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Information and Recommendations for the Engineer Fan Bearing
Maintenance & Troubleshooting
Lubrication
Proper lubrication and maintenance are essential for long
possible, the proper level with the fan on must be bearing life. An adequate supply of clean lubricant must marked on the level also. Oil can be blown out of bear- be present at all times to prevent damaging, metal-to- ings installed in the airstream, or near a non-sealed shaft metal contact.
hole. For this reason, Twin City Fan Companies does Twin City Fan Companies' experience has shown that not recommend oil lubrication on arrangement 3 fans grease is the best type of lubricant for fan applications. under any conditions. For ball and spherical roller bearings a NLGI Grade 2 Another advantage of grease is that it provides better or 3 grease is recommended. Special high temperature protection for the bearing against corrosion during stor- greases are recommended if the bearing temperature age and other idle periods.
exceeds 160°F.
Avoid mixing greases with different bases. This can Installation Effects on Bearing Life
cause hardening of the grease and inadequate lubrica- tion. The grease used at the factory is a lithium-based The belt drives used on fans have a large effect on grease. When using a grease constructed with a different bearing life. The combination of the sheaves selected base (such as sodium or a synthetic base), keep adding and their mountings can make the difference between the new grease until all of the lithium based grease having a bearing last its expected life and having it fail purges out of the bearing.
Fans are shipped with labels on them showing the Figure 1 shows a typical fan selected to use a sheave relubrication schedule for normal operating conditions. If with a six inch diameter on the fan shaft. With the the operating condition is relatively clean and at ambient sheave mounted at the end of the shaft, as far away temperature, use the relubrication interval shown on the from the bearing as possible, the L-10 life calculates to sticker as a starting point. Frequently, however, operat- be 32,000 hours. (See ED-1200 for an explanation of ing conditions are dirty or humid and require more fre- L-10 life.) By moving the sheave 1.25" closer to the quent relubrication. If there is high moisture, high tem- bearing, the life is increased to 40,000 hours. When the perature, or if the shaft is in the vertical position, start sheave is moved in another 1.25" so that it is as close out relubricating twice as often as shown on the sticker to the bearing as possible, the life increases to 50,000 and adjust the interval as required. The best way to see hours. The lesson learned by this example is to maxi- if a relubrication interval is adequate is to check the mize bearing life by mounting the sheave as close to condition of the grease purged from the bearing when the bearing as possible.
adding new grease. If darkened and badly oxidized Most belt drives are selected using computer pro- grease purges, relubricate more frequently. If the purged grams. The user puts in the information and the com- grease looks about the same as new grease, a longer puter selects several sheave and belt combinations that interval may work. will work, and usually highlights the least expensive. But High ambient temperatures can make bearing and the combination selected has a significant effect on lubricant selection difficult. The viscosity of the oil con- bearing life. Large sheaves result in less belt pull and tained in the grease is the property that prevents metal- longer bearing life. Figure 2 shows the L-10 life for dif- to-metal contact. With most common lubricants, the ferent fan sheave diameters. All of them are viable drive viscosity starts to fall at temperatures over 200°F. selections. The L-10 life varies from 19,000 hours with Because the temperature inside the bearing is always a 4.2 inch diameter sheave, to 90,000 hours with an 8.2 higher than the ambient temperature, bearing and lubri- inch diameter sheave. Avoid the selections with the cant selection is difficult if the ambient temperature rises smallest sheave sizes. They are probably not the least over 125°F. External means of cooling the bearing may expensive anyway since they require more belts. The be required in these situations. best drive selection is one with a reasonable sheave size Static or circulating oil should only be used with split and moderate cost.
pillow block bearings on high speed or high temperature Fan orientation affects bearing life. A fan designed to applications where grease is unsuitable. Oil lubricated operate with its shaft positioned horizontally will have a bearings require more rigorous maintenance. High oil lower bearing life when positioned with a vertical shaft. levels can prevent the bearing from venting the air inside This is due to the additional thrust loads put on the the bearing as it heats up. The built-up pressure can bearings from the shaft and impeller weight. Figure 3 blow all of the oil out of the bearing in only a few shows an example. An axial fan designed for horizontal minutes. Low oil levels do not provide adequate lubrica- duty has an L-10 life of 49,800 hours. Tipping the fan tion. If sight gauges are used to monitor the oil level so that the shaft is vertical lowers the bearing life to an they must be adjusted to read properly. Since the indi- unacceptable level of 13,400 hours. If a fan is to oper- cated level changes when the fan is turned on, it is best ate with a vertical shaft, make sure to order it for this to check the oil levels when the fan is off. If this is not 2000 Twin City Fan Companies, Ltd.




Figure 1. Effect of Sheave Figure 2. Effect of Sheave Figure 3. Effect of Shaft Position on Bearing Life Location on Bearing Life Diameter on Bearing Life 1400 RPM, 15 HP MOTOR HEAVY DUTY BALL BEARINGS 17/16" HEAVY DUTY 17/16" HEAVY DUTY SHEAVE L-10 LIFE SHEAVE L-10 LIFE
POSITION
L-10 LIFE
L-10 = 49,800 HRS.
