Selecting a lubricant

Q. How can I determine whether to use oil or grease in a particular machine application?
A. First, it helps to understand the differences between these lubricants. Lubricating oil consists of either mineral or synthetic oil combined with additives, such as rust or oxidation inhibitors, and antiwear or extreme pressure (EP) additives. Grease is a combination of oil, additives, and a thickener, which usually contains metallic soaps or clay.

A third type, solid lubricants, comes in the form of powders, pastes, suspensions, and bonded coatings. The most common types are graphite, molybdenum disulfide (moly), and PTFE (Teflon).

Oils. Lubricating oils are commonly used to reduce friction and wear and optimize the life of PT components such as bearings and gears.

In dirty environments, contaminants often find a way into the lubricant where they damage bearings and shorten their life. In such applications, oil lubrication can wash the contaminants away from the bearings.

Lubricating oils also serve as heat transfer fluids. In very hot applications, circulating oil draws excess heat from the lubricated parts. In some cases (calendaring operations), roller-support bearings are lubricated with the same oil that heats the rollers. This approach simplifies the lubrication system.

Bearings that operate at high speeds often require oil lubrication. When speed factors exceed 1.3 million nDm (or 800,000 DN), internal heat generated by bearings must be removed with oil mist lubrication. (Speed factor, nDm, equals bearing pitch diameter in mm, multiplied by maximum operating speed in rpm.)

Greases. Oil has one major drawback — it doesn’t stay where you put it. Thus, a grease lubricant is often used because it is more adhesive. Grease should be applied to equipment that is not oil tight, where oil contamination can’t be tolerated, or where oil delivery systems would be too cumbersome or costly.

Like oil, grease is often pumped through bearings to wash out contaminants. This situation is common where bearings are subject to water contact (steel-mill continuous casters). When grease is used in components that require periodic relubrication, you must ensure that an excess of solids doesn’t accumulate in the bearing cavity over time. Grease can also be used for long periods without relubrication or even in equipment that is “sealed-for-life.”

Greases generally have higher load capacities than oils of similar viscosity. They work well in areas that are subject to moisture, dust, and chemicals because the thickener helps to seal out these elements.

Fluid greases. Another type of lubricant, fluid grease, lies between oil and grease from a functional standpoint. Fluid greases have low levels of thickener so they act like oils. However, they are less prone to migration than oil and have better resistance to water, dust, and chemicals. Fluid grease can be used in poorly sealed gearboxes or in low-temperature areas where NLGI 2 (a National Lubricating Grease Institute classification) greases can’t flow.

Q. My equipment operates under high loads at high temperatures. Would solid lubricants help?
A. Solid lubricants are used chiefly for high temperatures and high loads. Most oils and greases can’t operate at temperatures below –55 F or above 500 F. But, some solid lubricants operate at temperatures as low as –300 F or as high as 2,200 F.

Solid lubricants are suitable for highload applications such as slow-moving bearings where the load factor P/C (applied load divided by rated dynamic load) is more than 30%, but only in conjunction with other lubricants (oil or grease).

Where contamination of machinery or products by the lubricant can’t be tolerated, solid lubricants may be the best choice. In optical systems, for example, such lubricants are bonded to the friction points to prevent contamination of lenses and mirrors. Because such lubricants don’t attract airborne contaminants, they are useful in areas where oil and grease would accumulate debris.

Table 1 is a general guide to lubricant selection. The additives can significantly alter a lubricant’s performance. For this reason, it is best to consult your lubricant supplier for specific recommendations.

Next: Improving machine efficiency.

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