by Drew D. Troyer

In my last column, I wrote about a proactive method for selecting a lubricant supplier. Let’s expand that conversation and focus on the process for purchasing the lubricant. In this column, I’ll explain lubricant performance properties. Next issue, I’ll explain the mechanics of creating generic specifications to support its purchase and management.

While performing consulting activities, I often ask clients to describe the lubricant they use in a particular machine and to explain, in technical terms, why they use it. The most common response to the first question is “Brand XYZ’s type ABC lubricant.” Brand or product is the most common method for describing a lubricant. The next most common response is based upon some common physical property scheme, like the ISO viscosity grade for oil or the NLGI consistency grade for grease. However, at its essence, a lubricating oil or grease is a bundle of performance properties.  Neither the brand-based descriptor nor the general physical property descriptors effectively expresses the lubricant’s performance properties.

The most common responses I get to the second (application) question are “we’ve always used that lubricant” and “the lube salesperson recommended it.” You have a good match if the product that always is used or is recommended by the lubricant salesperson possesses the performance properties to match the machine requirements relative to its operating application and environment. However, in practice, this isn’t always the case.

As a result of changing suppliers over time and ad hoc changes to the lubricant specification (not always thought-through initiatives), we sometimes find the lubricant that is in service has drifted away from the required performance characteristics. This sometimes results in a subpar lubricant. Other times, we are paying for a bunch of unneeded, and often very expensive, performance properties. In either case, you need managerial control.

For the most part, performance properties are easy to understand. They tend to relate intuitively to the application’s demands. Take oxidation resistance, for example. Some base oils resist oxidation better than others. Certain additives enhance this property; others diminish it. Oxidation stability is very important in applications where the lubricant is used for a long period of time. It is not so important for once-through (total loss) applications or applications where the lubricant is changed or re-administered frequently.

Other performance properties commonly used to describe oils include thermal stability, hydrolytic stability, lubricity (anti-wear/extreme pressure performance), rust and corrosion protection, demulsibility (separates form water), emulsibility (doesn’t separate from water), filterability (passes easily through filter material), biodegradability, etc.

For greases, evaluate properties like pumpability, slumpability, water washout resistance, dropping point, worked stability, bleed and separation resistance, etc. As with oil, the application should drive grease performance characteristics, and don’t pay a premium price for unnecessary characteristics. For instance, in very dry environments, there’s no reason to pay for excellent water washout resistance.

Notice that the word “ability” shows up in many performance descriptors? Performance properties describe the oil’s ability to: protect component surfaces against mechanical and chemical damage; control, eliminate or quarantine contamination; be delivered to the required location without damage; and, resist degradation.

It’s the performance properties that count. Physical and chemical properties are only important in as  much as they positively affect performance properties. Brand names are merely nominal identifiers.

Fortunately, there are hundreds of standardized tests devised by the American Society for Testing and Materials (ASTM) and others to help you understand a lubricant’s performance on one or more of these properties and compare lubricants to one another.

Because a lubricant is a bundle of performance properties, not just one, you must recognize that excellent performance on one criteria can, by definition, adversely affect the performance on other criteria. For instance, the physical and chemical properties that produce excellent demulsibility produce poor emulsibility. Those properties are polar opposites. Other influences aren’t so obvious. Excellent extreme pressure performance, for instance, can adversely affect a lubricant’s ability to resist corrosion of copper-based alloys. Trade-offs are common.

The moral to the story? Think of your lubricants as a bundle of performance properties. s

Drew Troyer is the senior editor of Machinery Lubrication Magazine. If you have a lubrication or oil analysis question, contact Coach Troyer at 800-597-5460 or e-mail dtroyer@noria.com. 

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