L-10 = 13,400 HOURS Bearing Failures and Their Causes
Figure 5. Normal Fatigue Failure The L-10 life discussed earlier is an estimate of the fatigue life of a bearing. However, there are many other factors that determine how long a bearing lasts. Figure 4 shows the results of one study that looked at the reasons for bearing failure. What is interesting about this study is that it shows that normal fatigue is not the normal cause of bearing failure, since only 34% of the bearings failed this way. Half of the bearing failures were due to either inadequate lubrication or contamination. Other sources list fatigue to be the cause of only 10% of bearing failures.
Figure 4. Bearing Failure Causes 36% - Inadequate Lubrication 34% - Normal Fatigue 14% - Contamination 16% - Other Causes Such as: Static Vibration Figure 6. Spall Electric Currents Source: Rolling Bearing Digest, 1985 If bearings that are failing can be taken out of service before they are completely destroyed, they can reveal information about the cause of failure. Different modes of failure leave unique evidence in the bearing compo- nents. The rest of this article shows how the more common types of fan bearing failures affect bearing Figure 5 shows an inner race that has had a normal fatigue failure. The repeated loading and unloading of the race every time a roller passes eventually causes microscopic fatigue cracks to form under the surface. In time, these cracks grow to the point where small Figure 7. Spalls & Wear Patterns in the Race portions of the surface flake off. This process is called "spalling" and the flaws left in the surface are called With continued operation, the spalls grow until the bearing becomes noticeably noisy. Figure 6 shows an inner race that has a severe spall. In addition to being noisy, it is also likely that this bearing would show high levels of vibration. The time required for the spalling to cause catastrophic bearing failure will vary with the load on the bearing and its speed. Usually the bearing will run for a while after it becomes noisy, but once noise is heard, prepare to have the bearing changed before it fails suddenly.
Inadequate lubrication can lead to spalling as seen in Figure 7. This bearing also shows signs of excessive Fan Engineering FE-1300






wear due to inadequate lubrication. The rollers have Figure 10. Smearing worn a very deep wear pattern in the race. Inadequate lubrication does not mean that there is no lubrication, only that the lubricant is not preventing metal-to-metal contact. This can be due to adding an insufficient amount of lubricant when relubricating, too long of a period between relubrications, using dirty lubri- cant, or using a lubricant that does not have the viscos- ity needed to maintain an oil film between the parts. Figure 8 shows the progressive failure of a bearing due to inadequate lubrication. At first there are traces of waviness, or a slight roughening of the race. With time small cracks develop, which lead to a modest amount of spalls and eventually the severe amount of spalling shown on the lower right.
Figure 8. Progressive Failure of a Bearing Due to Inadequate Lubrication Figure 11. Skid Smearing Figure 12. Highly Polished Surfaces Due to If the bearings continue to operate with inadequate lubrication, the evidence that shows this to be the mode of failure becomes destroyed. Notice how the spherical bearing rollers shown in Figure 9 become damaged with continued operation.
Figure 9. Damaged Spherical Bearing Rollers If the bearing had been taken out of operation when the roller was in the condition shown on the left, it would unloaded to being under load. With the light load it be possible to determine the cause of failure. By the takes them some distance to roll properly, and they skid time the bearing completely failed, it had run so hot that against the race causing smearing.
the steel softened and the bearing became deformed. At Inadequate lubrication can sometimes cause the bear- this point it is impossible to tell what caused the initial ing surfaces to become highly polished as Figure 12 failure. If you are having unexplained bearing failures, it shows. Even though the surface looks nice, this is an is best to take the bearing out of service when you first indication of trouble. With continued operation the surface notice noisy, hot, or vibrating operation. The evidence will change to a frosty appearance and eventually spall.
showing the initial cause of failure is probably still visible, Material that flakes out of spalls and other contami- and it will give you an idea as to what action to take nants from other sources such as dirty grease or worn to prevent additional failures. seals dents the races when caught between the rollers Under the right conditions the heat generated by and races. Figure 13 shows an inner race with severe metal-to-metal contact causes the two surfaces to particle denting. Figure 7 also shows signs of particle adhere for an instant. Small portions tear out from one denting. Each dent is a stress concentrator, which surface and adhere to the other surface. This is "smear- increases the stress in the area around the dent and ing" and is shown in Figure 10.
accelerates the fatigue failure process.
"Skid smearing," shown in Figure 11, occurs on Keeping moisture out of bearings is necessary to lightly loaded bearings. As the rollers come around and achieve long life. Figure 14 shows a bearing severely enter the lower half of the bearing, they go from being damaged from water corrosion. This much damage was Fan Engineering FE-1300 Figure 13. Severe Particle Denting Figure 14. Water Corrosion probably due to seal failure, possibly in a "wash down" environment. High moisture applications require the use of grease that has rust inhibiting properties. Frequent Figure 15. Bearing Roller with Rusty Ends relubrication is necessary, to insure that all bearing sur- Due to Moisture in the Lubricant faces are coated with a protective layer of grease and that the rust inhibitors are replenished. Figure 15 shows a spherical roller bearing roller that has rusty ends from moisture in the lubricant.
