Bearing Life Calculation

Bearing Life Extension Practical Guidelines for Maintenance and Reliability Engineers

Summary Reliability and Maintenance Engineers are often given the task of improving the reliability of bearing arrangements in a variety of applications. Lubrication, contamination levels, and seals are key areas to explore for improvement opportunities. For this article, we use the New Life Method to review typical bearing applications. The New Life Method is the latest calculation technique for predicting the effects of lubrication and contamination on bearing life. Practices such as proper lubrication schedules, choosing correct lubricant viscosities and film thickness, identifying and removing contamination sources, and temperature control are advanced as potential strategies for bearing life extension.

Joe Conyers Reliability Maintenance Institute 12 pages May 2002 SKF Reliability Systems @ptitudeXchange 4141 Ruffin Road San Diego, CA 92123 United States tel. +1 858 244 2540 fax +1 858 244 2555 email: [email protected] Internet: www.aptitudexchange.com

Use of this document is governed by the terms and conditions contained in @ptitudeXchange.

Bearing Life Extension

Introduction p = exponent of the life equation Reliability and Maintenance Engineers are often given the task of improving the reliability of bearing arrangements in a variety of applications. Lubrication, contamination levels, and seals are key areas to explore for improvement opportunities. For this article, we use the New Life Method to review typical bearing applications. The New Life Method is the latest calculation technique for predicting the effects of lubrication and contamination on bearing life. Practices such as proper lubrication schedules, choosing correct lubricant viscosities and film thickness, identifying and removing contamination sources, and temperature control are advanced as potential strategies for bearing life extension. Moreover, it allows engineers to select cost-effective, practical solutions from available practices - without guesswork. This can be the first step in planning a strategy for implementing a successful program plant-wide.

p=3, for ball bearings

p=10/3 for roller bearings

n = speed, revolutions per minute

Two general conclusions we can draw from the Basic Life Equation are:

1. Bearing life varies inversely with speed (double the speed, life is reduced by half).

2. Bearing life varies exponentially with applied load ( P ) (double the load, life is reduced to 0.125 of its original value for ball bearings and more for roller bearings).

The effects of these conclusions can be dramatic operationally. Consider a conveyor belt system whose speed is increased by 25%. Perhaps we can accept a bearing life reduction of 25% due to the speed increase. Are there any other considerations? More material will be conveyed per unit time, which increases weight on the system. Loads on the bearings also increase. If sheaves are changed to achieve the speed increase, the V-belts may begin to slip, which are tightened to compensate. If the combined effect of these changes increases the overall load only 10%, life for the ball bearings in this application is reduced by an additional 25%. The overall loss of life may be unacceptable. Significant design changes may be required to offset life reduction from something that appears, at the outset, as a straightforward speed increase.

Basic Life Equation The basic method for determining bearing life (by Lundberg and Palmgren) is well known. Bearing life is a function of the applied load, versus the basic dynamic load rating.

p

PCL

=10

p

h PC

nL

=

60000,000,1

10

Adjusting the Basic Life Equation With: The advent of cleaner bearing steels in the late 1950s had a dramatic improvement on bearing life. Bearing manufacturing processes continued to improve, and the basic life equation was extended to reflect these improvements. The Adjusted Life Equation was the result:

10L = basic rating life, millions of revolutions

hL10 = basic rating life, hours of operation

C = basic dynamic load rating

P = equivalent dynamic load 2002 SKF Reliability Systems All Rights Reserved 2


p

na PCaaL

= 231

p

nah PCaa

nL

= 23160

000,000,1

with:

naL = adjusted rating life, revolutions

nahL = adjusted rating life, hours

1

1

a = reliability adjustment factor

23a = material / lubrication adjustment factor

The factor allows us to adjust the basic life rating equation for reliability greater than 90% (Table 1). This factor can be used to down-rate applications where safety is critical. Another use of the a

a

1 factor is high reliability applications. For example, bearings in aircraft turbine engines require extreme reliability. The weight penalty from enlarging bearings and support components to meet reliability needs is unacceptable. In this case, a more cost-effective decision is to use smaller

bearings and change them out at an operational life well below the predicted fatigue failure life to achieve an adequate safety margin.

Reliability naL 1a

90% aL10 1

95% aL5 0.62

96% aL4 0.53

97% aL3 0.44

98% aL2 0.33

99% aL1 0.21

Table 1. The factor Allows us to Adjust the Basic Life Rating equation for Reliability Greater Than 90%.

