Hardness, Lubrication, Are Keys To Gear Inspection

To realize maximum life from the gears and reducers in your plant, pay attention to their metallurgies and the operating viscosities of their lubricants.

By Neville Sachs, P.E.

In a typical facility, gears are usually the most common method of transmitting power and changing shaft speeds. Vast numbers of equipment systems, from cooling towers to paper machines, will rapidly grind to a halt if their gears aren’t kept in good condition. Gear hardness and lubricant viscosity are two factors in the health of these components.

Metal hardness concerns

Understanding gear hardness is an important first maintenance step. This is necessary because of the very different metallurgies commonly used in these components and their varying damage tolerances.

Case-hardened gears (also called surface-hardened gears) have an extremely hard outer layer over a tough, yet softer, core. This case is usually somewhere between 0.015 and 0.125-in. thick, and as hard as bearing steel. Because their cases are so hard, these types of gears have great wear resistance and should run for many years with no visible pitting.

On a case-hardened gear, any pitting you can see is cause for concern. If the load is strong enough to break down a gear’s hard case, the lifespan for the underlying metal is guaranteed to be shorter.

Through-hardened gears reflect the same hardness all the way through the tooth. Some are very soft steel; others are about as hard as a Grade 8 bolt. Although through-hardened gears aren’t anywhere near as hard as case-hardened types, and their hardness will vary based on application, they are designed to withstand significant wear. (Note: Sixty years ago, almost all North American gears, whether open designs, such as those found on a kiln, or enclosed designs, such as small reducers, were through hardened. Because of economics and market pressures, however, almost all enclosed reducer gears today are case hardened.)

In short, case-hardened gears should not show any wear, i.e., pitting. Through-hardened gears, however, can take a tremendous amount of wear before you need to begin worrying about failure.

How do you know whether a gear is case or through hardened? Perform a hardness test. No expensive equipment is required (although a good hardness tester is a valuable tool in a maintenance department). The procedure is simple.

If you don’t have a hardness tester, simply rub a file over the corner of a gear tooth:

• If the file skids across the tooth, the gear is case hardened.
• If the file cuts the tooth, the gear is through hardened.

(Note: You can skip this hardness test for enclosed reducers made in the past 20 years. Almost all of them incorporate case-hardened components.)

Lubricant-viscosity concerns

Prior to shutting gears down, measure the lubricant temperature while the components are running at close-to-the-peak loads—preferably on a hot day. Then, referring to a viscosity chart for that oil, determine if the actual operating viscosity meets the manufacturer’s specifications.

This test is crucial for enclosed reducers purchased in the past 20 years. It was during this time that suppliers began downsizing the casings and increasing the power density of these units. The result is that normal heat generated by the gear and seal action has been transmitted out to the environment through ever-smaller surfaces, leading, in turn, to reducers that tend to run hot. The presence of a thin layer of dust—a common occurrence in plants—acts as insulation, which can make the problem even worse.

Improved maintenance procedures

Hardness and lubricant viscosity are two key factors to consider in your inspections of gears and reducers. Tips for improving the maintenance of these components, which can vary somewhat based on specific type, will be discussed in future Maintenance + Reliability Center sections. MT

Neville Sachs has spent many years working in the field of machinery reliability and lubrication for a wide range of industries. The author of two books on failure analysis and a contributor of sections to others, he has also written more than 40 articles on these topics. A Registered Professional Engineer, Sachs holds STLE’s CLS certification, among others. Contact him at sachscracks@att.net.