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Wednesday, 19 September 2012

How To Prevent Premature Rolling Bearing Failure.

Premature Rolling Bearing Failure Is Not Common.

Although premature rolling bearing failure is not a common occurrence, there are definite steps that can be taken to ensure longevity.

Under normal operating conditions, bearings have an impressive service life, which, according to research, less than 1 percent of rolling bearings fail prematurely.

What leads to premature rolling bearing failure.

When a rolling bearing does fail prematurely, it’s usually
due to factors that could have been avoided. Therefore, it’s important to correct the causes in order to prevent future failures resultant costs.

Most premature rolling bearing failure such as flaking, pitting, spalling, unusual wear patterns, rust, corrosion, creeping, skewing, etc., are usually attributed to a relatively small group of causes that are often interrelated and correctable.

These causes include lubrication, mounting, operational stress, bearing selection and environmental influence.

Proper Lubrication plays a crucial role in crucial in premature rolling bearing failure.

The correct lubrication of a bearing covers the rolling and sliding contact surfaces with a thin oil film to avoid direct metal-to-metal contact. This reduces friction and abrasion, conducts heat generated by friction, prolongs service life, prevents rust and corrosion, and keeps foreign objects and contamination away from rolling elements.

Grease is typically used for lubricating bearings, because it is easy to handle and simplifies the sealing system; while oil lubrication is more suitable for high-speed or high-temperature operations.

Generally lubrication failure, resulting in premature rolling bearing failure is due to:
  • Using the wrong type of lubricant 
  • Too little grease/oil 
  • Too much grease/oil
  • Mixing of grease/oil 
  • Contamination of the grease/oil by dirt or water

Premature rolling bearing failure is influenced by the Service Life of the grease used.

The grease service life is defined as; the time over which proper bearing function is sustained by a particular quantity and category of bearing grease. This is especially crucial in pump, compressor, motor and super-precision applications.

Correct Mounting and Installation of Bearings.
The mounting and installation process can have a role to play in premature rolling bearing failure, therefore, it’s critical to use the proper tools and ovens/induction heaters; as well as using a sleeve to impact the entire inner ring face being press fit.

Also, verify the shaft and housing tolerances. If the fit is too tight, you will create too much preload. If the fit is too loose, you will generate too little preload, which may allow the shaft to rotate or creep in the bearing. Don’t forget to check for proper diameters, roundness and chamfer radius.

Be sure to avoid misalignment or shaft deflection. This is particularly significant in mounting bearings that have separable components such as cylindrical roller bearings where successful load bearing or premature rolling bearing failure can be already be established.

Reducing Operational Stress through correct Bearing Selection.
Generally, it is the exception to find premature rolling bearing failure because a bearing has been improperly designed for the application.

However, factors within the larger application may change. If loads become too high, overloading and early fatigue may follow. If they are too low, skidding and improper loading of the rolling elements occur. Premature rolling bearing failure will follow in each situation. Similar issues arise with improper internal clearances.

The first sign of premature rolling bearing failure will be unusual noises and/or increased temperatures. Bearing temperatures typically rise with start-up and stabilize at a temperature slightly lower than at start-up (normally 10 to 40 degrees C higher than room temperature). A desirable bearing temperature is below 100 degrees C.

There are typical abnormal bearing sounds that reveal premature rolling bearing failure. While this is a subjective test, it is helpful to know that a screech or howling sound usually indicates too large an internal clearance or poor lubrication on a cylindrical roller bearing, while a crunching felt when the shaft is rotated by hand normally suggests contamination of the raceways.

Environmental Influences on premature rolling bearing failure. 
Even with the best design, lubrication and installation, premature rolling bearing failure will occur if the operating environment is not taken into consideration. While there are many potential issues, the main issues are:
  • Dust and dirt contaminating a bearing. Special care should be given to using proper sealing techniques to prevent bearing failure.  
  • Aggressive media or water. Once again, sealing is paramount. The use of specialty-type seals that do not score the shaft is recommended.
  • External heat. The ambient operating temperature mandates many choices in radial internal clearance, high-temperature lubricants, intermittent or continuous running and other factors that affect bearing life.
  • Current passage or electrolytic corrosion. If current is allowed to flow through the rolling elements, sparks can create pitting or fluting on the bearing surfaces. This can be corrected by creating a bypass circuit for the current or by using insulation on or within the bearing. 
In summary: to prevent premature rolling bearing failure it’s important to ensure correct bearing lubrication, mounting, tolerable operational stress, correct bearing selection and take into consideration environmental influence.

The experts at Habot Synthetic Lubricants are there to help you select the correct grease for your application, so that you never have to loose downtime and money due to premature rolling bearing failure.


Randy orton said...
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Various grades and types of Bearing Greases are available to suit the specific requirement of a particular bearing. Selection of the Bearing Grease is very important and the maintenance engineers should strictly follow the advice of the original equipment manufacturer in the selection of Bearing Grease.

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