At first glance, bearing failures may not be what they seemFor companies across the country, in every industry, equipment reliability and uptime are critical to keeping operating costs in line and unnecessary expenses in check. So when a machine fails, it’s imperative to identify the root cause and assure that the same problem doesn’t occur again. Sometimes, that investigative process yields surprising results — as is often the case with bearing failures.

     In fact, many bearing failures actually result from trouble elsewhere in the “system,” as the footprint can indicate, according to Chuck Worden, SKF Applications Engineer. Through years of accumulated data, SKF’s failure analysis group identifies trends in bearing failures, then uses those trends to advise customers on the most effective ways to avoid bearing trouble. One such trend underscores the fact that you often have to look beneath the surface for an answer.

     According to Worden, nearly 3 out of 4 bearing failures
actually result from difficulties with the shaft. Problems such as improper shaft fit, form or finish can trigger bearing
problems — resulting in an unexpected machine failure and unplanned downtime.

     “The primary function of a bearing is to reduce friction,” Worden says. “But if the bearing isn’t applied on the shaft correctly or the shaft is faulty in some way, the result is typically increased friction. Clearly, management of the shaft from start to finish is critical to maximizing bearing uptime and reliability.”

     Shaft management is a comprehensive effort that involves specifying the right shaft, lubrication, bearing and seal; assuring those specifications are met; and then implementing a condition monitoring program to identify early warning signs of an impending problem. Much like SKF’s Trouble-Free Operation (TFO) program, shaft management is a preventive effort that attempts to avoid bearing problems before they occur.
Quality by design

One of the first areas to consider in managing the quality of the shaft system — and the resulting operation of the bearing — is shaft design. According to Worden, several factors come into play during the design phase. The form of the shaft
is a key design issue and can involve several considerations. For instance, most bearings require a specific tolerance
for proper cylindricity, or how circular the bearing seat is in shape. If the shaft form doesn’t meet the required tolerance for cylindricity, the inner ring and the bearing seal will adapt to the improper form of the shaft. This, in turn, can cause excess operating noise, uneven stress distribution, and ultimately early bearing failure.

     “When it comes to shaft form, even an inaccuracy of .001 millimeters can contribute to premature bearing failure,” Worden notes.

     The surface finish of the shaft is another design factor which must be considered in order to avoid unnecessary bearing problems. A smoothly finished shaft is the ideal, since even small discrepancies in finish tolerance can be enough to cause bearing trouble. Worden explains that an improper shaft finish keeps the bearing from gripping the shaft correctly.

     “Without the proper contact between the bearing’s inner ring and the shaft, the ring will tend to creep, causing wear and corrosion,” he says. “Wear is essentially metal-to-metal rubbing, which generates heat and debris, contributing
to early bearing failure.”

     Proper sizing of the shaft also is essential. An improperly sized shaft can contribute to the same type of problems as improper finish — resulting in poor contact with the bearing’s inner ring and degradation as
a result. To help customers in selecting the proper size shaft, SKF’s General Catalog specifies the required shaft size for each bearing and application.

     Finally, proper shaft alignment is key to the effective operation of a bearing. Some SKF bearings are designed specifically to accommodate misalignment in the shaft, Worden says. But in other instances, the bearing required for a particular application can not, by design, allow for misalignment in the shaft.

     “The misalignment can be very slight — less than ½ °
— yet still contribute to
machinery operation problems,” Worden says.

     In addition to affecting the bearing directly, the quality of the shaft also can impact the seal, which plays an important role in overall bearing operation and useful life. Seals essentially protect the bearing by keeping lubricants in and contaminants out. If the seal
is compromised in any way, early bearing failure becomes a more likely event.

     Problems with the shaft can contribute to seal trouble in many different ways. If the shaft has the incorrect finish, the seal can be subject to excessive wear. If the shaft is too small for the seal specified, it can’t maintain the proper contact with the bearing. If an improper shaft finish causes friction and excessive heat,
the seal will break down over time. All of these potential problems compromise the seal’s ability to perform its function of retaining lubricant and excluding contamination.
Minimizing trouble

The first line of defense against shaft problems is
specifying the right bearing, seal and shaft for the job. SKF specialists work closely with customers to help at the early stages of the process and can advise customers in selecting the right specifications for components for the application in which they will be used.

     SKF specialists also offer help in the proper installation and maintenance of bearings and the impact of the shaft, through a unique training course — Bearing Maintenance Institute (BMI) in York, Pennsylvania. BMI provides three-day seminars for maintenance personnel interested in learning more about bearings and how to extend their useful life. Worden says the seminars cover issues such as how to choose the proper size and form for the shaft, the importance of surface finish, and other key considerations.

     Once the shaft is in use, the best way to assure it doesn’t compromise the operation of the bearing or seal is through a condition monitoring program. Condition monitoring provides a snapshot of equipment operating condition, while the equipment is running — allowing companies to focus time and resources on machinery that needs attention. If a condition monitoring program signals a potential problem, maintenance staff can use various diagnostic tools to sort out the cause and correct it. Most condition monitoring tools can check variables such as temperature, velocity, vibration, acoustics, and lubricating oil condition — all of which can signal a problem.

     If condition monitoring does signal impending trouble, the SKF USA Internet site can help. Experienced Application Engineers who are familiar with bearing selection, operation and maintenance can answer your questions via the Virtual Engineering Hotline. These engineers provide advice and troubleshooting help in areas such as bearing fit and selection, shaft finish and form, and the proper maintenance and lubrication of bearings. To access the site, visit www.skfusa.com.

      “Many times, customers with condition monitoring programs contact us to ask about the appropriate limits for certain operating parameters, such as speed or temperature,” says Worden. “To provide that help, we tap into our database and provide answers based on SKF’s experience with similar equipment.”

     Downtime resulting from an unexpected machinery failure can be an expensive proposition for any company. But by paying close attention to the shaft and its effects on other components, organizations can maximize bearing and seal life — and reduce the odds of a costly, unscheduled shutdown.