Fans play a key role in manufacturing applications, recirculating air, supplying combustion air and moving air through processes and pollution control equipment. Because they are essential components in these and other areas, it is vital they always operate reliably and at peak efficiency. Reliability avoids interruptions that lead to downtime and product damage.
While motor or belt failures sometimes contribute to fan malfunctions, the bearings are the most frequent point of breakdown. The bearings are the critical link between the rotating fan shaft and its stationary drive base-two of the most highly stressed components of the fan.
Bearing selection is critical
Selecting the right bearings is crucial in avoiding failure. Fans in high-temperature applications are either direct-connected or V-belt driven. Fans commonly use one of three kinds of bearings-single-row ball bearings, self-aligning spherical roller bearings or sleevoil bearings. Sleevoil bearings are generally used on large, heavy-duty applications, while the other two tend to be used with small- and medium-sized fans in a wide range of plant settings. Ball bearings are used for relatively light loads and high speeds, while spherical roller bearings are best suited for heavier loads and lower speeds.
There are other criteria to be considered when selecting the proper bearing, including L10 life, bearing operating temperature, ease of maintenance and cost. It is just as important to understand the most likely causes of failure, which include:
- Low inherent L10 life.
- Heat buildup from internal and external sources.
- Improper lubrication.
- Imbalance or mechanical deficiencies that produce vibration.
Analyzing L10 life
The L10 life is the calculated life of a bearing and is defined as the number of operating hours that 90 percent of a group of apparently identical bearings will exceed under a given set of conditions. It is a function of radial load, axial load, speed and the basic load rating of the bearing. The weight of the fan wheel and shaft plus the belt tension in the V-belt drive determine the radial loading.
On higher horsepower applications, the load imposed by belt pull is often greater than the force derived from the wheel and shaft weight. Understanding this and accounting for it is essential. Axial load is calculated from the thrust inherent in a single-width centrifugal fan, while basic load ratings can be obtained from bearing catalogs.
The L10 life should range from 50,000 to 100,000 hours for bearings in high-temperature fan applications. If the L10 life is less than 50,000 hours, the bearing will too quickly become compromised. An L10 life in excess of 100,000 hours is wasteful and counterproductive. Costs for bearings with high L10 life increase geometrically beyond 100,000 hours. Also, overrated bearings can actually increase wear and failure.
The engineers at one major building products company specified spherical roller bearings with very high L10 life for their exhaust fans, where temperatures routinely exceeded 600° F. However, these heavy-duty bearings were supporting a relatively light load. This insufficient loading did not provide enough force to effectively "push down" on the rollers, so the rollers skidded. This eventually caused the fan to fail. In this case, ball bearings with a lower L10 life actually did a better job and improved reliability.
The best way to determine optimal L10 life is to calculate normal load patterns, factor in parameters such as expected total operational hours, and then weigh those against previous practical experience. For example, when industrial fan bearings are in continuous operation, they often operate up to 8,000 hours per year. If the projected equipment life cycle is 10 years, then an L10 life of 80,000 hours would be required. But, again, the needs should be weighed against practical experience and the specific application of the fan.
Causes and solutions of heat buildup
Operating temperature is another important consideration in maximizing the reliability of fan bearings. The operating temperature of roller bearings should never exceed 200° F. Excessive temperature damages bearing components, particularly the cages and seals, and degrades the lubricant.
- The four sources of heat that normally affect bearings are:
- Heat conducted through the shaft from the process gas in the fan.
- Radiation from the fan casing.
- Self-generated heat from friction within the bearing.
- Hot ambient air.
Several approaches minimize heat conducted through the shaft. Select shaft material with a relatively low thermal conductivity. Stainless steel has significantly lower conductivity than carbon steel. Attaching a heat flinger-a small centrifugal fan wheel made of material with high thermal conductivity such as aluminum-to the shaft between the fan casing and non-drive end bearing also dissipates heat. This absorbs heat from the shaft and dissipates it into the atmosphere.
