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.
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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Other agents facilitate wetting or lubrication of bearing ball tracks. Wetting agents can be especially helpful in applications involving reciprocating motion, where rolling elements are unlikely to make full revolutions, rocking back and forth over the same contact area without redistributing the surrounding lubricant.
Keep it on
No lubricant is effective if it doesn’t stay where it belongs. In the case of grease, as opposed to oil, maintaining a steady supply is relatively easy.
The simplest way to maintain a grease supply is to seal it in. Sealed, or shielded, bearings use this approach, and are lubricated for life. Another method, used in more demanding applications, relies on oil that bleeds from a partially filled reservoir of grease (located in the exterior of the housing) that leads to a shield plate in the bearing.
An even more aggressive approach is to use an open bearing with grease reservoirs on both sides. External seals incorporated into the machine structure keep the grease next to the bearing. The cavities are usually only half-filled and must not be overloaded.
The toughest applications usually require continual maintenance, which includes periodically replacing the grease. Old and worn grease is forced out as new grease is added. To prevent overheating, the machine is run until the surplus grease is expelled before the cavity is closed. Overfilling the bearing can cause problems particularly at high speeds. Contamination is also a concern.
Many rolling-element bearings can operate at speeds of up to several thousand rpm with grease lubrication. In general, higher speeds call for more frequent lubrication.
When adding or replacing grease, it’s important to ensure that incompatible greases aren’t blended together. The chart above gives an account of ten greases mixed in every possible dual combination.
The properties of an incompatible mixture vary. Typically, the viscosity breaks down. This is especially problematic where grease thickness is relied on to keep lubricant pasted in with the bearing action. There are cases where the other extreme occurs, and the mixed-grease concoction hardens – this happens in some mixtures containing either lithium or clay grease.
An unsuitable mix may also separate into oily and greasy components, as in some blends containing barium grease, possibly a case of the barium staying intact while the second grease liquifies. Mixed greases can also become acidic and pose a corrosion hazard to the bearing.
Steve Masters is a sales engineer with NSK Corp., Ann Arbor, Mich.