Lubricants in general 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. The choice is still up to you, however. When choosing a grease, consider the severity of the application, design of the unit, operating conditions, and maintenance procedures.
Grease is basically a combination of soap and oil. Additives may be included for stabilization, anti-oxidation, and other special performance needs. Variations in greases are determined by the characteristics of the soap, the oil viscosity, the percentage of oil, and the additives used.
You need to consider many properties when selecting greases:
• Lubrication properties of the grease determine how well it prevents wear of bearing surfaces.
• Penetration value refers to the stiffness or movability of grease and is affected by the percentage of oil in the grease.
• Oxidation rate of the grease is important; oxidized greases are poor lubricants and tend to accelerate corrosion. Increased resistance to oxidation is needed at high temperatures, and during long storage or service life.
• Bleeding rate measures how fast the oil tends to separate from the soap.
• Emulsification properties are important, especially in humid conditions. A grease that is easily emulsified can be flushed from the bearing readily in wet applications. However, this grease best dissipates small quantities of moisture.
• Viscosity is important because higher-viscosity greases are less apt to leak out of a bearing, and lower-viscosity greases allow higher speeds due to lower friction and heat generation caused by working the grease.
Differences in soaps
A soap acts as a kind of sponge to keep oil in the mixture. Soaps differ according to type, hardness, and percentage used. There are several types of soaps with distinct characteristics. Table 1 lists many of them.
Soda-based soaps are used in dry applications and where it is necessary to absorb and dissipate small amounts of moisture, such as condensation. Moisture is dissipated throughout the grease and expelled when the temperature increases during operation.
Water-repellent soaps are not easily displaced by water. However, they can trap water, making it difficult to expel. Common water-repellent greases are calcium or lithium-based. Calcium-based greases are used for bearings in constant service at temperatures to 175 F, but should not be used at high speeds. Lithium soaps can be used at higher temperatures and speeds.
Other soaps and synthetic soap replacements are used in special applications with abnormal operating conditions. For instance, some aircraft applications require a special grease that can be used over a wide range of temperature and atmospheric pressure.
Additives are used to further modify greases. Anti-oxidants retard deterioration. Fillers, including sulfide compounds and graphite, are used for special applications, frequently involving extreme pressures that result from high loads. Wetting agents facilitate wetting or lubrication of the ball track, especially in applications involving reciprocating motion.
Maintaining a grease supply
Compared with the problems of oil lubrication, maintaining a grease supply is simple. The simplest device is the sealed bearing, Figure 1, or the shielded bearing, that is lubricated for life. In more severe cases, oil bleeds from a partially filled reservoir of grease in the housing exterior that leads to a shield plate in the bearing.
For even more severe applications, an open bearing is used with grease reservoirs on both sides. External seals incorporated into the machine structure keep the grease next to the bearing. The cavities are only half filled and must not be overloaded.
In the toughest applications, the grease must be replaced periodically. Old and worn grease is forced out as new grease is added. To prevent overheating, the unit is run until the surplus grease is expelled before the cavity is closed. Overfilling the bearing with grease can cause problems at high speed in particular. Also, contamination is always a potential problem with grease relubrication.
Many rolling-element bearings can operate to speeds of several thousand rpm with grease lubrication. In general, higher speeds call for more frequent relubrication.
Steve Masters is an application engineer with NSK Corp., Ann Arbor, Mich.