The heat radiated from a hot fan casing can be reduced with an insulated fan casing and a radiation shield. Insulation reduces the external temperature of the casing wall thereby minimizing radiation. A radiation shield, however, mounts between the bearing and casing, protecting the bearing assembly from direct radiation. This shield can be incorporated into the guard for the flinger. Improperly maintained, or even non-existent, insulation is one of the most common problems seen in higher temperature applications. If the temperature inside the fan housing can exceed 1,000° F, installing proper insulation is vital to controlling bearing temperature.
Ball bearings generate less heat than spherical roller bearings, but changing from spherical roller to ball bearings requires conducting a load analysis to ensure adequate L10 life.
Using a low-viscosity lubricant reduces heat generated by friction within the bearing. Unfortunately, the viscosity reduction required to reduce the temperature sufficiently may result in a grease that is too light for adequate lubrication. Generally, the minimum required viscosity at the bearing operating temperature is 70 SUS for ball bearings and 100 SUS for spherical roller bearings. In addition, temperature gradients between the shaft and bearing can cause excessive tightening of the bearing clearance as the shaft heats and expands. To compensate, bearings should have a higher than normal clearance.
Finally, bearing operating temperature varies almost directly degree-for-degree with ambient temperatures. So, the single most effective and simple way to control temperatures is by proper fan placement. Where possible, locate the fan in a relatively cool area. Avoid ceilings or corners where heat normally concentrates and circulation is inadequate. These locations make the fans difficult and uncomfortable to maintain, increasing chances for improper maintenance.
Avoiding ceiling or corner placement, unfortunately, is not always practical. Kiln recirculation fans, for example, must be on the roof to perform properly without adding excessive ductwork. In situations such as this, it is important to insulate properly. Increasing the velocity of ambient air near the bearing assembly also helps cool it.
Maintaining proper lubrication
Lubrication is one of the most important factors that ensures reliable bearing operation. It also is the most ignored. The lubricant minimizes friction at contact points in the bearing, protects the internal components from corrosion and keeps out dust and other contaminants.
Grease and oil are the two primary types of lubricant, each having its own benefits. Grease clings to surfaces better and improves the bearing seal. It also is easier to maintain and is easier to retain inside the bearing because there is less leakage. Oil makes higher bearing speed possible. It generates less heat and is easier to replace because it requires a simple drain and refill.
Oil allows more precise control of the lubricant, and it can be recirculated for improved cooling and cleaning. Of the two, grease is the more common lubricant. With that in mind, avoid these common mistakes:
- Wrong grease. It is essential to use the grease recommended by the bearing supplier.
- Mixing greases. Although different greases may be satisfactory, they rarely are compatible when mixed.
- Too much grease. The extra bulk of grease causes excessive temperatures.
Too little grease. If not replenished, grease loses its viscosity and the bearing wears prematurely.
Vibration is another factor that can lead to frequent fan failure. Vibration affects the fan rotor and shaft, causing cracking and ultimately catastrophic failure. It also increases bearing wear. Excessive vibration is a sign that the bearing may be carrying an excessive load and is being subjected to hardships that eventually will cause a breakdown. Monitoring and analyzing the fan's vibration signature occasionally helps avoid this problem.
Having considered the various factors that affect bearing failure, it is helpful to see how these issues can be handled in one type of application, a kiln heat recovery system. The kiln heat recovery fan was designed to operate at a speed of 797 rpm to move gas at a temperature of 600° F. In this application, the radial loads demanded spherical roller bearings.
The company first specified carbon steel, but an analysis showed this would conduct heat excessively. The material was changed to Type 416 stainless steel. A heat flinger is a standard accessory for fans in this temperature range. If one had not been included, the temperature of the non-drive bearing would have been in excess of 200° F. Another standard accessory is a guard with an integral radiation shield.
Since the fan would operate at temperatures above 300° F, we recommended high-performance, synthetic-base fluid grease with a lithium complex soap thickener having a viscosity of 220 CST at 40° C (1,100 SUS at 100° F). This holds up well and has excellent viscosity at high temperatures.
This combination of design elements based on an analysis of operating needs and environment ensured a highly reliable and efficient fan. Such considerations are essential in other applications to ensure optimum productivity and minimal repair costs.