Detecting Premature Bearing Failure

Bearing manufacturers have long been aware of the relationship of heat to bearing life

and have designed formulas to accurately calculate safe operating temperatures. The

results show a temperature band in which both bearings and lubricants will operate at

peak performance with the least stress. Once outside the ideal temperature range, they

will degrade at an accelerated rate. So how do you interpret temperature readings, and

how should they affect your maintenance procedures? This article describes some

temperature-oriented methods for determining bearing health and life expectancy, both in

the plant and in the field.

Figure 1. Heat Ranges of Bearings1

Figure 1 shows the thermal range of a typical rolling element bearing. Note that bearing

metal temperature is often higher (10 to 25 degrees Celsius) than the oil temperature in

the bearing within an oil circulation system. The green zone represents the sweet spot for

bearing and lubrication temperature; operating in the yellow zone reduces lubricant and

bearing life; and if your bearings are in the red zone, expect both the bearing and the

lubricant to be destroyed rapidly. There are different temperature bands for different

combinations of bearing and lubricant, but they will have the same general trend

regarding the best operating temperature and its effect on accelerated wear and failure. In

most standard lubricants, for every 15°C1 increase in temperature above 70°C, the

lubricant life is more than halved and there is a negative effect on bearing life. Any

mineral oil operating at a temperature above 80°C or 90°C will have a greatly diminished

life. In no case should bearing temperature ever exceed the maximum rating of either the

bearing or the lubricant.

Monitoring Bearing Condition

Machine bearings are generally monitored through vibration analysis, oil analysis and/or

ultrasound techniques. Through these, it is possible to compare current data to historical

data and accurately assess the life of the bearings. Temperature increase due to increased

friction is not even considered as a symptom of bearing failure in many bearing analysis

texts2 until Stage 3 bearing failure occurs.

If temperature is a reliable method of bearing life prediction, why is it ignored until after

it's too late? The monitoring of temperatures with thermography has been considered

unreliable because so many variables such as ambient temperature, speed, load and

runtime all have a pronounced influence on bearing temperature. This article compares

identical or near-identical bearings on the same shaft which cancels the effects of these

variables because they are common to both bearings. The remaining temperature

differences between the two bearings on a common shaft with the same load, can only be

the result of friction, an indicator of bearing problems.

Perhaps subtle changes are hidden because there are so many variables that can

potentially contribute to bearing temperature. In addition to friction, other factors that can

contribute to temperature variation are load, speed, ambient environment temperature and

runtime duration. If these conditions could be predicted and accounted for accurately,

then increases in temperature would reliably indicate bearing problems.

In most cases vibration analysis and oil analysis are still the best ways to determine

bearing health. Unfortunately, it's not always possible or affordable to use these methods

in hostile production environments. Any environments where staff or technicians cannot

easily access the machine without taking it offline, or cannot access the machine due to

hazardous conditions or inconvenient locations make vibration or oil analysis expensive

at best, or even impossible. There are many industries and production environments

where bearing failure represents catastrophic loss, yet vibration analysis is not practical.

Photographic film and paper manufacturing, chemical processing and metalworking

plants are a few examples of industries that depend greatly on bearings, but where

bearing accessibility can be a major problem. Most manufacturers have at least some vital

equipment in areas that are not easily accessible.

Infrared Sensors

Outside of manufacturing, other industries are similarly affected. Wheel bearings on

railroad cars are a specific case in point: they are underneath trains, which are not

hospitable places for performing these tests while the cars are moving. Secondly, there is

no practical method of checking all such wheel bearings in a timely manner because of

the sheer number of them in use. Currently in the railroad industry, the only practical

method for detecting impending bearing failure is to use so-called hotbox detectors.

These are infrared sensors located along the train tracks that detect high temperatures in

wheel bearings as the train passes. These are extremely limited and can detect bearings

only in Stage 4 failure where catastrophic failure (a wheel burn-off) is imminent. An

alarm requires immediate emergency action, disrupts train schedules, and is extremely

costly. When these detectors miss bad bearings or the alarm is not tended to immediately,

the resulting wheel burn-off may lead to a train derailment, widespread hazardous

chemical spill, loss or severe damage of client merchandise, or loss of human life.

Because the consequences are so dire, every false positive results in delays and

inspections that can negatively affect the entire industry.

Studying Temperatures

If bearing temperature changes due to maintenance-related problems could be isolated

from all the other factors that contribute to bearing heat, a properly designed monitoring

device could detect bearing failure at the Stage 2 level. A newly patented technique is

being evaluated that could have significant benefit to industries that need to analyze

bearing and lubrication life in difficult-to-reach areas, such as under train cars. The heart

of the patent is a technique that cancels all thermal variables except the increase in

bearing temperature due to wear or lubrication failure.

The procedure takes heat data from each bearing on a common shaft and compares the

data. Because the load, speed, ambient temperature, and run duration are common to all

the bearings common to the shaft, their effects on temperature are canceled. Any

recorded temperature variation is the result of unwanted maintenance- or repair-related

conditions such as over- or underlubrication, bearing damage, misalignment, or loose-

foot condition. If one bearing is more than 15°C greater than another on the same shaft,

the bearing health is in question and the root cause of the increased bearing temperature

must be determined. The bearing comparison is accomplished with electronic

temperature sensors and comparators powered by a self-contained power supply that

recharges its battery through the motion of the equipment.

The methodology involves the following:

•

• Temperature sensors are attached in close proximity to all the bearings on a

common shaft or axle.

• The sensors apply input to a sensing unit that is self-contained and has wireless

technology for communication with warning devices.

• The temperature data of each unit is analyzed and compared electronically.

• If any bearing temperature varies more that 15°C, an alarm is transmitted and an

LED indicator will light.

• The system is self-powered by a small power supply which is actuated by the

movement of the equipment.

• There is a maximum allowable temperature in case all bearings are out of normal

operating range.

This technique will never be as accurate as oil or vibration analysis, but in remote or

hazardous locations where these tools are not an option, it will provide an increased level

of condition monitoring that was not available in the past.

References 1. SKF bearing manual.

2. Entek IRD Volume I Vibration Analysis, p. 6-48.

Four Stages of Bearing Failure

Stage 1. Earliest detectable indication of bearing failure using vibration analysis. Signals

appear in the ultrasonic frequency bands around 250 KHz to 350 KHz. At this point,

there is approximately 10 to 20 percent remaining bearing life.

Stage 2. Bearing failure begins to "ring" at its natural frequency, (500 to 2,000 Hz) signal

appears at the first harmonic bearing frequency. Five to 10 percent remaining bearing

life.

Stage 3. Bearing failure harmonics of the fundamental frequency are now apparent.

Defects in the inner and outer race are now apparent and visible on vibration analysis of

the noise signal. Temperature increase is now apparent. One to five percent of remaining

bearing life.

Stage 4. Bearing failure is indicated by high vibration. The fundamental and harmonics

begin to actually decrease, random ultrasonic noise greatly increases, temperatures

increase quickly. Remaining life one hour to one percent.

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