Bearings exposed to high temperature, moisture, corrosive chemicals, or dirt, and which operate under high speed or high load conditions, will achieve their expected design life only if you select the correct lubricant and apply it properly.
Poor lubrication causes over 40% of bearing failures, and these failures are generally attributed to one of the following:
• Deterioration caused by prolonged service without sufficient relubrication intervals.
• Insufficient quantity or viscosity of lubricant.
• Contamination of lubricant and bearing by foreign materials.
• Deterioration of lubricant caused by operating in conditions beyond its capabilities.
• Selecting grease when operating conditions indicate oil.
• Selecting the wrong grease base for an application.
Extreme environmental or operational conditions under which bearing lubricants may be called upon to operate, Table 1, include:
• High temperatures.
• High vibration.
• Heavy dirt or other contamination.
• Low temperatures.
• High-velocity air flow.
• Heavy loads and high speeds.
• Semi-vacuum atmospheres.
Bearings may be exposed to high temperatures in equipment such as furnaces, ovens, fans, and blowers. Other applications include steel mill and foundry equipment, such as continuous casters and roll-out tables, plus dryers, electric motors, and generators.
Operating temperatures are a major factor in selecting the grade of oil or grease and the method of bearing lubrication. Figure 1 illustrates the useful temperature ranges for several common lubricants.
Conventional greases and oils usually withstand temperatures up to 200 F without a reduction in life. But lubricants used above their temperature limits deteriorate rapidly. This deterioration begins with evaporation of the oil, or oil additive in the grease, and continues until the grease loses its lubricity. Evaporation, coupled with oxidation of the oil and soap base, eventually transforms grease into a hardened state.
Temperatures above 200 F require special lubricant types and lubrication methods. For equipment operating at temperatures up to 250 F continuously, or 275 F intermittently, use a high-temperature or highly refined turbine-type petroleum oil. These oils are more stable and evaporate more slowly than most universal oils.
The use of static oil — a pool or sump in the housing reservoir where a bearing is partially submerged — should be confined to low-speed applications at high temperatures. Hold the speed to a DN value (bore, mm x speed, rpm) not to exceed 50,000 to 75,000 to avoid high deterioration of the oil caused by churning and aeration.
At continuous temperatures of 250 to 320 F, synthetic greases work well. They offer more stability than petroleum grease at elevated temperatures, Figure 2. Some synthetic oils are used at low-tomoderate operating speeds (10 to 60% of maximum rated speed) where temperatures range up to 400 or 430 F.
Synthetic greases and oils may require speed and load restrictions. As a general guide for high-temperature usage, restrict loads to about 10% of bearing dynamic capacity and speeds to 50% or less of the normal speed limit listed in the bearing manufacturer’s catalog. Operating a bearing beyond these limits causes rapid wear of raceways and other components.
A circulating oil system is the best approach for continuous operating temperatures above 250 F, or where high reliability is desired for temperatures over 200 F.
You can also protect bearings by locating them out of the immediate heat zone or reducing the temperature at which they operate. This can be accomplished by insulating furnace walls or fan casings to reduce radiant heat, for example. Heat flingers or cooling wheels and disks, coupled with high nickel-chrome, heat-resisting shaft material, also reduces heat conducted to bearings. For extreme hightemperatures, use water-cooled housings.
Though these steps incur higher installation costs, they reap long-term dividends by reducing lubrication and maintenance problems often encountered with hightemperature bearing applications.
Equipment such as vibrating screens, fans, or gyratory crushers, subject bearings, housings, and lubricants to high vibration. This condition tends to workshear grease, causing it to soften or liquefy so that it either leaks past the seals or causes churning in the bearing. Churning raises operating temperatures, especially at high speeds (over 60% of maximum rated speed), accelerating breakdown of the grease and causing premature bearing failure. Generally, you should specify greases with minimal work-shear tendencies for bearings on vibrating equipment.
High-vibration applications such as vibratory shaker screens, tampers, shaker tables, or vibratory rollers require grease with enough mechanical stability to prevent its collapse back into the path of the bearing rolling elements. Such a grease channels away from the rolling element path, preventing churning.
Many users prefer NLGI No. 2 or 3 grease with a lithium or lithium-complex soap base for vibratory applications because of their stability, which minimizes churning and lubricant deterioration.
The required relubrication interval in vibratory applications depends on operational factors such as rotational speed, vibration frequency and magnitude, and service cycle, plus environmental factors such as temperature, moisture, dirt, and abrasives. The seal’s ability to exclude contaminants and retain the lubricant also affects the relubrication interval.
Dirt and abrasive contamination is especially severe in applications such as foundry and steel mill equipment, aggregate or cement-processing machinery, and taconite handling conveyors. Equipment used in agriculture, logging, construction, and utility fields is also subject to such contamination.
In severely contaminated environments, selecting the right lubricant is only part of the solution to good bearing performance. After selecting a lubricant based on operating and environmental factors, consider the sealing and relubrication frequency.
With contamination, sealing is often the key to prolonging bearing life. Seals must keep contaminants out and still retain lubricant in the bearing. In many cases, a grease-flushable seal gives added protection and longer bearing life.
Frequent relubrication often enhances sealing effectiveness by expelling any contaminants that get past the seals. A fresh purge of grease also cleans the seal contact area, prolonging seal life.
Very severe environments may require continuous lubricant replenishment, while heavy to moderate levels may require purging every 8 to 10 hr of service. Relubrication intervals for specific applications can be determined by checking the consistency of used lubricant purged from the bearing after the initial interval. Significant changes in consistency indicate the need for more frequent lubrication.
