Tooth Tips: Donald R. McVittie P.E.

Discover why the toughest equipment at work in the world today uses carburized gearing.


Sometimes there simply is no easy fix for a misalignment problem. Maybe the box is bored out of line, the gears weren't matched originally, or the parts are so slender that they bend under load. In any case, the load is so heavy at one end of the teeth that wear is unacceptable.

If it can be removed, the whole gear box should be sent to a good gear shop. There the cause of the bad contact can be found and the part/s causing the misalignment reworked.

The gears will first be checked on true center distance in a test fixture. If the contact pattern in the fixture looks good, but the contact under load is bad, the box will be checked to make sure that the bores are parallel. The theoretical bending and twisting deflections are calculated to estimate the amount of deflection under load.

With this information, a correction can be made to the helix angle of the gear or pinion, creating an intentional mismatch at no load. When the gears wind up under load, they match and their capacity is increased. If they have to run a variety of loads, the required tooth form can be curved, or "crowned."

Sometimes the gear box can't be removed, so only the gears are available for checking. The repair is still possible, using the test fixture to simulate the gear box, but it's still difficult to know whether the fault lies in the gears, the box, the deflection, or all three. If only the gears are available for a modification, it may take more than one try to get it right.

The Benefits of Carburization
Unlimited hydroplanes, mining trucks, helicopters, automobile transmissions, tunnel-boring machines, and rock crushers have one thing in common: They all use carburized gears to carry high shock loads under tough conditions. What makes these gears so much better than others?

In a cross-section of a carburized tooth a dark outer structure can be seen, which is a high-carbon case created by soaking the gear in a carbon-rich atmosphere at high temperature. The surface has the same high carbon as spring steel, and it's file hard. Carbon content tapers off with depth, so the core of the tooth remains ductile and tough. This seamless metallurgical structure is ideal; the hard surface carries high loads and resists wear, while the core provides strong support for the case.

Carburized gears can carry about twice the loads of the best through-hardened gears in the same space (center distance and face width). This makes them ideal for applications where the first try didn't work, the load was underestimated, horsepower was later increased, or operations need to be speeded up.

Making good carburized gears isn't easy. The process must be carefully controlled to get the required metallurgical structure, and the finished parts or samples must be checked to be sure that the desired result has been achieved.

The carburizing process is done after the teeth are cut, so any distortion caused by heating or quenching affects the accuracy of the gears. Sometimes the distortion is within reasonable geometrical limits for the part, but for the highest accuracy, the teeth must be finished in a special gear-grinding machine after heat treatment.

So for the toughest applications, ground-tooth carburized gears are the best.


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is the president of Gear Engineers, Inc., and longtime board member of The Gear Works-Seattle, Inc. The Gear Works-Seattle, Inc. is a full-service gear manufacturing facility providing precision gear products and power transmission services. For more information on "Tooth Tips", contact Donald R. McVittie at 206-783-3919, or send email to or Roland Ramberg at 206-762-3333, or send email to Visit online at []