative velocity between the mating pinion and ring gear teeth at this location is zero, or "point contact"; therefore, the pitch line carries a very brief static stationary load. As can be seen, the pinion literally pushed the ring gear metal aside; another term for this condition is "plastic flow".

This condition is not desirable. When buying a gear arrangement such as this, ask the vendor just how much harder the pinion is than the ring gear. If he doesn't know, thank him very much for his time and hang up the phone.

When a vendor talks about a "matched set of gears" it means several good things from an engineering standpoint. Hopefully, one of those things is the correct hardness ratio.