Forensics for bearings

Bearing health can be monitored through the use of infrared temperature measurements, vibration and acoustical signals, and oil analysis. Failure mode analysis by itself is especially useful in diagnosing root causes of problems after the damage is done.

Through the prudent use of temperature and vibration-monitoring equipment, routine oil analysis, lubrication system evaluations and machine operational performance reviews, bearing distress may be identified and evaluated before catastrophic failure occurs.

Being able to properly identify damage resulting from improper installation, pitting, fatigue, abrasion and corrosion is the key to diagnosing root causes of bearing problems. Some typical problems include false brinelling, true brinelling, misalignment, fretting corrosion, contamination and fatigue.

False brinelling: Caused by vibration resulting in movement of the rolling element against the raceway in stationary bearings. Damage appears as linear or elliptical wear marks in an axial direction at each ball position. It can be controlled by eliminating axial movement and using lubricants containing solid additives.

True brinelling: Appears as indentations in the raceway at the same interval spacing as the balls. It will result in increased bearing vibration and can lead to fatigue failure. It is different from false brinelling in that the indentations are not just surface marks. True brinelling is caused by a severe impact to the bearing, such as by dropping it or striking it with a hammer. Static overload can also be a cause. Proper installation will cure most causes of true brinelling. Paying attention to static load ratings is important too.

Misalignment: Appears as wear path not parallel to the raceway edges. It is caused by many things, most commonly bent shafts or shafts that are not properly aligned, or improper installation. It is best fixed by properly aligning shafts and through good installation practices, including the use of precision locknuts.

Fretting corrosion: Characterized by micro-motion between parts where fit is just short of tight enough. It appears as brown rust, usually between the housing and the outer ring or the shaft and the inner ring. It can be prevented by assuring proper fit upon installation. Fretting corrosion can also affect shaft and hub-and-spline shaft connections. In these applications, the use of solid lubricants can cure the problem.

Contamination: Raceways and/or rolling elements will bear slight indentation marks in random patterns. It will often lead to early spalling failure. It is caused by dust or dirt particles or other abrasive substances introduced from the environment, or by a contaminated work area or lubricant. Clean tools, work areas and lubricants will solve the problem. Keep bearings in the original packaging until it is time to install them.

Normal fatigue (spalling failure): Indicated by pitting of raceway or rolling element surfaces. Progressive wear results in increasing vibration and leads to bearing failure. It is caused by excessive preloading, tight inner ring fits and excessive load. Normal fatigue is also the mode of failure for the 10% of bearings that operate for their calculated service life.

Preventive maintenance tips

1. Oil analysis – a quick visual examination of the oil and oil filter may be all that is required to determine that a problem exists. A thorough oil analysis can provide useful data to assist in diagnosing bearing or machine distress. As a minimum, the following should be supplied:

• Particulate density and particulate breakdown

• Viscosity and water contamination

• Chemical breakdown.

2. Operational data – another important source of diagnostic information is unit operational data, such as:

• Period of load or speed changes, recent maintenance

• Performance of related machinery, vibration data/analysis.

In a perfect world, hydrodynamic bearings theoretically have an infinite life. Equipment operators know that their world is far from perfect. By taking a forensic approach to plain bearing failures, the operator can uncover and correct system-related problems, and ultimately increase machine availability and output, and thereby decrease costs.

This article has been adapted from Smart Lubrication Series from the Molykote newsletter published by Dow Corning Corp., Midland, MI. For more information, visit www.dowcorning.com.