1a

The factors (material adjustment) and (operating conditions) are interdependent,

and are combined as . These adjustments make corrections for the effects of modern steels lubricant film thickness (Figure 1).

2a

3a

23a

2002 SKF Reliability Systems All Rights Reserved 3


Factor a23Use this chart to choose the value of a23from a known viscosity ratio, Kappa (k).

The darkened area under the curve represents better performance that may be achieved with the addition of EP additives to the lubricant.

Note: This chart applies only to standard 52100 bearing steels. Contact your manufacturer if using stainless, ceramics or other bearing materials.

Figure 1. The Factor as a Function of Viscosity and EP Additives.23a

In order to use factor (Figure 1), we must first determine the viscosity ratio, (kappa). Kappa is the ratio of actual viscosity ( v ) to

required viscosity ( ) of the application at the operating temperature. These viscosities can be calculated or selected from charts (Figures 2 and 3).

23ak

1

v



Figure 2. Minimum Required Oil Viscosity. Pitch Diameter dm is Defined as the Mean of Bore and Outer Diameter.



Figure 3. Viscosity-Temperature Chart. (Viscosity classification numbers are according to international standard ISO 3448-1975 for mineral oils with a viscosity index of 95. Approximate SAE viscosity grades are shown in parentheses.)



Applying the Adjusted Life Equation To illustrate the effects of the Adjusted Life Equation, we examine a 6210 ball bearing in an oil-lubricated application. (This is an example from the catalog reference [1].)

Example one: 6210 Single Row Deep Groove Ball Bearing

Bearing: 6210

Bearing Boundary Dimensions: 50mm bore x 90mm Outside Diameter x 20mm wide

Basic Dynamic Load Rating: 35,100 N

Operating Conditions:

Applied Load: 4,000 N Radial Load

Speed (n): 3,600 rpm

Bearing Operating Temp: 70C

hours

PC

nL

p

h

130,34000

100,35360060

000,000,1

60000,000,1

3

10

=

=

=

=

In this case, life can be increased slightly when the effects of oil film are included. To determine the adjusted rating life, we need the a1 and a23 factor. Assume no change in desired reliability above the standard 90%, so a1 = 1. To select a23, we need to know the required oil viscosity at the operating temperature of 70C. We use good quality, lightweight mineral oil (ISO VG32, VI 95). Figures 2 and 3 give the required oil viscosity for a 6210 bearing as 9 mm2/s. The actual viscosity of our ISO VG 32 oil at bearing operating temperature is about 11 mm2/s, which makes the viscosity ratio, kappa, ~1.2. Figure 1 gives us a resulting a23 value of 1.15. The expected life becomes:

hours

PCaa

nL

p

nah

600,34000

100,3515.11360060

000,000,1

60000,000,1

3

231

=

=

=

=

New Life Method In many cases, using the Adjusted Life Equation is a good starting design point. But some lightly loaded, clean applications have operational lives far in excess of the life predicted by the Adjusted Rating Life Equation. Contaminated applications are tough design problems (and extremely dirty applications defy accurate life prediction). The SKF New Life Method is an extension of the Adjusted Rating Life Equation and predicts bearing life much more accurately when lubrication and contamination conditions are better known. It allows designers and manufacturers to take advantage of controlled downsizing, exploit the enhanced life potential of modern bearings, and recognize the significance of contamination. A simplified equation that illustrates the relationship to the two ISO (or ABMA) life equations was derived:

p

skfnaa PCaaL

= 1

p

skfnaah PCaa

nL

= 160

000,000,1

with:

naaL = adjusted rating life, new life method, millions of revolutions

naahL = adjusted rating life, new life method, hours

skfa = SKF adjustment factor



General conclusions from Figure 4: The factor brings in two new concepts regarding bearing life:

skfa

1. The thickness of lubricant film has a significant role in bearing life. 1. There is a minimum load for every bearing

below which fatigue failure will not occur. In other words, the bearings stress levels are too low to produce subsurface cracking.

2. Oil film thickness greater than four times the required oil film thickness provides marginal benefit.

3. Reducing the contamination in applications greatly extends life. 2. The effects of solid particle contamination

on bearing life are quantifiable. The relationship is complex, but is simplified through the use of charts (Figure 4).

