Extend Bearing Life

Extend Baldor• Dodge Bearing Life by Evaluating Common Failure Modes

Anti-friction bearings are designed to meet a desired L10 life, which is the estimated length of time that 90% of a given group of bearings will successfully operate before failure.

This equation assumes that a bearing will operate within a specific operating speed range and be properly maintained.

However, anti-friction bearings rarely achieve the predicted L10 life, primarily due to improper preventative maintenance practices.

When maintained properly, bearing life is extended and production downtime is minimized.

80% of bearing failures can be attributed to ineffective lubrication, contamination, or a combination of the two.

Click here to Read More on Common Bearing Failures

Ineffective Lubrication

Lubrication is essential for bearings because it prevents metal to metal contact between rolling elements and raceways.

55% of all bearing failures are attributed to ineffective lubrication, including insufficient, unsuitable, or old lubrication.

If fresh grease is not added to the bearing on a regular basis, then the bearing life is limited to the service life of the grease. Without a proper protective film barrier, the bearing will wear, overheat, and fail. 

Many facilities, even those with a preventative maintenance program, fail to perform routine or consistent bearing lubrication. Some plants over grease, but the biggest culprit is a lack of lubrication. 

Lubricant life is effected by speed, load, and environmental influences such as temperature, humidity, and contamination.

When the lubricant breaks down the bearing will fail.

Choosing the wrong lubricant is another cause for premature failure.

Insufficient lubrication failure mode is most commonly evidenced by severe cage and roller damage within a dry bearing cavity.

Too often, users lubricate bearings with the most readily available grease gun from the storage room, focusing little attention on the specific type of grease or the amount of grease that it releases.

Problems arise because there are a variety of grease formulations, and not all of them are compatible. A high quality grease may actually do damage to a bearing if an incompatible grease is already present in the bearing cavity.

Using grease with the incorrect consistency or base may also harm a bearing:

• Grease with a soft consistency (low NLGI ranking) may not be ideal for high speed applications.
• Grease with a harder consistency (high NLGI ranking) may not be ideal for slow speed applications.

The viscosity of the lubrication plays one of the most important roles in the selection of lubricants.

Viscosity is defined as the property of a fluid that resists internal flow by releasing counteracting forces.

The viscosity of a lubricant is generally measured at 40 degrees Celsius in CentiStokes (cSt) or equivalent mm2 /s units.

In most applications a middle of the road viscosity of roughly 100 cSt at 40 degrees Celsius will suffice. In cases of extreme speed or temperature, either high or low, it becomes critical to select a lubricant with the proper viscosity.

Below are the steps needed to determine the required viscosity of a lubrication:

1. Identify the Type of Bearing i.e.: Ball or Roller

2. Calculate the Mean Bearing Dia.: dm = (D+d)/2

3. Determine the Operating RPM(n)

4. Determine the Operating Temperature in Celsius

5. Determine the Kappa Value: K = Operating viscosity(v)/Rated Viscosity(v1)

 K = 1 For Ball Bearings

 K = 2 For Spherical and Tapered Roller Bearings

 K = 2.5 For Slow Turning Applications or when Ndm < 10,000

Once the above information is known the required viscosity of a lubricant can be determined using the following diagrams.

Fan Application Example:

A 22220 Spherical Roller Bearing rotating at 2,000 RPM has an operating temperature of 90° Celsius.

• Step #1 Identify the Type of Bearing: 22220 Spherical Roller Bearing

• Step #2 Calculate the Mean Bearing Diameter:

             dm = (D+d)/2 Where D= O.D. of bearing in mm

             dm = (180 + 100)/2 d= I.D. of bearing in mm

             dm = 140mm

• Step # 3 & 4 Determine Operating RPM and Temperature

             Given: Operating RPM (n) = 2000

             Temperature = 90° C

• Step 5 Determine the Kappa Value (K)

             Given: Spherical Roller Bearing

             Therefore K = 2

Click here for more info: ABB Baldor Whitepaper "Oil Viscosity Selection"

Bearing Contamination

Contamination includes the ingress of either solid particles or fluids into the bearing cavity.

In all, contamination accounts for nearly 25% of premature bearing failures.

Environmental factors have a major influence on contamination.

Bearings mounted in dirty, dusty, or moist environments are at highest risk of contamination failures.

Sealing systems help, but even the most effective bearing seals will not completely protect a bearing from contamination.

Solid Particle Contamination is the most common contamination failure mode. Solid particles can enter a bearing cavity three ways:

1. Particles can bypass the sealing area. SOLUTION: Decrease the odds of contamination by increasing lubrication frequency; a fresh flow of grease away from the bearing helps prevent contaminants from entering the bearing cavity while forcing out any contaminants which may be present. You can also purchase bearings with multiple contacting lip seals.

2. Contaminants can enter during lubrication by using contaminated lubrication ports and tools. SOLUTION: Cleaning grease nipples and grease gun heads help prevent the manual addition of harmful contaminants to the bearing cavity.

3. Contamination can occur during installation. Handling naked bearings in dirty environments or installing them with dirty tools and hardware increases the potential for contamination failures. SOLUTION: Best bearing installation practices include using clean tools and, when possible, installing in a clean area. If clean areas are not practical for the application then consider installing pre-sealed or unitized bearing cartridges.

Fluid Contamination is caused by moisture accumulation in the bearing cavity. Fluids from muddy or moisture laden environments can bypass most bearing seals. 

Additionally, as bearings pump air through the seals, humid air combined with temperature changes will deposit condensation within the bearing cavity.

The best strategy to combat moisture contamination is frequent lubrication intervals.

If the fluid contamination is water, lubricate regularly with grease with emulsifying properties, such as a lithium based grease. Emulsifying greases will absorb the water rather than repel it.

Bearings are most often damaged by fluid contamination during shutdown periods.

Without emulsifying grease, the water would stagnate on the raceways. During these periods, moisture can puddle on raceways resulting in corrosion or moisture stains.

Lubricating prior to shut down will drive out the moisture and provide fresh grease to critical areas.

SUMMARY

Ineffective lubrication and contamination account for 80% of premature bearing failures.

In other words, 80% of downtime associated with bearing failures can be prevented by proper preventative maintenance techniques.

Adding regular lubrication schedules to the preventative maintenance plan, and addressing potential contamination issues will extend bearing life and prevent costly downtime preventative.

CONTENT CREDIT: ABB Baldor Dodge Whitepaper "Extend Baldor• Dodge Bearing Life by Evaluating Common Failure Modes"