Water from condensation causes corrosion in idle fans. This is particularly true for fans in high moisture areas, located outdoors, or improperly stored outdoors. Figure 16 shows an outer race damaged by puddle cor- rosion. Moisture or dew condensing in the bearing settles to the bottom of the race in puddles and causes rusting. Twin City Fan Companies packs ball and unit roller bear- ings completely full of grease prior to shipment. Split pillow block spherical roller bearings are shipped with Figure 16. Puddle Corrosion the bearing insert packed full. This minimizes the chanc- es of having puddle corrosion occur during shipment and storage. This practice covers all components with grease, and insures there are no pockets of air where water can condense. At startup the excess grease purges out of the bearing, which gets a little messy but is better than having a damaged bearing.
Figure 17 shows damage to an outer race from a bearing subjected to vibration while the unit is not rotat- ing. Notice the even spacing of the marks, which cor- respond to the roller spacing. You can also see marks where the rollers came to rest in different locations. This type of damage, called "false brinnelling," occurs when Figure 17. False Brinneling small portions of the race get rubbed away. The remain- ing area is ideal for starting fatigue, and the abrasive material rubbed off contaminates the grease which accelerates bearing wear.
"True brinnelling" occurs when a bearing is subjected to impact and the rollers dent the races. Figures 18 and 19 show damage caused by improper mounting. Impact from hammering on the outer race or the bearing hous- ing causes true brinnelling of the races. If it is necessary to tap on a bearing to get it into position, tap on the inner race so that the rolling elements do not carry the load. Dropping a bearing prior to mounting can also cause true brinnelling. Like false brinnelling, the marks are evenly spaced and the damage will eventually Figure 18. True Brinneling develop into spalls. Figure 20 shows spalls resulting from true brinnelling. Mounting a bearing pillow block on a warped or crowned surface, or improperly shimming a pillow block can pinch the outer race. As seen in Figure 21, this causes two load zones on the outer race 180° apart.
As the rolling elements pass through the pinched area, the load between them and the races increases. If the severity of the pinch is high, a drastic reduction in the life of the bearing will result.
Too much clearance between the bore of a bearing and the shaft, or too tight of a fit, can lead to bearing Fan Engineering FE-1300 failures. Figure 22 shows the surface of a bearing bore Figure 22. Smearing on a Bearing Race that had too much clearance. The shaft turned in the bore, smearing the surface.
Insufficient tightening of the setscrews on a set- screw mounted bearing causes this type of failure. It can also be caused on a tapered adapter mounted bearing by not tightening the adapter lock nut enough. The damage to the surface resulted in a crack. The bore of the bearing shown in Figure 23 shows signs of serious fretting corrosion. Microscopic move- ment be-tween the shaft and the bore causes the rust colored patches to develop. Small amounts of fretting are common with setscrew mounted bearings and are not detrimental to bearing life. Advanced cases, caused by loose mounting, can lead to cracks.
Tightening the locknut of an adapter mounted bearing Figure 23. Fretting Corrosion on Bearing Bore too much can crack the inner race as shown in Figure 24, or the outer race as shown in Figure 25.
This also occurs by exposing the bearing on a high temperature fan to excessive temperatures. This easily happens when shutting down high temperature fans when they are hot, or by exposing them to high tem- peratures while idle.
Figure 19. True Brinneling Figure 24. Cracked Inner Race Figure 20. Spalls Resulting from True Brinneling Figure 25. Cracked Outer Race Figure 21. Two Load Zones on the Outer Race Figure 26. Pits Caused By Electrical Current Fan Engineering FE-1300 Allowing electrical current to pass through a bearing can cause damage. Figure 26 shows a spherical roller with pits in its surface caused by electrical current. This type of damage commonly occurs when welding balance weights on a fan wheel with improper grounding. To avoid having the welding electrical current pass through the bearings, attach the grounding clamp to the wheel or shaft when adding balance weights.
Summary
There are several different types of rolling element bearings used for fan applications. The best choice is the one
that provides adequate L-10 life with reasonable cost. Proper care during mounting, shipment, storage, and operation is required in order for the bearings to achieve the theoretical L-10 life they were selected for.
References
Bearing Installation and Maintenance Guide, SKF USA Incorporated, King of Prussia, Pennsylvania, 1992
SKF General Catalog, SKF USA Incorporated, King of Prussia, Pennsylvania, 1991
Bearing Technical Journal, PTC Components Incorporated, Indianapolis, Indiana, 1982
Bearing Failure: Causes and Cures, The Barden Corporation, Danbury, Connecticut, 1992
Dodge Engineering Catalog, Volume 1.1R, Reliance Electric Industrial Company, Greenville, South Carolina, 1993
Sealmaster Bearing Products, Morse Industrial, Emerson Power Transmission Corporation, Ithaca, New York, 1988
Eugene R. Hafner, "Selecting Bearings for Fans and Blowers" Machine Design, April 29, 1990
Photographs in Figures 5 through 26 are courtesy of SKF USA Incorporated TwIN cITy FAN & blowER www.TcF.com
5959 Trenton lane N minneapolis, mN 55442 Phone: 763-551-7600 Fax: 763-551-7601

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