Extremely low-temperatures pose special problems for bearings and lubricants. As temperature drops, the lubricant thickens (viscosity increases). Where equipment operates in both summer and winter, temperatures may vary as much as 140 F, causing a substantial change in lubricant viscosity. For example, the viscosity of SAE 30 oil increases ten times when the temperature drops 140 F.
Many low-temperature greases and oils, both petroleum and synthetic, operate from 267 to 200 F. But almost all of these lubricants have limited load-carrying capabilities.
Heavily loaded bearings that operate within a wide temperature range include those used on pulley shafts of coal or ore handling conveyors. Typically, these bearings require a lubricant with an SUS viscosity of 2500 at the upper range of 100 F. But this high-viscosity grease transforms to a solid or near solid-state at 240 F.
Such a transformation causes high starting torque requirements on the equipment and could lead to damage of bearing components, housings, and seals. You can use a compromise lubricant (mid-range viscosity) under high loads and temperature extremes, but at a sacrifice in optimum bearing life.
Where lubricant can’t be changed on a seasonal basis, select one with adequate viscosity at the highest expected temperature and incorporate auxiliary heating in the winter.
Bearings repeatedly come in contact with moisture in applications such as farm machinery, equipment that sits idle in open storage, steel mill quench rolls, and food machinery that must be washed down periodically.
Corrosion-causing moisture gets into bearings in two ways; ingress of water or liquid chemicals from direct impingement, and moisture condensation or chemical vapors. To reduce corrosion, use high-quality grease with corrosion inhibitors and good moisture control characteristics.
For industrial and agricultural equipment not involved in handling foods, greases with a lithium base or corrosion inhibitors offer excellent moisture- resistance.
FDA laws require that lubricants on food handling or processing equipment must not contain soaps or additives that may be injurious to human health. Special food-grade lubricants are available for this type of equipment.
Where speed is relatively low, you can further protect bearings by filling the bearing and housing cavities with grease to evacuate air and prevent condensation. Bearings on high-speed equipment can be protected in the same way during storage, but remove some of the grease from the housing before full-speed operation to avoid overheating.
High-velocity air flow
Some bearings are exposed to high-velocity air flow caused by fans and blowers. Although there is no precise definition of high-velocity air flow, take special precautions where the air flow rate and direction, combined with the bearing housing geometry, causes a pressure differential across the bearing. A pressure gradient on one side of the bearing or housing can siphon or force oil through the seal on the low-pressure side. In such cases, either switch to grease, if speeds and loads permit, or use special air-baffled and pressure-equalized bearing housings, Figure 3.
Heavy loads and high speeds
For bearings exposed to heavy loads, high speeds, or high temperatures, a positive- pressure circulating oil system is a reliable and effective lubrication method. Applications that justify the cost of circulating systems include high-temperature fans and blowers, rolling mills, and other steel mill equipment. Other applications where the need for dependable performance may justify a circulating oil system are annealing line rolls, line shafts, printing press rolls, shafts in power generation equipment, and many transmissions and gearboxes.
A circulating oil system can remove heat, either generated by the bearing or transmitted to it from an external source; remove impurities from the oil in conjunction with a filtration device; and maintain an adequate oil supply to the bearings.
A pressure circulating system consists of an oil reservoir, positive-pressure displacement pump, heat exchanger, feed and drain lines, filter, and safety monitoring devices, Figure 4. The pump delivers oil from the reservoir to the filter and passes it through a heat exchanger for cooling. Then the pump circulates oil to the bearings where it is either suctioned or drained back to the reservoir.
Achieving optimum lubricant life requires adjusting the system’s oil flow to maintain the bearing and oil outlet temperature at no more than 170 to190 F. Typical flow rates range from ½ pint a minute for small bearings to several quarts a minute for large ones, depending on the heat load and operating speed.
Occasionally, bearings are used on equipment that operates in a semivacuum atmosphere, such as vacuum pumps, axial-flow fans and blowers, and wind tunnels. Very low vacuum pressures require oils or greases that have low vapor pressure, oxidation stability, good water-separation properties, and are free of moisture and gas.
Many oils and greases become ineffective at lower atmospheric pressures, due to high evaporation loss of the oil or oil components in the grease. This loss is accelerated with light-molecular-weight oils, and with higher temperatures.
The largest evaporation loss tends to occur at first, with the rate subsequently diminishing. Because of this time element, the relubrication frequency largely determines the lubricant life.
Generally, you can use conventional greases and oils where pressure reduction is limited to about 10-7 torr (One torr is the pressure needed to support a column of mercury 1-mm high at 0 C) and operating temperatures do not exceed 190 to 200 F (87 to 93 C). For bearings operating at low vacuum pressures, consult a lubricant or bearing manufacturer for lubricant recommendations.
At the low end of the vacuum pressure range (10-3 to 10-10 torr), the free flow of gaseous molecules dictates oil evaporation losses. You can control these losses to a large degree by selecting adequately fitted housing or bearing contact seals. Make sure the seals are compatible with other application conditions, including speed and bearing temperature.
Certain systems must operate in a partial vacuum at elevated temperatures, while others require no lubricant leakage, which could affect the end product. Solid film lubricants may be suitable for lubricating bearings in both these cases, but only for low speeds and light-to-moderate loads.
David A. Boyer is manager of product development engineering, Link-Belt Bearing Div., Rexnord Corp., Indianapolis, Ind.