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Lubricants are used in machinery because of their ability to reduce friction, dissipate heat, and help prevent corrosion. Grease is used when the lubricant must seal against contamination or when it must stay in place because relubrication is difficult or impossible.

Many new greases have been developed recently, and the variety grows constantly. When choosing a grease, there are several things to consider, including the severity of the application, bearing design, operating conditions, and maintenance procedures.

Get a grip

Grease is basically a combination of soap and oil. Additives may be included to help stabilize the grease and prevent oxidation.

In general, variations among greases are determined by the properties of the soap, oil viscosity (and percentage), and the qualities of the additives incorporated. Naturally there are several things to consider when selecting greases:

• Lubrication properties determine how well a grease can prevent bearing surface wear.

• Penetration value refers to the stiffness or movability of a particular grease, and is significantly affected by the percentage of oil in the grease.

• Oxidation rate is critical in many applications; oxidized greases are poor lubricants and tend to accelerate corrosion. Increased resistance to oxidation is a must if a grease is expected to endure high temperatures, extended periods of storage, or prolonged service life.

• Bleeding rate measures how fast the oil tends to separate from the soap.

• Emulsification properties are always a concern, particularly for bearings operating in humid or moist environments. Greases that are easily emulsified, for example, can be flushed readily from bearings in wet applications. On the other hand, such a grease best dissipates small quantities of moisture.

• Viscosity is another property requiring careful consideration. In bearings, higher viscosity greases are less apt to leak out, but lower viscosity greases reduce friction and heat, a huge plus when machines are pushed to higher speeds.

Know your soaps

In grease, soap acts as a sponge of sorts, keeping oil in the mixture. Soaps differ according to type, hardness, and the percentage used. Today there are several types of soaps used in bearing grease, each offering a different set of advantages.

Soda-based soaps are used in dry applications and where it’s necessary to absorb and dissipate small amounts of moisture (as in condensation). Moisture is absorbed throughout the grease and expelled when the grease heats up during operation.

Calcium and lithium-based soaps, on the other hand, are water-repellent. Greases based on calcium soaps are normally used in low-speed, constant-service applications where temperatures remain below 175°F. Lithium soaps can be used at higher temperatures and speeds. One potential problem with water- repellent soaps is that they can trap water.

Other soaps and synthetic soap replacements are used in special applications with unusual operating conditions. For instance, some aircraft applications require a special grease that can be used over a wide range of temperatures and atmospheric pressures.

Additive concerns

Additives are used in greases to further modify their properties. Anti-oxidants retard deterioration. Fillers, including sulfide compounds and graphite, are incorporated for special applications, frequently involving extreme pressures that result from high loads.

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