Determining a more precise value c requires a firm knowledge of application conditions. However, we can successfully examine the effect on life when changes are made to existing conditions.

Figure 4. Factor for Radial Ball Bearings. skfa

Using Tables to Explore the New Life Method Our life for the ball bearing application was relatively short (approximately three months, perhaps three and a half months using adjusted life). What can be done to extend the life of this application?



Figure (6): Guideline values for factor c for different degrees of contamination

Condition Values c 1)Very clean 1Debris size of the order of the lubricant film thicknessClean 0,8Conditions typical of bearings greased for life and sealedNormal 0,5Conditions typical of bearings greased for life and shieldedContaminated 0,5 ... 0,1Conditions typical of bearings without integral seals; coarselubricant filters and/or particle ingress from surroundingsHeavily contaminated 0(under extreme contamination values of c can be outside the scaleresulting in a more severe reduction of life than predicted by theequation for Lnaa )

1) The scale for c refers only to typical solid contaminants.Contamination by water or other fluids detrimental to bearing life is not considered.

Figure 5. Guideline Values.

One technique is to use a table of values. We can explore the effects of varying a single controllable factor at a time and observe the result on bearing life. This assists in selecting the best practical solution for extending life. Exploring alternatives is made easier with the use of calculation programs (Reference [2]). The user can easily change operating parameters such as bearing size, oil film thickness, operating temperature, and contamination conditions and record the results for a variety of cases. Let's explore the example case again.

Example 1: Ball Bearing

Example one: 6210 Single Row Deep Groove Ball Bearing

Bearing: 6210 Bearing Boundary Dimensions: 50mm bore x 90mm

Outside Diameter x 20mm wide

Basic Dynamic Load Rating: 35,100 N

Operating Conditions:

Applied Load: 4,000 N Radial LoadSpeed (n): 3,600 rpmBearing Operating Temperature: 70C

NOTE: We ignored secondary effects that may occur as a result of changing a single parameter. For example, increasing oil viscosity may result in an increase in friction and lower oil viscosity. For more accurate results, actual bearing temperatures can be estimated - contact your manufacturer.



Table I: Extending Life: 6210 Single Row, Deep Groove Ball Bearing

Case Conditions Temp. c (Kappa) askf

a23 L10h L10ah L10aah

No. (oC) Hours

1 Existing Dirty Application 70 0.2 (1.2)1.1

1.15 3130 3600 3410

2 Use Heavier Oil (VG 46) 70 0.2 (1.6)1.3

1.4 3130 4380 4060

3 Reduce Temperature 50 0.2 (2.4)1.6

1.8 3130 5630 5000

4 Increase Bearing Size to 6211 70 0.2 (1.3)1.5

1.2 6000 7200 9000

5 With Sealed Bearings 70 0.8 (1.2)6.8

1.15 3130 3600 21,300

Modified Values are indicated in bold

Table 2. Extending Bearing Life (Ball Bearing).

Analysis Sealing the bearings (Case 5) gives the most dramatic life improvement: about six times that predicted by the adjusted life calculation. Controlling the contaminants entering the bearing nets a huge improvement in overall life for very little cost. Secondary benefits with seals are controlled lubrication and prevention of over lubrication.

The other cases give significant life improvement, but may or may not be practical in the application. For example, increasing the oil viscosity (Case 2) may be precluded by other components in the application that require the original oil, such as a compressor. Reducing the temperature (Case 3) or redesign to accept a larger bearing (Case 4) may be cost

prohibitive. Adding cost analysis to the tables reveals your best choice.

Example 2: Roller Bearing

Example Two: Paper Machine Drying Cylinder

Bearing: 22244 Bearing Boundary Dimensions: 220mm bore x 400mm

Outside Diameter x 108mm wide

Basic Dynamic Load Rating: 1,760,000 N (Ref. (3))

Operating Conditions:Applied Load: 200,000 N Radial LoadSpeed (n): 250 rpmBearing Operating Temperature: 110C



Table II: Extending Life: 22244 CCK Spherical Roller Bearing

Case Conditions Temp. c (Kappa) askf

a23 L10h L10ah L10aah

No. (oC) hours

1 Existing Application 110 0.2 (0.6)0.3

0.4 93800 35000 28000

2 Filter to Improve Contamination

110 0.74 (0.6)0.8

0.4 93800 35000 75000

3 Use Heavier Oil (VG 320) 110 0.2 (0.7)0.4

0.6 93800 53200 37500

4 Add Lube with EP additives 110 0.2 (0.6)2.2*0.3

(doubtful because c < 0.5 )

0.8 93800 73400 62000

5 Reduce Temperature 90 0.2 (1.0)0.7

1.0 93800 94000 66000

Modified Values are indicated in bold

Table 3. Extending Bearing Life (Spherical Roller Bearing).

Analysis This applications adjusted rated life is severely degraded by the lack of adequate oil film thickness. The basic rated life of 10.7 years is reduced to just 4 years due to poor oil film thickness (and resultant low kappa values). Generally, SKF recommends hc values between 0.1 and 0.3 for paper mill applications. Readjusting with the New Life Method gets us back to 3.2 years, quite low for a papermaking machine.

Reducing the application temperature (Case 5) might be accomplished by increasing oil flow rates. This may be impractical, as the manufacturer generally optimizes drying cylinder oil flow rates.

Changing to oils with EP additives (Case 4) must be undertaken with extreme care. Some EP additives used in applications over 80oC can have a detrimental effect on bearing steels. They may chemically react with the steel, which can cause surface distress and premature failure. Also for low hc values SKF does not recommend to increase askf because of the additives (See SKF General Catalogue).

Heavier oils (Case 3) could be considered. A thorough analysis (contact your manufacturer) should be made of the operating temperature change that may occur when thicker oil is used. The temperature increase may offset the beneficial effect of the thicker oil on kappa values and askf.



Filtration (Case 2) is a good area to explore for improvement. This case introduced a 12-micron absolute filter with filter efficiency 200 to improve the c value to 0.74, which resulted in a life improvement to 8.6 years. For new paper machines, a 6-micron filter with efficiency 200 is recommended by SKF. Exploring different filtration cases may help you choose the most cost effective strategy.

Additional Strategies You can extend the value of these tables by factoring in the cost of applying the solutions and the expected return on your investment. The objective is to ensure the practicality of the applied solutions to extend life:

1. Cleaner mounting processes. For example, simply putting up plastic barriers or moving the bearing assembly area to a cleaner atmosphere

2. Grease and oil quality sampling as part of your supplier acceptance process

3. Oil Pre-filtering

4. Cleaning and flushing new applications after run-in. Portable filter carts are also available for temporary use during the run-in period.

5. Consider portable and fixed water removal systems. The New Life Method does not account for contamination other than solid particles. The effects of water and other chemical contaminants must be addressed to achieve acceptable life extension using the New Life Method. Anecdotal evidence reveals that as little as 0.1% water in oil (approximately one teaspoon water in one gallon of oil) can reduce the effective viscosity of oils by up to 50%. Strive for 200-500 ppm water in your applications.

After applying your solution, take time to verify your results. Lubrication condition monitoring can be prime sources for data. Particle counting, ferrography, and

spectroscopy can determine the size, number and composition of contamination particles before and after you implement your improvement strategy. Collect failure statistics. Examine failed bearings for root cause and correlate the results with your life improvement tactics.

Conclusion The New Life Method can be used to explore practically deployable strategies to successfully extend bearing life in rotating mechanical equipment. Using case analysis tables with the New Life Method allows us to choose the most cost-effective course of action to extend bearing life.

References [1] SKF USA, Catalog 4000 US Third Edition, 1999-01.

[2] SKF Interactive Engineering Catalogue

[3] SKF Publication 4401/I E, SKF spherical roller bearings.

Acknowledgements Dan Snyder, Director, SKF USA Applications Engineering

Mark Cutler, North American Engineering Manager, SKF USA Industrial Division

About RMI The Reliability Maintenance Institute (RMI) is a comprehensive offering of training courses designed to help eliminate machinery problems and achieve maximum reliability and productivity. When you attend an RMI class, you learn about the latest in precision maintenance techniques, skills, and technologies.

Contact RMI: http://www.skfusa.com/rmi Toll free +1 866 753 7378 (US only) or +1 717 646 2900


IntroductionBasic Life EquationAdjusting the Basic Life EquationApplying the Adjusted Life EquationNew Life MethodUsing Tables to Explore the New Life MethodExample 1: Ball BearingAnalysisExample 2: Roller BearingAnalysis

Additional StrategiesConclusionReferencesAcknowledgementsAbout RMI

Bearing Life Calculation

Documents

Transcript of Bearing Life Calculation